SELECTION OF SPREAD FOOTINGS ON SOILS
 

TO SUPPORT HIGHWAY BRIDGE STRUCTURES
 

Publication No. FHWA-RC/TD-10-001 February 2010

Revised December 2017
Change Log – November 2017

Selection of Spread Footings on Soils to Support Highway Bridge Structures

Publication No. FHWA-RC/TD-10-001

Page 1 of 1

Global change

The following global change was made to make symbols (notations) consistent with those in

AASHTO LRFD Bridge Design Specifications.

1. Use of symbol “S” in lieu of “ ” when vertical movement (i.e., settlement) is discussed.

Specific changes

1. In Table of Contents, updated the title of Section 3.3 to make it consistent with the change

noted in Item 4a below.

2. Updated List of Symbols and Abbreviations in the Table of Contents to reflect the change in

symbols noted herein.

3. Section 3.1, page 14, first bullet item

a. Modified paragraph to explain relationship between “Accuracy” and “Bias.” Included a

new reference to Tan and Duncan (1991).

4. Section 3.3

a. Included “THE S-0 CONCEPT OF” at the start of the title for this section.

b. Modified Figure 3-2 based on the global change noted above.

c. Modified Figure 3-3 to show Mode 1 and Mode 2 patterns.

d. Deleted Table 3-1 and included it as part of Figure 3-3.

e. Modified write-up related to Figure 3-2 and Figure 3-3.

f. Renumbered Table 3-2 as Table 3-1 and made changes at all locations in the document

where Table 3-2 was previously referenced.

5. Section 4.1

a. Changed symbols in Figure 4-1, updated caption, and adjusted first paragraph to be

consistent with revised notations.

b. Modified Figure 4-3 to show Mode 1 and Mode 2 to be consistent with the modified

Figure 3-3. Modified write-up in second full paragraph of page 23 to reflect modification

of Figure 4-3.

c. Repaginated other text in this section due to Item 5a.

6. Section 4.2

a. In Item 4 on page 27, referenced information in Appendix C.3.3.3 to better define

structural fill.

7. Appendix E

a. Updated the limiting eccentricity criteria in Decision 2 box in Figure E.3-1, from “Is eBSTR < Bf-SER/4?” to “Is eB-STR < Bf-SER/3?” to reflect the latest AASHTO LRFD provisions.

b. Made the above limiting eccentricity criteria update in Step 2 of the example problem in

Section E.4.1.

c. In Figure E.4-2, the symbol S2 was used twice. Corrected the second occurrence to S3.

8. Bibliography

a. On page 33, corrected the title and report number for FHWA (1997) reference.

b. On page 38, added reference of Tan and Duncan (1991) that was introduced in Item 3a.
 

 

 

 

 

FOREWORD
 

The FHWA believes that spread footings on soils are underutilized because designers encounter 

one or more of the following obstacles: (a) limited knowledge of AASHTO/FHWA technical 

references that pertain to spread footings on soils to support bridges; (b) limited knowledge of 

adequate performance data for spread footings; (c) unrealistic tolerable settlement criteria; (d) 

overestimation of loads used to calculate settlement; and (e) the use of conservative settlement 

prediction methods. These obstacles have resulted in institutional biases and overly conservative 

designs that lead to the unnecessary use of costlier deep foundation systems. The primary goal of 

this report is to promote the use of spread footings bearing on competent natural soils, improved 

soils, and engineered fill materials as a routine alternative to deep foundations for support of 

highway structures by addressing the factors identified above. The introduction of Load and 

Resistance Factor Design (LRFD) permits a rational approach to the consideration of spread 

footings on soils as a viable alternative to deep foundations. Using documented performance 

data, the report presents powerful concepts such as the use of construction-point analysis and 

angular distortions to demonstrate the efficacy of using spread footings. The report presents 

sources of information that agencies and designers can use as references in their project 

applications. It is hoped that this report will provide the impetus for agencies and transportation 

designers to evaluate the use of spread footings for all conditions other than those where their use 

is precluded (e.g., scour conditions). 

Scott A. Anderson, PhD, PE 

Federal Highway Administration, Resource Center 

Geotechnical & Hydraulics Technical Services Team Leader 

Notice 

This document is disseminated under the sponsorship of the U.S. Department of Transportation 

in the interest of information exchange. The U.S. Government assumes no liability for the use of 

the information contained in this document. The contents of this report reflect the views of the 

authors, who are responsible for the facts and accuracy of the data presented herein. The 

contents do not necessarily reflect policy of the U.S. Department of Transportation. This report 

does not constitute a standard, specification, or regulation. 

The U.S. Government does not endorse products or manufacturers. Trademarks or 

manufacturers’ names appear in the document only because they are considered essential to the 

objective of the document. 

Quality Assurance Statement 

The Federal Highway Administration (FHWA) provides high-quality information to serve 

Government, industry, and the public in a manner that promotes public understanding. Standards 

and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its 

information. FHWA periodically reviews quality issues and adjusts its program and processes to 

ensure continuous quality improvement. 
 

 

 

 

 

 

 

 

 

 

 

Technical Report Documentation Page 

1. Report No.

 FHWA-RC/TD-10-001 

2. Government Accession No. 3. Recipient's Catalog No. 

4. Title and Subtitle

 Selection of Spread Footings on Soils to Support Highway Bridge 

Structures 

5. Report Date

 February 2010 

6. Performing Organization Code

 RD2009-01 

7. Author(s) 

Naresh C. Samtani, PE, PhD, Edward A Nowatzki, PE, PhD and 

Dennis R. Mertz, PE, PhD 

8. Performing Organization Report No. 

9. Performing Organization Name and Address

 NCS GeoResources, LLC 

640 West. Paseo Rio Grande

 Tucson, AZ 85737 

10. Work Unit No. (TRAIS) 

11. Contract or Grant No. 

DTFH-57-09-P-00030 

12. Sponsoring Agency Name and Address

 Federal Highway Administration

 Resource Center

 4749 Lincoln Mall Drive, Suite 600

 Matteson, IL, 60443 

13. Type of Report and Period Covered

 Final Report

 September 2009 – February 2010 

14. Sponsoring Agency Code

 HFTS-16.4 

15. Supplementary Notes 

Advisory Panel Members: Naser Abu-Hejleh, PhD, PE; Scott A. Anderson, PhD, PE; Michael T. Adams, PE;

 Silas Nichols, PE 

COTR: Scott A. Anderson, PhD, PE; Funded by the Resource Center Technology Deployment Program 

16. Abstract 

The FHWA believes that spread footings on soils are underutilized because designers encounter one or more 

of the following obstacles: (a) limited knowledge of AASHTO/FHWA technical references that pertain to 

spread footings on soils to support bridges; (b) limited knowledge of adequate performance data for spread 

footings; (c) unrealistic tolerable settlement criteria; (d) overestimation of loads used to calculate settlement; 

and (e) the use of conservative settlement prediction methods. These obstacles have resulted in institutional 

biases and overly conservative and excessively costly institutional processes that lead to the unnecessary use 

of costlier deep foundation systems. 

The primary goal of this report is to promote the use of spread footings bearing on competent natural soils, 

improved soils, and engineered fill materials as a routine alternative to deep foundations for support of 

bridges by addressing the factors identified above. Documented performance data is used to make this case. 

The report also presents powerful concepts such as construction-point analysis and angular distortions to 

demonstrate the efficacy of using spread footings. Implementation of these concepts requires only that 

conventional computations be taken one step further without any requirement for advanced computational 

skills. The report presents sources of information that agencies and designers can use as references in their 

project applications. The report contains comprehensive appendices that treat in detail many of the topics 

discussed in the report. For example, one such appendix provides an introduction to Load and Resistance 

Factor Design (LRFD) that permits a rational approach to the consideration of spread footings on soils as a 

feasible alternative to deep foundations. 

17. Key Words 

Spread Footings, FHWA, AASHTO, LRFD, 

Settlement, Angular Distortion, Limit States,

 Serviceability, Construction-point, Bearing

 Resistance Chart 

18. Distribution Statement

 No restriction. This document is available to the

 public from the sponsoring agency. 

19. Security Classif. (of this report) 

Unclassified 

20. Security Classif. (of this page) 

Unclassified 

21. No. of Pages 

98 

22. Price 

-

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized 
 

 

SI* (MODERN METRIC) CONVERSION FACTORS 

APPROXIMATE CONVERSIONS TO SI UNITS 

Symbol When You Know Multiply By To Find Symbol 

in 

ft 

yd 

mi 

in2 

ft2 

yd2 

ac 

mi2 

fl oz 

gal 

ft3 

yd3 

oz 

lb 

T 

°F 

fc 

fl 

lbf 

lbf/in2 

LENGTH 

inches 25.4 millimeters 

feet 0.305 meters 

yards 0.914 meters 

miles 1.61 kilometers 

AREA 

square inches 645.2 square millimeters 

square feet 0.093 square meters 

square yard 0.836 square meters 

acres 0.405 hectares 

square miles 2.59 square kilometers 

VOLUME 

fluid ounces 29.57 milliliters 

gallons 3.785 liters 

cubic feet 0.028 cubic meters 

cubic yards 0.765 cubic meters 

NOTE: volumes greater than 1000 L shall be shown in m3 

MASS ounces 28.35 grams 

pounds 0.454 kilograms 

short tons (2000 lb) 0.907 megagrams (or "metric ton") 

TEMPERATURE (exact degrees) 

Fahrenheit 5 (F-32)/9 Celsius 

or (F-32)/1.8 

ILLUMINATION 

foot-candles 10.76 Lux 

foot-Lamberts 3.426 candela/m2 

FORCE and PRESSURE or STRESS 

poundforce 4.45 newtons 

poundforce per square inch 6.89 kilopascals 

mm 

m 

m 

km 

mm2 

m2 

m2 

ha 

km2 

ml 

l 

m3 

m3 

g 

kg 

Mg (or "t") 

°C 

lx 

cd/m2 

N 

kPa 

APPROXIMATE CONVERSIONS FROM SI UNITS 

Symbol When You Know Multiply By To Find Symbol 

mm 

m 

m 

km 

mm2 

m2 

m2 

ha 

km2 

ml 

L 

m3 

m3 

g 

kg 

Mg (or "t") 

°C 

lx 

cd/m2 

N 

kPa 

LENGTH 

millimeters 0.039 inches 

meters 3.28 Feet 

meters 1.09 yards 

kilometers 0.621 miles 

AREA 

square millimeters 0.0016 square inches 

square meters 10.764 square feet 

square meters 1.195 square yards 

hectares 2.47 acres 

square kilometers 0.386 square miles 

VOLUME 

milliliters 0.034 fluid ounces 

liters 0.264 gallons 

cubic meters 35.314 cubic feet 

cubic meters 1.307 cubic yards 

MASS 

grams 0.035 ounces 

kilograms 2.202 pounds 

megagrams (or "metric ton") 1.103 short tons (2000 lb) 

TEMPERATURE (exact degrees) 

Celsius 1.8C+32 Fahrenheit 

ILLUMINATION 

lux 0.0929 foot-candles 

candela/m2 0.2919 foot-Lamberts 

FORCE and PRESSURE or STRESS 

newtons 0.225 poundforce 

kilopascals 0.145 poundforce per square inch 

in 

ft 

yd 

mi 

in2 

ft2 

yd2 

ac 

mi2 

fl oz 

gal 

ft3 

yd3 

oz 

lb 

T 

°F 

fc 

fl 

lbf 

lbf/in2 

ii 
 

 

 

 

TABLE OF CONTENTS
 

TABLE OF CONTENTS 

CHAPTER 1 – INTRODUCTION ................................................................................... 1

 1.1 Goal ....................................................................................................................... 2
 

1.2 Scope of the Work ................................................................................................. 2
 

1.3 Past Studies and the Current Study ......................................................................... 2
 

1.4 Available Resources............................................................................................... 3

 1.5 Summary ............................................................................................................... 3
 

CHAPTER 2 –PERFORMANCE...................................................................................... 5
 

2.1 Reported Experience of State DOTs ....................................................................... 5
 

2.2 Documented Performance of Bridges .................................................................... 5
 

2.3 Characteristics of Load-Deformation Performance ................................................. 7
 

2.4 Cost Considerations................................................................................................ 10
 

2.4.1 Use of Deep Foundations where Spread Footings are Technically Viable ... 11

 2.5 Summary ............................................................................................................... 11
 

CHAPTER 3 – EVALUATION OF TOLERABLE SETTLEMENTS FOR
 

HIGHWAY BRIDGE DESIGN ................................................................................... 13

 3.1 Settlement Prediction Methods ............................................................................. 13
 

3.2	 FHWA Recommended Methods for Calculating Settlements ................................ 14
 

3.3 The S-0 Concept for Evaluation of Calculated Settlements in the Context of
 

Bridge Design ...................................................................................................... 15
 

3.3.1 Tolerable Angular Distortions ..................................................................... 17
 

3.3.2 Footings Proportioned for Equal Settlement................................................ 18

 3.4 Summary ............................................................................................................... 19
 

CHAPTER 4 – LOAD CONSIDERATIONS IN EVALUATION OF 21 SETTLEMENTS ..........................................................................................................
 

4.1 Construction-point Concept ................................................................................... 21
 

4.2 Other Considerations ............................................................................................. 25

 4.3 Summary................................................................................................................ 27
 

CHAPTER 5 – LRFD AND SPREAD FOOTINGS .......................................................... 29
 

5.1 Analysis of Spread Footings using the LRFD Approach......................................... 29
 

5.2 Opportunity For New LRFD-Based Guidance ........................................................ 30
 

5.3 Summary .............................................................................................................. 30
 

CHAPTER 6 – SUMMARY AND CONCLUSIONS ........................................................ 31
 

BIBLIOGRAPHY .............................................................................................................. 33
 

APPENDIX A – AVAILABLE RESOURCES .................................................................. 39
 

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APPENDIX B – FIELD PERFORMANCE DATA FOR LARGE SIZE SPREAD
 

FOOTINGS ON SOILS........................................................................... 41
 

APPENDIX C – GENERAL DESIGN CONSIDERATIONS FOR SPREAD
 

FOOTINGS ................................................................................................................... 45

 C.1 Terminology – General Types of Spread Footings.................................................. 45
 

C.2 Use of Spread Footings for Piers, Abutments and Retaining Walls ........................ 48
 

C.2.1 Use of Spread Footing Abutments on Top of MSE Walls ........................... 49
 

C.2.2	 Use of Spread Footings for Integral Abutments .......................................... 50
 

C.2.3 Quality Control and Drainage ..................................................................... 51
 

C.3 Feasibility Evaluation ............................................................................................ 52
 

C.3.1 Unfavorable Situations ............................................................................... 52
 

C.3.2	 Subsurface Investigations ........................................................................... 53
 

C.3.3 Categories for Use of Spread Footings on Soils for Highway Bridges. ........ 53
 

C.3.3.1 Spread Footings on Competent Native Soils................................... 54
 

C.3.3.2 Spread Footings on Improved In-Situ Soils .................................... 54
 

C.3.3.3 Spread Footings on Compacted Engineered Embankment Fill
 

Materials........................................................................................ 55
 

C.4 Horizontal Deformations........................................................................................ 58
 

C.4.1 Horizontal Deformations Due to Lateral Loads ........................................... 58
 

C.4.2	 Horizontal Deformations Due to Lateral Squeeze ....................................... 59
 

APPENDIX D – CONSTRUCTION INSPECTION, MONITORING AND
 

INSTRUMENTATION ................................................................................................ 61
 

D.1 Structural Fill Materials.......................................................................................... 61

 D.2 Monitoring ............................................................................................................ 61
 

D.3 Instrumentation...................................................................................................... 63
 

APPENDIX E – LRFD GUIDANCE FOR SPREAD FOOTINGS .................................. 67

 E.1 Limit States and Spread Footings ........................................................................... 67
 

E.2 Spread Footing Design Based on Geotechnical Considerations ............................. 69
 

E.3 Spread Footing Design Using LRFD Methodology ................................................ 73
 

E.4 Numerical Example With Detailed Calculations .................................................... 76
 

E.4.1 Step-by-Step Procedure for Sizing a Spread Footing at Service and
 

Strength Limit States.................................................................................. 79
 

E.4.2 Construction-Point Analysis ....................................................................... 81
 

E.5 Spread Footing Design Based on Structural Considerations ................................... 83
 

E.5.1 Contact Pressure Distributions .................................................................... 83
 

E.5.2	 Maximum and Minimum Load Factors....................................................... 83
 

E.5.3 Flexural and Shear Resistance of Footings .................................................. 84
 

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LIST OF FIGURES 

Figure 2-1. Schematic. Typical axial load versus vertical displacement curve (Q-) 

for spread footing of width Bf. ................................................................... 8 

Figure 3-1. Graph. Comparison of measured and calculated settlements based on 

data in FHWA (1987). ............................................................................... 13 

Figure 3-2. Schematic. Concept of settlement and angular distortion in bridges............ 16 

Figure 3-3. Schematic. Estimation of maximum angular distortion in bridges............... 17 

Figure 4-1. Schematic. Construction-point concept for a bridge pier. (a) 

Identification of critical construction points, (b) Conceptual loaddisplacement pattern for a given footing width. .......................................... 22 

Figure 4-2. Figure. Examples of data from measurements of contact pressure and 

settlement at various stages of bridge construction (Sargand, et al., 1999) 

[100 kPa = 1 tsf; 25.4 mm = 1 inch]. [Note: Figure (a) is for a pier, and 

(b) is for an abutment]. ............................................................................... 24 

Figure 4-3. Schematic. Angular distortion in bridges based on construction-point 

concept. ..................................................................................................... 24 

Figure C.1-1. Schematic. Geometry of a typical spread footing foundation (AASHTO 

2002) ........................................................................................................ 45 

Figure C.1-2. Schematic. Isolated spread footing ............................................................ 47 

Figure C.1-3. Schematic. Continuous strip footing ......................................................... 47 

Figure C.1-4. Schematic. Combined footing. .................................................................. 47 

Figure C.2-1. Schematics. (a) Continuous footing for a cantilever stemwall, (b) 

Combined footing for spill-through abutment, (c) Continuous footing for 

perched abutment, (d) Continuous footing for a cantilever retaining wall. .. 48 

Figure C.2-2. Schematic. “True” bridge abutment with continuous footing on top of a 

mechanically stabilized earth (MSE) wall. ................................................ 49 

Figure C.2-3. Schematic. “Diaphragm-on-Footing” type integral abutment on spread 

footing in an end slope used by Nevada Department of Transportation. ..... 50 

Figure C.3-1. Figure. Computation of vertical stresses beneath centerline of bridge 

piers and earth embankments (NCHRP, 1971). ......................................... 55 

Figure C.3-2. Schematic. Suggested approach embankment details for spread footing 

application (FHWA, 2006). ........................................................................ 57 

Figure C.4-1. Schematic. Horizontal deformations at the superstructure level due to 

settlement and rotation of spread footings for the case of soft layers 

within DOSI. (a) at piers, (b) at abutments. ................................................ 59 

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TABLE OF CONTENTS
 

Figure D-1. Schematic. Typical instrumentation setup for a spread footing 

foundation (Sargand and Masada, 2006). ................................................... 64 

Figure E.1-1. Schematic. Strength limit states for spread footings (a) Sliding, (b) 

Limiting eccentricity, (c) Bearing resistance .............................................. 68 

Figure E.1-2. Schematic. Service limit states for spread footings (a) Settlement, (b) 

Overall stability ......................................................................................... 68 

Figure E.2-1. Schematic. Bearing resistance chart showing strength limit state and 

service limit state criteria as a function of effective footing width. ............. 70 

Figure E.2-2. Schematic. Axial load versus vertical displacement curve for a spread 

footing having a given effective width B'f showing significant points of 

interest with respect to the LRFD approach. ............................................... 71 

Figure E.2-3. Schematic. Alternative bearing resistance chart in the format of stresssettlement curves for a range of effective footing widths and settlements. .. 72 

Figure E.3-1. Flowchart. Design process for footings using LRFD approach. .................. 74 

Figure E.4-1. Graph. Example of a factored bearing resistance chart for a footing of 

length, L′f = Lf = 150-ft (no eccentricity) and depth of embedment, Df = 

6-ft with base elevation of 994-ft. The resistance factor of b= 0.45 is 

included in the strength limit state curve. For the service limit state b= 

1.00 and it is included in the service limit state curves. Therefore all 

curves are for factored resistance. “S” in the legend refers to immediate 

settlement. ................................................................................................. 77 

Figure E.4-2. Graph. Example of a factored bearing resistance chart in terms of stresssettlement curves for a range of effective footing widths. ........................... 82 

vi 
 

 

 

 

 

 

TABLE OF CONTENTS
 

LIST OF TABLES
 

Table 2-1. Vertical displacement values for Points I and N in Figure 2-1 (after 

Akbas and Kulhawy, 2009). ...................................................................... 8
 

Table 2-2. Ratio of loads for Points I and N in Figure 2-1 (after Akbas and
 

Kulhawy, 2009). ........................................................................................ 8
 

Table 2-3. Range of typical /Bf for spread footings on soils for highway bridges. ...... 9
 

Table 3-1. Tolerable movement criteria for highway bridges (AASHTO 2002, 2007
 

with 2009 Interims) .................................................................................... 17
 

Table B-1. Data established by FHWA (1987). ............................................................ 42
 

Table B-2. Data compiled from literature in FHWA (1987). ........................................ 42
 

Table B-3. Data established by Sargand et al. (1999) and Sargand and Masada
 

(2006). ....................................................................................................... 43
 

Table B-4. Data established by Baus (1992). ................................................................ 44
 

Table B-5. Data established by FHWA (1997) [Same as by Briaud and Gibbens,
 

1999]. ......................................................................................................... 44
 

Table B-6. Data from FHWA archives. ........................................................................ 44
 

Table C.3-1. Typical specification for compacted structural fill material used by
 

WSDOT (FHWA 2006). . ........................................................................... 56
 

Table D-1. Inspector responsibilities for construction of spread footings (FHWA,
 

2006). ........................................................................................................ 62
 

Table E.4-1. Terminology and notation used in example problem ................................... 76
 

Table E.4-2. Pertinent parameters for example abutment footing (L'f=150-ft). ................ 78
 

Table E.4-3. Pertinent parameters for Service I Limit State related to significant
 

construction points for example abutment footing (L'f=150-ft).................... 78
 

Table E.4-4. Common significant construction points for an abutment footing................ 81
 

Table E.4-5. Summary of computations of settlements at significant construction
 

points for the example abutment footing. .................................................... 82
 

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TABLE OF CONTENTS
 

LIST OF SYMBOLS AND ABBREVIATIONS 

AASHTO American Association of State Highway and Transportation Officials 

ASCE American Society of Civil Engineers 

ASD Allowable Stress Design 

ASTM American Society for Testing and Materials 

A1 Calculated angular distortion in Span 1 

A2 Calculated angular distortion in Span 2 

A3 Calculated angular distortion in Span 3 

A4 Calculated angular distortion in Span 4 

A'f Effective footing area 

B Direction of lesser plan dimension of footing 

Bf Total footing width 

B'f Effective footing width 

CPT Cone Penetration Test 

DA1-1 Design angular distortion in Span 1 for Mode 1 

DA1-2 Design angular distortion in Span 1 for Mode 2 

DA2-1 Design angular distortion in Span 2 for Mode 1 

DA2-2 Design angular distortion in Span 2 for Mode 2 

DA3-1 Design angular distortion in Span 3 for Mode 1 

DA3-2 Design angular distortion in Span 3 for Mode 2 

DA4-1 Design angular distortion in Span 4 for Mode 1 

DA4-2 Design angular distortion in Span 4 for Mode 2 

Df Depth of footing base below finished grade 

DMT Dilatometer Test 

DOSI Depth of Significant Influence 

DOT Department of Transportation 

DS Design differential settlement within a given span 

DS1-1 Design differential settlement in Span 1 for Mode 1 

DS1-2 Design differential settlement in Span 1 for Mode 2 

DS2-1 Design differential settlement in Span 2 for Mode 1 

DS2-2 Design differential settlement in Span 2 for Mode 2 

DS3-1 Design differential settlement in Span 3 for Mode 1 

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TABLE OF CONTENTS
 

DS3-2 Design differential settlement in Span 3 for Mode 2 

DS4-1 Design differential settlement in Span 4 for Mode 1 

DS4-2 Design differential settlement in Span 4 for Mode 2 

e Eccentricity 

eB Eccentricity in B-direction 

eL Eccentricity in L-direction 

F Factored Load (Strength Limit State)
 

FHWA Federal Highway Administration
 

ft foot (feet)
 

ft2 square feet
 

GS Geotechnical Specialist 

in inches 

ksf kips per square foot 

kPa kilo Pascal 

L (in context) Distance between 2 points over which the angular distortion is calculated 

L (in context) Direction of greater plan dimension of footing 

Lf Total footing length 

L1 Length of Span 1 

L2 Length of Span 2 

L3 Length of Span 3 

L4 Length of Span 4 

LRFD Load and Resistance Factor Design 

M Moment 

MCFT Modified Compression Field Theory 

MSER Moment at Service Limit State I 

MSTR Moment at Strength Limit State I (max) 

m meter 

m2 square meters 

MPa Mega Pascal 

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MSE Mechanically Stabilized Earth 

NA Not Available 

NCHRP National Cooperative Highway Research Program 

NDOT Nevada Department of Transportation 

NIST National Institute of Standards and Technology 

N SPT N-value (uncorrected) 

N60 SPT N-value corrected for hammer efficiency 

Ncorr SPT N-value corrected for overburden pressure 

N160 SPT N-value corrected for overburden pressure and hammer efficiency 

OCR Over Consolidation Ratio 

ODOT Ohio Department of Transportation 

qmax Maximum contact pressure 

qnn Net nominal bearing resistance 

qteuv Net equivalent uniform vertical bearing resistance 

qR Factored net bearing resistance 

qteuv Total equivalent uniform vertical bearing resistance 

Q Estimated dead load 

QI Load at initial limit 

QF Load at factored limit 

QS Load at service limit 

QN Load at nominal limit 

S (in context) Settlement 

S (in context) Service Load (Service Limit State) 

S1 Calculated settlement at construction-point 1 

S2 Calculated settlement at construction-point 2 

S3 Calculated settlement at construction-point 3 

S4 Calculated settlement at construction-point 4 

SA1 Total settlement at Abutment 1 

SA2 Total settlement at Abutment 2 

SI Settlement at initial limit 

SF Settlement at factored limit 

Sm Measured settlement 

SN Settlement at nominal limit 

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TABLE OF CONTENTS
 

SP1 Total settlement at Pier 1 

SP2 Total settlement at Pier 2 

SP3 Total settlement at Pier 3 

SS Settlement at service limit 

Stol Tolerable Settlement 

SW Settlement under load W 

SX Settlement under load X 

SY Settlement under load Y 

SZ Settlement under load Z 

SE Load factor for SE Load type in AASHTO LRFD 

SPT Standard Penetration Test 

SS Structural Specialist 

TRB Transportation Research Board 

US United States 

V Vertical component of the resultant load 

VSER Vertical component of the resultant load at Service Limit State I 

VSTR Vertical component of the resultant load at Strength Limit State I (max) 

W Load after footing construction 

WSDOT Washington State Department of Transportation 

X Load after pier column/wall construction 

Y Load after superstructure construction 

Z Load after wearing surface construction 

p Load factor for permanent earth load 

s Unit weight of soil within the depth of footing embedment, Df 

 Calculated foundation movement 

b Resistance factor for bearing 

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ACKNOWLEDGMENTS 

The task of developing this document would have been significantly more demanding for the 

authors without the cooperation of several people whose help is acknowledged below. 

	 The contributions of the Federal Highway Administration (FHWA) advisory panel (Naser

Abu-Hejleh, PhD, PE; Scott A. Anderson, PhD, PE; Michael T. Adams, PE and Silas

Nichols, PE) are gratefully acknowledged.

	 The effort of Scott A. Anderson, PhD, PE, in preparing Appendix A and in the role of

FHWA’s Contracting Officer’s Technical Representative (COTR) is appreciated.

	 The authors wish to thank Naser Abu-Hejleh, PhD, PE, for his insights on the obstacles to

wider use of spread footings for highway bridge foundations and for contacting various state

DOTs to collect information about their practices related to the use of spread footings.

	 The willingness of Michael T. Adams, PE, in sharing relevant literature from his library is

appreciated.

	 The support provided by Bill Karras, PE, and Nathan Palmer, PE, of Parsons Brinckerhoff in

developing the data in Table E.4-3 in Appendix E is acknowledged.

	 The support of Amin Islam, PhD, PE, of the Bridge Group of the Arizona Department of

Transportation by providing some of the data in Appendix E is acknowledged.

	 Permission by John M. Kulicki, PhD, PE, of Modjeski & Masters, Inc., to synergistically

share concepts developed as part of on-going Project R19B of Strategic Highway Research

Program 2 (SHRP2) is appreciated.

 Finally, the authors wish to thank Jerry A. DiMaggio, PE, for his input and review of the

document as well as discussions regarding the subject matter.

xii 
 

 

 

 

 

CHAPTER 1 – INTRODUCTION
 

CHAPTER 1 – INTRODUCTION 

In general terms, foundations for bridges and their appurtenant structures such as retaining walls 

can be categorized as shallow or deep. The duty of a foundation design specialist is to establish 

the most economical design that safely satisfies prescribed structural criteria and properly 

accounts for the intended function of the structure. However, in reality the foundation specialist 

often chooses a deep foundation system because of a variety of reasons that include institutional 

culture and biases and/or lack of knowledge about rational methods for analysis and design. 

Based on the research of Department of Transportation (DOT) practices, it appears that the 

primary obstacles to the wider use of shallow foundations on soils for highway bridges are: 

1.	 Limited knowledge of documents from the Federal Highway Administration (FHWA) and 

the American Association of State Highway and Transportation Officials (AASHTO) that 

provide guidelines for the selection, and safe and economical design and construction of 

shallow foundations on soils to support bridges. 

2.	 Limited knowledge of adequate performance information of spread footings on soils from (a) 

documented results of axial load tests, and (b) case histories of the successful and economical 

use of spread footings on soils to support bridges. 

3.	 Use of conservative settlement prediction methods. 

4.	 Restrictive tolerable settlement criteria, which have led some agencies to set unrealistic 

performance criteria for foundations (e.g., zero or 1⁄4 inch of settlement). 

5.	 Overestimation of loads that generate settlement. 

6.	 Institutional culture and biases; for example, “It’s the way we have always done it.” 

The cumulative effect of these obstacles often results in the indiscriminate use of costlier and 

more complex deep foundations. FHWA (1997) estimated a potential cost savings to the public 

of approximately $135 million, if one assumes that only 25% of the approximately 600,000 

existing bridges in the United States were to use spread footings when they are replaced due to 

age or other reasons. If that amount is adjusted for inflation, the potential cost savings in terms 

of 2009 dollars would be more than $180 million. Because of this significant cost saving the 

FHWA is making efforts to ensure that spread footings are considered as a viable foundation 

alternative for all bridges. 

The introduction of the Load and Resistance Factor Design (LRFD) methodology permits a 

rational approach to the consideration of spread footings on soils as a viable alternative to deep 

foundations. Using documented performance data, this report presents powerful concepts, such 

as the use of construction-point analysis and angular distortions, to demonstrate the benefits of 

using spread footings. Implementation of these concepts requires only that conventional 

computations be taken one step further without any requirement for advanced computational 

skills. This report also presents sources of information that agencies and designers can use as 

references in their project applications. The hope is that this report will provide the impetus for 

agencies to evaluate the use of cost-effective spread footings for all conditions other than those 

where their use may be suspect or is clearly precluded (e.g., scour conditions). 

1
 
 

 

 

 

CHAPTER 1 – INTRODUCTION
 

1.1 GOAL 

The primary goal of this publication is to promote the mainstream use of spread footings on soils 

as a routine foundation alternative to deep foundations for the support of bridges and other 

highway structures. To attain this goal, the following chapters will demonstrate that spread 

footings may be an excellent foundation alternative for fast and low cost, low risk construction 

on competent native soils, improved in situ soils, and compacted engineered embankment fill 

materials. Spread footings are not always appropriate, and this publication can help one identify 

those situations as well. 

1.2 SCOPE OF THE WORK 

The scope of the work for the present study to attain the stated goal is as follows: 

 Provide performance data that demonstrate the successful use of spread footings, 

 Provide guidance for calculating settlements based on an appropriate loading sequence, 

 Provide references to resources that can be used to evaluate spread footings and overcome 

the factors listed above, and 

 Recommend mechanisms to analyze and design spread footings in the context of the LRFD 

methodology. 

The authors assume that the reader has a basic understanding and knowledge of the analysis and 

design of shallow foundation systems as well as the LRFD approach. The reader is referred to the 

bibliography chapter that contains an extensive list of references for additional information. 

1.3 PAST STUDIES AND THE CURRENT STUDY 

The topic of shallow foundations is not new and neither is the subject matter of this study. 

Several studies have been sponsored by the FHWA and other agencies such as the Transportation 

Research Board (TRB) through their National Cooperative Highway Research Program 

(NCHRP), the Departments of Transportation (DOT) of various states (e.g., Ohio, South 

Carolina), the National Institute of Standards and Technology (NIST), and the American Society 

of Civil Engineers (ASCE). Internationally, there are other similar studies. A list of the studies 

relevant to highway bridge structures is provided in the Bibliography of this report, which is 

organized according to primary and secondary sources. Primary sources include publications of 

the FHWA, TRB, NCHRP, AASHTO, ASTM, NIST, DOTs and various university 

transportation research organizations. Secondary sources include articles published in peerreviewed technical journals or in conference proceedings. 

This effort differs from past studies in that it is phrased in the context of the Load and Resistance 

Factor Design (LRFD) approach, which is now mandatory for the design of highway bridge 

structures that are built with Federal funds. The full benefit of the LRFD methodology will be 

realized in time through achieving better performance with less overall resources by ‘balancing’ 

the design for desired reliability and by improving communication between design specialists 

2
 
 

 

 

 

CHAPTER 1 – INTRODUCTION
 

working on different parts of the design. Part of the communication improvement will come 

through the use of a common language; one that relies on defined limit states and criteria for 

performance. As practice moves towards this goal, now is the time to reevaluate the 

performance of spread footings on soils and the process of foundation selection and design for 

highway bridge structures. This study provides guidance in an LRFD language common to both 

geotechnical and structural specialists. 

1.4 AVAILABLE RESOURCES 

Limited knowledge of available resources was identified earlier as the first obstacle to greater 

use of spread footings on soils. The FHWA has a reference manual (FHWA, 2001), a 

Geotechnical Engineering Circular (FHWA, 2002), and several research reports and training 

courses that provide guidance on the appropriate use of spread footings on soils. The primary 

resources are listed in the bibliography. However, most of these were developed based on the 

formerly used Allowable Stress Design (ASD) methodology. The NHI LRFD Substructures 

Course and Reference Manual (FHWA, 2005), as well as the manual for mechanically stabilized 

earth (MSE) walls (FHWA, 2009) provide guidance for shallow foundations in the context of the 

LRFD methodology. The AASHTO LRFD specification (AASHTO, 2007 with 2009 Interims) 

itself provides valuable commentary on the use of spread footings on soils. Appendix A 

describes other resources that are also available from FHWA. In this report, comprehensive 

guidance for the use of the LRFD methodology is included in Appendix E, which concentrates 

on the evaluation of spread footings in the LRFD context. The Appendix pulls together previous 

guidance and is intended to mitigate the obstacle of limited knowledge of available resources. 

1.5 SUMMARY 

Chapter 1 identifies six obstacles that may be impediments to the wider use of spread footings on 

soil for highway bridge structures. The first obstacle pertains to the availability of FHWA and 

other resources to designers. Section 1.4 of this chapter and Appendix A provide information in 

this regard. A number of the FHWA publications currently available are listed in the 

Bibliography. Thus, the first obstacle is addressed explicitly in this chapter. Similarly, the 

remaining five obstacles will be systematically addressed in the following chapters. 

3
 

 

 

 

 

CHAPTER 2 –PERFORMANCE
 

CHAPTER 2 - PERFORMANCE 

Limited understanding of documented performance is the second obstacle listed in Chapter 1 as 

an impediment to wider use of spread footings. Performance is documented here through 

reported experience from state DOTs, documented data, and cost considerations. As shown in 

this chapter, spread footing performance for highway bridges is determined by deformation 

criteria, not soil bearing failure. Furthermore, spread footings have generally been less 

expensive than other foundations, and the bridges founded on them have performed very well. 

2.1 REPORTED EXPERIENCE OF STATE DOTs 

As part of this study two independent surveys were performed to obtain a broad view of the 

extent to which state DOTs used spread footings in the past for highway bridge foundations and 

the extent of their use in current practices related to pier and abutment foundations. Abu-Hejleh 

(2009) surveyed predominantly geotechnical specialists while Mertz (2009) concentrated on 

bridge specialists. A combined total of more than 40 states participated in the surveys. 

Following are the key findings based on these surveys: 

	 The results of both surveys were remarkably similar and revealed that, for the most part, 

structural specialists currently use spread footings for bridge substructures only at non-stream 

crossings and when the substructures can be founded on bedrock or intermediate 

geomaterials (IGMs), i.e., geomaterials whose engineering properties are intermediate 

between soil and rock. 

 Use of spread footings varied significantly between states with some states reporting that 

they "never" use spread footings. 

 Even if only anecdotally, it was found that bridges on spread footings were performing as 

well as those on deep foundations. 

 None of the respondents reported significant problems with bridges founded on spread 

footings when the footings were technically viable and designed and constructed properly. 

 Many state DOTs have started migrating towards the use of integral or semi-integral 

abutments founded on driven piles. 

	 Spread footings were reported to be used in a variety of configurations and for conditions 

ranging from fills, granular soils, overconsolidated clays, on top of MSE walls (i.e., as “true” 

bridge abutments) and, to a lesser degree, for integral abutments. 

2.2 DOCUMENTED PERFORMANCE OF BRIDGES 

Significant documentation for the successful performance of bridges is available. The 

Bibliography provides a list of references where such information can be found. This section 

presents some pertinent findings from previous reports on spread footing use for highway bridge 

structures. The key point is that highway bridges can tolerate significant deformations and that 

spread footings have been used successfully for highway bridge structures. 

5
 
 

 

 

 

 

 

CHAPTER 2 –PERFORMANCE
 

	 FHWA (1985) presents the results of work that was based on a nationwide study of 314

bridges and arrives at the following conclusions:

“The results of this study have shown that, depending on type of spans, length and 

stiffness of spans, and the type of construction material, many highway bridges can 

tolerate significant magnitudes of total and differential vertical settlement without 

becoming seriously overstressed, sustaining serious structural damage, or suffering 

impaired riding quality. In particular, it was found that a longitudinal angular distortion 

(differential settlement/span length) of 0.004 would most likely be tolerable for 

continuous bridges of both steel and concrete, while a value of angular distortion of 

0.005 would be a more suitable limit for simply supported bridges." 

	 NCHRP (1983) concluded the following:

"In summary, it is very clear that the tolerable settlement criteria currently used by most 

transportation agencies are extremely conservative and are needlessly restricting the use 

of spread footings for bridge foundations on many soils. Angular distortions of 1/250 of 

the span length and differential vertical movements of 2 to 4 inches (50 to 100 mm), 

depending on span length, appear to be acceptable, assuming that approach slabs or 

other provisions are made to minimize the effects of any differential movements 

between abutments and approach embankments.” 

	 FHWA (1982) and the Washington State Department of Transportation conducted a

performance evaluation of 148 highway bridges supported by spread footings on engineered

fills. This review, in conjunction with detailed survey investigations of the foundation

movement of 28 selected bridges, concluded that spread footings can provide a satisfactory

alternative to deep foundations, especially when high embankments of good quality borrow

materials are constructed over satisfactory foundation soils. Studies of foundation movement

showed that bridges easily tolerated differential settlements of 1 to 3 inches without

significant distress.

In addition to the above studies, there are numerous instances of the successful use of spread 

footings on top of MSE walls at abutments (so-called “true bridge abutments”) with resultant 

cost savings. For example, true bridge abutments were used on 30 bridges as part of the 

reconstruction of the “BIG-I” traffic interchange at the intersection of I-25 and I-40 in 

Albuquerque, New Mexico. In service since 2000, these bridges are performing well. In 

Colorado, true bridge semi-integral abutments were constructed at the Founders/Meadows 

Bridge over I-25. Geogrids were used for soil reinforcements in these structures, which were 

extensively instrumented (Abu-Hejleh et al., 2001). Another successful application is in Tucson, 

Arizona, where use of true bridge abutments for a 5-span bridge over a local parkway and 

multiple tracks of the Union Pacific Rail Road resulted in significant cost savings for the owner. 

In service since 2004, these abutments are performing well based on the results of monitoring 

over 500 survey markers placed on the MSE abutments. 

6
 
 

 

 

 

 

 

 

 

CHAPTER 2 –PERFORMANCE
 

Unfortunately, many agencies continue to disregard spread footings as a viable foundation 

alternative to deep foundations for highway structures. Section 2.3 presents data in a format that 

demonstrates the basic behavior of spread footings leading to their successful performance as 

foundation systems for highway bridge structures. 

2.3 CHARACTERISTICS OF LOAD-DEFORMATION PERFORMANCE 

The reported excellent performance of spread footings for highway bridge structures can be 

understood better by looking at instrumented test cases and the characteristics of measured loaddeformation behavior for spread footings. Note that due to the large size of spread footings for 

highway bridges, soil bearing failure is not likely (FHWA, 2006). Therefore, the performance of 

spread footings in highway bridge design is evaluated primarily on the basis of vertical 

displacement, i.e., settlement, and how differential settlements affect angular distortion. Figure 

2-1 shows the schematic of a typical axial load (Q) versus vertical displacement (S curve for a 

spread footing of width Bf. The Q-S curve contains three distinct parts as follows: 

 Initial (Part O-I)

 Transition (Part I-N)

 Nominal (Beyond N)

The Q-S curve is linear within the initial and nominal parts, while in the transition part it is 

non­linear as shown in Figure 2-1. Point I is often called the elastic limit because the curve is 

linear up to that point. However, soils very rarely act elastically upon initial loading even though 

their Q-S curve may be linear. Therefore, in this report, Point I will be referred to as the 

initial limit. Point N corresponds to the nominal resistance that is based on the shear 

resistance of the soil on which the footing of width Bf is bearing. In conventional terms, this 

point corresponds to the ultimate bearing capacity, which in LRFD is referred to as the nominal 

bearing resistance. 

Based on an evaluation of 167 axial compression field load tests on spread footings of widths 

(Bf) ranging from approximately 0.8-ft to 13-ft, Akbas and Kulhawy (2009) report the values of 

vertical displacement, , in terms of footing width, Bf, at Points I and N as shown in Tables 2-1 

and 2-2. Table 2-3 presents the ratio of loads, QN/QI, at Points I and N based on the same work. 

All the data in Tables 2-1 to 2-3 were developed by Akbas and Kulhawy (2009) based on the L1­

L2 method of analysis of a load settlement curve, which is described in their paper. 

Following are the notable points from the information in Tables 2-1 and 2-2: 

 The mean value of settlement at the initial limit (Point I), SI, in Figure 2-1 is 0.26% Bf 

 The mean value settlement at the nominal limit (Point N), SN, in Figure 2-1 is 5.88% Bf 

 The mean value of QN/QI is 6.74, i.e., the load at nominal limit (Point N in Figure 2-1) is

6.74 times the load at initial limit (Point I in Figure 2-1)

7
 
CHAPTER 2 –PERFORMANCE
 

Figure 2-1. Schematic. Typical axial load versus vertical displacement curve (Q-δ) for 

spread footing of width Bf. 

Table 2-1. Vertical displacement values for points I and N in Figure 2-1 (After Akbas and 

Kulhawy, 2009). 

 

Point 

S/Bf (%) Coefficient of 

Variation, COV 

(%) 

Maximum 

(Upper Bound) 

Minimum 

(Lower Bound) Mean 

At Point I, SI 1.33[0.34] 0.05[0.12] 0.26[0.23] 71[23] 

At Point N, SN 14.9[6.63] 3.30[3.99] 5.88[5.39] 26[13] 

Values in [ ] parenthesis are for uncemented transported soils. Other values include data from 

residual, cemented as well as uncemented soils. 

Table 2-2. Ratio of loads for points I and N in Figure 2-1 (After Akbas and Kulhawy, 2009). 

Axial Load, Q QN/QI 

Maximum 14.3 [10.3] 

Minimum 1.70 [3.67] 

Mean 6.74 [6.88] 

Coefficient of Variation, COV (%) 38.0 [24.0] 

Values in [ ] parenthesis are for uncemented transported soils. Other values 

include data from residual, cemented as well as uncemented soils. 

8
 
 

 

 

 

 

 

 

 

 

 

CHAPTER 2 –PERFORMANCE
 

Table 2-3 presents calculated values of S/Bf for common ranges of sizes of spread footings and a 

range of settlements. By comparing the values in Table 2-3 with the mean values in Table 2-1, it 

can be observed that on the load-settlement curve shown in Figure 2-1, most of the footing 

designs would be performed in the vicinity of Point I. This clearly shows the extreme 

conservatism in typical spread footing designs for highway bridges. 

Table 2-3. Range of typical S/Bf for spread footings on soils for highway bridges. 

Footing 

Width, Bf (ft) 

S/Bf (%) 

S= 0.50-in S= 0.75-in S= 1.00-in S= 1.5-in S= 2.00-in 

10 0.42 0.63 0.83 1.25 1.67 

15 0.28 0.42 0.56 0.83 1.11 

20 0.21 0.31 0.42 0.63 0.83 

25 0.17 0.25 0.33 0.50 0.67 

30 0.14 0.21 0.28 0.42 0.56 

35 0.12 0.18 0.24 0.36 0.48 

The geotechnical literature contains a large amount of field performance data for spread footings 

for constructed facilities. However, much of the data are related to smaller size footings for 

buildings. As part of the development of this report, an attempt was made by the project team to 

identify case histories of large-scale field load tests and/or instrumented footings in actual 

highway bridges. Appendix B provides a summary of case histories that were identified in the 

open literature (English language) that contains data related to large spread footings typical of 

highway bridges. It is acknowledged that there are other data bases that are not available in the 

public domain or in languages other than English that are not included in Appendix B. However, 

the information presented in Appendix B is sufficient to develop the following general 

observations: 

 The typical contact pressure under a highway bridge footing varies from 3 to 6 ksf. 

 When expressed in terms of ratio of measured settlement, Sm, to total footing width, Bf, the 

Sm/Bf values range from 0.1% to 1.0% for footing widths greater than 10-ft. For smaller 

footing widths values of Sm/Bf as large as 2.50% can occur. 

Comparing this range of observed Sm/Bf values with those cited in Table 2-1 for /Bf, it can seen 

that in the context of Figure 2-1 the large spread footings for highway bridges are generally 

designed at or near Point I or the initial limit. This observation was also made with respect to the 

information in Table 2-3. 

Based on the information in this chapter, it is clear that spread footings on soils for highway 

bridges perform well with respect to settlement and have a large margin of safety with respect to 

soil strength since the loads are well below the nominal resistance (Point N in Figure 2-1). 

However, this does not mean that larger settlements will be acceptable from the viewpoint of 

structural integrity of the bridge superstructure or aesthetics. The issue of settlements with 

respect to their effects on bridge superstructure will be further evaluated in later chapters. 

9
 
 

 

 

 

 

 

 

 

CHAPTER 2 –PERFORMANCE
 

2.4 COST CONSIDERATIONS 

Except for the cases discussed in Section 2.4.1, it is generally accepted that spread footings are 

more cost-effective than deep foundations. There are many reasons for this but perhaps the most 

pertinent reasons are that the construction of spread footings does not require specialty 

contractors or equipment and that all the construction is performed at shallow depths. 

Even though spread footings are more cost-effective, there are not many published detailed cost 

analyses documenting the cost savings associated with the spread footing alternative. Following 

are some observations from three significant formal cost analyses: 

	 FHWA (1982) presents 3 case histories of bridges constructed by Washington State DOT 

(WSDOT) which demonstrate that the spread footing alternative was 46% to 67% less 

expensive compared to the deep foundation alternative. 

	 FHWA (1987) indicates that for bridge abutment and pier loads of less than 1,000 tons 

(2,000 kips), which are typical of highway overpass structures, the cost of pile foundations in 

1986 dollars is on the order of $40 to $80 per ton of pier load depending on the pile length, as 

compared with a spread footing cost on the order of $15 per ton. For heavier loads the cost 

of the pile foundation is likely to be on the order of 2 to 3 times that of spread footings. 

	 Based on their work for Ohio DOT (ODOT), Sargand and Masada (2006) report that cost 

savings of 58% can be realized by ODOT by using spread footings instead of pile 

foundations. 

From these studies, it is evident that spread footings, where technically feasible, can provide 

significant cost-savings. Therefore, it is recommended that for all projects where spread footings 

are feasible, i.e., where unfavorable situations as identified in this report do not exist, owners 

perform a detailed cost analyses to justify not using spread footings. Following are guidelines 

for performing the cost analyses so that costs are not biased towards use of deep foundations 

(FHWA, 2006): 

	 Express the total cost of a foundation system in terms of dollars per kip (or ton) of load that 

will be supported; this is the approach used in the FHWA (1987) study cited above. 

	 For an equitable comparison, the total foundation cost should include all costs associated 

with a given foundation system including the need for excavation or retention systems, 

environmental restrictions on construction activities, e.g., vibrations, noise, disposal of 

contaminated spoils, pile caps and cap size, etc. 

	 For major projects, if the estimated costs of alternative foundation systems during the design 

stage are within 15 percent of each other, the alternate foundation designs should be 

considered for inclusion in the contract documents. If alternate designs are included in the 

contract documents, both designs should be adequately detailed. Otherwise, material costs 

and not the installed foundation cost will likely determine the low bid. 

10
 
 

 

 

 

 

CHAPTER 2 –PERFORMANCE
 

As noted in Chapter 1, FHWA (1997) estimated a potential cost savings to the public of 

approximately $135 million, if one assumes that only 25% of the approximately 600,000 existing 

bridges in the United States were to use spread footings when they are replaced due to age or 

other reasons. If that amount is adjusted for inflation, the potential cost savings in terms of 2009 

dollars would be more than $180 million. This is likely a lower bound estimate of the real costsavings. 

2.4.1 Use of Deep Foundations where Spread Footings are Technically Viable 

As part of the surveys reported in Section 2.1, it was found that some states such as Texas and 

Colorado often use drilled shafts even though spread footings are technically feasible. Such 

decisions are made based on local conditions such as significant competition among drilled shaft 

contractors and the generally favorable soil and groundwater conditions for open-hole shaft 

construction. In these situations, drilled shafts may be cost-competitive and could be considered 

feasible. Site constraints and construction scheduling are other considerations that could favor 

the use of deep foundations where spread footings would be otherwise technically feasible. For 

example, on the I-10/I-19 traffic interchange in Tucson, AZ, spread footings were viable, but 

drilled shafts were used because the large open excavations for spread footings in close 

proximity to each other created site constraints from a construction viewpoint. 

2.5 SUMMARY 

Chapter 2 provides information to mitigate the second obstacle to the wider use of spread 

footings on soil for highway bridge structures. As noted in Chapter 1, the second obstacle is 

“Limited knowledge of adequate performance information of spread footings on soils from (a) 

documented results of axial load tests, and (b) case histories of the successful and economical 

use of spread footings on soils to support bridges.” There is ample evidence of successful use 

and performance of spread footings on soils in highway bridge design practice. The key points 

in this regard are as follows: 

(a)	 Most spread footing designs are very economical and very conservative in that the design 

load is far less than the nominal resistance, and 

(b)	 Spread footings perform as well as deep foundations for conditions where both are 

technically feasible. 

11
 

 

 

 

 

CHAPTER 3 – EVALUATION OF TOLERABLE SETTLEMENTS FOR HIGHWAY BRIDGE DESIGN
 

CHAPTER 3 – EVALUATION OF TOLERABLE SETTLEMENTS FOR HIGHWAY
 

BRIDGE DESIGN
 

The controlling factor in the design of spread footings for bridges is usually tolerable settlement. 

Use of conservative methods for the estimation of settlement was identified in Chapter 1 as the 

third obstacle to the wider use of spread footings on soils for highway bridge structures. In 

addition, the overly restrictive and unrealistic tolerable settlement criteria often used by agencies, 

e.g., zero or 1⁄4 inch of settlement was identified in Chapter 1 as the fourth obstacle to the wider 

use of spread footings on soil for highway bridge structures. To help mitigate these two 

obstacles, Chapter 3 discusses settlement prediction methods and the settlement criteria that 

should be used in the context of bridge design. 

3.1 SETTLEMENT PREDICTION METHODS 

The settlement of a footing is the result of the compression of soils within the "depth of 

significant influence" (DOSI), i.e., the depth below the base of the footing to which significant 

applied stresses are experienced by the soil. The DOSI varies from 2 to 4 times the effective 

width of the footing depending on the shape of the footing. Appendix C contains further 

discussion of the DOSI. The total settlement under a load is comprised of two major 

components: (a) immediate (so-called elastic) settlements, and (b) long-term time-dependent 

consolidation (primary and secondary) type of settlements. While the methods to determine 

long-term consolidation type settlements are fairly standard (FHWA, 2006), there are a number 

of different settlement prediction methods available for computing immediate settlement. Each 

analytical method used for calculating immediate settlements is based on a set of assumptions 

specific to that method. Therefore, there is an inherent uncertainty associated with the calculated 

values of movements. Several studies have been undertaken by various researchers that have 

compared measured settlements to calculated settlements, e.g., Burland and Burbridge (1984), 

FHWA (1987), Baus (1992), FHWA (1997), Sargand and Masada (2006), etc. Figure 3-1 shows 

the data from the study performed by FHWA (1987). Other studies show similar trends. 

 

 

 

 

 

 

 

 

 

Figure 3-1. Graph. Comparison of measured and calculated settlements based on data in
 

FHWA (1987).
 

13
 
 

 

 

 

 

 

 

 

CHAPTER 3 – EVALUATION OF TOLERABLE SETTLEMENTS FOR HIGHWAY BRIDGE DESIGN
 

The key points to be noted from the data in Figure 3-1 are: 

	 The more accurate the method, the closer the data points will plot to the 1:1 diagonal line. 

Two definitions, Accuracy (also sometimes referred to as Settlement Ratio) and Bias, can 

be used to evaluate different methods. Accuracy, which is often used in the geotechnical 

literature, is defined by Tan and Duncan (1991) as the value of the ratio of calculated to 

measured settlements. Bias, which is often used in LRFD context, is defined as the value of 

the ratio of the measured to the calculated settlements. Thus, Bias is the inverse of Accuracy 

and in either case the closer the value of these ratios to 1, the more accurate the calculation 

method. The average (arithmetic mean) values of accuracy and bias data are defined as 

Accuracy Factor and Bias Factor, respectively. Since both the definitions are with respect to 

the average of the data points, the method which has the value of these factors closest to 1.0 

may result in calculated values being more or less than the actual (measured) value. 

	 Some methods are more reliable than others. Reliability should not be confused with 

accuracy. Reliability can be considered as the probability that the measured (actual) 

settlements would be less than those calculated by a specific method. It is a measure of 

conservativeness of a settlement prediction method. With respect to Figure 3-1, the method 

that yields the most data points below the 1:1 diagonal line is the most reliable, but not 

necessarily the most accurate. 

From the above discussion, it can be surmised that a method that is accurate is not necessarily as 

reliable as the method that is less accurate. For example, a method with a an Accuracy Factor of 

2.0 (or alternatively a Bias Factor of 0.5) is more reliable than that with an Accuracy Factor or 

Bias Factor of 1.0 because it would give a conservative estimate of settlement, i.e., the measured 

value would be half the calculated value as opposed to being equal to the calculated value. 

3.2 FHWA RECOMMENDED METHODS FOR CALCULATING SETTLEMENTS 

While accuracy and reliability considerations are important, the choice of a method should also 

be based on whether or not it offers a rational evaluation of stress and strain distribution with 

depth for various footing shapes and the elastic properties of the foundation soils. Based on all 

of these considerations, FHWA (2006) recommends the use of the method proposed by 

Schmertmann et al. (1978) for computing immediate settlements. Since AASHTO (2007 with 

2009 Interims) recommends the use of specific correlations to estimate the elastic properties of 

the soils, it is recommended that the version of the method as presented in FHWA (2006), which 

is calibrated to the AASHTO correlations, be used for calculating immediate settlement. Use of 

the FHWA modified version of the method of Schmertmann et al. (1978) is illustrated in FHWA 

(2006). 

If long-term, time-dependent consolidation settlements are anticipated, then FHWA recommends 

the use of the conventional consolidation theory to estimate the long-term settlements. These 

long-term settlements would have to be added to the immediate settlements to obtain total 

settlements. Procedures for computing the long-term consolidation type of settlements can be 

14
 
 

 

 

 

 

 

 

 

CHAPTER 3 – EVALUATION OF TOLERABLE SETTLEMENTS FOR HIGHWAY BRIDGE DESIGN
 

found in FHWA (2006) or other well-known geotechnical textbooks, such as Perloff and Baron 

(1976), Lambe and Whitman (1979), Holtz and Kovacs (1981) and Terzaghi et al. (1996). 

It is important to note that it is simply not enough that the designer is using the FHWA 

recommended methods for computing settlements. Incumbent in this process is the need to 

perform a minimum standard of investigations as outlined in FHWA (2006) and Section 10 of 

AASHTO (2007 with 2009 Interims) to develop the appropriate geotechnical parameters for use 

in settlement analyses. In the context of LRFD, AASHTO allows the use of larger resistance 

factors if better investigation techniques, such as cone penetration tests or plate load tests, are 

used. For critical highway bridges on life-line routes (e.g., interstate highways), consideration 

may be given to increasing the level of field and laboratory investigations particularly in those 

areas where prior information and/or experience is poor or not available. 

3.3 THE S-0 CONCEPT OF EVALUATION OF CALCULATED SETTLEMENTS IN 

THE CONTEXT OF BRIDGE DESIGN 

Although calculation of total settlement is an important and necessary exercise, it is only the first 

step in evaluating the feasibility of using spread footings on soils for bridge foundations. 

Whether the calculated settlements will be detrimental to a bridge structure will depend on the 

configuration of the bridge and the materials used to construct the bridge, e.g., steel or concrete. 

This is because differential settlements and angular distortions are more pertinent parameters in 

the evaluation of damage to bridge facilities than the absolute settlements alone. In a larger 

context, the issues of accuracy and reliability as well as the use of a specific method of 

calculating settlements are mitigated when settlements are expressed in terms of differential 

settlements and angular distortions. Using the concepts introduced in earlier chapters, the 

published performance data, and knowledge of the general behavior of bridges, this section and 

the remainder of this report will demonstrate that the use of spread footings on soils will often 

lead to an acceptable behavior of bridge structures in terms of deformations. 

While all methods have a certain degree of uncertainty in estimating settlements, the uncertainty 

of the calculated differential settlement is larger than the uncertainty of the calculated total 

settlement at each of the two support elements used to calculate the differential settlement, e.g., 

between an abutment and a pier, or between two adjacent piers. For example, if one support 

element actually settles less than the amount calculated while the other support element actually 

settles the amount calculated, the actual differential settlement will be larger than the difference 

between the two values of calculated settlement at the support elements. 

Based on the above considerations and guidance in FHWA (2006) and NCHRP (1991), the 

following criteria are suggested to estimate a realistic value of differential settlement and angular 

distortion: 

	 The actual settlement of any support element could be as large as the value calculated by 

using a given method, and 

15
 
 

 

 

CHAPTER 3 – EVALUATION OF TOLERABLE SETTLEMENTS FOR HIGHWAY BRIDGE DESIGN
 

	 The actual settlement of the adjacent support element could be less, taken as zero in the limit, 

instead of the value calculated by using the same given method. 

The above criteria lead to the “S-0” concept, with a value of S representing full settlement at one 

support of a span and a value of “0” representing zero settlement at an adjacent support. Use of 

the S-0 concept would result in an estimated maximum possible differential settlement equal to 

the larger of the two total settlements calculated at either end of any span. The use of S-0 

approach may be prudent for cases where site variability is “high” based on judgment, criteria 

noted in commentary to Article 10.5.5.2.3 of AASHTO (2007 with 2009 Interims), and/or 

discussions of subsurface variability in Article 10.4 of AASHTO (2007 with 2009 Interims). 

The application of the S-0 concept can be illustrated by considering a hypothetical case shown in 

Figure 3-2 of a 4-span bridge structure with 5 support elements (2 abutments and 3 piers) 

wherein the calculated settlement, S, at each support is different (SA1 < SP1 > SP2 < SP3 < SA2). 

Differential settlements induce bending moments and shear in the bridge superstructure when the 

spans are continuous over supports. These moments and shears can potentially cause structural 

damage. To a lesser extent, differential settlements can also cause damage to a bridge consisting 

of simple spans. However, the major concern with simple span bridges is the quality of the 

riding surface and aesthetics. Due to a lack of continuity over the supports, the changes in slope 

of the riding surface near the supports of a simple span bridge induced by differential settlements 

may be more severe than those in a continuous span bridge. 

Support Settlement 

Abutment 1 SA1 

Pier 1 SP1 

Pier 2 SP2 

Pier 3 SP3 

Abutment 2 SA2 

Figure 3-2. Schematic. Concept of settlement and angular distortion in bridges. 

For the hypothetical case shown in Figure 3-2, the differential settlements and angular distortions 

for design are evaluated as shown in Figure 3-3 based on the S-0 concept. As shown, two viable 

modes of deformation shapes, Mode 1 and Mode 2, are possible depending on which support 

settlement is assumed to be zero. The symbols for design differential settlement and angular 

distortion are DSi-j and DAi-j, respectively, where i represents the span number (1 to 4) and j 

represents the mode (1 and 2). The hypothetical settlement profile assumed for computation of 

16
 
CHAPTER 3 – EVALUATION OF TOLERABLE SETTLEMENTS FOR HIGHWAY BRIDGE DESIGN
 

the maximum design angular distortion for each span is represented by the dashed lines in Figure 

3-3. It should not be confused with the calculated total settlement profile that is represented by 

the solid lines. The values of design differential settlements and design angular distortions in 

Figure 3-3 are the maximum values for each span according to the criteria above. In AASHTO 

LRFD context, the “design” values are based on factored settlement values using the SE load 

factor. 

Figure 3-3. Schematic. Estimation of maximum angular distortion in bridges. 

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CHAPTER 3 – EVALUATION OF TOLERABLE SETTLEMENTS FOR HIGHWAY BRIDGE DESIGN
 

3.3.1 Tolerable Angular Distortions 

Based on the FHWA (1985) cited earlier in Section 2.2 of Chapter 2, AASHTO presents the 

guidance summarized in Table 3-1 for the evaluation of angular distortions in a bridge structure. 

Table 3-1. Tolerable movement criteria for highway bridges (AASHTO 2002, 2007 with 

2009 Interims) 

Limiting Angular Distortion, DS/L (radians) Type of Span 

0.004 Continuous span 

0.008 Simple span 

The criteria in Table 3-1 suggest that for a 100-ft span, a differential settlement of 4.8-inches is 

acceptable for a continuous span and 9.6-inches is acceptable for a simple span. Such relatively 

large values of differential settlement create concern for structural designers, who often 

arbitrarily limit the criteria to one-half to one-quarter of the values listed in Table 3-1. While 

from a structural integrity viewpoint, there are no technical reasons for structural designers to set 

such arbitrary additional limits to the criteria listed in Table 3-1, there are often practical reasons 

based on the tolerable limits of deformation of other structures associated with a bridge, e.g., 

approach slabs, wingwalls, pavement structures, drainage grades, utilities on the bridge, 

deformations that adversely affect quality of ride, etc. Thus, the relatively large differential 

settlements based on Table 3-1, should be considered in conjunction with functional or 

performance criteria not only for the bridge structure itself but for all of the associated facilities. 

The following steps are suggested in this regard (FHWA 2006): 

Step 1: Identify all possible facilities associated with the bridge structure, and the tolerance of 

those facilities to movements. 

Step 2: Due to the inherent uncertainty associated with estimated values of settlement, determine 

the differential settlement by using conservative assumptions. It is important that the 

estimation of angular distortion is based on a realistic evaluation of the construction 

sequence and the magnitude of loads. 

Step 3: Compare the angular distortion from Step 2 with the various tolerances identified in Step 

1 and in Table 3-1. Based on this comparison identify the critical component of the 

facility. Review this critical component to check if it can be relocated or if it can be 

designed to more relaxed tolerances. Repeat this process as necessary for other facilities. 

In some cases, a simple re-sequencing of the construction of the facility based on the 

construction sequence of the bridge structures may help mitigate the issues associated 

with intolerable movements. 

The above approach will help to develop project-specific limiting angular distortion criteria that 

may differ from the general guidelines listed in Table 3-1. 

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CHAPTER 3 – EVALUATION OF TOLERABLE SETTLEMENTS FOR HIGHWAY BRIDGE DESIGN
 

With respect to the hypothetical case shown in Figure 3-2, although the criteria for limiting 

angular distortions listed in Table 3-1 may be exceeded in one or more spans as estimated in 

Figure 3-3, they will generally be satisfied if the construction-point concept discussed in Chapter 

4 is applied and the design angular distortions are computed based on loads placed after the 

construction of the substructure units. It will be shown that angular distortion after construction 

of the substructure units may be within tolerable limits. 

3.3.2 Footings Proportioned for Equal Settlement 

Often geotechnical and structural specialists will try to proportion footings for equal settlement. 

In this case, the argument is made that there will be no differential settlement or angular 

distortion. While this concept may work for a building structure because the footprint is 

localized, it is a fallacy to assume zero differential settlement for a long linear highway structure 

such as a bridge or a retaining wall because the properties of the geomaterials along the length of 

the structure will inevitably vary too much. Furthermore, as noted earlier, the prediction of 

settlements from any given method is uncertain by itself. Thus, for highway structures even 

when the footings are proportioned for equal settlement it is advisable to evaluate differential 

settlement assuming that the actual settlement of any support element could be as large as the 

value calculated by using a given method while at the same time, the actual settlement of the 

adjacent support element could be zero. 

3.4 SUMMARY 

Chapter 3 provides information that will help mitigate the third and fourth obstacles to the wider 

use of spread footings on soil for highway bridge structures. As noted in Chapter 1, the third 

obstacle is, “Use of conservative settlement prediction methods” and the fourth obstacle is, 

“Restrictive tolerable settlement criteria, which have led some agencies to set unrealistic 

performance criteria for foundations (e.g., zero or 1⁄4 inch of settlement).” The key points in this 

regard are as follows: 

(a)	 Guidance on the selection and use of appropriate settlement prediction methods is 

available. 

(b)	 Since a bridge structure is much more tolerant to deformations than a conventional 

building structure, larger total footing settlements may be acceptable for highway bridges. 

Thus, there are no technical reasons for using restrictive and often unrealistic criteria for 

tolerable settlement such as zero or 1⁄4 inch of settlement. 

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CHAPTER 4 – LOAD CONSIDERATIONS IN EVALUATION OF SETTLEMENTS
 

CHAPTER 4 – LOAD CONSIDERATIONS IN EVALUATION OF SETTLEMENTS 

The estimation of settlement may be routinely accomplished with adequate geotechnical data and 

a realistic estimation of the structural loads that cause potentially damaging settlements. The 

overestimation of loads in the evaluation of settlements was identified in Chapter 1 as the fifth 

obstacle to the wider use of spread footings on soil for highway bridge structures. Chapter 4 

attempts to mitigate this obstacle by discussing the construction-point concept to illustrate how 

settlements develop during construction of a bridge structure. Chapter 4 also discusses various 

other considerations that must be taken into account by both the geotechnical specialist and the 

structural specialist to reduce unnecessary conservatism in the estimation of settlements. 

4.1 CONSTRUCTION-POINT CONCEPT 

Most designers use the criteria for tolerable movements described in Chapter 3 as if a weightless 

bridge structure is instantaneously wished into place and all the loads are applied at the same 

time. In reality, loads are applied gradually as construction proceeds. Consequently, foundation 

movements will also occur gradually as construction proceeds. There are several critical 

construction points or stages during construction that should be evaluated separately by the 

designer. For the case of vertical loads and settlements, Figure 4-1a shows the critical 

construction stages (1, 2, 3, and 4) and Figure 4-1b shows the associated load-settlement 

behavior. In this case, the total foundation settlement, St, is equal to the settlement corresponding 

to the total construction Load, Z (= Load 1 + Load 2 + Load 3 + Load 4). Thus, St = SZ. The 

settlements that occur before placement of the superstructure may not be relevant to the design of 

the superstructure. Thus, the settlements between application of loads in construction stages 2 

and 4 are the most relevant. For continuous-span bridges the settlement, SX, corresponding to 

Load X (= Load 1 + Load 2) may be applicable for computing relevant total settlement, Str. In 

this case, the total foundation settlement prior to construction of bridge superstructure, Stp, is 

equal to the settlement, SX, corresponding to Load X. Thus, for this example, Str= SZ - SX. 

Similarly, for simple-span bridges where settlement, SY, corresponding to Load Y (= Load 1 + 

Load 2 + Load 3) may be applicable for computing relevant total settlement, Str= SZ – SY. 

Formulation of settlements in a manner shown in Figure 4-1b permits an assessment of 

settlements up to that point that can affect the bridge superstructure. Although Figure 4-1 

illustrates the construction-point concept for the case of a pier, vertical loads and settlements 

(vertical movement), the concepts apply to other elements of bridge structure (e.g., abutments), 

load types (shears, moments, etc.) and deformation types (lateral movements, rotations, etc.). 

Consideration of relevant settlements in conjunction with the type of superstructure and bearings 

can lead to a more rational consideration of spread footings on soils for highway bridges rather 

than an uninformed decision to select a costlier deep foundation system. Alternatively, if a deep 

foundation system is chosen due to reasons noted in Section 2.4.1 and Appendix C.3.1, the size 

of the deep foundation can be optimized, e.g., reduction in number of piles, reduction in size of 

piles, reduction in pile spacing with associated reduction in size of pile cap, and/or reduction in 

length of piles. 

21
 
CHAPTER 4 – LOAD CONSIDERATIONS IN EVALUATION OF SETTLEMENTS
 

 

Legend 

W 

X

 Y 

Z 

Load after footing construction 

Load after pier column/wall construction 

Load after superstructure construction 

Load after wearing surface construction 

SW

SX 

SY

SZ 

 Settlement under load W 

Settlement under load X

 Settlement under load Y 

Settlement under load Z 

Figure 4-1. Schematic. Construction-point concept for a bridge pier, vertical loads, and 

vertical movement (settlement). (a) Identification of critical construction points, (b) 

Conceptual load-displacement pattern for a given footing width. 

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CHAPTER 4 – LOAD CONSIDERATIONS IN EVALUATION OF SETTLEMENTS
 

Sargand et al. (1999) and Sargand and Masada (2006) report the measurements of contact 

pressure, settlements, and tilting at various significant construction points for over 50 spread 

footings at 7 highway bridges in Ohio. Based on their study, Sargand and Masada (2006) made 

the following observations: 

 The average contact pressure was typically less than 1 ksf at the end of footing construction, 

less than 4 ksf after placement of the beams and less than 5 ksf shortly after the construction 

of the bridge deck. 

 The average settlement was typically less than 0.7 inches by the time the footing is 

backfilled, less than 1 inch after placement of beams, and less than 1.5 inches shortly after 

the bridge deck construction. 

 The post-deck settlement varies from 0 to 0.4 inches with an average of 0.14 inches, which 

corresponds to 0 to 37.5% (average 19.0%) of the total settlement recorded in the field. 

 The average degree of rotation was typically less than 0.15 degrees (0.0026 radians) by the 

time the beams were placed and less than 0.3 degrees (0.0052 radians) after the bridge deck 

construction. 

Figure 4-2 presents example plots from measurements of contact pressure and average settlement 

at various stages of construction of one of the bridges in the study by Sargand et al. (1999). It 

can be seen that the settlement between placement of beams and end-of-construction is less than 

50 percent of the total settlement. This percentage is not unique and can vary from 25 to 75 

percent depending on the type of the superstructure and the construction sequence. With respect 

to the example of the 4-span bridge and the angular distortions in Table 3-1, the use of the 

construction-point concept would result in smaller angular distortions to be considered in 

structural design. This will be true of any bridge evaluation. Using Figure 3-3 as a reference, 

Figure 4-3 shows a comparison of the profiles of the calculated settlements (solid lines), 

hypothetical maximum angular distortions (dashed lines) and the range of actual angular 

distortions (hatched pattern zones) based on the construction-point concept. For a given project 

and site-specific conditions, the actual relevant angular distortion profile will be represented by a 

dashed line within the hatched pattern zone. FHWA (2006) presents a detailed example for a 

bridge where application of this concept permitted the use of spread footings even though a total 

settlement of 7.5-inches was calculated. 

Evaluation of incremental displacements between various construction points, when taken in 

conjunction with guidance on angular distortions provided in Table 3-1, can permit a more 

efficient design of the substructure as well as the superstructure. For example, adjusting the 

bearing levels can generally compensate for the settlements that occur before the placement of 

the superstructure. Therefore, such settlements may be irrelevant with respect to their effect on 

the design of the superstructure itself. Properly accounting for such settlements will lead to 

smaller settlements for the construction stages that follow, which may be of more interest from 

the viewpoint of differential settlement and angular distortion, e.g., the angular distortion 

between end-of-construction of a pier and after placement of the superstructure. Such 

considerations may lead to more efficient designs for both the substructure and the 

superstructure. 

23
 
CHAPTER 4 – LOAD CONSIDERATIONS IN EVALUATION OF SETTLEMENTS
 

(b) (a) 

Figure 4-2. Figure. Examples of data from measurements of contact pressure and settlement 

at various stages of bridge construction (Sargand, et al., 1999) [100 kPa = 1 tsf; 25.4 mm = 1 

inch]. [Note: Figure (a) is for a pier, and (b) is for an abutment]. 

 

 

 

 

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CHAPTER 4 – LOAD CONSIDERATIONS IN EVALUATION OF SETTLEMENTS
 

While using the construction-point concept it is important to realize that the various quantities 

are being measured at discrete construction stages and the associated settlements are considered 

to be immediate. However, the evaluation of total settlement and the maximum (design) angular 

distortion as discussed previously must also account for long-term settlements. For example, 

significant long-term settlements may occur if footings are founded on saturated clay deposits or 

if a layer of saturated clay falls within the DOSI of the footing, even though the footing itself is 

founded on competent soil. In such cases long-term settlements will continue under the total 

construction load (Z) as shown by the dashed line in Figure 4-1. Continued settlements during 

the service life of the structure will tend to reduce the vertical clearance under the bridge with 

associated problems of high profile vehicles impacting the bridge superstructure. Therefore, the 

geotechnical specialist must estimate and report to the structural specialist the magnitude of the 

long-term settlement that will occur during the design life of the bridge. As noted in Chapter 3, 

the geotechnical specialist can estimate the long-term settlement by using consolidation theories 

described in texts such as Perloff and Baron (1976), Lambe and Whitman (1979), Holtz and 

Kovacs (1981) and Terzaghi et al. (1996). Finally, a key point in evaluating settlements at 

critical construction points is close coordination between the structural and geotechnical 

specialists. 

4.2 OTHER CONSIDERATIONS 

The accuracy of calculated settlements, regardless of the method used, is only as good as the 

quality of the geotechnical data and the estimation of the actual loads. Settlements of spread 

footings are frequently overestimated by designers for the following reasons: 

1. The structural load causing the settlement is not estimated properly 

	 In the absence of actual structural loads during the design process, geotechnical specialists 

conservatively assume that the bearing pressure is equal to the allowable bearing capacity 

(or factored bearing resistance in LRFD terminology). This scenario occurs when the 

geotechnical specialist develops the settlement estimate for a given bearing pressure. Use 

of the bearing resistance chart approach discussed in Appendix E alleviates this problem 

since the structural specialist can then use the bearing resistance chart as necessary at any 

stage of his/her design process when load refinements may be done. The geotechnical 

specialist is no longer justified to provide the structural specialist with only a single value 

of bearing resistance and an associated calculated settlement expressed as being less than 

a certain amount. 

	 Structural specialists often estimate the loads at the upper plane of the footing. As noted 

earlier, spread footings for bridge structures are large and the weight of the footing can be 

significant. By estimating the loads on the upper plane of the footing, the weight of the 

footing is neglected, which has the effect of increasing the computed eccentricity. This 

method of estimating loads affects the effective width of the footing and the calculated 

settlements since the DOSI is a function of the effective footing width. Although 

computing loads at the upper plane of the footing is convenient to the structural specialist 

25
 
 

 

 

 

 

 

 

 

 

 

 

 

 

CHAPTER 4 – LOAD CONSIDERATIONS IN EVALUATION OF SETTLEMENTS
 

because the initial footing size is not known, some simple guidelines may help in 

identifying an initial footing size as follows (FHWA, 2006): 

- For piers, assume a total footing width equal to 1⁄3 of the pier column height. 

- For abutments, assume a total footing width equal to 1⁄2 of the abutment height. 

As an alternative, the initial footing width may be assumed to be equal to 6 times the 

computed eccentricity based on service (working) loads at the upper plane of the footing. 

For initial load estimation purposes, the thickness of the footing for both of the above 

cases may be assumed to be 3- to 4-ft. 

	 Structural specialists should consider the net pressure at the base of the footing when 

estimating bearing pressures. Considering that the base of the footing for highway bridge 

structures is often embedded 5-ft to 6-ft below finished grade, the net pressure at the base 

of the footing is approximately 0.60 ksf to 0.72 ksf less than the gross footing pressure 

because of the effect of removing the soil. Since settlements occur under net bearing 

pressure, accounting for the effect of embedment will result in reduced calculated 

settlements. For spread footings in fill, the use of the net pressure is a function of whether 

the footing is placed in an excavated slot after the final configuration of the fill has been 

constructed or whether the final configuration of the fill is established after the footing 

has been constructed at a certain elevation within the fill. In the latter case the use of net 

pressure is not advisable. 

2. Preloading of the subsoil is not accounted for in the analysis. 

Preloading may be due to a geologic load applied in the past or to the removal of significant 

amounts of soil in construction prior to placement of the foundation. Not accounting for 

preloading can cause a grossly overestimated settlement. This is true for both granular soils 

and fine-grained cohesive soils. In the latter case, the preload effect is expressed in terms of 

over-consolidation ratio (OCR) and the effect of preloading is explicitly accounted for in 

consolidation theory (FHWA 2006). For sands, Schmertmann et al. (1978) recommended a 

reduction in settlement after preloading or other means of compaction of half the predicted 

settlement. Alternatively, in the case of preloaded soil deposits, the settlement can be 

computed by using the method proposed by D’Appolonia (1968, 1970), which includes 

explicit consideration of preloading. 

3. Settlement occurring during construction is not subtracted from total predicted amounts. 

By adoption of the construction-point concept discussed earlier, this consideration can be 

easily accounted for in the bridge design. 

26
 
 

 

 

 

 

 

 

CHAPTER 4 – LOAD CONSIDERATIONS IN EVALUATION OF SETTLEMENTS
 

4. Proper parameters are not used for estimation of settlement of footings on structural fills. 

Calculation of the settlement of a spread footing supported in or on an engineered fill requires 

an assumption about the compressibility of the fill material. Because structural fills should 

be constructed of good-quality granular materials and by following good construction 

techniques, the estimation of settlement lends itself to the application of the methods 

discussed in this report. To estimate settlements of footings in structural fills, an assumption 

must be made about the SPT N-value that is representative of the engineered fill. For fills 

constructed using good-quality granular materials, as discussed in Section C.3.3.3 of 

Appendix C, FHWA (2006) suggests a corrected SPT N-value (N160) of 23 blows per foot as 

a representative value for estimating settlement in structural fills if a relative compaction of 

95% based on Modified Proctor (ASTM D 1557) compaction energy is used. The N160 value 

can be increased to 32 blows per foot if 97% relative compaction based on Modified Proctor 

compaction energy is used. 

5. Construction Inspection and Monitoring 

As is the case with any foundation system, the successful in-service performance is dependent 

on the care taken during construction. It is important that the project plans and specifications 

carefully detail any specific items that need to be inspected and monitored. Appendix D 

presents information on construction inspection and monitoring for spread footings. 

4.3 SUMMARY 

Chapter 4 provides information that will help mitigate the fifth obstacle to the wider use of 

spread footings on soil for highway bridge structures. As noted in Chapter 1, the fifth obstacle is 

“overestimation of loads that generate settlement.” The key points in this regard are as follows: 

(a)	 Settlements occur progressively and a significant portion of the immediate (elastic) 

settlement may be realized before it becomes an issue with respect to the structural 

performance of the bridge superstructure. 

(b)	 In the absence of long-term (time-dependent) consolidation settlements, structural 

designers should properly estimate the loads that are consistent with the construction 

sequence of the bridge structures. Only the immediate settlements resulting from loads 

imposed by the superstructure construction sequence should be used to evaluate the effects 

of settlement and angular distortion on the bridge superstructure. 

27
 

 

 

 

 

 

 

 

 

CHAPTER 5 – LRFD AND SPREAD FOOTINGS
 

CHAPTER 5 –LRFD AND SPREAD FOOTINGS 

In Chapter 1, institutional culture and bias was listed as the sixth obstacle to the wider use of 

spread footings on soil for highway bridge structures. This obstacle is likely a result of the 

cumulative effect of the first five obstacles listed in Chapter 1, and a long period of doing 

“business as usual.” However, the mandatory implementation of LRFD methodology since 

October 1, 2007 for states seeking federal funding for new highway bridge projects is forcing 

designers to re-evaluate their “business as usual” current practices. The change in design 

platform from allowable stress design (ASD) to LRFD is not easy and requires significant efforts 

by agencies, their consultants and the contractors doing the construction work to learn about 

LRFD and document new approaches. The complete revision to LRFD also provides agencies, 

consultants and contractors an ideal opportunity to revisit past practice and to make appropriate 

changes. With this in mind, the purpose of this chapter is to introduce significant guidance on 

spread footing design using the AASHTO LRFD approach. This guidance will also permit the 

designers to take a fresh look at current practice for evaluating and selecting foundation 

alternatives based on performance experience and expectations. 

5.1 ANALYSIS OF SPREAD FOOTINGS USING THE LRFD APPROACH 

The LRFD concepts and methodology provide a rational mechanism to incorporate into the 

design process most of the remedies to the obstacles to the wider use of spread footings on soil 

for highway bridge structures listed in Chapter 1. As discussed previously in this report, these 

remedies included a proper understanding of the use of spread footings based on performance 

data, appropriate settlement estimation methods, the use of the construction-point analysis, and a 

realistic estimate of angular distortions. In addition to presenting information on each of these 

remedies, a major component of the scope of work of this study is to provide guidance for the 

evaluation of spread footings in the LRFD context. Appendix E provides a comprehensive 

treatment of the analysis of spread footings based on the LRFD approach. The following 

information can be found in Appendix E. 

 Discussion of the LRFD-based limit states applicable to spread footings. 

 Streamlined evaluation of bearing resistance and settlement through use of a convenient 

bearing resistance chart that incorporates LRFD limit states. 

 A step-by-step design process flow chart. 

 A comprehensive numerical example that demonstrates through detailed calculations the 

various concepts introduced in this report. This numerical example is based on an actual 

bridge structure. 

Upon review of Appendix E, it will be readily apparent to a designer that now there are explicit 

mechanisms available through the LRFD approach to address the concerns of structural 

specialists regarding geotechnical data presentation and design input developed by geotechnical 

specialists. In the past the geotechnical specialist's input was generally limited to a broad 

evaluation of settlements and the effect of footing configurations, i.e., width and depth, on such 

settlements. The LRFD approach requires much closer collaboration between the geotechnical 

29
 
 

 

 

 

 

 

 

 

CHAPTER 5 – LRFD AND SPREAD FOOTINGS
 

and structural specialists than has existed in the past and thereby assures opportunity for a more 

realistic and reliable design of feasible foundation elements, especially in the case of spread 

footings on soils. 

5.2 OPPORTUNITY FOR NEW LRFD-BASED GUIDANCE 

As state DOTs and others implement the LRFD platform, they will be revisiting their procedures 

for foundation design. This entire report, and in particular Appendix E, provides guidance on 

how this can be done for spread footings on soils. The performance data, previous publications, 

and the experiences of state DOTs reported here provide the technical support to develop LRFDbased guidance that considers the widest possible use of spread footings on soils. Indeed, this is 

an excellent opportunity for states to move away from past practice that is based perhaps too 

heavily on “It’s the way we have always done it” to a practice that is based on a wider body of 

performance data. 

5.3 SUMMARY 

Chapter 5 provides information that will help mitigate the sixth obstacle to the wider use of 

spread footings on soil for highway bridge structures. As noted in Chapter 1, the sixth obstacle 

is “institutional culture and bias.” The key points in this regard are as follows: 

(a)	 This report provides a clear and rational approach to the evaluation of spread footings by 

the LRFD methodology through inclusion of comprehensive LRFD guidance in Appendix 

E. This approach provides opportunities to designers for overcoming institutional culture 

and biases that have been and continue to be an obstacle to the wider use of spread 

footings on soil for highway bridge structures. 

(b)	 The migration from allowable stress design (ASD) to LRFD is a great opportunity for 

agencies to redraft their foundation design guidance to ensure that spread footings are 

given due consideration in the foundation selection process. 

30
 
 

 

 

 

 

 

 

 

 

 

 

 

CHAPTER 6 – SUMMARY AND CONCLUSIONS
 

CHAPTER 6 – SUMMARY AND CONCLUSIONS 

The primary goal of this report is to promote spread footings bearing on natural soils (native or 

improved) and compacted engineered fill materials as a routine alternative to deep foundations 

for the support of highway bridges. The following approach was taken to attain that goal: 

1.	 Identify the obstacles that create an impediment to the wider use of spread footings on soil 

for highway bridge structures, 

2.	 Recognize that the obstacles can be overcome without extensive research. Rather they can 

be mitigated by studying past work, conducting careful analyses, and having good 

communication between geotechnical and structural specialists, and 

3.	 Provide documentation and guidance here that can help mitigate and overcome those 

obstacles. 

In the above framework, using documented performance data, the report presents powerful 

concepts such as the use of construction-point analysis and angular distortions to demonstrate the 

efficacy of using spread footings on soils. Implementation of these concepts requires only that 

conventional computations be taken one step further without any requirement for advanced 

computational skills. The report presents sources of information that agencies and designers can 

use as references in their project applications. Based on the information presented in Chapters 1 

to 5 and Appendices A and B the following key points and conclusions can be summarized: 

	 Significant resources are available to bridge designers to evaluate the performance of spread 

footings on soils and determine the feasibility of their use for highway bridge structures. 

	 The size of spread footings on soils for highway bridges is controlled by settlement 

considerations including differential settlements and associated angular distortions. Field 

measurements of actual bearing pressures suggest that spread footings have a large margin of 

safety with respect to soil strength since the loads are well below the nominal bearing 

resistance in LRFD terminology or ultimate bearing capacity in ASD terminology. 

	 By properly accounting for the construction sequence of a bridge structure, settlements that 

are relevant to the performance of the bridge structure are often only 25 to 50 percent of the 

total settlements. This statement is supported analytically and by measurements on actual 

bridge footings in the field. The implications are that actual angular distortions will be 

similarly reduced so that the AASHTO tolerable movement criteria for highway bridges will 

rarely be exceeded for conventional bridge loads carried by spread footings founded on soils. 

	 Significant documentation for the successful implementation of spread footings on soils is 

available. For example, based on field measurements it has been shown that the typical 

contact pressure under a highway bridge footing varies from 3 to 6 ksf. Also, when 

expressed in terms of the ratio of measured settlement, Sm, to total footing width, Bf, the 

Sm/Bf values range from 0.1% to 1.0% for footing widths greater than 10-ft. For smaller 

footing widths values of Sm/Bf as large as 2.50% can occur. These values have been 

31
 
 

 

 

 

 

CHAPTER 6 – SUMMARY AND CONCLUSIONS
 

independently verified by a number of researchers. Thus, the field performance data clearly 

demonstrate that the spread footings for highway bridges operate in a safe manner. 

Significant additional information regarding the use of spread footings on soils is presented in 

Appendices C and D. Based on the information presented in these appendices, the following 

significant points and conclusions can be summarized: 

	 Spread footings in structural fills at abutments have been constructed successfully and are 

currently performing satisfactorily. Appendix C of the report presents material specifications 

and suggested fill configurations based on successful applications by various agencies. 

	 The successful use of spread footings to support abutments on top of MSE walls is 

documented for many projects. This concept can result in significant savings besides 

mitigating the “bump at the end of the bridge.” 

	 Spread footings can and have been used to support semi-integral and integral abutments. 

Section C.2.2 discusses these applications and provides an example to demonstrate the 

successful application of spread footings for such applications. 

	 Detailed construction inspection, monitoring and instrumentation guidelines are available in 

Appendix D that can be implemented on a routine basis in a cost-effective manner. 

Finally, a detailed approach for the evaluation of spread footings in the LRFD framework is 

presented in Appendix E of the report. This stand-alone appendix presents procedures requiring 

that only conventional computations be taken one step further without the need for advanced 

computational skills or analytical tools. Using the methods in Appendix E, bridge designers can 

now rationally evaluate spread footings on soils when making decisions on the choice of the 

most cost-effective foundation systems for bridges. 

In conclusion, this report has tackled the obstacles (perceived or otherwise) that are an 

impediment to the wider use of spread footings on soils for highway bridge structures. Using 

documented performance data and the now mandatory LRFD methodology, the report presents 

detailed guidance on the evaluation of spread footings on soils in a rational and unbiased manner. 

FHWA resources are identified and a comprehensive bibliography is presented to assist 

structural designers in the evaluation of spread footings on soils. Between the resources, 

bibliography and comprehensive documentation and guidance, this report provides a clear 

avenue for agencies and designers to make an informed evaluation of the use of cost-effective 

spread footings for all conditions other than those where their use may be suspect or is clearly 

precluded (e.g., scour conditions). This type of evaluation should be done as a matter of routine 

as part of the foundation type selection process. 

32
 
 

 

 

 

 

 

 

BIBLIOGRAPHY
 

BIBLIOGRAPHY 

The bibliography presented here contains references that are organized according to primary and 

secondary sources. Primary sources are considered to be publications of transportation agencies 

(e.g., FHWA, NCHRP, state DOTs, etc.) and codes and standards related to highway bridge 

engineering (e.g., AASHTO, ASTM, etc.). Secondary sources are journal articles, books, and all 

other publications that do not classify as primary sources. References that are cited in the report 

are identified with an asterisk (*) while other references are listed as a resource for the reader 

interested in further information on the items discussed in the report. It must be noted that the 

bibliography presented herein focuses on large spread footings and material directly relevant to 

the report. It is acknowledged that the literature is replete with many more references related to 

spread footings. The interested reader is referred to the comprehensive “Bibliography of Case 

Histories of Shallow Foundations,” prepared by ASCE GeoInstitute Committee on Shallow 

Foundations in 1999. 

PRIMARY SOURCES 

Federal Highway Administration (FHWA) 

*FHWA (1982). Performance of Highway Bridge Abutments on Spread Footings on Compacted 

Fill. Report No. FHWA RD-81-184, Author: DiMillio, A. F., Federal Highway 

Administration, U.S. Department of Transportation. 

*FHWA (1985). Tolerable Movement Criteria for Highway Bridges. Report No. FHWA RD-85­

107, Authors: Moulton, L. K., GangaRao, H. V. S., Halvorsen, G. T., Federal Highway 

Administration, U.S. Department of Transportation. 

*FHWA (1987). Spread Footings for Highway Bridges. Report No. FHWA RD-86-185, Author: 

Gifford, D. G., Kraemer, S. R., Wheeler, J. R., McKown, A. F., Federal Highway 

Administration, U.S. Department of Transportation. 

*FHWA (1997). Large-Scale Load Tests and Data Base of Spread Footings on Sand. Report No. 

FHWA RD-97-068, Authors: Briaud, J-L., and Gibbens, R., Federal Highway Administration, 

U.S. Department of Transportation. 

*FHWA (2001). Shallow Foundations Reference Manual. Report No. FHWA-NHI-01-023, 

Authors: Munfakh, G., Arman, A., Collin, J.G., Hung, J.C.-J., and Brouillette, R.P., Federal 

Highway Administration, U.S. Department of Transportation. 

*FHWA (2002). Geotechnical Engineering Circular 6, Shallow Foundations. Report No. 

FHWA-SA-02-054, Author: Kimmerling, R.E., Federal Highway Administration, U.S. 

Department of Transportation. 

33
 
 

 

BIBLIOGRAPHY
 

*FHWA (2005). LRFD for Highway Bridge Substructures and Earth Retaining Structures – 

Reference Manual. Report No. FHWA-NHI-05-094, Federal Highway Administration, U.S. 

Department of Transportation. 

*FHWA (2006). Soils and Foundations. Volumes I and II, Report No. FHWA-NHI-06-088 and 

FHWA-NHI-06-089, Authors: Samtani, N. C. and Nowatzki, E. A., Federal Highway 

Administration, U.S. Department of Transportation. 

*FHWA (2006a). Ground Improvement Methods, FHWA NHI-06-019 (Vol. I) and FHWA NHI06-020 (Vol. II) Authors: Elias, V., Welsh, J., Warren, J., Lukas, R., Collin, J.G. and Berg, 

R.R., Federal Highway Administration, U.S. Department of Transportation. 

*FHWA (2009). Design and Construction of Mechanically Stabilized Earth Walls and Reinforced 

Soil Slopes, FHWA NHI-09-083, Authors: Berg, R.R., Christopher, B. R. and Samtani, N. C., 

Federal Highway Administration, U.S. Department of Transportation. 

Transportation Research Board (TRB) and National Cooperative Highway Research 

Program (NCHRP) 

Bozozuk, M. (1978). “Bridge Foundations Move.” Transportation Research Record (Journal of 

Transportation Research Board), No. 678, National Research Council, Washington, 

D.C., pp. 17-21. 

*NCHRP (1971). Construction of Embankments. Synthesis of Highway Practice 8, 

Transportation Research Board, National Research Council, Washington, D.C. 

NCHRP (1983). Shallow Foundations for Highway Structures. NCHRP Report 107, Author: 

Wahls, H. E., Transportation Research Board, National Research Council, Washington, 

D.C. 

NCHRP (1990). Design and Construction of Bridge Approaches. NCHRP Report 159, Author: 

Wahls, H. E., Transportation Research Board, National Research Council, Washington, 

D.C. 

*NCHRP (1991). Manuals for the Design of Bridge Foundations : Shallow Foundations, Driven 

Piles, Retaining Walls and Abutments, Drilled Shafts, Estimating Tolerable Movements, 

Load Factor Design Specifications, and Commentary. NCHRP Report 343, Authors: 

Barker, R.M., J.M. Duncan, K.B. Rojiani, P.S.K. Ooi, C.K. Tan and S.G. Kim, 

Transportation Research Board, National Research Council, Washington, D.C. 

NCHRP (1997). Settlement of Bridge Approaches (The Bump at the End of the Bridge). NCHRP 

Report 234, Authors: Briaud, J-L, James, R. W., and Hoffman S. B., Transportation 

Research Board, National Research Council, Washington, D.C. 

34
 
 

BIBLIOGRAPHY
 

Shields, D. H., Deschenes, J.H., Scott, J. D., and Bauer, G. E. (1980). “Advantages of Founding 

Bridge Abutments on Approach Fills.” Transportation Research Record (Journal of 

Transportation Research Board), No. 749 (Embankment Stabilization and Soil 

Mechanics). National Research Council, Washington, D.C., pp. 39-42. 

Walkinshaw, J. L. (1978). ‘‘Survey of Bridge Movements in the Western United States.’’ 

Transportation Research Record (Journal of Transportation Research Board), No. 678, 

National Research Council, Washington, D.C., pp. 6–11. 

State DOTs and University Transportation Research Organizations 

*Abu-Hejleh, N., Zornberg, J. G., Wang, T., McMullen, M., and Outcalt, W., (2001). 

Performance of Geosynthetic-Reinforced Walls Supporting the Founders/Meadow Bridge 

and Approaching Roadway Structures. Report No. CDOT-DTD-R-2001-12, Colorado 

Department of Transportation. 

Abu-Hejleh, N., Hanneman, D., White, D. J., Wang, T. and Ksouri, I., (2006). Flowfill and MSE 

Bridge Approaches: Performance, Cost, and Recommendations for Improvements. 

Report No. CDOT-DTD-R-2006-2, Colorado Department of Transportation. 

*Baus, R. L. (1992). Spread Footing Performance Evaluation in South Carolina. Report No. 

F92-102, South Carolina Department of Highways and Public Transportation and the 

Federal Highway Administration, Columbia, SC. 

*Sargand, S. M., and Masada, T. (April 2006). Further Use of Spread Footing Foundations for 

Highway Bridges, State Job Number 14747(0) Final Report FHWA-OH-2006/8 Final 

Report to Ohio Dept. of Transportation, 332 p. 

*Sargand, S. M., Hazen, G. A., and Masada, T. (1997). Field and Laboratory Performance 

Evaluations of Spread Footings for Highway Bridges. Report No. FHWA/OH-98/017, 

Final Report to Ohio Dept. of Transportation, Civil Engineering Dept., Ohio University, 

Athens, OH, 234+ p. 

Yokel, F. Y. (1990). Proposed Design Criteria for Shallow Bridge Foundations. Report No. 

NISTIR 90-4248, U.S. Department of Commerce, National Institute of Standards and 

Technology, Prepared for U. S. Department of Transportation, Federal Highway 

Administration, Turner Fairbanks Highway Research Center, McLean, VA 

Codes and Standards 

*AASHTO (2002). Standard Specifications for Highway Bridges. American Association of State 

Highway and Transportation Officials, Washington, D.C. 

35
 
BIBLIOGRAPHY
 

*AASHTO (2007 with 2009 Interims). AASHTO LRFD Bridge Design Specifications. 4th Edition, 

American Association of State Highway and Transportation Officials, Washington, D.C. 

*ASTM (2005). Annual Book of ASTM Standards. American Society for Testing and 

Materials, West Conshohocken, PA. 

SECONDARY SOURCES 

Journal and Conference Papers 

*Anderson, S. A. and LaFronz, N. (2007). “ Rockfill Embankment Settlement – Sugarloaf 

Mountain Bridge Abutment and Hoover Dam Bypass (US-93),” Transportation Research 

Record: Journal of the Transportation Research Board, No. 2106, Transportation 

Research Board of National Academies, Washington, D. C., pp. 3-12. 

*Akbas, S. O., and Kulhawy, F. H. (2009). “Axial Compression of Footings on Cohesionless 

Soils. I: Load-Settlement Behavior,” Journal of Geotechnical and Geoenvironmental 

Engineering, Vol. 135, No. 11, American Society of Civil Engineers (ASCE), pp. 1562­

1574. 

*Bergdahl, U. and Ottosson, E. (1982). “Calculation of Settlements on Sands from Field Test 

Results,” Proceedings of the Second European Symposium on Penetration Testing, 

Amsterdam. 

*Briaud, J-L., and Gibbens, R. (1999). “Behavior of Five Large Spread Footings in Sand.” 

Journal of Geotechnical and Geoenvironmental Engineering, Vol. 125, No. 9, American 

Society of Civil Engineers (ASCE), pp. 787-796. 

Briaud, J-L., and Lepert, P. (1990). “WAK Test to Find Spread Footing Stiffness.” Journal of 

Geotechnical Engineering, Vol. 116, No. 3, American Society of Civil Engineers (ASCE), 

pp. 415-431. 

Briaud, J-L. (2007). "Spread Footings in Sand: Load Settlement Curve Approach," Journal of 

Geotechnical and Geoenvironmental Engineering, Vol. 133, No. 8, American Society of 

Civil Engineers (ASCE), pp. 905-920. 

*Burland, J. B., and Burbidge, M. C. (1984). “Settlement of Foundation on Sand and Gravel.” 

Proceedings, Institution of Civil Engineers, Part 1, Vol. 78, pp. 1,325-1,381. 

*D’Appolonia, D. J., D’Appolonia, E., and Brissette, R. F. (1968). “Settlement of Spread 

Footings on Sand.” Journal of Soil Mechanics and Foundations Division, Vol. 94, No. 

SM3, American Society of Civil Engineers (ASCE), pp. 735-760. 

36
 
 

 

 

 

BIBLIOGRAPHY
 

*D’Appolonia, D. J., D’Appolonia, E., and Brissette, R. F. (1970). "Closure to Discussion of 

‘Settlement of Spread Footings on Sand’,” Journal of the Soil Mechanics and 

Foundations Division, ASCE, Vol. 96, No. SM2, pp. 754-762. 

*DeBeer, E. (1948). “Settlement Records of Bridges Founded on Sand,” Proceedings, 2nd 

International Conference on Soil Mechanics and Foundations Engineering, Vol. 2. 

*DeBeer, E. and Martens, A. (1956). “Method of Computation of an Upper Limit for the 

Influence of the Heterogeneity of Sand Layers on the Settlements of Bridges,” 

Proceedings, 4th International Conference on Soil Mechanics and Foundations 

Engineering, London. 

*Dunker, K. F. and Liu, D. (2007). “Foundations for Integral Abutments.” Practice Periodical on 

Structural Design and Construction, Vol. 12, No. 1, American Society of Civil Engineers 

(ASCE), pp. 22-30. 

Grant, R., Christian, J. T., and Vanmarcke, R. H. (1974). “Differential Settlement of Buildings,” 

Journal of Geotechnical Division, ASCE, Vol. 100, No. GT9, pp. 973-991. 

*Levy, J. F. and Morton, K. (1974). “Loading Tests and Settlement Observations on Granular 

Soils,” Settlement of Structures Conference, British Geotechnical Society, Cambridge. 

*Meyerhof, G. G. (1953). "The Bearing Capacity of Foundations under Eccentric and Inclined 

Loads." Proceedings of the 3rd International Conference of Soil Mechanics and Foundation 

Engineering, Volume I Zurich, 440-445. 

*Sargand, S. M., Masada, T., and Engle R. (1999) “Spread Footing Foundation for Highway 

Bridge Application.” Journal of Geotechnical and Geoenvironmental Engineering, May, 

pp 373-382. 

Sargand, S. M., Masada, T., and Abdalla, B. (2003). “Evaluation of Cone Penetration Test-Based 

Settlement Prediction Methods for Shallow Foundations on Cohesionless Soils at 

Highway Bridge Construction Sites.” Journal of Geotechnical and Geoenvironmental 

Engineering, Vol. 129, No. 10, American Society of Civil Engineers (ASCE), pp. 900­

908. 

*Schmertmann, J. H., Hartman, J. P., and Brown, P. R. (1978). "Improved Strain Influence 

Factor Diagrams." American Society of Civil Engineers, Journal of the Geotechnical 

Engineering Division, 104 (No. GT8), 1131-1135. 

Skempton, A. W., and MacDonald, D. H. (1956). “Allowable Settlement of Buildings,” 

Proceedings, Institution of Civil Engineers, Part III, Col 5, London, U.K. 

37
 
BIBLIOGRAPHY
 

Tan, C., and J. Duncan. 1991. “Settlement of Footings on Sands: Accuracy and Reliability.” 

Proceedings, Geotechnical Engineering Congress 1991, ASCE Geotechnical Special 

Publication No. 27, Vol. 1, pp. 446–455. 

Books 

*ASCE (1999). Bibliography of Case Histories of Shallow Foundations. Prepared by 

GeoInstitute Committee on Shallow Foundations, Reston VA. 

*Bowles, J. E. (1996). Foundation Analysis and Design. 5th Edition, The McGraw Hill 

Companies Inc., New York, NY 

*Holtz, R. D., and Kovacs, W. D. (1981). An Introduction to Geotechnical Engineering. PrenticeHall, Englewood Cliffs, N.J. 

*Lambe, T. W., and Whitman, R. V. (1979). Soil Mechanics: SI Version. John Wiley & Sons, NY. 

*Perloff, W. H. and Baron, W. (1976), Soil Mechanics: Principles and Applications, Ronald 

Press Co., New York. 

*Terzaghi, K., Peck, R. B., and Mesri, G. (1996). Soil Mechanics in Engineering Practice. 3rd 

Edition, John Wiley & Sons, New York, NY. 

Others 

*Abu-Hejleh, N. (2009). Personal Communication – Unpublished Report. 

*Mertz, D. R. (2009). Personal Communication – Unpublished Report. 

*NCS (2007). In-House Files. NCS Consultants, LLC, Tucson, AZ. 

38
 
 

 

 

 

 

 

 

 

 

 

APPENDIX A – AVAILABLE RESOURCES
 

APPENDIX A – AVAILABLE RESOURCES 

This work was sponsored by the FHWA Resource Center’s Technology Deployment program 

and the Geotechnical and Hydraulics Technical Services Team (TST). Geotechnical specialists 

from the TST are engaged with the FHWA Division offices in each state and territory, and with 

the Divisions of the FHWA Federal Lands Highway program, as well as with state highway 

agencies and local agencies through the Local and Tribal Technology Assistance Programs 

(LTAP and TTAP). Through this engagement the team has an understanding of standards of 

practice (SOP), regional practices, and successes and failures, and is in a unique position to pass 

on these observations and foster deployment of good technologies where they are not currently 

used. 

Working with both the FHWA Headquarters Office and the FHWA Turner Fairbank Highway 

Research Center, the TST is also involved with ongoing research, and in setting current policy 

and guidance. Research is being conducted internally through federal funded programs, through 

pooled fund studies, and through NCHRP. FHWA geotechnical engineers serve in at least a 

liaison role in all of these efforts to guide and stay appraised of developments. Policy and 

guidance is set through the Headquarters Office of Infrastructure in the form of policy 

memoranda or technical guidance documents, such as Geotechnical Engineering Circulars 

(GECs), and National Highway Institute (NHI) training materials and reference manuals. Policy 

and guidance is set through collaboration with AASHTO, TRB and national industry groups such 

as ASCE, and geotechnical team members are active in the committees of these organizations. 

Readers of this report are encouraged to use the FHWA resources to help implement the findings 

of this work as appropriate for their state or region. The TST members and resources can be 

accessed at the following site (or by internet search for FHWA, geotechnical, resource center): 

http://www.fhwa.dot.gov/resourcecenter/teams/geoK\GUDXOLFV

The TST can help states and Divisions, as well as LTAPs and TTAPs through delivering online 

or in person training, through technical assistance at the foundation selection stage of a project, 

or through conducting what is called a ‘Process Review’ of the foundation selection 

process. The review would consider the current process in light of the experiences of others, and 

might reveal specific obstacles for the fair evaluation and selection of spread footings on soils 

that could be overcome, or it might confirm that the current process is reasonable and 

appropriate. 

Readers are also directed to the agency home page for geotechnical engineering at the following 

site (or by internet search for FHWA geotechnical): 

http://www.fhwa.dot.gov/engineering/geotech/ 

This site provides links to references, and to the home pages of the Resource Center and Turner 

Fairbank Highway Research Center, for example. 

39
 

 

 

 

 

 

 

APPENDIX B – FIELD PERFORMANCE DATA FOR LARGE SIZE SPREAD FOOTINGS ON SOILS
 

APPENDIX B – FIELD PERFORMANCE DATA FOR LARGE SIZE SPREAD
 

FOOTINGS ON SOILS
 

As part of this study, an attempt was made to locate field performance data for large size spread 

footings on soils because footing sizes for highway bridges are large as noted in Chapter 2. For 

such large size footings, settlement controls the footing design. Although there are several 

significant data bases in the geotechnical literature that present performance data, most of these 

data are related to building foundations that are comparatively smaller in size. For smaller size 

footings bearing resistance is the controlling factor rather than settlements. Examples of these 

data bases include the work performed by Burland and Burbridge (1984), FHWA (1987), Baus 

(1992), FHWA (1997), Sargand and Masada (2006), etc. 

After screening the available open literature, 84 cases from 6 data bases were found to contain 

useful documented information relevant to this study. It is acknowledged that there are several 

other good data bases for the performance of large spread footings, but these are not available in 

the public domain or are published in languages other than English. In addition, there are 

anecdotal case histories, but they do not have documented data that is useful. 

A summary of the relevant data from the sources found in the open literature is included in this 

Appendix in a tabular format. The summary includes the dimensions of the footings (Bf, Lf, Df), 

maximum contact pressure (qmax), the measured (uncorrected) SPT N-values (N), overburden 

corrected N-values (Ncorr), and measured settlement (Sm). The computed values of Sm/Bf 

expressed as a percentage are presented in the last column of Tables B-1, B-2, B-4 and B-5. The 

range of computed values of Sm/Bf expressed as a percentage is presented in the last two columns 

of Table B-3. These values of Sm/Bf are used in Chapter 2 to evaluate the performance data. 

41
 
APPENDIX B – FIELD PERFORMANCE DATA FOR LARGE SIZE SPREAD FOOTINGS ON SOILS
 

Table B-1. Data established by FHWA (1987). 

Structure Bf Lf/Bf Df/Bf Ncorr qmax Sm Sm/Bf 

(ft) (dim) (dim) (dim) (ksf) (inch) (%) 

1 Bridge 1 – Abutment 1 17.0 3.7 NA 44.0 3.2 0.35 0.17 

2 Bridge 1 – Abutment 2 17.0 3.7 NA 58.0 2.6 0.67 0.33 

3 Bridge 2 – Abutment 1 15.3 3.4 NA 43.0 2.4 0.94 0.51 

4 Bridge 2 – Abutment 2 16.8 3.1 0.2 19.0 2.4 0.76 0.38 

5 Bridge 2 – Pier 12.5 3.3 0.4 12.0 1.8 0.61 0.41 

6 Bridge 3 – W. Abutment 11.0 6.8 NA 34.0 1.8 0.42 0.32 

7 Bridge 3 – E. Abutment 18.5 4.3 0.3 22.0 2.4 0.61 0.27 

8 Bridge 3 – Pier 1 North 21.0 1.0 0.2 18.0 2.0 0.28 0.11 

9 Bridge 3 – Pier 1 South 21.0 1.5 0.2 18.0 1.6 0.26 0.10 

10 Bridge 3 – Pier 2 North 16.0 1.7 0.3 20.0 2.4 0.29 0.15 

11 Bridge 3 – Pier 2 South 16.0 1.2 0.3 22.0 2.4 0.25 0.13 

12 Bridge 3 – Pier 3 North 21.0 1.6 0.2 15.0 1.4 0.97 0.38 

13 Bridge 3 – Pier 3 South 21.0 1.4 0.2 25.0 1.6 0.98 0.39 

14 Bridge 4 – S. Abutment 8.1 5.3 NA 21.0 3.4 0.46 0.47 

15 Bridge 4 – N. Abutment 8.1 5.3 NA 8.0 3.4 0.34 0.35 

16 Bridge 5 – N. Abutment 16.8 4.6 0.4 42.0 2.4 0.23 0.11 

17 Bridge 5 – S. Abutment 15.3 5.0 0.4 24.0 2.4 0.44 0.24 

18 Bridge 6 – Abutment 1 15.3 4.0 0.6 55.0 1.8 2.26 1.23 

19 Bridge 6 – Abutment 2 15.3 4.4 0.6 39.0 1.8 0.83 0.45 

20 Bridge 7 – Abutment 2 28.0 1.0 0.0 24.0 2.2 0.64 0.19 

21 Bridge 8 – Abutment 1 20.0 5.0 1.1 23.0 3.0 0.46 0.19 

22 Bridge 8 – Abutment 2 20.0 5.0 0.3 38.0 3.2 0.66 0.28 

23 Bridge 9 – Abutment 1 21.8 2.0 NA 39.0 3.6 0.61 0.23 

24 Bridge 9 – Abutment 2 16.0 2.8 0.0 49.0 3.4 0.28 0.15 

Note: NA – Not Available 

Table B-2. Data compiled from literature in FHWA (1987). 

1 Bergdahl & Ottosson (1982) 16.4 1.7 0.5 24.0 3.8 0.50 0.25 

2 DeBeer (1948) 19.0 4.2 0.4 17.0 1.6 0.50 0.22 

3 DeBeer (1948) 19.7 2.7 0.5 42.0 3.2 0.30 0.13 

4 DeBeer (1948) 19.7 2.7 0.6 42.0 4.4 0.20 0.08 

5 DeBeer (1948) 23.0 5.1 0.3 42.0 2.8 0.50 0.18 

6 DeBeer (1948) 17.0 5.4 0.4 42.0 2.0 0.40 0.20 

7 DeBeer & Martens (1956) 9.8 3.4 1.0 50.0 4.8 0.80 0.68 

8 DeBeer & Martens (1956) 8.5 8.1 0.8 9.0 4.2 1.30 1.27 

9 Levy & Morton (1974) 13.0 1.8 1.3 32.0 10.6 0.50 0.32 

42
 
APPENDIX B – FIELD PERFORMANCE DATA FOR LARGE SIZE SPREAD FOOTINGS ON SOILS
 

Table B-3. Data established by Sargand et al. (1999) and Sargand and Masada (2006). 

Structure Bf Lf/Bf Df Ncorr (a) qmax Sm Sm/Bf 

(ft) (dim) (ft) (dim) (ksf) (inch) (%) 

1 Bridge A – Panel A&B 

12.0 – 

16.0 

3.00 – 

3.25 3 39/48 3.0 

0.40 0.28 - 0.21 

2 Bridge A – Panel C 0.60 0.42 - 0.31 

3 Bridge A – Panel D 0.70 0.49 - 0.36 

4 Bridge A – Panel E 0.40 0.28 - 0.21 

5 Bridge A – Panel F 0.60 0.42 - 0.31 

6 Bridge A – Panel G/H 0.80 0.56 - 0.42 

7 Bridge B – Abutment 1 

16.8 3.80 – 

4.60 4 41/44 3.4 

0.90 0.45 -

8 Bridge B – Center Pier 0.70 0.35 -

9 Bridge B – Abutment 2 1.00 0.50 -

10 Bridge C (b) – West Footing 4.0 12.20 12 18/16 4.8 1.20 2.50 -

11 Bridge C (b) – East Footing 1.00 2.08 -

12 Bridge D – Pier 1-N 

7.50 – 

10.50 

2.50 – 

3.60 4 37/35 6.4 

0.90 1.00 - 0.71 

13 Bridge D – Pier 1-S 1.00 1.11 - 0.79 

14 Bridge D – Pier 2-N 1.20 1.33 - 0.95 

15 Bridge D – Pier 2-S 1.10 1.22 - 0.87 

16 Bridge D – Pier 3-N 0.60 0.67 - 0.48 

17 Bridge D – Pier 3-S 0.60 0.67 - 0.48 

18 Bridge D – Pier 4-N 0.70 0.78 - 0.56 

19 Bridge D – Pier 4-S 0.60 0.67 - 0.48 

20 Bridge D – Pier 5-N 0.40 0.44 - 0.32 

21 Bridge D – Pier 5-S 0.45 0.50 - 0.36 

22 Bridge E – Rear Abutment 

11.0 – 

15.0 1.90 4 22/22 4.0 

1.40 1.06 - 0.78 

23 Bridge E – Pier 1 0.80 0.61 - 0.44 

24 Bridge E – Pier 3 0.55 0.42 - 0.31 

25 Bridge E – Pier 4 0.60 0.45 - 0.33 

26 Bridge E – Pier 5 0.20 0.15 - 0.11 

27 Bridge E – Pier 6 0.34 0.26 - 0.19 

28 Bridge E – Pier 7 0.42 0.32 - 0.23 

29 Bridge E – Pier 8 0.50 0.38 - 0.28 

30 Bridge E – Forward Abutment 0.48 0.36 - 0.27 

31 Bridge F – Center Pier 8.0 5.00 8 50/49 4.0 0.20 0.21 -

32 Bridge G – Pier 18 21.0 – 

24.0 

2.10 – 

3.20 9 

27/38 

2.8 

0.70 0.28 - 0.24 

33 Bridge G – Pier 19 47/51 0.96 0.38 - 0.33 

34 Bridge G – Forward Abutment 1.68 0.67 - 0.58 

Notes: 

(a) Two values of Ncorr are presented. The first value is the average corrected N-value over a 

depth of 1Bf and the second value is the average corrected N-value over a depth of 2Bf. 

(b) Bridge C is a 3-sided culvert with side walls on footings. For this structure as well with other 

structures with footing widths smaller than 10-ft, the Sm/Bf values are larger. 

43
 
 

 

 

 

 

APPENDIX B – FIELD PERFORMANCE DATA FOR LARGE SIZE SPREAD FOOTINGS ON SOILS
 

Table B-4. Data established by Baus (1992). 

Structure Bf Lf/Bf Df N(a) Q(b) Sm Sm/Bf 

(ft) (dim) (ft) (dim) (kips) (inch) (%) 

1 Bridge 1, Bent 3 – Footing 1 15.0 1.000 5.9 29 678 0.426 0.24 

2 Bridge 1, Bent 3 – Footing 4 15.0 1.000 4.7 39 678 0.520 0.29 

3 Bridge 2, Bent 3, EBL – Footing 1 12.5 1.080 6.2 23 496 0.408 0.27 

4 Bridge 2, Bent 3, EBL – Footing 3 12.5 1.080 6.3 23 496 0.468 0.31 

5 Bridge 2, Bent 3, WBL – Footing 1 12.5 1.080 5.6 23 496 0.300 0.20 

6 Bridge 2, Bent 3, WBL – Footing 3 12.5 1.080 5.7 23 496 0.324 0.22 

7 Bridge 2, Bent 4, EBL – Footing 1 12.0 1.125 9.0 25 495 1.728 1.20 

8 Bridge 2, Bent 4, EBL – Footing 3 12.0 1.125 9.0 25 495 1.631 1.13 

9 Bridge 3, Bent 2, SBL – Footing 1 11.5 1.348 15.0 22 520 0.336 0.24 

10 Bridge 3, Bent 2, NBL – Footing 1 11.5 1.348 9.0 34 520 0.636 0.46 

11 Bridge 3, Bent 2, NBL – Footing 4 11.5 1.348 9.0 47 520 0.444 0.32 

Notes: 

(a) N-value is an average measured (uncorrected) N-value within a depth of approximately 

2Bf. Baus (1992) also report CPT results. 

(b) Q is the estimated dead load. 

Table B-5. Data established by FHWA (1997) [Same as by Briaud and Gibbens, 1999]. 

Structure Bf Lf/Bf Df N(a) Q(b) Sm Sm/Bf 

(ft) (dim) (ft) (dim) (kips) (inch) (%) 

1 Footing 1 (3-m North Footing) 9.85 1.00 2.50 20/50 1011 1.00 0.85 

2 Footing 2 (1.5-m footing) 4.89 1.01 2.50 20/50 337 1.00 1.70 

3 Footing 3 (3-m South Footing) 9.89 1.00 2.92 20/50 1011 1.00 0.85 

4 Footing 4 (2.5-m Footing) 8.19 1.00 2.50 20/50 809 1.00 1.02 

5 Footing 5 (1.0-m Footing) 3.25 1.00 2.33 20/50 191 1.00 2.56 

Notes: 

(a) Two values of measured (uncorrected) SPT N-value are presented. The first value is the 

average N-value over a depth of 30-ft and the second value is the average N-value below 30­

ft to approximately 36-ft which was the bottom of exploration. The report also presents 

subsurface data from a number of different methods, e.g., pressuremeter, CPT, DMT, etc. 

Full load settlement curves are also reported. Refer to the report for details. 

(b) Q is the applied load and is obtained from load-settlement measurements based on 

application of a concentric load and corresponds to a measured settlement, Sm, of 1.00-in. 

Table B-6. Data from FHWA archives. 

Structure Bf Lf/Bf Df/Bf Ncorr qmax Sm Sm/Bf 

(ft) (dim) (dim) (dim) (ksf) (inch) (%) 

1 I-359, Tuscaloosa, Alabama 12.0 1.00 NA NA 8 0.10 0.07 

Note: NA – Not Available 

44
 
 

 

 

 

 

 

APPENDIX C – GENERAL DESIGN CONSIDERATIONS FOR SPREAD FOOTINGS
 

APPENDIX C – GENERAL DESIGN CONSIDERATIONS FOR SPREAD FOOTINGS 

The purpose of this appendix is to present general considerations for the design of spread 

footings. The appendix is organized to present information that addresses the following items: 

	 Terminology (Section C.1) – this section presents definitions and terminology for spread 

footings in general, e.g., isolated, strip, depth of significant influence (DOSI), etc. 

	 Uses (Section C.2) – this section presents the various potential uses of spread footings for 

highway bridges. 

	 Feasibility evaluation (Section C.3) – this section presents various considerations for 

evaluating the feasibility of using spread footings on soils for highway bridges. 

	 Horizontal deformations (Section C.4) – this section presents mechanisms that lead to 

horizontal deformations of spread footings for highway bridges. 

C.1 TERMINOLOGY – GENERAL TYPES OF SPREAD FOOTINGS 

The geometry of a typical spread footing foundation is shown in Figure C.1-1. As the name 

suggests, a spread footing spreads the applied load over the area of contact between the footing 

and the soil and eventually into the subsurface soils beneath the footing. Thus, the spread footing 

relies on bearing resistance in contrast to deep foundations where frequently a large component of 

the applied load is carried by side resistance. The depth of embedment, Df, of spread footings is 

small compared to their cross-sectional size (width, Bf, or length, Lf). 

Figure C.1-1. Schematic. Geometry of a typical spread footing foundation (AASHTO 

2002). 

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APPENDIX C – GENERAL DESIGN CONSIDERATIONS FOR SPREAD FOOTINGS
 

Common types of spread footings are shown in Figures C.1-2 through C.1-4. Following are certain 

size definitions and considerations that help in characterizing spread footing foundations (FHWA 

2006): 

	 Footings with Lf/Bf less than 10 are considered to be isolated spread footings. As shown in 

Figure C.1-2, isolated spread footings are designed to distribute the concentrated loads 

delivered by a single column to prevent shear failure of the soil beneath the footing. The size 

of the footing is a function of the loads imposed by the supported column and the stiffness 

(strength and compressibility) characteristics of the bearing materials beneath the footing. For 

bridge columns, Bf is typically greater than 10 ft (3 m) and Lf/Bf < 10. Bf, and/or Lf increase 

when eccentric loads are applied to the footing. The depth of significant influence (DOSI) for 

settlement analysis, i.e., the depth below the base of the footing to which applied stresses are 

significantly felt in the soil, varies from 2*Bf for Lf/Bf = 1 to 4*Bf for Lf/Bf ≥ 10. Structural 

design of isolated spread footings includes consideration for flexural resistance at the face of 

the column as well as shear and punching around the column. 

	 Footings with Lf/Bf ≥ 10 are considered to be continuous or strip footings. As shown in 

Figure C.1-3, strip footings typically support a bearing wall to reduce the pressure on the 

bearing materials. Sizing and structural design considerations are similar to those for isolated 

spread footings with the exception that plane strain conditions are assumed to exist in the 

direction parallel to the long axis of the footing. This assumption affects the DOSI for 

settlement analysis. In contrast with isolated footings where the DOSI is between 2*Bf to 4*Bf, 

the DOSI in the case of strip footings will always be 4*Bf. The structural design of strip 

footings is generally governed by beam shear and bending moments. 

	 Combined footings are similar to isolated spread footings or continuous footings except that 

they support two or more columns and are rectangular or trapezoidal in shape (Figure C.1-4). 

They are used primarily when the column spacing is non-uniform (Bowles, 1996) or when 

isolated spread footings become so closely spaced that a combination footing is simpler to 

form and construct. Due to the frame action that develops with combined footings, they can 

be used to resist large overturning or rotational moments in the longitudinal direction of the 

column row. When Lf/Bf of the combined footing is ≥ 10, it should be treated analytically like 

a continuous footing; when Lf/Bf of the combined footing is < 10, it should be treated 

analytically like an isolated footing. 

46
 
APPENDIX C – GENERAL DESIGN CONSIDERATIONS FOR SPREAD FOOTINGS
 

Figure C.1-2. Schematic. Isolated spread footing. 

Figure C.1-3. Schematic. Continuous strip footing. 

Figure C.1-4. Schematic. Combined footing. 

47
 
 

 

 

 

 

APPENDIX C – GENERAL DESIGN CONSIDERATIONS FOR SPREAD FOOTINGS
 

C.2 USE OF SPREAD FOOTINGS FOR PIERS, ABUTMENTS AND RETAINING 

WALLS 

Spread footings have been used in a variety of configurations to support bridge abutments and 

piers. The common uses of spread footings for abutments at highway bridges and cantilever 

retaining walls are shown in Figures C.2-1. At piers, the spread footings can be isolated footings 

as shown Figure C.1-2 or combined as shown in Figure C.1-4. Other applications of spread 

footings, e.g., footings on top of MSE walls and for integral (or semi-integral) abutments on soils 

for highway bridges are discussed in Section C.2.1 and Section C.2.2, respectively. 

Compared to spread footings used for buildings, the size of spread footings for highway bridge 

applications is large. For example, the width of spread footings for continuous strip applications 

may range from 10- to 15-ft with lengths as great as 60- to 120-ft or more. For pier applications 

the size of isolated square footings (Lf/Bf = 1) may range from 15-ft x 15-ft to 30-ft x 30-ft with 

rectangular footings in the same size range. The thickness of footings for highway bridges can 

range from 3- to 6-ft. 

 

 

Figure C.2-1. Schematics. (a) Continuous footing for a cantilever stemwall, (b) 

Combined footing for spill-through abutment, (c) Continuous footing for perched 

abutment, (d) Continuous footing for a cantilever retaining wall. 

48
 
 

 

 

 

 

 

APPENDIX C – GENERAL DESIGN CONSIDERATIONS FOR SPREAD FOOTINGS
 

C.2.1 Use of Spread Footing Abutments on Top of MSE Walls 

Often referred to as a “true” bridge abutment, this configuration recognizes the significantly 

increased bearing resistance offered by the properly constructed reinforced backfill of an MSE 

wall acting as the founding material for a spread footing. A typical configuration is shown in 

Figure C.2-2. 

Figure C.2-2. Schematic. “True” bridge abutment with continuous footing on top of a
 

mechanically stabilized earth (MSE) wall.
 

Where the native ground below the MSE wall prism is not anticipated to experience long-term 

settlements, the following are the pertinent criteria for the evaluation of spread footings on top of 

MSE walls (FHWA 2009): 

 Factored bearing resistance for strength limit state = 7 ksf 

 Service limit state bearing resistance at 1⁄2-inch settlement = 4 ksf 

These values of bearing resistance are larger than (often double) the values that are 

conventionally reported by geotechnical specialists for footings on native soils or improved 

ground. As a matter of fact, these values are similar to those reported for IGMs or weak rock, 

because the soil is reinforced with mechanical inclusions such as steel or geosynthetics. As 

noted earlier, typical bearing pressures under footings for highway bridges range from 3 to 6 ksf. 

With the bearing resistance values noted above, these bearing pressures can be easily 

accommodated. 

Use of true bridge abutments can result in significant cost savings. True bridge abutments also 

have significant advantages over conventional abutments as follows: 

	 The proverbial bump at the end of the bridge is eliminated because the footing settles along 

with the MSE wall in contrast to a deep foundation that does not settle at the same rate as the 

surrounding MSE walls. 

	 Approach slabs are not necessary because of the elimination of conditions that would lead to 

the bump at the end of the bridge. The elimination of approach slabs results in significant 

cost savings. 

49
 
 

 

 

 

 

 

APPENDIX C – GENERAL DESIGN CONSIDERATIONS FOR SPREAD FOOTINGS
 

C.2.2 Use of Spread Footings for Integral Abutments 

Integral and semi-integral abutments are being increasingly used in an attempt to eliminate the 

maintenance-intensive and troublesome joints and bearings. There are also no abutment seats to 

collect debris and no damage caused by leaking expansion joints. In the case of integral 

abutments, the upper portion of the abutment, sometimes called an end-wall, encases the beam 

ends and is integral with the bottom part of the abutment or pile cap. For semi-integral 

abutments, the upper portion of the abutment is entirely isolated by expansion material and/or a 

bearing pad, allowing it to move relative to the bottom portion. 

Dunker and Liu (2007) present a summary of various foundation types for integral abutments. 

Virtually all integral abutments currently rely on flexible deep foundations in the form of H-piles 

with the weaker axis in the direction of the longitudinal axis of the bridge to permit the necessary 

thermal movement at the end of the bridge. Such is not the case with semi-integral abutments, 

where the end-wall can move independently of the bottom portion of the abutment and they can 

be founded on shallow or deep foundations. Instead of placing the integral abutment on a deep 

foundation, the Nevada Department of Transportation (NDOT) successfully places it on a spread 

footing as shown in Figure C.2-3. This proven method should be considered by other agencies to 

realize the significant synergistic cost savings of integral abutments and spread footings. The 

Colorado Department of Transportation has had success with a true bridge semi integral 

abutment on an MSE wall at the Founders/Meadows Bridge over I-25 (Abu-Hejleh et al., 2001). 

 

 

Figure C.2-3. Schematic. “Diaphragm-on-footing” type integral abutment on spread 

footing in an end slope used by Nevada Department of Transportation. 

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APPENDIX C – GENERAL DESIGN CONSIDERATIONS FOR SPREAD FOOTINGS
 

C.2.3 Quality Control and Drainage 

The two applications of spread footings described in sections C.2.2 and C.2.3 involve 

construction of spread footings on or in compacted fill materials. Improperly compacted fill 

materials will have variable stiffness that may result in uneven settlement and rotation of a 

footing founded in or on the fill. In addition, improper drainage will permit ingress of moisture 

into the subsoil under the footing thereby leading to softening of the subsoil and associated 

settlement problems. Therefore, for all applications of spread footings on or in compacted fill 

materials, it is critical to ensure proper quality control of compaction during construction and 

implementation of proper surface and subsurface drainage measures. It should be noted that 

these recommendations do not constitute special or extraordinary requirements, but are simply 

the best practice for successful applications of the spread footings to MSE wall and integral 

abutments. Further discussion on the recommended properties of compacted fill materials is 

provided in Section C.3.3 of this appendix. 

51
 
 

 

 

 

 

 

 

APPENDIX C – GENERAL DESIGN CONSIDERATIONS FOR SPREAD FOOTINGS
 

C.3 FEASIBILITY EVALUATION 

As indicated in Section C.2, spread footings can be used to support bridge elements a number of 

different ways. In all such uses the foundation soils must be competent enough to support the 

bridge loads without undergoing catastrophic soil shear failure or excessive deformations that 

would result in unacceptable settlements. Section C.3 first discusses the unfavorable situations 

in which spread footings are not viable or should not be considered. Then, as a prelude to the 

discussion of the favorable situations for spread footing applications, a summary is presented of 

the subsurface investigations required to perform a proper evaluation of the load-displacement 

characteristics of the footing-soil system. As indicated in Chapters 3 and 4 of this report, the 

load-displacement characteristics are of primary importance in assessing the feasibility of using 

spread footings for a given application. Finally, the use of spread footings on competent native 

soils, improved soils, and compacted fill materials is discussed in detail. Thus, this section 

presents information for the bridge designer to evaluate the feasibility of using spread footings 

on soils for highway bridges. 

C.3.1 Unfavorable Situations 

Similar to any foundation type, spread footings have limitations with respect to their applications 

for highway bridges. For a feasibility evaluation, the following two key points must be 

understood: 

1.	 Since the base of a spread footing is generally within a few feet of the finished grade, 

consideration should be given to all external conditions that can alter the conditions within 

the depth of embedment. 

2.	 As noted in Section C.1, the DOSI below the footing is a function of the footing width. 

Therefore, any adverse geologic conditions or potential future changes within the DOSI 

should be considered while evaluating the feasibility of spread footings. 

Based on these considerations, spread footings are not feasible for the following situations: 

 Stream crossings where scour is a concern 

 Liquefiable soils 

 Deep collapsible soil deposits 

 Soils with swell pressure larger than footing pressure 

 Karstic deposits 

 Permafrost areas 

 Deep frost penetration 

 Areas of tidal fluctuations 

 Possibility of future unsupported excavations below the base of the footing 

 Significant long-term settlements that would affect the structural integrity of the bridge 

52
 
 

 

 

 

 

APPENDIX C – GENERAL DESIGN CONSIDERATIONS FOR SPREAD FOOTINGS
 

Other conditions which might limit the use of spread footings are as follows: 

 Limited right-of-way which would control the size of the footing 

 Excavation of contaminated soils 

 Significant dewatering for cases where the water table is within the depth of embedment 

 Situations where groundwater may rise within the DOSI in the future (e.g., in areas of 

groundwater recharge or adjacent significant unlined water bodies/facilities, which may leak) 

C.3.2 Subsurface Investigations 

Once the unfavorable situations are screened for a given project and a spread footing system has 

been identified as a potential candidate for foundations of a bridge structure, then an adequate 

subsurface investigation program must be implemented to evaluate the continued feasibility of 

the spread footing system. As noted in Section C.1, a “spread footing” spreads the applied load 

over the area of contact between the footing and the soil and eventually into the subsurface soils 

beneath the footing. This is in contrast to deep foundations where a substantial portion of load is 

commonly supported by side friction, which is concentrated in the immediate vicinity of the deep 

foundation element. The contact pressure under a spread footing is spread vertically and 

horizontally below the base of the footing with its magnitude decreasing with depth in multiples 

of the footing width. The performance of a spread footing will be directly affected by the 

stiffness characteristics of the subsurface soils within the DOSI. Therefore, to evaluate the 

feasibility of spread footings particular care should be taken in evaluating the subsurface soil 

characteristics within the DOSI. This report assumes that a minimum standard of investigations 

as outlined in FHWA (2006) and Section 10 of AASHTO (2007 with 2009 Interims) is 

rigorously implemented. For critical highway bridges, such as those on life-line routes (e.g., 

interstate highways), consideration may be given to increasing the level of field and laboratory 

investigations particularly in those areas where prior information and/or experience is poor or not 

available. 

Performing an adequate subsurface investigation program does not guarantee proper 

performance of a spread footing. As with any foundation system, it is incumbent on the owner to 

implement an adequate construction inspection and monitoring program. Appendix D provides 

information on such a program and this report assumes that the minimum standard of care noted 

in Appendix D is implemented. Thus, the aspects of subsurface investigations, construction 

inspection and monitoring should be part of the feasibility evaluation of spread footings. It 

should be noted that these recommendations do not constitute special or extraordinary 

requirements, but are simply best practice for routine applications. 

C.3.3 Categories for Use of Spread Footings on Soils for Highway Bridges 

Once the subsurface investigations have been performed, the stiffness of the soils within the 

DOSI should be evaluated to verify if the spread footing system can be feasible with respect to 

the tolerable deformations established by the structural specialist. In this context the use of 

spread footings can be categorized based on the types of ground as follows: 

53
 
 

 

 

 

 

 

 

 

 

 

APPENDIX C – GENERAL DESIGN CONSIDERATIONS FOR SPREAD FOOTINGS
 

 Spread footings on competent native soils – in this category the stiffness of the native soils is 

deemed to be adequate to support the loads within tolerable settlements. 

 Spread footings on improved in situ soils – in this category it is deemed that the in situ soils 

need mechanical and/or chemical treatment to improve their stiffness so that the loads can be 

supported within tolerable settlements. 

 Spread footings on compacted engineered embankment fill materials – in this category the 

footings are supported in fills constructed of select soils placed under engineered conditions. 

C.3.3.1 Spread Footings on Competent Native Soils 

Existing native soils generally suitable for the use of spread footings include dense to very dense 

sands, gravels, and sand and gravel mixtures; medium- to highly-cemented silty sands that are 

not susceptible to hydro-compaction; glacial tills and highly overconsolidated lean clays. Spread 

footings are feasible in such soils provided they exist to a depth below the base of the footing at 

least equal the DOSI as discussed in Section C.1. To assure uniformity below the base of the 

footing, these soils are generally scarified at least 12-inches at the foundation base elevation and 

compacted to a minimum relative compaction of 95% based on Modified Proctor compaction 

energy. Spread footings are also feasible for conditions where native soils are underlain at 

shallow depth by competent rock that is not subject to severe degradation by weathering, e.g., 

shales. In such cases the overlying soil is usually excavated and the spread footings are founded 

on the competent rock. The use of spread footings for the support of bridge structures in the 

soils described above is feasible only if there is no danger of significant erosion or scour. 

C.3.3.2 Spread Footings on Improved In-Situ Soils 

In general, in-situ soils that are not suitable for the use of spread footings include: alluvial soils 

(fans), fluvial soils (flood plain), colluvial soils, collapse-susceptible soils including loess, 

expansive soils, loose sands and silts, soft or organic clays, formations where large near-surface 

voids are present (karst topography, sinkholes), former landfills, disposal sites for building 

rubble and construction debris and other non-engineered fills. In order to carry the loads 

imposed by bridge structures the load-displacement characteristics of such soils must be 

improved mechanically or chemically. 

The feasibility of using spread footings on improved in-situ soils generally depends upon the 

thickness of the soils that need to be improved and the depth to which the improvement can be 

undertaken economically. There is a practical limit to this depth that depends upon a number of 

factors including footing size, loading, the ability of the unsuitable soils to be stabilized, the 

economy of the stabilization method versus the cost of bypassing the poor soils entirely by deep 

foundations, etc. The methods most commonly used to stabilize near surface soils so that 

shallow spread footings are a feasible option for bridge abutments and piers are: 

 surcharging, 

 vibrocompaction, 

54
 
 

 

 

 

APPENDIX C – GENERAL DESIGN CONSIDERATIONS FOR SPREAD FOOTINGS
 

 deep dynamic compaction,
 

 excavation and recompaction,
 

 excavation with removal and replacement, and
 

 chemical stabilization by mixed-in-place admixtures such as lime and Portland cement.
 

Information on the above methods for ground stabilization can be found in FHWA (2006a).
 

C.3.3.3 Spread Footings on Compacted Engineered Embankment Fill Materials 

Figure C.3-1 shows a comparison of stress distribution under earth embankment and pier loads. 

As can be clearly seen, the weight of the embankment is predominant except at very shallow 

depths. Based on this comparison, geotechnical specialists have long recognized the feasibility 

of placing footings on engineered fills. If adequate time is allowed for the foundation soils to 

settle under the fill load, subsequent application of a smaller structural load will result in 

negligible settlement of the structure. In bridge construction, common practice is to build the 

approach embankment excluding the area to be occupied by the abutment and allow settlement to 

occur prior to abutment construction. Details of the evaluation of settlement of approach 

embankment fills are presented in FHWA (2006). 

Figure C.3-1. Figure. Computation of vertical stresses beneath center lines of bridge 

piers and earth embankments (NCHRP, 1971). 

55
 
 

 

 

 

 

APPENDIX C – GENERAL DESIGN CONSIDERATIONS FOR SPREAD FOOTINGS
 

Compacted structural fill materials used for supporting spread footings should be a select and 

specified material that includes sand- and gravel-sized particles. Furthermore, the fill material 

should be compacted to a minimum relative compaction of 95% based on Modified Proctor 

compaction energy. The fill material should extend through the entire embankment below the 

footing. Suggested approach embankment details for spread footing applications are shown in 

Figure C.3-2. The soil zoning shown in Figure C.3-2 also controls the internal embankment 

deformations (FHWA, 2006). FHWA (2002c) notes that the Washington State Department of 

Transportation (WSDOT) successfully used the gradation listed in Table 4.1 to design spread 

footings for the I-5 Kalama Interchange. WSDOT limited the maximum bearing pressures to 6 

ksf. The measured settlements were found to be less than 1.5-inches within the fill. 

In addition to WSDOT, the Nevada Department of Transportation (NDOT) commonly uses 

spread foundations founded within compacted structural embankment fills (FHWA, 2006). A 

successful case history of the use of an abutment spread footing on a 130-ft high welded wire 

reinforced rockfill embankment is also presented in Anderson and LaFronz (2007). 

Direct shear testing of materials such as those described in Table C.3-1 is not practical on a 

project-by-project basis since such materials require large specialized test equipment. Therefore 

the design of spread footings on compacted sand and gravel is based on a combination of 

experience and the results of infrequent large-scale laboratory testing on specified gradations of 

select fill materials. Material specifications are then developed based on the specified gradations 

to ensure good quality control during construction. This procedure helps ensure that the 

conclusions from the laboratory tests are valid for the construction practices used to place the 

fills. 

Table C.3-1. Typical specification for compacted structural fill material used by WSDOT 

(FHWA, 2006). 

Sieve Size Percent Passing 

4” (100 mm) 100 

2” (50 mm) 75 – 100 

No. 4 (4.75 mm) 50 – 80 

No. 40 (0.425 mm) 30 max 

No. 200 (0.075 mm) 7 max 

Sand Equivalent (ASTM D2419) 42 min 

56
 
 

APPENDIX C – GENERAL DESIGN CONSIDERATIONS FOR SPREAD FOOTINGS
 

Figure C.3-2. Schematic. Suggested approach embankment details for spread 

footing application (FHWA, 2006). 

57
 
 

 

 

 

 

APPENDIX C – GENERAL DESIGN CONSIDERATIONS FOR SPREAD FOOTINGS
 

C.4 HORIZONTAL DEFORMATIONS 

Chapters 2 to 5 of the report concentrated on the issue of vertical deformations or what is 

commonly called settlement. Although settlements are important, horizontal deformations can 

often cause more severe and widespread damage than an equal magnitude of vertical movements 

(FHWA, 1985). For spread footings, horizontal deformations can occur because of the following 

two mechanisms: 

 lateral loads

 lateral squeeze of the foundation soil

C.4.1 Horizontal Deformations Due To Lateral Loads 

Assuming that adequate drainage features are in-place and functioning satisfactorily, the primary 

source of lateral loads at abutments is earth fill and any surcharges behind the abutment. If 

appropriate drainage is not provided, then additional lateral loads can occur due to the build-up 

of hydrostatic pressures and frost action. Assuming that the abutment walls are free to displace 

laterally and the foundation soils are competent, the minimum movement that can be anticipated 

for design is the movement required to mobilize the active earth pressure. Such lateral 

movements can occur by sliding at the base of the spread footing and/or by rotation of the 

abutment stem wall. In the later case, the spread footing is subjected to rotation (tilting). In 

either case, the primary concern is the horizontal movement at the superstructure level. 

Generally granular fills are used at abutment locations. For these types of materials, the typical 

horizontal movements that can be anticipated are in the range of 0.001 to 0.004 times the height 

of the abutment wall. Thus, for example, if the abutment is 20-ft tall, horizontal movements in 

the range of 1⁄4- to 1-inch may be anticipated. In a general construction sequence, the earth fill 

behind the abutment is substantially complete prior to placement of the superstructure. In this 

case, the horizontal movement at the superstructure level is virtually complete and should not be 

of concern assuming that the vertical joint between the end of the superstructure and the 

abutment back-wall was designed properly to accommodate the movement. However, the lateral 

movements caused by lateral loads due to surcharges, such as live loads and thermal effects, 

experienced by the abutment after the placement of the superstructure should be considered in 

the design of the bridge structure. 

At pier locations, the primary source of lateral loading is from thermal effects, braking forces 

and forces due to unequal spans if any exist on either side of the pier. Assuming that the pier 

substructure has sufficient structural resistance, these lateral loads are primarily resisted by 

sliding resistance at the base of the spread footing. Where the foundation soils are weak in shear 

strength, e.g., fine-grained clayey soils, the interface shear strength may be small which increases 

the potential for sliding. Once the interface shear strength is overcome by the horizontal forces, 

large sliding movements can occur that can cause significant damage to the superstructure. 

Where such conditions exist, consideration should be given to excavating the weaker soils and 

58
 
 

 

APPENDIX C – GENERAL DESIGN CONSIDERATIONS FOR SPREAD FOOTINGS
 

replacing them with select granular fill materials. Consideration should also be given to 

incorporating a shear key at the base of the spread footing. 

C.4.2 Horizontal Deformations Due To Lateral Squeeze 

When the width of the footing is larger than the thickness of the compressible layer beneath it or 

when there is a finite soft layer within the DOSI below the loaded area, significant lateral stresses 

and associated lateral deformations can occur within the soil. These deformations are transmitted 

to the superstructure. Figure C.4-1 shows schematics of such movements at pier and abutment 

locations. The lateral squeeze phenomenon is due to an unbalanced load at the surface of the soft 

soil. The lateral squeeze behavior may be: (a) short-term undrained deformation that results in 

horizontal deformation from a local bearing resistance type of failure, or (b) long-term drained, 

creep-type deformation. Creep refers to the slow deformation of soils under sustained loads over 

extended periods of time and can occur at stresses well below the shear strength of the soil. The 

bridge abutment may tilt forward or backward depending on a number of factors including the 

relative configuration of the fill and the abutment, the relative stiffness of the footing and the soft 

deposit, the strength and thickness of the soft layer, and the rate of construction of the fill. 

In all cases, these types of movements will likely occur only if inadequate subsurface 

investigations were performed that missed the presence of the softer layers within the DOSI. 

Where such softer layers exist within the DOSI, spread footings should not be used without some 

ground improvement. 

 

Figure C.4-1. Schematic. Horizontal deformations at the superstructure level due to 

settlement and rotation of spread footings for the case of soft layers within DOSI. (a) at 

piers, (b) at abutments. 

59
 

 

 

 

 

 

 

 

 

 

APPENDIX D – CONSTRUCTION INSPECTION, MONITORING AND INSTRUMENTATION
 

APPENDIX D – CONSTRUCTION INSPECTION, MONITORING AND
 

INSTRUMENTATION
 

Construction inspection requirements for spread footings are similar to those for other concrete 

structures. In some cases, agencies may have inspector checklists for construction of shallow 

foundations. Table D-1 provides a summary of construction inspection check points for spread 

footings on soils. Throughout construction, the inspector should check submittals for 

completeness before transmitting them to the engineer. 

D.1 STRUCTURAL FILL MATERIALS 

Fill requirements should be strictly adhered to because the fill must perform within expected 

limits with respect to strength and, more importantly, within tolerance for differential settlement. 

Sometimes the area for construction of the fill is small, such as behind abutments and wingwalls. 

In such situations, the use of hand compactors or smaller compaction equipment may be 

necessary. 

When the construction of structural fills that will support shallow foundations is being 

monitored, particular attention should be paid to the following items: 

	 The material should be tested for gradation and durability at sufficient frequency to ensure 

that the material being placed meets the specification. 

	 The specified level of compaction must be obtained in the fill. Testing, if applicable, should 

be performed in accordance with standard procedures and at the recommended intervals or 

number of tests per lift. 

If a surcharge fill is required for pre-loading, it should be verified that the unit weight of the 

surcharge fill meets the value assumed in the design. 

D.2 MONITORING 

The elevations of constructed foundations should be checked before and after the structural load 

is applied. The measurements made at those times will serve as a baseline for the long-term 

monitoring of the bridge. Subsequently, additional survey measurements should be made to 

confirm satisfactory performance or to identify whether potentially harmful settlements are 

occurring. If the fill was constructed over soft compressible soils, it may be important to check 

that the fill settlements are complete before foundation construction begins. Instruments such as 

settlement plates, horizontal inclinometers, or other types of instrumentation are typically 

installed in such cases. In some instances, the lateral displacement potential can be greater than 

the vertical movements; therefore, if conditions warrant, monitoring may also include complete 

survey coordinates and possibly more accurate instrumentation. Instrumentation is discussed in 

Section D.3. 

61
 
 

 

 

APPENDIX D – CONSTRUCTION INSPECTION, MONITORING AND INSTRUMENTATION
 

Table D-1. Inspector responsibilities for construction of spread footings (FHWA, 2006). 

CONTRACTOR SET UP 













Review plans and specifications. 

Review contractor’s schedule. 

Review test results and certifications for pre-approved materials, e.g., cement, coarse and fine 

aggregate. 

Confirm that the contractor’s stockpile and staging area are consistent with locations shown on 

plans. 

Discuss anticipated ground conditions and potential problems with the contractor. 

Review the contractor’s survey results against the plans. 

EXCAVATION 















Verify that excavation slopes and/or structural excavation support is consistent with the plans. 

Confirm that limits of any required excavations are within right-of-way limits shown on the plans. 

Confirm that all unsuitable materials, e.g., sod, snow, frost, topsoil, soft/muddy soils, are removed 

to the limits and depths shown on the plans and the excavation is backfilled with properly 

compacted granular material. The in-place bearing stratum of soil or rock should be checked 

to verify the in-situ condition and the degree of improvement achieved by the contractor’s 

preparation approach. Some soil types can become remolded and weakened from 

disturbance. If the conditions deviate from those anticipated in the geotechnical report 

and/or the plans and specifications, the geotechnical specialist should be consulted to 

determine if additional measures are necessary. 

Confirm that leveling and proof-rolling of the foundation area is consistent with the requirements 

of the specifications. Probing is recommended for verification of subgrade. 

Confirm that contractor’s excavation operations do not result in significant water ponding. 

Confirm that existing drainage features, utilities, and other features are protected. 

Identify areas not shown on the plans where unsuitable material exists and notify engineer. 

SPREAD FOOTING 



















Approve footing foundation condition before concrete is poured. 

Confirm reinforcement strength, size, and type consistent with the specifications. 

Confirm consistency of the contractor’s outline of the footing (footing size and bottom of footing 

depth) with the plans. 

Confirm location and spacing of reinforcing steel consistent with the plans. 

Confirm water/cement ratio and concrete mix design consistent with the specifications. 

Record concrete volumes poured for the footing. 

Confirm appropriate concrete curing times and methods as provided in the specifications. 

Confirm that concrete is not placed on ice, snow, or unsuitable ground. 

Confirm that concrete is being placed in continuous horizontal layers and that the time between 

successive layers is consistent with the specifications. 

POST INSTALLATION 

 Verify pay quantities. 

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APPENDIX D – CONSTRUCTION INSPECTION, MONITORING AND INSTRUMENTATION
 

D.3 INSTRUMENTATION 

The three primary parameters of interest with respect to performance monitoring of a spread 

footing are as follows: 

 Settlement 

 Bearing pressure 

 Rotation (or tilting) 

A typical layout of instrumentation is shown in Figure D-1(a). The instrumentation needed to 

achieve the measurements of the three parameters listed above is as follows (after Sargand and 

Masada, 2006 and FHWA 1987): 

	 Settlement – Settlement monitoring points are placed over the top surface of the footing to 

allow vertical displacement measurements with respect to a stationary benchmark located 

nearby. A minimum of 5 monitoring points are recommended with one point located at or 

near the center of the footing and the remaining four points located symmetrically near the 

footing corners. The monitoring points can be in the form of stainless steel eyebolts 

anchored into the footing and encased in a 4-inch diameter PVC riser pipe. Survey methods 

for leveling can be used to detect the vertical displacement at each point with respect to the 

stationary benchmark. 

	 Bearing pressure – Pressure cells are positioned at the base of the footing to measure the 

magnitude and distribution of the contact pressure. A minimum of five pressure cells are 

used and their position is similar to that of the settlement points except that the cells are 

located at the base of the footing. If the footing supports more than one column then at least 

one pressure cell should be located directly under the center of each column. 

The pressure cell should be of the vibrating-wire type with a minimum diameter of 9-inches. 

Each pressure cell is typically precast in a concrete block which is 12” wide x 24” long x 2” 

thick. This precasting is necessary to prevent the cell from being disturbed during footing 

construction. In the field, each precast pressure cell block is placed carefully at the 

predetermined location with the sensing disk pressed against a 2” thick compacted sand 

layer. A nonwoven geotextile is inserted between the sand layer and the bearing soil to keep 

the two dissimilar materials separate. The pressure cell in each block should be carefully 

calibrated before the concrete for the footing is placed. The range of each pressure cell 

should be from 0 to 100 psi with a sensitivity of 0.1 psi. Provisions should be made for 

extending the pressure cell wires out of the footing to a convenient and safe location where a 

readout unit can be attached to obtain the measured values. 

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APPENDIX D – CONSTRUCTION INSPECTION, MONITORING AND INSTRUMENTATION
 

(a) General layout of instrumentation for monitoring settlement, bearing pressure and 

tilting 

 

 

Figure D-1. Schematic. Typical instrumentation setup for a spread footing foundation. 

(Sargand and Masada, 2006) 

64
 
 

APPENDIX D – CONSTRUCTION INSPECTION, MONITORING AND INSTRUMENTATION
 

	 Rotation (or tilting) – Rotation of the spread footing is inferred from the tilt of the 

substructure unit (pier column or abutment) above the footing. Tilting is measured by 

accelerometers attached to the substructure unit. Tilt stations are established on the 

substructure unit as shown in Figure D-1(a). At each monitoring station, two stainless steel 

reference points are grouted at least 2-inches deep into the substructure unit approximately 

2.5-ft apart vertically as shown in Figure D-1(b). For taking tilt measurements, a stainless 

steel ball joint is screwed into each reference point and a reference plate is set to rest freely 

against the ball joints. An accelerometer is then attached to the side of the reference plate as 

shown in Figure D-1(c). Data is collected from a readout box that is connected to the 

accelerometer. The sensor should have a range of ±30° and a sensitivity of 0.003°. 

A full suite of readings for the three parameters should be obtained at every significant 

construction-point as noted in Chapter 4. Where deformations are anticipated to occur after the 

end of construction, a suitable frequency of measurements should be established on a projectspecific basis. 

65
 
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APPENDIX E – LRFD GUIDANCE FOR SPREAD FOOTINGS
 

APPENDIX E – LRFD GUIDANCE FOR SPREAD FOOTINGS 

The Load and Resistance Factor Design (LRFD) methodology is currently being implemented 

across the United States, particularly for federally funded transportation facilities. The American 

Association of State Highway and Transportation Officials (AASHTO) recently released the 4th 

Edition of the LRFD Bridge Design Specifications in 2007. Since October 1, 2007, the 

AASHTO-LRFD approach must be fully implemented by states seeking federal funding for new 

transportation projects. It is important that the structural and geotechnical specialists involved in 

the design of such transportation facilities properly understand the basics of the LRFD approach 

as included in AASHTO’s specifications (AASHTO, 2007 with 2009 Interims). 

The purpose of Appendix E is to present detailed guidance for the analysis of spread footings in 

the LRFD framework. To achieve this purpose the appendix is organized as follows: 

	 Limit states and spread footings (Section E.1) – this section presents a summary of the 

primary limit states in the LRFD framework and the categories of the various failure modes 

for spread footings with respect to the LRFD-based limit states. 

	 Spread footing design based on geotechnical considerations (Section E.2) – this section 

introduces and discusses the concept of the bearing resistance chart to evaluate the strength 

and service limit states for spread footings. 

	 Spread footing design using LRFD methodology (Section E.3) – this section presents and 

discusses a detailed design process flowchart for the design of spread footing using LRFD 

methodology. 

	 Numerical example for spread footing analysis using LRFD methodology (Section E.4) – this 

section presents the calculations for a comprehensive numerical example using the various 

concepts introduced in the report and described in the previous sections of Appendix E. 

E.1 LIMIT STATES AND SPREAD FOOTINGS 

In the AASHTO-LRFD framework, there are four distinct limit states: (a) strength (or ultimate) 

limit states, (b) service limit states, (c) extreme event limit states and (d) fatigue limit states. For 

most routine cases, the design of a bridge or a component is generally governed by either the 

strength or the service limit states. These two commonly analyzed limit states are briefly 

described below: 

	 Strength (or ultimate) limit states are limit states that pertain to structural safety and the loss 

of load-carrying capacity. These limit states may be reached through either geotechnical or 

structural failure. Evaluation of strength limit states is based on inelastic behavior of the 

structure, which is accomplished by using increased or factored loads, and on modification of 

soil behavior, which is accomplished by using reduced or factored strengths. From a 

geotechnical viewpoint, strength limit states are reached when they involve the partial or total 

collapse of the structure due to sliding, bearing failure, etc. Figure E.1-1 shows the three 

67
 
 

 

 

 

APPENDIX E – LRFD GUIDANCE FOR SPREAD FOOTINGS
 

strength limit states for spread footings. For well-designed structures strength limit states 

have an extremely low probability of occurrence. 

	 Service limit states are the limiting conditions affecting the function of the structure under 

expected service conditions. Thus, service limit states address serviceability and include 

conditions short of complete collapse that may restrict the intended use of the structure, e.g., 

excessive total or differential settlements, cracking, local damage, poor ride quality, etc. 

Evaluation of service limit states is usually performed by using expected service loads, 

nominal strengths and elastic analyses. Compared to strength limit states, the service limit 

states have a higher probability of occurrence, but, if exceeded, involve less likelihood of 

loss of life. Figure E.1-2 shows the two service limit states for spread footings. An 

associated service limit state that involves horizontal movement and rotation (due to coupling 

with vertical and horizontal movements) is also possible. 

Due to the rather large size of spread footings for highway bridges, deformations at service limit 

states are generally the controlling factors in footing design. Finally, extreme event limit states 

are also evaluated by considering each of the failure modes shown in Figure E.1-1 for strength 

limit state. Settlements are not a consideration at extreme event limit state since the expectation 

is to preserve life and not necessarily the serviceability of the structure. 

 

Figure E.1-1. Schematic. Strength limit states for spread footings. (a) Sliding, (b) Limiting 

eccentricity, (c) Bearing resistance. 

 

Figure E.1-2. Schematic. Service limit states for spread footings. (a) Settlement, (b)
 

Overall stability.
 

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APPENDIX E – LRFD GUIDANCE FOR SPREAD FOOTINGS
 

E.2 SPREAD FOOTING DESIGN BASED ON GEOTECHNICAL 

CONSIDERATIONS 

The design of a spread footing based on geotechnical considerations is a two part process. First 

the factored soil bearing resistance must be established to ensure stability of the foundation and 

determine if the proposed structural loads can be supported on a reasonably sized foundation. 

Second, the amount of settlement due to the actual structural loads must be calculated and the 

time of occurrence estimated. Experience has shown that settlement is usually the controlling 

factor in the decision to use spread footings for highway bridges. This is not surprising since 

structural considerations usually limit tolerable settlements to values that can be achieved only 

on competent soils not prone to a bearing resistance failure. Thus, the factored net bearing 

resistance of a spread footing is defined as the lesser of: 

 The applied stress that is equal to the net nominal bearing resistance (qnn) multiplied by a 

suitable resistance factor (b). This criterion is based on a strength limit state as discussed 

in Section E.1. 

or 

 The applied stress that results in a specified amount of settlement (Stol). Stol is generally 

dictated by structural considerations and/or institutional culture. This criterion is based on a 

service limit state as discussed in Section E.1. 

Both of these criteria are a function of the least lateral dimension of the footing, typically called 

the effective footing width and designated as B'f. The effective footing width is defined as the 

total footing width, Bf, minus two times the eccentricity in the direction of the width of the 

footing, eB, (Meyerhof, 1953). Thus, B'f = Bf – 2eB and, if there is no load eccentricity, B'f = Bf. 

The influence of effective footing width on factored net bearing resistance and settlement is 

shown conceptually in Figure E.2-1. The factored net bearing resistance of a footing is usually 

limited by soil shear-failure considerations for narrow footing widths as shown in Zone A in 

Figure E.2-1. As the footing width increases, the factored net bearing resistance is generally 

controlled by the value of (Stol) with respect to the settlement potential of the soils supporting 

the footing, as expressed by the contours of net bearing resistance for given settlements shown in 

Zone B in Figure E.2-1. The contour of net bearing resistance for a given settlement varies with 

effective footing width because the DOSI is a function of footing width as discussed in Chapter 

3.1 and Section C.1 of Appendix C. Stated another way, for a given applied load, as the footing 

width increases the applied stress decreases; therefore, the stress increase experienced by the soil 

below the footing may also decrease. However, the effect of the applied stress will extend 

deeper below the footing base, i.e., the DOSI increases. Therefore, a stress increase will be 

experienced by soils that had not been influenced previously so that settlements may actually 

increase depending on the types of soils within the deeper DOSI. The result of this 

interrelationship is shown schematically in Zone B in Figure E.2-1. 

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APPENDIX E – LRFD GUIDANCE FOR SPREAD FOOTINGS
 

The concept of decreasing factored bearing resistance with increasing footing width for the 

settlement-controlled cases is an important concept to understand. In such cases, the factored net 

bearing resistance is the value of the applied stress at the footing base that will result in a given 

settlement. Since the DOSI increases with increasing footing width, the only way to limit the 

settlements to a certain desired value (Stol) is by reducing the applied stress. The more stringent 

the settlement criterion (Stol) the less the stress that can be applied to the footing, i.e., the factored 

net bearing resistance is correspondingly less. This concept is illustrated in Figure E.2-1, which 

shows that decreasing the settlement, i.e., going from 3S to 2S to S decreases the factored 

bearing resistance at a given footing width. More details regarding the process for development 

of charts similar to that shown in Figure E.2-1 can be found in FHWA (2006). A numerical 

example of the use of the bearing resistance chart is presented in Section E.4 of this appendix. 

 

 

 

Figure E.2-1. Schematic. Bearing resistance chart showing strength limit state and 

service limit state criteria as a function of effective footing width. 

While the format of the bearing resistance chart shown in Figure E.2-1 is convenient for the 

purpose of establishing the design footing width as demonstrated in Section E.4 of this appendix, 

the chart can be easily re-organized in the format of load settlement curves for a range of footing 

widths to permit a rational evaluation of deformations at the service limit state in relation to the 

strength limit state and extreme event limit state. For a given footing width, Figure E.2-2 shows 

the load-settlement (Q-S) curve that was first introduced in Figure 2-1 in Chapter 2. In Figure 

E.2-2, two additional points on the Q-S curve are introduced; these are points S and F. Point S 

corresponds to the service load (QS) at the appropriate service limit state while Point F 

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APPENDIX E – LRFD GUIDANCE FOR SPREAD FOOTINGS
 

corresponds to the factored load (QF) at the appropriate strength limit state. By definition, the 

following conditions are necessary: 

	 Condition 1: QF > QS. 

	 Condition 2: Point F corresponds to the factored bearing resistance (the steeply ascending 

solid line in Figure E.2-1) because otherwise stability at the strength limit state will not be 

satisfied. 

	 Condition 3: Point N corresponds to the condition of a bearing resistance failure. Thus, it 

represents the nominal bearing resistance for a footing of effective width B'f. 

	 Condition 4: Based on Conditions 2 and 3, for stability of the footing, QF = b(QN) where b 

is the bearing resistance factor from Section 10 in AASHTO (2007 with 2009 Interims) and 

QF and QN are as defined before. 

	 Condition 5: Since Point N corresponds to the nominal bearing resistance, the deformation at 

this point can be expected to occur at the extreme event limit state since b= 1 and nominal 

resistance is used at this limit state. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure E.2-2. Schematic. Axial load versus vertical displacement curve for a spread 

footing having a given effective width B'f showing significant points of interest with 

respect to the LRFD approach.. 

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APPENDIX E – LRFD GUIDANCE FOR SPREAD FOOTINGS
 

By using the concepts introduced in Figure E.2-2, the data in Figure E.2-1 can be re-plotted as a 

series of stress-settlement curves for a range of footing widths as shown in Figure E.2-3. 

Basically, the plots in Figure E.2-3 represent vertical sections at discrete footing widths through 

the various curves in Figure E.2-1. This construction merely takes conventional computations 

one step further and does not require any advanced computational skills. Section E.4 of this 

appendix shows the development of this format of the bearing resistance chart by using a 

numerical example. The advantage of this format is that it readily allows an evaluation of 

settlements that will be experienced at various stages of bridge construction. Since a range of 

effective footing widths is used in Figure E.2-3, the Y-axis is in terms of stress (resistance) units. 

If the chart were to be developed for a given effective footing width then the Y-axis can be 

expressed in terms of stress units or force units. In the latter case, the Q-S curve up to Point F 

shown in Figure E.2-2 will occur. The key point is that by manipulating the same loadsettlement data, a number of different design aspects can be explored in a rational manner. 

Before the use of curves such as those shown in Figure E.2-3 is demonstrated, the design process 

for spread footings using the LRFD approach will be discussed so that the need for close 

collaboration between the geotechnical and structural specialists can be better understood. 

 

 

 

Figure E.2-3. Schematic. Alternative bearing resistance chart in the format of stresssettlement curves for a range of effective footing widths and settlements. 

72
 
 

 

 

 

 

APPENDIX E – LRFD GUIDANCE FOR SPREAD FOOTINGS
 

E.3 SPREAD FOOTING DESIGN USING LRFD METHODOLOGY 

The design of a spread footing in the LRFD approach is done in a systematic manner that 

accounts for all of the limit states shown in Figures E.1-1 and E.1-2 while considering all 

applicable load combinations. The design process for spread footings using the LRFD approach 

is discussed in this section and a design process flow chart is presented and explained. 

Design of a spread footing must provide adequate resistance against geotechnical and structural 

limit states, i.e., "failure" modes. The geotechnical limit states are identified in Section E.1 of 

this appendix and include the following: 

 Strength limit state 

o Bearing resistance 

o Limiting eccentricity 

o Sliding 

 Service limit state 

o Settlement 

o Global stability 

 Extreme Event limit state 

o Bearing resistance 

o Limiting eccentricity 

o Sliding 

The structural design includes consideration of limit states for the following: 

 Flexural resistance (strength limit) 

 Shear resistance (strength limit) 

 Crack control (service limit) 

All the above limit states are analyzed in a systematic manner as shown by the flow chart in 

Figure E.3-1. The first 8 steps establish the necessary geotechnical and structural information to 

evaluate the various limit states. As noted in sections E.1 and E.2, the size of the footings for 

highway bridges is large and is almost always governed by tolerable settlement, Stol. Therefore, 

the first limit state to be evaluated is the service limit state as shown in Step 9 in the flow chart. 

After the plan dimensions of the footing are established based on settlement (service limit state) 

considerations in “Decision 1,” they are verified with respect to the various failure modes in 

strength limit states as shown in Steps 11 through 14 in the flow chart. If the footing is found to 

be acceptable from the viewpoint of strength limit state, then the dimensions are verified for the 

extreme event limit state as shown in Step 15 in the flow chart. After the evaluation of the 

extreme event limit state, the design process moves on to structural design, which establishes the 

thickness of the footing and the size and placement of the reinforcement within the footing. 

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APPENDIX E – LRFD GUIDANCE FOR SPREAD FOOTINGS
 

Figure E.3-1. Flowchart. Design process for spread footings using LRFD approach. 

74
 
 

 

 

APPENDIX E – LRFD GUIDANCE FOR SPREAD FOOTINGS
 

In terms of responsibilities, the geotechnical specialist is responsible for performing the tasks in 

Steps 1 to 5. The remaining steps are best performed by a structural specialist since s/he has the 

knowledge of the various loads and load combinations. It is strongly recommended that the 

structural specialist interact with the geotechnical specialist as necessary to obtain more 

information rather than making assumptions. 

The geotechnical and structural aspects of the design of spread footings are included in Section 

10 (Foundations) and Section 5 (Concrete Structures) of AASHTO (2007 with 2009 Interims). 

75
 
 

 

 

 

 

 

 

 

APPENDIX E – LRFD GUIDANCE FOR SPREAD FOOTINGS
 

E.4 NUMERICAL EXAMPLE WITH DETAILED CALCULATIONS 

The bearing resistance chart developed by the geotechnical specialist provides the structural 

specialist with a powerful tool for studying the interrelationships among effective footing widths, 

uniform bearing pressures (or resistances) and settlements. The procedure for developing a 

bearing resistance chart is found in FHWA (2006). This section of Appendix E presents a stepby-step procedure that the structural specialist should follow when using such a chart and 

illustrates the procedure through an example problem based on an actual bridge in Arizona. The 

presentation herein assumes that the reader is familiar with AASHTO (2007 with 2009 Interims) 

and the general procedures for spread footing analysis. Table E.4-1 presents the terminology and 

notation used in the example problem that follows. 

Example Problem Statement: 

For a 2-span post-tensioned box girder viaduct bridge located in a non-seismic zone in Arizona, 

the abutments will be founded on spread footings similar to the configuration shown in Figure C.2­

1(a). The span lengths are 105-ft and 123-ft. The length, Lf of the abutment footings is 150-ft. 

The minimum embedment depth of embedment, Df, of the footing base is 6-ft. Using the 

procedures in FHWA (2006), the geotechnical specialist (NCS, 2007) developed a bearing 

resistance chart based on site-specific subsurface data. The chart is shown in Figure E.4-1. The 

pertinent parameters for the example abutment footing are included in Tables E.4-2 and E.4-3. 

Table E.4-1. Terminology and notation used in example problem. 

Parameter Notation 

Footing length Lf 

Footing width Bf 

Footing depth of embedment Df 

Vertical component of resultant load V 

Moment M 

Eccentricity e = M/V 

Eccentricity in Bf direction eB 

Eccentricity in Lf direction eL 

Effective width L' f = Lf - 2eL 

Effective length B' f = Bf - 2eB 

Effective footing area A'f = B'fL'f = (Bf - 2eB) (Lf - 2eL) 

Equivalent total uniform bearing stress qtveu = V/A'f 

Equivalent net uniform bearing stress qnveu = qtveu- - p (s Df) 

Factored net bearing resistance qRn 

Resistance factor of bearing resistance b 

Load factor for permanent earth load p 

Unit weight of soil within Df s 

Note: p is the load factor for permanent vertical earth (“EV” designation) pressure load and 

can be obtained from Tables 3.4.1-1 and 3.4.1-2 of AASHTO (2007 with 2009 Interims). 

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APPENDIX E – LRFD GUIDANCE FOR SPREAD FOOTINGS
 

Figure E.4-1: Graph. Example of a factored bearing resistance chart for a footing of length, 

L′f = Lf = 150-ft (no eccentricity) and depth of embedment, Df = 6-ft with base elevation of 

994-ft. The resistance factor of b= 0.45 is included in the strength limit state curve. For 

the service limit state b= 1.00 and it is included in the service limit state curves. Therefore 

all curves are for factored resistance. “S” in the legend refers to immediate settlement. 

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APPENDIX E – LRFD GUIDANCE FOR SPREAD FOOTINGS
 

Table E.4-2. Pertinent parameters for example abutment footing (L'f=150-ft). 

Parameter Limit State (Notes 1 and 2) 

Service I Limit State Strength I (maximum) 

Vertical component of the resultant load VSER = 9,080 kips VSTR = 12,028 kips 

Moment MSER = 22,720 k-ft MSTR = 35,290 k-ft 

Eccentricity in the Bf-direction (Note 3) eB-SER = 2.50-ft eB-STR = 2.93-ft 

Notes: 

(1) Only one service limit state and strength limit state are used herein for illustration 

purposes. In actual design all applicable service and strength limit states must be 

considered. 

(2) The notations are appended by subscripted text to denote the appropriate limit state being 

considered. Thus, VSTR denotes the vertical component of the resultant load for the 

strength limit state while eB-SER denotes the eccentricity in the Bf-direction using service 

limit state loads. 

(3) The B-direction is the direction of the least lateral dimension of the footing. The 

eccentricity in the L-direction for an abutment footing is commonly negligible and is 

assumed to be zero for this example case, i.e., L′f = Lf. For cases where the footing has 

eccentricity in both directions, the eccentricity in the L-direction should also be evaluated. 

In the case of the eccentricity in both directions, the least lateral dimension is the smaller 

dimension of the footing after adjustment for the eccentricities. 

Table E.4-3. Pertinent parameters for Service I Limit State related to significant 

construction points for example abutment footing (Lf=150-ft). 

Quantity Units 

Construction-point 

1 2 3 4 

End of 

construction 

of footing 

End of 

construction of 

stem, backwall 

and wingwalls 

Completion 

of earth fill 

behind 

abutment 

Placement of 

Superstructure 

and open to 

traffic 

V k 1,310 3,310 6,446 9,080 

M k-ft 0 400 6,215 22,720 

eB = M/V ft 0.00 0.12 1.93 2.50 

Notes: 

1. As indicated in Table E.4-1, V is the vertical load, M is the moment due to the vertical and 

horizontal load, and eB is the eccentricity in the B-direction. 

2. Only Service I limit state is addressed since settlements will be evaluated using the 

construction-point concept. 

NOTE: This example problem uses only Service I and Strength I (max) limit states to 

illustrate the process of determining the plan size of a footing. In actual designs all 

applicable limit states must be evaluated using the procedures described in this appendix. 

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APPENDIX E – LRFD GUIDANCE FOR SPREAD FOOTINGS
 

E.4.1 STEP-BY-STEP PROCEDURE FOR SIZING A SPREAD FOOTING AT 

SERVICE AND STRENGTH LIMIT STATES 

1.	 Assume a total footing width, Bf-SER. Calculate effective footing width B'f-SER = Bf-SER - 2eBSER. Calculate qnveu-SER. Enter the bearing resistance chart with qnveu-SER and effective footing 

width, B'f-SER and determine the settlement, S. Compare S with a target tolerable total 

settlement value, Stol. If necessary iterate the footing width until S ≈ Stol. 

Example: Assume Stol = 0.90-in.1 Assume Bf-SER = 15-ft
 

Since eB-SER = 2.50-ft, B'f-SER = Bf-SER - 2eB-SER = 15-ft – 2(2.5-ft) = 10-ft
 

For L′f = 150-ft and B'f-SER = 10-ft, A'f-SER = (150-ft) (10-ft) = 1,500 ft2
 

qtveu-SER= VSER/A'f-SER = 9,080 kips / 1,500 ft2 = 6.05 ksf
 

From Table 3.4.1-1 of AASHTO (2007 with 2009 Interims) the load factor p for 

vertical earth pressure corresponding to Service I limit state is 1.0. Using the 

values provided in Note 12 of Table 1, the factored overburden stress at footing 

base level = p(sDf) = (1.0)(0.120 kcf) (6-ft) = 0.72 ksf 

qnveu-SER = qtveu-SER - p(sDf) = 6.05 ksf – 0.72 ksf = 5.33 ksf 

Enter Figure E.4-1 with B'f-SER = 10-ft from X-axis and qveu-SER= 5.33 ksf from the 

Y-axis and find the point of intersection on the chart which represents the 

estimated settlement for this particular set of B'f-SER and qveu-SER values. From 

Figure E.4-1, the estimated settlement, S, is 0.87-in which is slightly less than 

0.90-in. Since S ≈ Stol the assumed footing width is correct. Otherwise, repeat the 

process with another assumed footing width until S ≈ Stol is achieved. 

2.	 Check if eB-STR < Bf-SER/3. If yes, then denote the total footing width after this step as Bf-STR 

since it is based on comparison with strength limit state criterion for eccentricity. 

Example: For B f-SER = 15-ft, Bf-SER/3 = 5.00-ft 

From Table E.4-2, eB-STR = 2.93-ft. 

Since eB-STR < Bf-SER/3, a footing width of 15-ft is acceptable based on eccentricity 

consideration. 

Denote the footing width for strength limit state design as BSTR = 15-ft. This is 

the footing width that is also used for structural design and detailing. 

3.	 For strength limit state, determine the effective width of the footing B'f-STR = Bf-STR - 2eB-STR 

and qnveu-STR 

1	 The value of 0.90-in is used for illustration purposes and does not represent a standard or fixed value. In actual 

design, the value shall be based on the tolerable total settlement determined by the structural specialist. 

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APPENDIX E – LRFD GUIDANCE FOR SPREAD FOOTINGS
 

Example: For Bf-STR = 15-ft and eB-STR = 2.93-ft.
 

B'f-STR = Bf-STR - 2eB-STR = 15-ft – 2(2.93-ft) = 9.14-ft.
 

For L′f = 150-ft and B'f-STR = 9.14-ft, A'f-STR = (150-ft) (9.14-ft) = 1,371 ft2
 

qtveu-STR = VSTR/A'f-STR = 12,028 kips / 1,371 ft2 = 8.77 ksf
 

From Table 3.4.1-2 of AASHTO (2007), the load factor p for permanent vertical 

earth pressure corresponding to Strength I (maximum) limit state is 1.35 for 

“Retaining Walls and Abutments.” Using s= 120 pcf, the factored overburden 

stress at footing base level = p(sDf) = (1.35)(0.120 kcf) (6-ft) ≈ 0.97 ksf 

qnveu-STR = qtveu-STR - p(sDf) = 8.77 ksf – 0.97 ksf = 7.80 ksf 

4.	 For B'f-STR determine the factored net bearing resistance, qRn, from the steeply rising curve 

based on shear strength considerations. 

Example: Enter Figure E.4-1 with B'f-STR = 9.14-ft from X-axis and find the point of 

intersection with the steeply rising curve above the settlement curves. This point 

of intersection represents the factored net bearing resistance, qRn, for B'f-STR. 

From Figure E.4-1, for B'f-STR = 9.14-ft, qRn ≈ 8.4 ksf. 

5.	 If qRn > qnveu-STR then the footing width BSTR is adequate. 

Example: From Step 3, qnveu-STR = 7.80 ksf
 

From Step 4, qRn ≈ 8.4 ksf
 

Since qRn > qnveu-STR, the footing width Bf-STR is adequate.
 

Repeat the above steps for all applicable strength and service limit states and determine the 

governing spread footing size, i.e., total width (Bf) and total length (Lf). For every limit state, 

the spread footing size should also be checked for sliding as per the requirements of Article 

10.6.3.4 of AASHTO (2007 with 2009 Interims). 

There are several other ways to use factored bearing resistance charts. For example, one can 

conceivably establish a preferred footing size based on project space constraints and then enter 

the chart from the X-axis to design the footing. Alternatively, one can select a tolerable 

settlement contour curve and evaluate several alternative combinations of factored bearing 

resistance and effective footing width in an attempt to balance the settlements across several 

discrete footings at a given substructure element. 

Regardless of the way the data in a factored bearing resistance chart are evaluated, the structural 

specialist can perform parametric analyses to optimize the size of footings. It is anticipated that 

some level of iterative analysis will be required to determine a footing configuration that meets 

the requirements of the various limit states. Commonly, the service limit state is evaluated first 

to establish the size of the footing and then the footing is checked with respect to the strength 

limit state. 

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APPENDIX E – LRFD GUIDANCE FOR SPREAD FOOTINGS
 

Finally, it should be remembered that the structural design of the footing should be performed by 

using the total governing footing width (Bf) and length (Lf) with the appropriate bearing stress 

distribution as per Article 10.6.5 of AASHTO (2007 with 2009 Interims) - uniform if no 

eccentricity, trapezoidal or triangular if there is eccentricity. 

E.4.2 CONSTRUCTION-POINT ANALYSIS 

A factored bearing resistance chart can be used to develop a plot of net factored bearing 

resistance versus settlement for one or more effective footing widths of interest. Such a plot is 

useful for performing a construction-point analysis. To develop this plot the structural specialist 

enters the X-axis of a bearing capacity chart, such as that shown in Figure E.4-1, with the 

effective footing width(s) of interest and reads the values of net bearing resistance corresponding 

to the intersection points of the effective width line with the curves representing the various 

magnitudes of settlement. The results can then be plotted as shown in Figure E.4-2, which was 

developed by following the above procedure for selected footing widths with reference to the Xaxis in Figure E.4-1. 

Figure E.4-2 can then be used to perform a construction-point analysis for a footing having a 

given effective footing width by entering the chart on the Y-axis with values of the effective net 

bearing resistance corresponding to the construction points of interest and reading the 

corresponding settlements on the X-axis. For the example problem of an abutment footing, the 

significant construction-points are as noted in Table E.4-4. 

Table E.4-4: Common significant construction points for an abutment footing. 

Constructionpoint Description 

1 End-of-construction of the spread footing 

2 End-of-construction of the abutment stem-wall, back-wall, and 

structurally connected wing-walls 

3 End-of-placement of earth fill behind the abutment 

4 After placement of superstructure and opening to traffic 

Table E.4-5 summarizes the computations for the various construction points using the data 

provided in Table E.4-3 and Figure E.4-2 developed from Figure E.4-1. The settlements that are 

particularly relevant to the performance of the bridge superstructure are those that occur at 

construction points 3 and 4. Based on the data in Table E.4-5, the difference in settlement 

between construction point 4 (S4) and construction-point 3 (S3) is 0.36 inches (= 0.87-in–0.51­

in). Thus, the relevant settlement of 0.36 inches is only 41% of the total settlement of 0.87 

inches as calculated in Step 1 of the procedure above. The value of 41% is consistent with the 

observations and discussions in Chapter 4. Furthermore, for a footing width of 15-ft, the S/Bf 

value is 0.48% based on the total settlement (0.87 inches) and 0.20% based on the constructionpoint settlement (0.36 inches). These values of S/Bf are within the ranges of the documented 

data discussed in Chapter 2 and Appendix B. 

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APPENDIX E – LRFD GUIDANCE FOR SPREAD FOOTINGS
 

Figure E.4-2. Graph. Example of a factored bearing resistance chart in terms of stresssettlement curves for a range of effective footing widths. 

Table E.4-5: Summary of computations of settlements at significant construction points for 

the example abutment footing. 

Quantity Units 

Construction-point 

1 2 3 4 

End of 

construction 

of footing 

End of 

construction of 

stem, backwall 

and wingwalls 

Completion 

of earth fill 

behind 

abutment 

Placement of 

Superstructure 

and open to 

traffic 

V k 1,310 3,310 6,446 9,078 

M k-ft 0 400 6,215 22,720 

L'f = Lf ft 150.00 150.00 150.00 150.00 

Bf ft 15.00 15.00 15.00 15.00 

eB = M/V ft 0.00 0.12 1.93 2.50 

B'f = Bf – 2eB ft 15.00 14.76 11.14 10.00 

qtveu = V/[(B' f)(L'f)] ksf 0.58 1.50 3.86 6.05 

p(sDf) ksf 0.72 0.72 0.72 0.72 

qnveu = qtveu - p(sDf) ksf -0.14 0.78 3.14 5.33 

S (from Figure E.4-2) in S1 = 0.00 S2 = 0.12 S3 = 0.51 S4 = 0.87 

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APPENDIX E – LRFD GUIDANCE FOR SPREAD FOOTINGS
 

Conceivably, in cases where strength limit state may not be a concern, e.g., in dense soils, one 

could theoretically optimize the footing width by reducing it so that the theoretical settlement 

between construction points is closer to tolerable limits. 

This example problem clearly illustrates the importance of performing construction-point 

analyses when spread footings are being evaluated as a technically feasible and cost 

effective alternative foundation system for highway bridge structures. 

E.5 SPREAD FOOTING DESIGN BASED ON STRUCTURAL CONSIDERATIONS 

Although the construction-point concept is very significant in determining the effects of 

distortion of the spread footing on the superstructure and its final alignment, the concept has little 

or no effect on the structural design of spread footings themselves. While geotechnical 

considerations and service limit states govern the plan size of a footing, the thickness of a footing 

and its reinforcement is based on structural considerations and strength limit states. In the 

strength limit-state proportioning of the footing and its reinforcement, the weightless bridge 

structure can be instantaneously wished into place and all the loads applied at the same time. 

The distortion of the footing is ignored, and at the limit, the loads are re-distributed to be resisted 

non-linearly by the footing sections. This section of the appendix presents a brief overview of 

the contact pressure distributions, use of maximum and minimum load factors to determine the 

critical force effects, and evaluation of the flexural and shear structural resistance. 

E.5.1 Contact Pressure Distributions 

The contact pressure distribution at the base of a footing is a function of the relative stiffness of 

the footing with respect to the soil subgrade. While uniform contact pressure distribution over 

the effective width of the footing is assumed for the bearing resistance and settlement evaluation, 

the structural design is based on the more traditional triangular or trapezoidal contact pressure 

distributions, which are determined from factored loads. Thus, the pressure distribution is a 

factored pressure distribution. The proper calculation of the factored pressure distribution is 

therefore a key to the structural design of the footing. In this context, it is important to 

understand the proper application of the maximum and minimum load factors as discussed next. 

E.5.2 Maximum and Minimum Load Factors 

In the LRFD approach in which uncertainty is acknowledged, loads can be greater than the 

nominal loads or less than the nominal loads based upon the assumed uncertainty. Thus, 

maximum and minimum load factors are specified for permanent loads in LRFD. Following are 

the two basic rules for selection and application of the load factors: 

 If the particular load in question increases the load side of the LRFD equation, it must be 

factored up by the specified maximum load factor. 

 If the particular load in question decreases the load side of the LRFD equation, it must be 

factored down by the specified minimum load factor. 

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APPENDIX E – LRFD GUIDANCE FOR SPREAD FOOTINGS
 

In determining the factored moments and shears used to proportion the footing and its 

reinforcement, the factored pressure distributions are the reactions of the loads applied to the top 

of the footing used in the free-body diagrams to determine the critical force effects (moment or 

shear) for the structural design. To satisfy equilibrium, the loads applied to the top of the footing 

(including the footing’s body forces) must be factored by the very same load factors, maximum 

or minimum, before they are used to determine the pressure distribution at the base of the 

footing. 

E.5.3 Flexural and Shear Resistance of Footings 

As noted in Section C.1 of Appendix C, the structural design of a spread footing involves an 

evaluation of the flexural (bending) and shear resistance. While the proportioning of the footing 

and its reinforcement to resist moment has not changed with the LRFD approach in terms of the 

determination of the flexural resistance, the determination of the shear resistance offers many 

new options. In Section 5 (Concrete Structures) of AASHTO (2007 with 2009 Interims) no less 

than four sectional-model procedures and the strut-and-tie model can be applied for the 

determination of the shear resistance of a footing. Since the thickness of a footing is generally 

governed by shear it is important that the proper method be chosen for the structural design. 

The presentation of many options for the determination of shear resistance in Section 5 (Concrete 

Structures) of AASHTO (2007 with 2009 Interims) was in response to the complexity of the 

modified-compression field theory shear-resistance procedure (MCFT) of the original drafts of 

the first edition of the specifications. MCFT is a model that acknowledges the ability of 

diagonally-cracked concrete sections to resist shear. The design parameters needed to use this 

procedure were originally iteratively derived from tabularized values. The solution was thought 

to be time-consuming and non-unique. A recent interim change to the specifications has 

replaced this iterative procedure with a direct solution with equations for the design parameters. 

The simplest and most often used procedure found in published design examples is the simplified 

procedure for non-prestressed sections in which constant values of the design parameters are 

assumed consistent with older, less accurate shear-resistance models. However, this procedure is 

not recommended since, as noted in AASHTO (2007 with 2009 Interims), “these traditional 

expressions can be seriously unconservative for large members not containing transverse 

reinforcement.” Most spread footings are not reinforced with significant amounts of transverse 

reinforcement. The recommended procedure for determining the shear resistance of a spread 

footing is the modified-compression field theory (MCFT) because it is the most accurate and, 

with the addition of equations for the design parameters, it is easy to apply. The reader is 

referred to Section 5 (Concrete Structures) of AASHTO (2007 with 2009 Interims) for further 

details on the various procedures for evaluating the flexural and shear resistance of spread 

footings. 

84