AlllDO

NATL INST OF STANDARDS & TECH R j C.

A1 11 00992972

Lew H S (Hal Sang/Satety durlna const TA435 .U58 V80;19f6 C.1 NBS-pJb-C 1976 NBS BUILDING SCIENCE SERIES 80

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

Safety During Construction of

Concrete BuildingsA Status Report

1 CiX ^
The Building Science Series

I ho IJuildins Sciciuc Series ilisseminutes technical informalion developed at the National Bureau of Standards on

huilrlinjz materials, components, systems, and whole structures. The Series presents research results, test methods, and performance criteria related to the structural and environmental functions and the durability and safety charaelcristies of building elements and systems. These publications, similar in style and content to the NBS Building Materials and Structures Reports (1938-59),

are directed toward the manufacturing, design, construction, and research segments of the building industry, standards organizations, and officials responsible for building codes. The material for this Series originates principally in the Center for Building Technology of the NBS Institute

for A|)plied Technology. The publications are divided into three general groups: Building Systems and Processes;

Health, Safety and Comfort; and Structures and Materials. For further information regarding these publications

please contact the .Scientific and Professional Liaison Section, Center for Building Technology, Institute for Applied Technology, National Bureau of Standards, Washington, D.C. 20234.

I See mailing list announemienl itn pajse |
Safety During Construction of

Concrete Buildings- A Status Report

H. S. Lew

Center for Building Technology

Institute for Applied Technology

National linreaii of Standards

Washington, D.C. 20234

U.S. DEPARTMENT OF COMMERCE, Rogers C. B. Morton, Secreiary

James A. Baker, III, Under Secreiary

Dr. Betsy Ancker-Johnson, Assisiani Secreiary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Aciing Direcior

Issued January 1976
Library of Congress Cataloging in Publication Data

Lew. Hai Saiifi. Safety iluriii^ coii-truction of roiirrete buildings.

(National Ruieau of .Standard- i)uildinf: .science series: 80)

Ril)liof;ra[iliv : p. .Supt. of Docs. No.: C 1H.29/2:R0

1. (ioncrete construction—Safety measures. I. Title. II. Series: United States. National Bureau of Standards. Building Science

Series: 80. TA435.U58 No. 80 [TH1461] 602.1s 1693..^] 7.S-619.389

National Bureau of Standards Bviilding Science Series 80

Nat. Bur. Stand. (U.S.). Bldg. Sci. Ser. 80, 56 pages (Jan. 1976) CODEN: BSSNBV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1976

For *ale by llie Siiperiiileudenl of Oociinieiits, U.S. Government Printing Office, Washington. D.C 20402 (Order by SD Catalog No. C 13.29/2 :80). Price $1.15 (Add 25 percent additional for other than U.S. mailing).
CONTENTS

Page

SI CONVERSION UNITS iv

GLOSSARY V

1. INTRODUCTION 1

2. CAUSES OF CONSTRUCTION FAILURE 4

3. REGULATORY AND INDUSTRIAL STANDARDS 7 3.1. General 7 3.2. Federal Standards 8 3.3. State and Local Building Codes 8 3.4. National Standards 9 3.5. Recommended Practices 10

3.6. Comparison of Requirements in Existing Documents. 10

4. FACTORS AFFECTING CONCRETE CONSTRUCTION SAFETY 13

4.1. General 13

4.2. Design Loads for Falsework 14

4.2.1. Design Loads for Formwork 14

4.2.2. Design Loads for Shores 18

4.3. Strength of Falsework 20

4.3.1. Capacity of Formwork 20

4.3.2. Capacity of Shores -. . . 21

4.3.3. Capacity of Support 22

4.4. Construction Cycle 24

4.4.1. Rate of Concrete Strength Gain 24

4.4.2. Accuracy of Strength Determination 25

4.4.3. Required Concrete Strength for Formwork Removal ... 26

5. CONCLUSIONS 31

6. ACKNOWLEDGMENTS 32

7. REFERENCES 32

APPENDIX A. SUMMARY OF PROVISIONS OF MODEL BUILDING CODES AFFECTING

CONCRETE CONSTRUCTION SAFETY 3 6 APPENDIX B. SUMMARY OF PROVISIONS OF LOCAL BUILDING CODES AFFECTING

CONCRETE CONSTRUCTION SAFETY 3 8 APPENDIX C. SCAFFOLDING AND SHORING INSTITUTE RECOMMENDED PROCEDURE

FOR COMPRESSION TESTING OF SCAFFOLDS AND SHORES 42

iii
SI Conversion Units

In view of present accepted practice in this technological area, U. S.

customary units of measurements have been used throughout this report. It

should be noted that the U. S. is a signatory to the General Conference on Weights and Measures which gave official status to the metric SI system of

units in 1960. Readers interested in making use of the coherent system of SI

units will find conversion factors in ASTM Standard Metric Practice Guide,

ASTM Designation E 380-72 (Available from American Society for Testing and

Materials, 1916 Race Street, Philadelphia, Pennsylvania 19103). Conversion

factors for units used in this paper are:

Length

* 1 in = 0.0254 metre

1 ft = 0. 3048 metre

Area

1 in2 = 6.4516* x lO""* metre^

1 ft2 = 9.2903 X 10"^ metre^

Volume

1 ft3 = 2.832 X 10"^ metre^

Force

1 lb (Ibf) = 4.448 newton

1 kip = 4448 newton

Pressure , Stress

1 psi = 6895 pascal (Pa) , (N/m^)

1 psf = 47.88 pascal (Pa), (N/m^)

Mass

1 lb = 0.4536 kg

Moment

1 Ibf-ft = 1.3558 newton-metre

Temperature

°C = 5/9 (Temperature °F - 32)

* Exact value

IV
GLOSSARY

Falsework - Falsework is the temporary structure erected to support work

in the process of construction. It is composed of shores,

formwork and lateral bracing.

Formwork - Formwork is a part of the falsework which directly supports

(or forms) freshly placed concrete and molds the concrete to the desired

size and shape.

Lateral - Lateral bracing is a part of the falsework which consists of

Bracing usually diagonal members and resists lateral loads on the

falsework

.

Reshores - Reshores are shores placed firmly under a stripped concrete

member where the original formwork has been removed.

Shores - Shores are either vertical or horizontal members which support

directly the formwork.

V

SAFETY DURING CONSTRUCTION OF CONCRETE BUILDINGS - A STATUS REPORT

H . S . Lew

The current state-of-the-art of safety in concrete building

construction is reviewed and summarized. The material presented con- siders only the technical aspects of the construction safety. Safety

of the individual in using equipment and in following construction

procedures is not included in this study.

The report presents comparative accident frequencies in con- crete construction. Based on reported construction failures, the

relative vulnerability of various categories of concrete construction is estimated. The report examines causes of construction

failures and reviews major regulatory standards at the federal,

state, city and industry level affecting safety in concrete construction.

The factors which affect safety in concrete construction are examined relative to the state-of-the-knowledge and, where appropriate, recommendations are made for areas needing improved standards.

Key words: Building; codes; concrete; construction; falsework;

formwork; loads; reshoring; safety; shoring; standards.

1. INTRODUCTION

The construction industry, in general, is known to be one of the high risk

industries. Since 1960, annual construction death and disabling injuries have

been about 2700 and 220,000, respectively (table 1). These figures are higher

than those of the national average of all industries. Accident statistics

compared in table 2 reveals that the chance of accidental death in construction is four times greater than the overall occupational hazard average and

the chance of disabling accidents is twice that of the average. Construction

safety according to these statistics is lagging behind the overall safety of

other industries.

Data on frequency and severity of accidents in concrete construction is

difficult to assess. As a generic class, neither the National Safety Council

nor the Department of Labor publishes data for construction accidents which

single out concrete building construction. However, assuming that the

1
* Table 1 Construction Safety Statistics

Year

Estimated

Number of Workers Estimated Deaths

Estimated

Disabling Injuries

1960 3, 500, 000 2,400 213,000

1961 3,400,000 2,300 210, 000

1962 3, 250, 000 2,400 210,000

1963 3,400,000 2, 500 210,000

1964 3,550,000 2, 600 220,000

1965 3 ,700,000 2, 700 225,000

1966 3,800,000 2, 800 240,000

1967 3,800,000 2, 700 230,000

1968 3,800, 000 2,800 240,000

1969 4,000,000 2, 800 240,000

1970 3, 900, 000 2,800 240,000

1971 3,800,000 2,700 240, 000

1972 4,000,000 2,800 260,000

1973 4,100,000 2, 900 280, 000

Average - 3,700,000 - 2,700 - 220,000

* Source: National Safety Council

2
to *H

-P (H1 Q 1 c Q Q ,—1

CO -H

•P

(fi 0

Q) , 1 -H

_p

t/1

Q IJ

M

C/1 1 to C H

-P 0 iH CO

cn r-i XI 0) *H (X3 1—1

•H

-P CO •H ,—1

r-v 1—1 , 1 CO 4J <f

I>-i 4J r\<j

cu

CO

Q f~\ M-f -H Q CO \^ Q rd d M-i CU (SJ \jw _IJ H

•H fd

Q

H (-^ Tj -p 0 1 Qu :3 03 S-l 0 * -H rH CO \^ (-• CO Q

(11 Ul rC

r ) , 1 -P n fO

frt lU 1—1

nH

fT3 -p i) C r3

CU 03 c 0) H M •H U M 0 nH P < rH

<;

0) o o 00 O CN) CM

ro iH o^ r~ IDm iH 0

r-~ r- IX) >X5 VD <D <D 1X> <T> cr\ a^ a^

.H rH rH rH iH rH iH iH

n3COH

-P

rC 2C>H

cn

fO +J

T3CO

13

QJ

cn m

0)uo 3
frequency of accidents is related to the frequency of construction failures,

an estimate can be made from a comprehensive study of structural collapses and

failures during construction based on the report by a committee of the Institute

of Civil Engineers in England [l]"*". The study revealed that concrete construction constituted about 12 percent of failures in the construction and an additional 10 percent were temporary structures often related to concrete

construction. Taking into consideration that some 32 percent of the cases are

not identified, it is reasonable to estimate that 25 percent of all construction

failures probably involve concrete construction.

These unfavorable statistics clearly indicate that the concrete construction is a hazardous work. A close examination of all aspects of the construction process which contribute to accidents and failures is justified as well

as identification of possible solutions to unsafe construction practices.

This report examines causes of construction failures and reviews major regulatory standards at the federal, state, city and industry level governing safety

practices in concrete building construction. Where appropriate, research

recommendations are given in areas needing information in order to improve

existing safety standards applicable to concrete construction.

2. CAUSES OF CONSTRUCTION FAILURE

Failure of concrete construction stems from a number of causes. These

include failures related to formwork, premature removal of falsework, inadequate

amount of reinforcement, low concrete strength attributed to inferior cement

or use of insufficient cement, poor curing conditions during hot and cold

weather, etc. Individual case histories of concrete construction failures

resulting from various causes are compiled in references 4, 5, 6, and 7. All

of these case histories however, occured prior to 1967. Feld's books [4,5] are based on failures which occured from 1900 to 1966, McKaig's book [6] from

1895 to 1960, and Short's report [7] includes a number of failures which

occurred prior to 1967 in Great Britain.

Because both construction practices and standards have evolved in the

last decade and because there have been substantial revisions in concrete

design practice as a result of changes in the ACI Building Code Requirements

[8] in 1963 and 1971, a summary of concrete building construction failures

since 1963 was developed (table 3). This summary includes those 24 cases reported in Engineering News Record (ENR) from 1964 through 1974. Because the

articles reported in ENR are dependent on many factors including their news

value, the availability of local reporters, and cooperation of local authorities,

"'"Numbers in brackets refer to the literature references listed in section 6.

4
TABLE 3 Construction Failures of Concrete Buildings

Engineering News Record: Reported 1964-1974

DATE LOCATION STRUCTURAL TYPE APPARENT CAUSES ENR REFERENCE

1 . nVi i 1 n .1 o 1 nil i -1 PA 21-story sliprorm service core i'ormwork tire caused by salamender. 1-9-64 pi 7 for office building

1-64 pjew lOrK Lity, iny Flat slab hospital Formwork fire caused by heater 1-9-64 pl7

exp 1 OS ion

.

5

.

4-64 Madrid, Spain 350 car, 3-story parking garage Inferior cement suspected respon- 4-9-64 p29

sible for low concrete strength

of columns.

4 . 9-64 IVasliington, D.C. 6-story flat plate office Formwork collapsed during casting. 9-17-64 p61 bui Iding No lateral bracing for shores suspected

.

5-65 .Jacksonville, FL 4-story school Formwork collapsed due to over- 5-13-65 p29 loading of shoring with stacks

of metal pans.

6

.

5-65 Italian Riviera, 8-story apartment No information available. 5-20-65 p24

Italy

7. 7-65 Concord , Nil 5-story office building Shoring failure. 7-29-65 p 1

5

8. 11-65 Platteville, IVI Waffle slab roof ' Tubular steel shoring collapse 11-18-65 p67 during casting.

9. 4-66 Ottawa, Canada 12-story flat plate building Inadequate reshoring and premature 4-7-66 6 p72 removal of forms. S-5-66 pll

10. 5-66 Athens, Greece 2-story papermill Inadequate lateral bracing of 29- 5-19-66 p30

foot high shores.

11. 5-67 Ft. Worth, TX Waffle slab, library roof Metal shoring failure suspected. 5-18-67 pjj

12. 1-68 Chapel Hill, NC Roof section of student union Formwork collapsed during casting. 1-11-68 p31 Improper formwork placement blamed.

13. 6-68 Arlington, VA 12-story office building Formwork collapsed during casting. 6-13-68 6 p25 Insufficient lateral bracing blamed. 6-27-68

14

.

.5-69 Pittsburgh, PA Flat slab hospital addition 26-ft high falsework and shoring 3-6-69 pl3 failure

.

15. 8-69 Buenos Aires, Brazil 22-story apartment Premature removal of formwork 8-7-69 p37 caused collapse of balcones.

16. 9-69 Rockville, MD 7-story office building Heavy rains undermined shores. 9-25-69 p21

17. 2-70 Buenos Aires, Brazil 23-story apartment Nearly completed building totally 2-26-70 pl3 collapsed. Understrength material blamed

.

18. 1-71 Boston, MA 16-story flat plate apartment Progressive collapse of 16 stories. 1-20-71 p3 Insufficient concrete strength and 2-4-71 pl3 inadequate reshoring were causes of 7-15-71 pl9

failure. 8-19-71 n7P' 11-11-71 pl6 12-23-71 P3 1-27-72 pll

19. 3-71 Arlington, VA 3-story parking garage Formwork collapsed during casting. 3-11-71 pl2

20. 6-72 Vancouver, B.C.' Parking garage and 3-story Collapse of shoring blamed. 6-29-72 plO apartment . 21. 3-72 Bailey's Crossroad 26-story flat plate apartment Premature removal of shores caused 3-8-73 e, pl2

VA progressive collapse of 24-stories. 4-19-73 pll 5-3-73 pl6 5-31-73 pl3 6-14-73 pl5 7-19-73 pl6 10-4-73 p32 11-22-73 pis

22. 8-73 Frankfort , Germany 42-story office building Fire destroyed formwork and shores. 8-30-73 pll

23. 2-74 Cleveland, OH 21-story hotel Fire destroyed formwork causing 2-20-74 S pll progressive collapse of a portion 4-18-74 pll building.

24. 6-74 Miami , FL 8- level parking garage Failure of adjustable wooden shores 6-6-74 5 P5 caused collapse of 31 x 110 ft, 8 in 6-15-74 pll post-tension slab.

5
they are considered to constitute a biased sample of concrete construction

failures. However, they probably typify the most serious types of failures.

Thus, they can be regarded as a reasonable sample of the type of failures

which should receive immediate attention.

A summary of individual failures classified according to the major causes

is presented in table 4, While the proportion of individual causes of failure

to the number of total failures does not directly correlate to the proportion

of actual failures, it does indicate the major possible causes in concrete

construction. The major causes in descending order of frequency are:

54% Formwork Failure - This category includes failures due to

deficiences in design, erection and assembly of the formwork

and inadequate size and spacing of reshores and lateral

bracings

.

17% Fire - This category includes partial or total destruction of

wooden formwork due to fires originating in heating devices for

winter concreting.

Premature removal of shores - This category includes failures

resulting from premature stripping of forms and shores supporting

partially matured concrete, usually for economic purposes.

Faulty material - This category includes failures due to underdevelopment of the concrete caused by use of inferior and/or

inadequate amounts of cement.

4% Overloading - This category includes failures due to storage of

construction material in a concentrated area; thereby over- stressing either the concrete or the formwork.

4% Soil failure - This category includes failures resulting from

the instability of soil which supports shores and mudsills.

4% Others - This category includes those cases for which accurate

information for cause of failures is not available.

In addition to these categories other causes have been cited. These include errors in design of formwork, inadequate connection detail such as insufficient

nailing or failure to tighten the lock devices on shores, equipment impact on vertical shores causing a progressive collapse of the entire falsework and displacement of shores due to vibrations produced by men and equipment on the
* Table 4 Classified Summary of Construction Failures

Cause of Number '^of Percent of

Failure Failures Total Failures

Formwork Failure: 13 54%

Formwork collapse (2)

Shoring failure (6)

Inadequate reshoring (2)

Inadequate lateral

bracing (3)

Premature Removal

of Shores: 2 8%

Over Loading: 1 4%

Soil Failure: 1 4%

Faulty Material: 2 8%

Fire: 4 18%

Others

:

1 4%

TOTAL 24 100%

* Source: Engineering News Record (1964-1974)

formwork. As a group, however, more than 60 percent of failures were related

to improper falsework practices including premature removal of forms and shores.

Consequently, the most benefit can be derived by improving present requirements

for concrete construction in building codes and standards, and manuals of practice in this area.

3. REGULATORY AND INDUSTRIAL STANDARDS

3.1. General

For concrete construction, many sets of requirements and standards exist

ranging from those of federal standards to those of local building codes. These

include

:

(1) Federal Standards - Regulations promulgated under the Occupational

Safety and Health Act of 1970 (OSHA) which gives the Secretary of

Labor authority to promulgate and enforce occupational -safety and

health standards. All safety requirements relative to construction

are given in Construction Safety and Health Regulations [9] published

in the Federal Register of June 24, 1974.
(2) State and local building codes - Regulations enacted by local legislatures or councils.

(3) National standards - Consensus standards promulgated by organizations

such as the American National Standards Institute (ANSI).

(4) Recommended practices - Recommended practices and procedures promulgated by trade associations and professional organizations such as the Associated General Contractors of America, Inc. (AGC) , the

Scaffolding and Shoring Institute (S&SI) , and the American Concrete

Institute (ACI)

.

The requirements in these documents are not uniform and vary substantially

in certain subject areas. In the remaining part of this chapter, these regulatory and recommended practices are reviewed and the interrelationships of

these will be examined.

3.2. Federal Standards

The most widely cited federal standards for concrete construction safety

are given in Subpart Q (Concrete, concrete forms and shoring) of OSHA's Construction Safety and Health Regulations [9]. By reference the OSHA Standard,

section 1926.700(a) makes a blanket inclusion of all provisions of ANSI AlO.9-

1970, Safety Requirements for Concrete Construction and Masonry Work [10].

For emphasis. Subpart Q repeats, in some cases verbatim, some of the ANSI A10.9

provisions but a contractor must still refer to the ANSI A10.9 in order to com- ply with the OSHA regulations.

3.3. State and Local Building Codes

In the United States, most local building codes are patterned after the

so-called "model" building codes. They are the BOCA [11], National [12],

Southern Standards [13], and Uniform Building Codes [14]. Summaries of provisions in these model codes affecting concrete construction are given in

Appendix A. Treatment of this subject in these codes is minimal, referring

generally to the provisions in ACI 318 [8], ACI 347, Recommended Practice for

Concrete Formwork [15] , or ANSI A10.9 [10].

8
3.4. National Standards

In 1931 the American Standards Association Committee (now ANSI) on Standards for Safety in the Construction Industry issued the American Safety

Code for Building Construction . Since then, it has been updated and the part

on concrete and masonry construction is currently issued as American National

Standard Safety Requirements for Concrete Construction and Masonry Work , AlO . 9-1970 [10] . This is the most comprehensive standard currently available

in the U.S. and it is probably the most single important document, as it has

been included in its entirety by reference in OSHA's Construction Safety and

Health Regulations [9]

.

The ANSI AlO. 9 standard provides a comprehensive listing of many important

factors which must be considered in concrete construction. Its quantitative

treatment of many of its requirements, on the other hand, is not specific.

For example, section 6.3.4 requires that imposition of any construction loads

on the partially completed structure be approved by the engineer-architect.

However, it fails to provide the criteria which the engineer-architect should

consider. Similarly, section 8.1.30 requires that the allowable load on the

supporting slab should not be exceeded when reshoring. The standard does not

specify whether the allowable load is to be based on the strength of slab at

the time of reshoring, fully cured 28-day strength, the factored load, or unfactored design load. ANSI AlO. 9 is currently being revised by the subcommittee which is responsible for its updating. There are many ambiguities in

the current standard and they nee^ to be clarified.

Another national standard which influences concrete construction is ANSI

89.1-1972, better known as ACI-318-71, Building Code Requirements for Reinforced

Concrete [8]. Its construction coverage is far more comprehensive than the

model building codes. It provides a general set of guidelines for placing and

curing the concrete, design of formwork, and removal of forms and shores.

Quantitative values are not specified for any of the guidelines.

The most comprehensive standard for concrete construction is ACI 347-68,

Recommended Practice for Concrete Formwork [15] . This document is the basic source for many of the ANSI AlO. 9 provisions and consequently for others that

use ANSI AlO. 9 as a model. It provides clear statements which clarify the

role of the contractor and of the engineer-architect who issues project drawings

and specifications and who supervises certain aspects of the construction process.

The document contains many important detailed technical matters not covered in

ANSI AlO. 9. It also contains recommmended formwork practices for special

methods of construction, including preplaced aggregate concrete, slipform

9
construction, prestressed concrete, precast concrete, and underwater concrete.

An ACI publication, SP-4, Formwork in Concrete [16] which serves as a commentary

to ACI 347, has abundant detailed information relative to formwork practice

with design aids, examples, and illustrative figures. In fact, SP4 has been

cited as a part of the requirements in the U. S. Army Corps of Engineer's

Safety Manual [17]

.

3.5. Recommended Practices

There are two widely distributed recommended practice documents dealing

with safety in the construction industry. One is distributed by the Scaffolding

and Shoring Institute (S & SI) [18] and the other is by the Associated General

Contractors of America (AGO [19]

.

The S & SI document provides guidelines primarily for shoring and reshoring,

and generally follows closely those included in ANSI AlO.9. However, in some cases a few of the important requirements of ANSI A10.9 are not included in

this document. For example, section 2.5 of the document follows ANSI A10.9,

section 8.1.5 verbatim in describing vertical loads for shoring and formwork.

However, while ANSI AlO.9, further requires that design for lateral load be

not less than the greater of 100 lb per foot of floor edge or 2 percent of

total dead load of the floor, the S & SI recommendation is silent on this

important requirement for lateral stability of the shoring system. Similarly,

section 5.2 of the document duplicates ANSI AlO.9, section 8.4.2 for lateral

and diagonal bracing, except that it fails to include the requirement that

"bracing be installed as the shores are erected."

The AGC Manual covers concrete construction in Chapter 37. The coverage

on shoring seems to follow the recommendations of the Scaffolding and Shoring

Institute (S & SI). In general, the safety provisions contained in this

document is less than that in the S & SI recommendations. It also omits the

lateral load requirement for shoring. However, the AGC manual does strongly

recommend tests for job-cured cylinders to determine the adequacy of the

concrete strength prior to form removal.

3.6. Comparison of Requirements in Existing Documents

Coverage on concrete construction safety in building codes and standards

vary widely from a minimal coverage in local building codes (except for the

City of New York code) to a detailed coverage in national standards. Most of

10
the requirements found in these documents are patterned partially or wholly,

after those in ANSI A10.9, which, in turn, adopted many of the requirements in

ACI 347-68. Unfortunately, in the transition to ANSI A10.9, many of the

subtle but important features of ACI 347 are lost, including the definition

and role of the engineer-architect who is usually a representative of the owner rather than of the contractor.

Many of such subtle differences in the documents are difficult to compare

in detail on a one-to-one basis. In this section, design requirements and

dimensional tolerances which are found in five widely referenced documents are compared. The five documents include ACI 347-68, ANSI A10.9, OSHA 1926,

Recommended Safety Requirements of S & SI, and the AGC Manual. Salient features

of the requirements from each of the documents are listed in table 5.

For design of falsework, the treatment of the vertical load in all five

documents are similar in that it includes both dead (DL) and live loads (LL)

;

the dead load consists of weight of the formwork and concrete and the live

load consists of such loads as workmen, equipment, runway and impact. However,

only ANSI AlO.9 includes separate design load requirements for both formwork

and shoring. ACI 347 includes the requirements primarily for formwork, while

both the S & SI and AGC Manual include only the requirements for shoring.

Live load requirements in these documents vary from 50 psf to 20 psf including

the weight of formwork. Where 20 psf is specified, the live load allowance

would be 10 psf if the weight of formwork is assumed to be 10 psf. This is

far less than what is considered as a minimum allowance for the weight of

workmen, runways, equipment and impact. There is no apparent basis for these

figures.

Although rational base for the numerical values are not apparent, both

ACI 347 and ANSI A10.9 provide requirements for lateral loads. These include

a general lateral load requirement for the formwork system and a special

provision for walls and wall bracings. On the other hand, these specific

numerical requirements are omitted in the S & SI and AGC documents. It is

reasonable to assume that contractors would be more likely to use these documents than ANSI AlO.9 or ACI 347. The large number of construction failures

resulting from insufficient lateral bracings could be attributed to the omission of lateral load requirements.

Except for the AGC Manual, the other documents caution the contractor

about the imposition of any construction loads on the partially matured

structure unless such loading has been considered in the design and approved

by the engineer-architect. Storage of construction material over a small area

11
TABLE 5 Suramary of Construction Safety Provisions

In Regulatory and Industry Standards

ACI - 68 ANSI AIO.9& OSHA 1926

SCAFFOLDING

& SHORING INSTITUTE

ASSOCIATED GENERAL

CONTRACTORS

VERTICAL

LOADS DL + LL

FORMS: DL = Weight of fonnwork

and concrete LL = Weight of workmen,

runways, impact

'\ 50 psf

DL + LL

FORMS: DL = Weight of formwork

and concrete

LL = Weight of workmen,

equipment, runways,

impact

DL + LL

FORMS: None specified

DL + LL

FORMS: None specified

SHORES: None specifiedSHORES: DL + LL 1 100 psf

LL + Foimrork { 20 psf

SHORES: DL + LL -1: 100 psf

LL \ 20 psf

SHORES: DL + LL •( 100 psf LL + Formwork { 20 psf

When motorized carts are

used: Add 25 psf to design load

When motorized carts are

used: Add 25 psf to design load

When motorized carts are

used: Add 25 psf to design load

LATERAL

PRESSURE

In section 1.2.2 formulas are given for columns and walls in terms of rate of placement and temperature of concrete in the formsNone specified None specified None specified

LATERAL FORMS: FORMS: None specified None specified

LOADS Wind, dumping of concrete and equipment Greater of

100 lbs/ft of floor edge or d/o 01 u.b. 01 I loor

WALL" FORMS:

Wind, impact of concrete

and equipment Greater of

100 lbs/ft of floor edge or z/o 01 u.L. 01 I loor

WALL FORMS: WL \ 10 psf

Bracings for wall forms: 100 lb/lineal ft of wall

applied at the top

WL -f 10 psf

Bracings for wall forms: 100 lb/lineal ft of wall applied at the top

SHORES: None specified

SHORES: Wind, impact of concrete and equipment Greater of

100 lbs/ft of floor edge

or 2% of D.L. of floor

CONSTRUCTION

LOADS Not permitted unless approved by EngineerArchitect

Not permitted unless considered in design or approved by EngineerArchitectStrengthening or give

special consideration

Not specified

IMPACT Part of LL Part of LL None specified None specified

TOLERANCES FORMS: Suggested tolerances are given to ensure dimensional

control over finished

structural manbers

FORMS: None suggestedFORMS: None suggestedFORPIS: None suggested

SHORES: None suggestedSHORES:

1/8 inch in 3 ft SHORES:1/8 inch In 3 ft or not more than 1 inch

in 40 ft

SHORES: None suggested

REMOVAL OF . FORMS AND

SHORES

Based on Engineer-Architect

specification, or under ordinary conditions, use recomuended minimum period of curing

Based on engineer specifi- cation or local building

code, or under ordinary

conditions, use recommended minimum period of curing

Based on approval of qualified engineer

Based on job cured strength

DL = Dead Load

LL = Live Load

WL = Wind Load

12
has been a cause of construction failures. Collapse of a small area can often

lead to a progressive collapse resulting in the total collapse of a structure.

Extensive listing of formwork tolerances are given in ACI 347 for various

structural elements. The primary intent of these tolerances is to maintain

finished dimensions of structural members within specified limits. To guard

against instability of vertical supporting members, such as shores and reshores,

both ANSI A10.9 and the S & SI document recommend a tolerance of 1/8 inch for

out-of-plumbness in 3 feet. However, it is doubtful whether this requirement

is realistic from a practical standpoint in day-to-day field operation.

Neither ACI 347 nor AGC have any such requirement.

One of the most critical operations in concrete construction is correct

timing of removal of forms and shores so that no damage to the concrete results.

For this reason, all four documents stress that removal of the forms and

shores should be based on the strength of job-cured cylinder and/or the approval

of the engineer-architect.

It is apparent from the above comparison that a lack of uniformity

exists in present documents. In many cases, the basis for numerical values of

specific requirements are not apparent. Thus, one cannot make judicious

selection of quantitative requirements for design of falsework. In the following chapter, deficiencies in these requirements are reviewed and pertinent

recommendations are made for upgrading them.

4. FACTORS AFFECTING CONCRETE CONSTRUCTON SAFETY

4.1. General

This chapter reviews those factors which affect the safety of the structure

during construction. The basic factors are; design loads for falsework;

strength of the falsework which provides temporary support to the structure,

and the construction cycle of forming, stripping and reshoring processes.

These broad areas can be subdivided into a number of related components each

of which consists of several elements. These relationships are shown in

figure 1. For each of the subareas, the state-of-the-knowledge is discussed,

and inadequacies of requirements in existing regulatory documents are identified, and, where appropriate, recommendations for research to improve existing

knowledge are made.

13
Factors

Affecting

Concrete

Construction

Safety

Design Loads for

Falsework

Strength of

Falsework

Construction

Cycle —

Fofmwork

Design Loads

Shoring

Dfeslgn Loads

Formwork

Capacity

Shoring

Capacity

Support

Capacity

Rate of

Strength Gain

Accuracy of Strength

Determination

Required Concrete

Strength for

Formwork Removal

Figure 1. Factors Affecting Concrete Construction Safety

4.2. Design Loads for Falsework

Falsework is composed of formwork, which is in contact with the concrete,

shores, and lateral bracing. While gravity loads are supported by formwork

and shores, lateral loads are resisted by lateral bracing.

4.2.1. Design Loads for Formwork

are

:

The loads which formwork must carry may be divided into six types. These

(a) Dead Loads

(b) Live Loads

(c) Lateral Pressure from fresh concrete

(d) Impact

(e) Lateral Loads

(f) Other Loads

14
(a) Dead Loads - Included under this category are static loads which occur

for a considerable period of time during the life of the formwork. It

includes the weight of the form itself, the fresh concrete and reinforcement.

All regulatory standards mention these two kinds of loading as dead load.

It should be noted that both ACI 347 and ANSI A10.9 make reference to

concrete as "freshly placed concrete" without explicitly mentioning the

v;eight of reinforcement to be included in the dead load. However, in

most cases, use of 150 lb. per cu. ft. for normal weight concrete and 120

lb. per cu. ft. for lightweight concrete can be used to represent the

weight of reinforced concrete. Depending on the type of aggregates used,

however, the unit weight of concrete can range up to 400 lb. per cu. ft.

(b) Live Loads - Live load consists of the weight of workmen and equipment, runways and other temperary loads that are supported by the formwork

during concrete placing and finishing. ACI 347 recommends a minimum

value of 50 psf for live load. While this design value appears reasonable

because no failures have been attributed directly to excessive live load,

the basis for the 50 psf is not apparent. The present trend is to use somewhat less than 50 psf for design of formwork. Although this use is

not justified by actual measurements of live loads imposed on the formwork, in recent years the increased use of heavier and larger capacity

equipment would seem to suggest that live load be increased rather than

decreased. Until accurate assessment of actual live loads is made, 50

psf or greater for design live load would be desirable. Documentation of

live load variations in various types of concrete construction using

different placement procedures would be invaluable in establishing load

factors and design load criteria.

(c) Lateral Pressure - Two important factors influence the lateral pressure

exerted on the side of the form by the fresh concrete. They are the rate

of placement and the temperature of concrete. As the concrete is being

placed, lateral pressure at a given location increases. As it hardens,

the concrete changes from a plastic state to a solid state, gradually

eliminating the lateral pressure on the form. At low temperatures the

concrete takes longer to harden and therefore, concrete is placed in

greater depth before the lower portion becomes sufficiently firm to be

self-supporting. Because greater hydraulic head can be developed under

this condition, greater lateral pressure results.

There are other factors which affect the lateral pressure on the

form. These include the type of vibration (internal or external) which

is used to consolidate the concrete and which causes the concrete to

behave as a fluid along the full depth of vibration, thereby increasing

15
lateral pressure; impact loading caused by free fall of concrete when it

is discharged into or on the form; and consistency of concrete. However,

the effects of these factors on lateral pressure are relatively small

when compared with the other two main factors, and also their effects are considered in usual concrete practice.

ACI Committee 347 has developed workable formulas for maximum lateral

pressures (p) on the form, for prescribed conditions of temperature, rate

of placement, vibration, weight of concrete, and slump. The formulas

listed below are based on limited experimental data but are practical

for form designs.

For wall forms with rate of placement not exceeding 7 ft per hour

(max. 2000 psf or 1 whichever is less)

p = 150 + ^

(max. 2000 psf or 150 h.

For wall forms with rate of placement greater than 7 ft per hour

, 43,400 , 2800 R , ^ , p = 150 + — + — (max. 2000 psf or 150 h,

whichever is less)

For columns

90 0 R

p = 150 + —=— (max. 300 psf or 150 h, whichever is less)

Where

:

p = maximum lateral pressure, psf

R = rate of concrete placement, depth of concrete placed per

hour, ft per hour

T = temperature of concrete in the form, °F

h = maximum height of fresh concrete in the form, ft.

The load values obtained by these formulas have been verified for thos€

conditions which represent "good practice." Taylor [20] has suggested a need for more experimental research on placement rates of more than 10 ft

per hour and to investigate form pressures due to impact from dumping of

concrete in the forms and extreme variations in placing temperature.

(d) Impact - Impact is generally considered as part of the live load and

is that produced by dumping of concrete on the form. The magnitude of

impact on the form is affected by the quantity of concrete dumped per

load and the rate of dumping. Impact can not only produce temporary
overloading on the form but it can also cause uplift of supports in adjacent bays. Unless the falsework is securely fastened together, tilting

or sometimes loss of shores can occur causing collapse. At present, no data are available on impact measurements to establish quantified provisions. Dynamic measurements on impact load should be made in order to

establish a minimum value for design.

(e) Lateral Loads - The most serious deficiency in formwork is in the

consideration of lateral loads. Many practical design guides do not

include any lateral load provision to warn the designer of the importance

of adequate lateral bracing to insure the stability of the falsework.

Lateral loads include wind, cable tensions, inclined supports,

impact of placement of concrete, starting and stopping of equipment and

earthquake. Most standards and codes recognize these effects except for

the earthquake loading. Both ACI 347 and ANSI AlO.9 require that forms

be designed for lateral loads, in any direction, of not less than 100 lb

per ft of floor edge or 2 percent of the total dead load of the floor,'

whichever is greater.

This lateral force requirement seems to be largely a judgement

matter. The British [21] have implemented an increase in their lateral

load requirement to 3 percent of the dead load. This increase is based

on examination of a recent collapse which was triggered by failure of

lateral bracing. An investigation [22] on a California bridge falsework

collapse recommended a minimum of 5 percent for the lateral force coefficient. Others recommended as high as 8 percent for the same collapse.

In view of ambiguity in assigning proper values for lateral force, a comprehensive review of lateral force requirements for formwork should be

undertaken. The study should consider all factors which can induce

lateral forces. Both ACI 347 and ANSI AlO.9 use a value of 10 psf for

wind on vertical wall forms. This is inadequate at some geographical

location in the U. S. and is unrealistic in that the requirement does not

vary with factors such as building height, the mean recurrence interval

for the wind speed, and the shape of form, etc. It appears that general

upgrading of the lateral force requirements is needed.

(f) Other Loads - The design of formwork should also consider any special

conditions likely to occur, such as unsymmetrical placement of concrete,

uplift, concentrated loads of reinforcement and storage of construction

17
^materials, etc. Storage of construction materials has been more often

cited as a cause of failure of formwork than others conditions.

In high-rise constructions, excessive construction loads applied to

the formwork cause about 17 percent of all building construction accidents.

This is due mainly to the vertical spatial distribution problem for con- struction material in tall buildings. Both OSHA section 1926.700 (e) (1) (i)

and ANSI A10.9, section 8.1.1.6 require that when temporary storage of

reinforcing rods, material and equipment on top of formwork becomes necessary, those areas be strengthened to meet the intended loads. Although

the intent of this requirement is clear, it is probably not realistic in

application. Local strengthening of formwork demands careful control of

material storage placement at the job site, that often is absent in con- struction operations. Studies should be made of existing site practices

and estimates of actual material storage loads on the falsework.

4.2.2. Design Loads for Shores

Most of the load requirements previously discussed for the formwork also

apply to shores. However, ANSI AlO.9, Section 8, treats the loads on shores

separately, while it is not treated in ACI 347. Five types of loads are con- sidered below.

(a) Dead loads

(b) Live loads

(c) Impact

(d) Lateral loads

(e) Other loads

(a) Dead, loads - Dead load for shores include all the dead load considered

for the formwork plus the weight of shores and the lateral bracings. ANSI

A10.9, section 8.1.5 requires that the minimum total design dead and live

loads be not less than 100 psf. This suggests that if ACI 314 recommendation of 50 psf for live load and 10 psf for the falsework are subtracted

from this value, only 40 psf is left for dead load or less than a 4-inch

concrete slab, assuming the unit weight of 150 Ib/ft^ for normal weight

concrete. While the basis for this combined treatment is not apparent, it

would be realistic and less confusing to treat the dead and live load

seperately.

18
(b) Live loads - A minimum allowance of 100 psf for a combined dead and

live load is suggested in ANSI A10.9, in the S & SI recommendations and in

the AGC document. This provision would lead to much less live load than

the ACI 347 requirement of 50 psf. For example, for a 6-inch slab only 15

psf is available for live loads after deducting 10 psf for formwork and

shores. For an 8-inch slab, no allowance is left for live load. In

addition, both ANSI A10.9 and the AGC document require that the minimum

allowance for live load and formwork should be not less than 20 psf. The

S & SI requires that the live load allowance alone should not be less than

20 psf. Since "heaping" of only 4 inches of extra concrete would produce

50 psf, these smaller values represent an almost insignificant live load.

There is no apparent basis for such small values for live load requirement. Similar to the case for formwork, assessment of actual loads on shores should be made in order to arrive at realistic design values.

(c) Impact - Other than impact produced by "dumping" of concrete on the

form, direct lateral impact on shores is not common. When it occurs,

failure of shores is usually localized. Impact by the "dumping" is

usually shared by several shores, and when they are adequately braced,

failure of shores can be avoided. Lateral impact produced by starting and

stopping of equipment such as motorized buggies, is also resisted by the

lateral bracings. As in the case for the formwork, no data on actual

impact measurements are available to determine realistic values for design

at the present time.

(d) Lateral loads - Shores together with lateral bracings resist the

lateral loads transmitted from the formwork above and the wind and other

loads directly applied on the shore. Only ANSI A10.9 has a seperate

provision for the lateral load for shores, which is the same as that for

the formwork. All the comments relative to the later force requirements

for the formwork also apply for the shoring. In addition, it is also

important to consider forces on individual shores as well as overall

forces on the entire shoring system.

(e) Other loads - Special loads that arise in concrete construction are usually transmitted to shores through the formwork. Among the types of

loads mentioned in section 4.2.1 (f), the effect of concentrated con- struction loads on the shore is greater than the others, as inadequately

braced shores have a tendency to become unstable under excessive loads.

ANSI A10.9, section 8.1.16 cautions the designer about excessive loading.

19
since little information on the magnitudes of construction storage loads

is available, overall requirements for overload for which shores need to

be designed cannot be recommended at this time. A reasonably accurate

determination of overloading on shores is necessary to arrive at a realistic load factor for design live load.

4.3. Strength of Falsework

Overall strength of the falsework depends on the strength and stiffness of

the formwork, the shoring together with lateral bracings, and the support which

provides reaction to the loads transmitted through shores. Factors of safety

used in each' of these components vary from 1.5 to 4. Some of these factors of

safety are based on allowable stresses and others are based on ultimate load

tests. Therefore, the current design approach to the total falsework system is

not uniform, and thus, safety against collapse of the system cannot be assessed

readily and accurately.

Because it is considered a temporary structure, the falsework receives

bare minimum engineering attention during its design stage. In many instances,

the design and erection are left to the job foreman or superintendent. Because

of their limited knowledge of the importance of certain engineering aspects of

falsework, such as bracing requirements for lateral loads, the chances of

faulty or inadequate design would be high. Even small deviations from proper

construction of the falsework would result in a serious reduction in the safety.

In order to ensure the safety of the falsework, ACI 347, section 1.3.2

requires that a design analysis for all formwork, including analysis against

buckling of every member, be made in all cases. Little has been written in the

engineering literature regarding the analysis of falsework. Because a number of

failure studies mentioned numerous examples of buckling of falsework, a set of

guidelines for comprehensive analysis involving stability checks for the most common types of falsework needs to be developed.

4.3.1. Capacity of Formwork

Both wooden and metal forms are extensively used for floor slabs, walls

and columns. When prefabricated forms are used, ACI 347, section 1.3.1 recom- mends that manufacturers' recommendations for allowable loads be followed if

supported by test reports and successful experience records. On the other

hand, when wooden forms fabricated at the site are used, the design is to be

based on allowable stresses given in the applicable codes and standards.

20
Because allowable stresses depend on many factors, including the species grade,

size of cross section, moisture content, and duration of loading, one single

factor of safety cannot be specified realistically. For stress grade lumbers,

the factor of safety in allowable stresses design is about 2.5. Permitting the

allowable stress to increase 25 percent for light construction would reduce

this factor of safety to 1.87 [16]. In some cases, the factor of safety is not

known because of reliance on reference standards or codes to set these allowable stresses.

Where the material has deteriorated from its new condition or has been

damaged from previous use, allowable capacity should be reduced to accomodate

for the loss of strength. ACI 347, section 1.3.1, merely cautions the designer

about this. In Great Britain modification factors which could be used to

reduce the allowable stresses have been introduced [21]. Their interim provisions require the basic allowable stresses be multiplied by modification

factors

.

An in-depth study should be undertaken to evaluate the effects of the re- use of material on overall strength of falsework and develop modification

factors as described above. Such factors can then be incorporated into a limit

state approach in defining the overall safety of the falsework.

4.3.2. Capacity of Shores

The capacity of shores is affected by three main factors. They are strength variability in the material used for the shoring, the geometric con- figuration of shores, and construction deviations such as out-of -plumbness

,

eccentricity with respect to stringers, etc. While strength variation in the

material is an important factor, the capacity of shores is significantly

affected by the two latter factors. The effect of these parameters are greater

on single post shores than on frame-type shores as these are pre-engineered

products which frequently have lateral bracings built into the shoring system.

The actual load capacity of shores is usually determined by testing of

shores under simulated field conditions. ANSI A10.9 relies heavily on Safety

and Shoring Rules developed by the Scaffolding & Shoring Institute and on test

results following the S & SI test procedure (Appendix C) . The standard test

procedure appears reasonable within the tolerance range specified in ANSI

A10.9, section 8.1.24. However, a major question arises as to how well average

site conditions reflect the condition specified in ANSI AlO.9. In an extremely

informative British survey [23] of 40 construction sites, actual plumbness and

eccentricity were determined for a large number of shores. The ANSI AlO.9
allowable construction tolerance of 1/8 inch in 3 feet corresponds to a value

considerably less than the smallest deviation measured in the field assessment.

Sixty percent of the shores measured were found to be more than 5/16 inch out

of plumb in 3 feet; indicating that ANSI AlO.9 tolerance would be unrealistic

in British practice. Over 10 percent of the shores were as much as 1 inch out

of plumb in 3 feet. Twenty percent were found to have end eccentricities

greater than 1/2 inch. In view of such a large magnitude of out of plumbness

combined with large end eccentricities as found in the British study, the ANSI

construction tolerance should be reexamined after similar as-built tolerance

surveys are carried out in the United States.

4.3.3. Capacity of Support

The shoring is supported either by soil or floor slabs. The strength

gain of the slab will be treated in detail in section 4.4.3. In this section

attention is focused on foundation and soil support for the shoring.

One of the recurring causes of falsework failure is inadequate support for

the falsework. This inadequacy stems from soil deficiencies, improper falsework foundation load distribution and the effect that environmental factors

such as rain and frost have on falsework supports. ANSI AlO.9, sections 8.1.12

through 8.1.14 provide general guidelines relative to the input of soil mechanics or foundation engineering into the falsework support design on soils. In

general, the language focuses on the strength aspects which are important but

does not point out the need for consideration of settlement characteristics of

the soil. In this regard, the working bearing pressure should be limited by

the maximum allowable settlement.

In many instances, circumstances do not permit or warrent an extensive

foundation analysis for falsework. Under these situations a reasonable empirical design method is needed for temporary falsework foundation. An interesting

guideline is presented in reference 21, wherein permissible bearing pressures

for various type soils and rocks are given in relatively simple terms. Modification factors are presented for the case of incomplete soil information, for

special settlement restrictions, for exposure to high water table and for

various fill materials. These modification factors are listed in tables 6, 7,

and 8. Foundation design approach such as this one would be useful for many

medium to small constructions.

22
TABLE 6 Maximum Allowable Bearing Capacities for

Foundation on Fully Coirpacted Fill (Ref. 21)

Classification of fill Allowable bearing capacity

tonf/ft2 (kN/m2)

broken rock 2.0 (200)

well-graded sands

and gravels 1.5 (150)

uniform sands and hard

shaley clays 1.0 (100)

firm to stiff clays 0.75 (75)

TABLE 7 Modification Factors for Restricted Allowable

Settlement (Ref. 21)

Loading Cohesive Non-cohesive Rocks

period soils soils

Short-term

loading 0.75 0.75 1.0

Long-term

loading 0.67 0.75 1.0

TABLE 8 Modification Factors for Flooding or High Ground

Water Levels (Ref. 21)

Condition Cohesive Non-cohesive

soils soils

Rocks

1 Ground water i level at B, or 1

1 less, below

level of

foundation 1.0 0.5 1.0

(B=width of

foundation)

I Site liable

to flooding 0.67 0.5 1.0
4.4. Construction Cycle

The responsibility of establishing the construction cycle rests with the

contractor, because he has the responsibility for completing the construction.

However, while the contractor has responsibility for design and erection of

formwork, ACI 347 recommends that time of rem.oval of falsework, in part or whole, be specified by the engineer because of the possibilty of damage to the

concrete from overloading the incomplete structure. Similarly, ANSI A10.9

recommends that the length of time that forms should remain in place following

concrete placement be controlled by the engineer's specification or by local

building codes. When field operations are not controlled by the engineer's

specification, both ACI 347 and ANSI AlO.9 provide tables for determining the

time that forms and supports should remain in place under "ordinary" conditions.

In general, the decision as to when the falsework can be removed safely

depends on three interrelated factors. These are the rate of strength gain in

the concrete, the accuracy -of strength determination of in situ concrete and

the level of temporary stress and deformation that the structure can withstand.

4.4.1. Rate of Concrete Strength Gain '

Among the factors which affect the gain in strength of a concrete, temperature and moisture are the most pronounced. Extensive studies have been

made on the development of the compressive strength, primarily measured using

} the standard 6x12 inch cylinder [24, 25, 26]. The results of these studies

indicate that the effects of curing temperature, moisture, and age on compressive strength are generally predictable within reasonable limits. On the other

hand, a number of researchers have investigated the relationship between the

compressive strength of concrete and maturity [27, 28, 29, 30 and 31]; where

maturity is defined as a function of temperature and time. This relationship

is based on a concept that samples of the same concrete will have equal com- pressive strengths if their maturities are the same, regardless of their temperature histories. While this approach appears promising, further study is

needed to examine its applicability to cement with higher ranges of heat of

hydration and to higher curing temperature.

Although the compressive strength is an important factor in determining

the basic strength of a concrete member, the strength of a flexural member at

early ages is usually governed by either shear (diagonal tension) or bond

strength. Some investigations [31] have assumed that the relationship between

24
these two strength parameters and the compressive strength is the same at any

age. To date, little data are available to substantiate this assumption. In

view of this, a series of investigations should be made to confirm and to

verify the assumption that at early ages, flexural strength, shear and bond

strength, as well as the stiffness vary with the same function of the compressive strength as at later ages.

4.4.2. Accuracy of Strength Determination

Determination of the time of form removal is usually based on the results

of tests performed either on concrete specimens or on structural members. Both

ANSI A10.9, section 6.4.7 and ACI 347, section 2.7.2.1 recommend the use of

test results of field-cured cylinders. Although it is common to carry out

compression test on cylinders, neither ANSI nor ACI specifies the type of test,

e.g., compressive strength test or splitting tensile strength test. Recent

investigators [32,33] have shown that the results from cylinder tests, even

cured in the similar environmental condition to which the structure is subjected, often do not correlate consistantly with the concrete strength of the

structure. This inconsistancy stems from the fact that even when cured at job

site, the curing conditions of cylinders are not the same as those of beams,

slabs and walls, simply because of the difference in shape and the size of

structural sections.

In order to overcome this difference in curing condition, a special

curing procedure has been followed by a large construction firm in England. It uses a mobile environmental chamber to cure concrete test specimens. The

temperature condition in the chamber is controlled by output of thermocouples

which are inserted in the freshly poured concrete. This is to simulate as closely as possible the temperature at which the structural member is cured.

While this procedure appears reasonable, no research data are available to

substantiate its effectiveness.

There are three types of destructive test methods commonly used to determine the compressive strength of concrete. They are (1) testing of 6x12 inch

standard cylinders, (2) testing of cores of various sizes taken from actual

structural members and (3) testing of pushout cylinders. The pushout cylinders

are prepared at the same time structural members are cast. Concrete is placed

in plastic sleeves which are inserted in the respective structural members.

25
This enables the cylinders to cure in the same manner as the bulk concrete.

At the desired time, the cylinder is removed from the structural member and

its compressive strength measured. The, results of a study [33] show that the

coefficient of variation of the compressive strengths of 216 field cured

cylinder specimens was 2.4 percent compared with 6.0 percent for an equal

number of core- specimens and with 3.9 percent for push-out cylinder specimens'.

The study concluded that "field cured cylinders may provide useful information

but do not quantitatively reflect core strength and that push-out cylinders

cast in the slab provided , a fairly reliable measure, relatively, of core strength .

Since it is cumbersome either to mold and cure large numbers of cylinders

or to take cores from the structure and test, it would be desirable to have

reasonably accurate nondestructive test methods to determine the strength of

concrete for falsework removal. Numerous methods involving the use of impact

penetration of steel probes, rebound impact hammer measurements and ultrasonic

velocity measuring devices have been suggested. Detailed descriptions of these

methods with evaluation of their relative merits are presented in reference 34.

In general, individual nondestructive test methods do not measure directly the

compressive strength of concrete. They measure some properties of concrete,

and the measurements are correlated to the compressive strength empirically.

At the present time, the individual nondestructive test methods do not

appear to provide accurate prediction of the in situ strength of concrete.

Their applicability to "green" concrete for the purpose of determining the

time of form removal has not been thoroughly evaluated. The use of non(^estructive test methods for this purpose needs further investigation.

4.4.3. Required Concrete Strength for Formwork Removal

Because the falsework cost can range up to 60 percent of the total cost

of concrete construction, for desire of economy, forms and shores are usually

removed at an earliest possible time for reuse and, in their place, reshores

are placed to transfer the loads to previously cast floors. Reshores are shores placed firmly under a stripped concrete slab or structural members

whose original falsework has been removed. To guide this process, ACI 306-66

[35] suggests that shores can be removed and reshores used when the concrete

strength is 55 to 65 percent of design strength. ACI 347-68, section 2.7.2.1,

26
recommends that the supports of suspended structural members can be safely

removed if the ratio of actual concrete strength to design strength is equal

or greater than the ratio of total dead load and construction loads to total

design loads, with a minimum concrete strength of 50 percent of the design

strength required. For example, in an 8-inch thick flat plate building with

40 psf design live load and 20 psf construction load, this would be =

^ ' 100 + 40

0.86 design strength. In buildings with higher live load to dead load ratios,

this ratio would decrease. Thus, it seems apparent that the minimum concrete

strength for stripping should be considerably greater than 55 percent of

design strength as suggested in ACI 306-66.

Transfer of upper level construction loads to more mature lower floors is

feasible only if the lower floors have been designed to support the loads.

The number of floors which need to be reshored in order to distribute the

loads from above depends on the type of structural system, the ratio of design

live load to dead load, the magnitude of construction loads, the rate of

strength and stiffness gain of concrete and the construction cycle time.

Because the number of floors to be mobilized in reshoring is recognized as one

of most critical operations in concrete construction, ACI 347-68, section

2.8.3.1 recommends that reshoring operations be planned in advance and be

approved by the engineer.

Neilsen [36] was one of the first who studied distribution of loads on shores and on slabs in multistory structures. He made an extensive theoretical analysis of the interaction between the formwork and the slabs supporting

the falsework loads. In the analysis he assumed that (1) the slabs behave

elastically, (2) shrinkage and creep of the concrete can be neglected for the

purpose of analysis, (3) shores are represented by uniform elastic support and

(4) the torsional moments and the shearing forces in the formwork are neglected.

He expressed the total load carried by the slab which supports the load above

and by shores as a percentage of the dead weight of the concrete plus the

weight of the falsework on each story. On this basis the maximum calculated

load on a floor was 256 percent of the load on each story. His maximum measured load on an individual floor was 200 percent of the load. In his study,

Neilsen considered shoring on two or three floors only. The calculations

involved are lengthy and can not be readily applied to individual cases.

27
Grundy and Kabaila [37] developed a simplified analytical method to

predict the load on shores and supporting slabs. The significant difference

between their approach and that of Neilsen is the assumption that the shores

are infinitely stiff in comparison with that of the slabs. With rigid shores,

all slabs connected by the shores deflect identically. Consequently, the

total load applied to the system, either by casting of new slabs or by removal

of shores, is distributed between the slabs in proportion to their relative

flexural stiffness. Assuming construction of one floor per week, with shores

removed five days after a casting operation, they developed the load distribution sequences for slabs with constant flexural stiffness and for slabs with

varying flexural stiffness with age. These are shown in figures 2 and 3,

respectively. The load ratios shown are the ratio of the total load being

carried by a slab or a set of shores to the dead load of one slab plus the

weight of falsework. The load ratios shown on these figures reveal that magnitudes of floor loading between the two cases are very small. This indicates

that the error introduced by the assumption of equal slab stiffness is not

appreciable on the load distribution analysis. Bresford [38] also concluded

from his study that within practical limits, consideration of changes in the

stiffness of the slab with age did not change load ratios significantly.

The maximum load ratio obtained by Neilsen was 2.56 and by Grundy and

Kabaila was 2.36. This difference is due to the different construction cycles

considered in their examples. However, this difference suggests that the

effect of ignoring the flexibilty of shores is small. Agarwal and Gardner

reported [31] , based on their field measurements of loads on shores at two

different construction sites, that the simplified Grundy and Kabaila theory

predicted maximum construction loads within 15 percent of the measured values.

When the differences in stiffness of the conc^rete slabs were considered, the

theory predicted the load within 10 percent.

While the methods of Neilsen and of Grundy and Kabaila give slightly

different magnitudes of the load which individual slabs experience during

construction, the methods clearly show that the load imposed on slabs, particularly at lower levels, is more than twice the dead load of one slab at ages

28
o

</> o , _ CVJ — oo

8SDO

a) rj 1 CD * in O ^ • 06-6 — O O o ^ to *D O <0 « O K) ro

oj

— oa

0)co

ClEooc

<i> oo

o"o

a.

a.o O

O

O

—

—

O ^CO ^ CD o ^ m 0 — 6 — — 6 —

00 ^ O fO ro O I'm) K> ro

O

— -o - 6 —

&1

•rHc

USCO

CO <;

CO -p

to •H 0 u

•H

4-> c CO 0 03 •H rH

-p

H 0

13 M-l

i-l 0

-P

CO CO c00

M-l 0 0

CO P H

CO 03 >i -P rH CO c 0u

Cr. -H

O

O 10K f- o"

10

O

n

O 60-0- - O fO O 10

O

"9 ^ O ID K>

O

— 6 ——— CM

—

O ^ U> <D

® CD

O * iTKX) CD 00 o- o o -

00—0

— ' (D ?1 ? ' o - o

c\j ro ffi ff> 00 — o O O O o — o - : CD

C

•HEm

CO

CO T3

03 •H 0 u

r-i +J

C CO 0 03 •H -H

4-)

w

U3 14-1 U 0

-P

CO to 0 i-i u

-o0 0 s

CO (U •H rH

CO X3

>^ 03 iH •H

(d J-l 03 < >

OJ

•H

T3OO

O

—

O

WO IC

^1 -I

O ^ i

uil ^1 ro| - I ° l o

-aE

29
earlier than that at which the design strength of the concrete is attained.

Grundy and Kabaila showed that for a 10 inch thick flat plate, the construction

loads would exceed service loads by 53 percent and would govern design. For

load factor design (currently 1.4D + 1.7L) and using a load factor for con- struction load of 1.0, live load must exceed 56 percent of dead load in order

that the design be governed by (1.4D + 1.7L).

U = 1.4D + 1.7L >_ 1.0 (2.36D)

1.7L >_ 0.96D

L >_ 0. 56D,

The construction load factor of 1.0 would leave no margin for safety and would

rely on full strength development of the concrete member. ANSI AlO.9, section

8.1.30, certainly implies the load factor of 1.0 in speaking of the allowable

load. On this basis, if a load factor of 1.0 is used, without additional

construction load, live load must exceed 136 percent of dead load for safe

construction.

S r.OD + l.OL ^ 1.0 (2.36D)

l.OL > 1.36D

L >_ 1.36D

This condition is seldom realized in the design of office or apartment

buildings

.

Since partially matured concrete frames would carry some construction

loads, Feld [39] suggests the use of a load factor to increase the loadcarrying capacity by 33 percent. Under this consideration, live load must

exceed dead load by 77 percent.

1.33S = 1.33D + 1.33L >_ 2.36D

, 1.33L >^ 1.03D

L >^ 0.77D.

The above analysis clearly show that further study of construction load

factors need to be undertaken. The study should consider such factors as live

load to dead load ratio, the rate of strength gain of concrete, and construction

cycle

.

30
5. CONCLUSIONS

An attempt has been made to identify all factors relating to concrete

construction safety, that is, the statistics of deaths and injuries, causes of

construction failures, review of existing regulatory and industrial standards

as they affect the safety in construction, and review of the-state-of -knowledge

,

Based on this study the following conclusions are drawn.

(1) Safety in concrete construction is a severe problem in that some 25

percent of all construction failures are related to concrete con- struction .

(2) Survey of reported failures in concrete buildings under construction

indicate that about 62 percent of failures result from inadequate

practices in falsework design and erection. Many of the inadequacies,

however, can be attributed to numerous conflicts and shortcomings of

existing regulatory and industrial standards.

(3) Review of existing documents which deal with safety in concrete

construction reveals that there is a lack of consistant design and

construction philosophy and uniformity in the regulations. In

general, contrary to extensive treatment given to the completed

structure, virtually no guidance is given in existing standards and

codes as to allowable stresses to be used in relation to the rate of

strength development for the concrete and appropriate load factors or safety factors to be used for falsework design. There is no clear

distinction made in the regulatory documents as to the scope of

applicability of the requirements to different types of structures.

For example, all the requirements which may apply to a large multistory construction would be equally applicable to a single-story

dwelling construction.

The role of contractor and engineer-architect is not clearly defined.

Although it is stated that design and erection of falsework and

safe construction of the permanent structure is the responsibility of

the contractor, many critical phases of construction process such as placement of construction loads and removal time of falsework are left for the judgement of an engineer-architect.

(4) Review of existing documents and literature reveals that improvements

are needed in all categories which affect safety in construction.

The following are specific subject areas needing research.

31
a) Documentation of actual vertical live loads on falsework.

b) Formulation of a realistic set of lateral load requirements

considering geographical location of the structure, occurance

of strong wind* for short recurrence period, location of falsework with respect to height,

c) Documentation of construction material storage loads.

d) Determination of equivalent hydrostatic loads for rapid rates

of placement of concrete under various temperature conditions.

e) Determination of impact loads on falsework.

f) Determination of actual falsework construction tolerances from

field surveys.

g) Evaluation of the bracing requirements which reflect actual

construction tolerances.

h) Development of a simplified foundation design procedure for

falsework in relation to type and size of structure.

i) Documentation of strength and stiffness gain characteristics

of concrete with time under various curing conditions.

j) Development of improved nondestructive test procedures for

determining in situ strength of concrete.

k) Determination of the proper levels of concrete strength required

for falsework removal considering both strength and deformation.

1) Determination of construction load factors for design of falsework and for shoring and reshoring analysis.

6 . ACKNOWLEDGMENTS

The contribution of Professor John E. Breen of the University of Texas

of Austin to the study of safety in concrete building construction is gratefully acknowledged.

7. REFERENCES

[1] Report of the Committee on Safety in Civil Engineering, Proceedings,

Institute of Civil Engineers (London) , Vol. 42 (Jan. 1969)

.

[2] Accident Facts, National Safety Council, Chicago, Illinois (1974).

[3] Philo, H., Discussion, Association of General Contractors' National

Safety Conference, 77 (1968).

32
[4] Feld, J., Lessons from Failures of Concrete Structures, ACI Monograph

No. 1, American Concrete Institute, Detroit (1964).

[5] Feld, J., Construction Failure, John Wiley & Sons, Inc., New York (1968).

[6] McKaig, T. H., Building Failures, Case Studies in Construction and Design,

McGraw-Hill Book Co., New York (1962).

[7] Short, W. D. , Structural Collapse During Erection or Demolition,

Proceedings, Institute of Civil Engineers, Vol. 36, London (March 1967).

[8] Building Code Requirements for Reinforced Concrete (ACI 381) , American

Concrete Institute, Detroit (1971)

.

[9] Construction Safety and Health Regulation, Occupation Safety and Health

Administration, Department of Labor, Federal Register, Vol. 39, No. 122,

(June 24, 1974)

.

[10] Safety Requirements for Concrete Construction and Masonry Work, ANSI

A10.9 - 1970, American National Standard Institute, Inc., New York (1970).

[11] The BOCA Basic Building Code/1975, Sixth Edition, Building Officials & Code Administrator International, Inc., Chicago, 1975.

[12] National Building Code, 1967 Edition, American Insurance Association,

New York (1967) .

[13] Southern Standard Building Code, 1973 Edition, Southern Building Code

Congress, Birmingham, Alabama (1973).

[14] Uniform Building Code, 1973 Edition, International Conference of

Building Officials, Whittier, California (1973).

[15] Recommended Practice for Concrete Fromwork (ACI 347-68) , American Concrete Institute, Detroit (1968).

[16] Kurd, M. K., Formwork for Concrete, Special Publication SP-4, American

Concrete Institute, Detroit (1973).

[17] General Safety Requirements, U.S. Corporation of Engineers Manual EM385-

1-1, U.S. Government Printing Office, Washington, D.C. (1973).

[18] Recommended Safety Requirements for Shoring Concrete Formwork, Scaffolding

and Shoring Institute, Cleveland, Ohio (1973).

33
[19] Manual of Accident Prevention in Construction, The Associated General Contractors of America, Inc., Washington, D.C. (1972).

[20] Taylor, R. S., Pressure on Formwork Under Site Conditions, Structural

Concrete, Vol. 1, No. 10 (July-August 1963).

[21] Falsework-Interim Report of the Advisory Committee on Falsework, Department of Employment and Department of Environment, Her Majesty's Stationery

Office, London (1974).

[22] Hart, S., Falsework Dangers, Civil Engineering (ASCE) , 103 (May, 1974).

[23] Birch, N., Booth, J. G. and Walker, M. B. A., Effect of Site Factors on

the Load Capacities of Admustable Steel Props, CIRIA Report 27, London

(1971)

.

[24] Klieger, P., Effect of Mixing and Curing Temperature on Concrete Strength,

Journal of the American Concrete Institute, Vol. 54 (June 1958).

[25] Price, W. H. , Factors Influencing Concrete Strength, Journal of the

American Concrete Institute, Vol. 47 (February 1951).

[26] Winter Concreting, Theory and Practice, Proceedings of the RILEM Symposium,

Copenhagen (1956)

.

[27] Plowman, J. M. , Maturity and the Strength of Concrete, Magazine of Concrete Research, Vol. 8, No. 22 (March 1956).

[28] Sadgrove, B. M. , The Compressive and Indirect Tensile Strengths of

Concrete at Early Ages, CIRIA Project Record 21/1, London.

[29] Weaver, J., Temperature Development in hydrating Concrete, Unpublished

Ph.D disseration. University of London (1971)

.

[30] Sadgrove, B. M. , Prediction of Strength Development in Concrete Structures,

Paper presented at the 54th Annual Meeging of the Transportation Research

Board, Washington, D.C. (January 1975).

[31] Agarwal, R. K. and Gardner, N. J., Form and Shore Requirements for Multistory Flat Slab Type Buildings, Journal of the American Concrete Institute,

Vol. 71, No. 11 (November 1974).

34
[32] Bloem, D. L. , Concrete Strength Measurement - Core versus Cylinders,

Proceedings of the American Society for Testing Materials, Vol. 65 (1965)

[33] Bloem, D.L., Concrete Strength in Structures, Journal of the American

Concrete Institute, Vol. 65, No. 3 (March 1968).

[34] Clifton, J. R. , Nondestructive Tests to Determine Concrete Strength - A Status Report, NBSIR 75-729, the National Bureau of Standards,

Washington, D. C. (1975).

[35] Reocmmended Practice for Cold Weather Concreting (ACI 306-66), American

Concrete Institute, Detroit (1966)

.

[36] Neilsen, K. E. C, Loads on Reinforced Concrete Floor Slabs and Their

Deformations During Construction, Proceedings No. 15, Swedish Cement

and Concrete Research Institute, Stockholm (1952)

.

[37] Grundy, P. and Kabaila, A. , Construction Loads on Slabs Shored Formwork in Multistory Buildings, Journal of the American Concrete Institute,

Vol. 60, No. 12 (December 1963).

[38] Bresford, F. D., An Analytical Examination of Propped Floors In MultiStory Flat Plate Construction, Construction Review, Melbourne, Australia,

(November 1964)

.

[39] Feld, J., Reshoring of Multistory Concrete Buildings, Concrete Construction (May 1974) .

35
APPENDIX A

SUMMARY OF PROVISIONS OF MODEL BUILDING CODES

AFFECTING CONCRETE CONSTRUCTION SAFETY

Al. BOCA Basic Building Code - 1975

(Building Officials & Code Administrators International, Inc.)

Section. 841.1

Requires that the construction of reinforced concrete buildings on structures be followed in accordance with the provisions in ACI 347-68,

Recommended Practice for Concrete Formwork.

In addition, sections 710.4 and 1303 requires special provisions for construction

loads

.

A2 . Natonal Building Code - 1967 (American Insurance Association)

Section 913.1 and 1201.1

Requires that reinforced concrete construction be followed in accordance

with the provision in ACI 318-63 and USAS AID. 2 1944, Safety Code for

Building Construction. It should be noted that both of these references

are outdated. The latest edition of these references are ACI 381-71 and

ANSI AID. 9-1970, respectively.

Section 913.8

Requires that forms and shoring supporting structural members not be

removed until members have sufficient strength to support own weight and

"such loads as may come upon them."

Section 1210.6

Requires that the material for temporary construction such as platform,

support and tarpulins be made of fire retardent treated material.

36
A3. Southern Standard Building Code - 1973 and -1974 revision.

(Southern Building Code Congress)

Section 1601

Refers to the provisions in ACI 318-71.

Section 2101.9

Requires that all temporary equipment shall be substantially constructe

and erected to ensure the safety of workmen.

A4. Uniform Building Code - 1973

(International Conference of Building Oficials)

Section 2606 (a)

Forms "shall be properly braced or tied together so as to maintain

position and shape. Forms and their supports shall be designed so that

previously placed structures will not be damaged."

Section 2606 (b)

Prohibits construction loads exceeding design loads on unshored structur

Requires job-cured test specimens strength and structural analysis for

removal of shores and imposition of construction loads.

37
APPENDIX B

SUMMARY OF PROVISIONS OF LOCAL BUILDING CODES AFFECTING

CONCRETE CONSTRUCTION SAFETY

Bl. Atlanta

Sec. 901 requires structures to carry all permanent and construction

loads without exceeding allowable stresses. Further requires temporarysupports to be governed by such allowable stresses.

Sec. 913.8 - form provisions identical to National Building Code.

B2. Baltimore

Sec. 6006 requires registered engineer to design and supervise con- struction of all controlled concrete construction.

Sec. 6651 requires forms to be designed for a hydrostatic pressure

equivalent to a fluid weighing 150 Ib/cu.ft,

Sec. 6652 allows wood and metal form stresses 50 percent higher than for

permanent construction.

Sec. 6654 requires forms to be removed in a way to insure complete

safety of structure and after structure has sufficient strength to

support its weight and any loads imposed.

B3. Boston

Sec. 710.4 limits construction loads and stresses to normal design

limits for complete structures.

Sec. 842.0 requires approved inspection personnel for all concrete

construction

.

Sec. 1303.1 and 1303.2 - same as BOCA Code.

38
B4. Buffalo

1972 City Code still based on ACI 318-56 except in special cases 318-63

may be approved. Sec. 204 requires concrete work to be supervised by

engineer responsible for its design. Also requires complete records of

temperature and curing histories of concrete.

B5. Chicago

Sec. 68-1 requires construction stresses to meet same limits as finished

structure design stresses.

Sec. 69-3.1 requires inspection of work during construction to be under

a registered architect or structural engineer.

Sec. 76-2 requires stresses due to wind, dead, and material storage and

erection equipment loads during construction not to exceed allowable

stresses in design codes.

B6. Denver

Sec. 2302(b) requires temporary loads during construction be investigated

and provided for by the responsible person imposing such loads.

Sec. 5900(b) limits loading of structures, parts of structures, temporary

supports, etc., to safe carrying capacity.

B7 . District of Columbia

Requires building official inspection of forms and reinforcement before

concreting to insure forms are substantial. Specifies formwork to be

structurally designed.

B8. Jacksonville, Florida

Sec. 900-901.2 requires temporary supports to be limited to normal

design stresses.

Sec. 900-913,5 limits construction loads on unshored portions of structure

to design loads. Gives building official power to require job cured
test cylinder strengths prior to form removal. Construction loading and

shore removal to depend on member strength gain.

Sec. 900-913.7 requires building official -inspection of formwork.

B9. Los Angeles

Sec. 91.0311 requires approval of formwork by a structural inspector

selected by the engineer.

Sec. 91.2617 requires cylinder tests before removal of form shoring or bracing if forms are to be removed before 7 days for slabs and 15 days

for beams and girders.

BIO. New Orleans

Sey. 2610 requires forms to be properly braced and tied and not to

be removed until concrete has gained sufficient strength to support

safely its own weight and superimposed loads. Building Official may

require forms to remain in place for a specified time.

Bll. New York City

This city code is the most detailed and comprehensive of all local codes

examined regarding concrete construction safety.

Sec. 1900.8 specifies interim fire protection facilities, including

hoists for fire fighters and standpipes.

Sec. 1900.9 specifies design to be done by registered architect or engineer whenever "design" is required.

Sec. 1904.3 is a comprehensive requirement for formwork.

B12. New York State

Sec. 304-12 requires all structural members and temporary supports to be

strong enough to suffer no damage under construction loads.

40
B13. Ohio

Requires complex work to be under supervision of a registered engineer

or architect.

Sec. 1210.02 requires all construction operations to keep building

within allowable stresses and to insure stability of the structure.

B14. Philadelphia

Building official may require forming and shoring design and removal

sequence analysis.

B15. Pittsburgh

No building or building part or any temporary support shall be loaded in excess of its safe carrying capacity.

B16. Seattle

Identical to Uniform Building Code.

B17. South Florida

Sec. 2301.1(b) requires buildings to be designed to withstand construction

loads without exceeding design loads. Forms are to be removed in a way

to insure complete safety of structure. Beam bottom forms and shoring

for slabs, beams, and girders are not to be removed for 14 days unless

approved by building official after test results show sufficient strength

gain

.

Sec. 3305 requires storage of construction materials in a structure

during building operations to be done with due consideration of structural

stresses developed which may not exceed design stresses.

B18. Wisconsin

Requires construction of any building of over 500,000 cu ft volume to be

under supervision of an architect or engineer. Form removal must insure

complete safety of structure and critical forms and supports are not to

be removed until members have sufficient strength to carry dead load

plus any imposed construction loads.

41
APPENDIX C

SCAFFOLDING AND SHORING INSTITUTE RECOMMENDED PROCEDURE

for

COMPRESSION TESTING OF SCAFFOLDS AND SHORES

CI. Scope

This procedure is intended to cover the compression testing of equipment

used for scaffolding and vertical shoring.

C2. Definition of Terms

The definition of terms below, relating to the compression testing of

equipment used for scaffolding and shoring should be considered as applying

to the terms used in these methods of compression testing.

ACCESSORIES - Those items other than frames, braces, or post shores used

to facilitate the construction of scaffold and shoring.

ADJUSTMENT SCREW - A leveling device or jack composed of a threaded

screw and an adjusting handle used for the vertical adjustment of

the shoring and formwork.

ALLOWABLE LOAD - The ultimate load divided by factor of safety.

BASE PLATE - A device used to distribute the vertical load.

COUPLER OR CLAMP * - A device for locking together the component parts of

a tubular metal scaffold.

COUPLING PIN - An insert device used to connect lifts or tiers vertically.

CROSS BRACING - A system of members which connect frames or panels of

scaffolding laterally to make a tower or continuous structure.

DEAD LOAD - The load of forms, stringers, joists, reinforcing rods, and

the actual concrete to be placed.

EXTENSION DEVICE - Any device, other than an adjustment screw, used to

obtain vertical adjustment of .shoring towers.

FACTOR OF SAFETY - The ratio of ultimate load to the allowable load.

FORMWORK - The material used to give the required shape and support of

poured concrete, consisting primarily of:

Sheathing - material which is in direct contact with the concrete

such as wood, plywood, metal sheet or plastic sheet.

Joists - Members which directly support sheathing.

Stringers or ledgers - members which directly support the joists,

usually wood or steel load-bearing members.

* These terms may be used synonymously.

42
FRAME OR PANEL - The principal prefabricated, welded structural unit in

a tower.

HORIZONTAL SHORING - Metal or wood load-carrying beam or fabricated

trussed section used to carry a shoring load from one bearing

point, column, frame, post, or wall to another.

JOISTS - See Formwork.

LIFTS OR TIERS * - The number of frames erected one above each other in a vertical direction.

LIVE LOAD - The total weight of workmen, equipment, buggies, vibrators,

and other loads that will exist and move about due to the method of

placement, leveling and screeding of the concrete pour.

LOAD BEARING MEMBER - Any component of a scaffold structure which is

directly subjected to load.

LOCKING DEVICE - A device used to secure the cross brace to the frame or panel

.

POST SHORE or POLE SHORES * - Individual vertical member used to support

loads

.

a. Adjustable Timber Single Post Shore - Individual wooden timbers

used with a fabricated clamp to obtain adjustment and not

normally manufactured as a complete unit.

b. Fabricated Single Post Shores - Type I: single all-metal post,

with a fine adjustment screw or device in combination with pin

and hole adjustment or clamp.

Type II: Single or double wooden post members adjustable for a metal clamp or screw and usually manufactured as a complete unit.

c. Timber Single Post Shores - Wood timber used as a structural

member for shoring support.

RESHORING - A system used during the construction operation in which

the original shores are removed and replaced in a sequence planned

to avoid any damage to partially cured concrete.

SAFE LEG LOAD - The load .which can safely be directly imposed on the

frame leg. (See Allowable Load)

SHOCK LOAD - Impact of material such as the concrete as it is released

or dumped during placement.

SHORE HEADS - Flat or formed metal pieces which are placed and centered

on vertical members.

SHORING LAYOUTS - A properly designed drawing prepared prior to erection

showing arrangement of equipment for proper shoring.

SILL OR MUD SILL* - A footing (usually wood) which distributes the vertical

shoring loads to the ground or slab below.

These terms may be used synonymously.

43
SPAN - The horizontal distance between posts, columns, or upright support

members.

STRINGERS OR LEDGERS * - see formwork.

TESTING APPARATUS OR FIXTURE - A special purpose device fabricated for

the express purpose of testing scaffolding and shoring.

TESTING MACHINE - A compression testing machine of a type usually found

in Universities, Colleges, and reputable Testing Laboratories.

TIMBER STRESSES - Stress-grade lumber conforming to recommended tables in

"Wood Structural Design Data Book," by National Lumber Manufacturers

Association, Washington, D.C.

TOWERS - A composite structure of frames, braces, and accessories.

TUBE AND COUPLER SHORING - An assembly used as a load-carrying structure

consisting of tubing or pipe which serves as posts, braces, and ties,

a base supporting the posts, and special couplers which serve to

connect the uprights and join the various members.

ULTIMATE LOAD - The maximum load which may be placed on a structure before

its failure due to buckling of column members or failure of some component

.

C3. Calibration of Testing Devices

(a) The device used to determine loads applied shall be calibrated

and certified either immediately before or after the testing by a reputable testing laboratory.

(b) Testing machines used for compression testing shall be calibrated

in accordance with ASTM Specification E4 of current revision during

the preceeding 12 month period.

C4. Test Specimens

(a) Scaffold and shoring components shall be selected at random from

inventory and shall exhibit approximately the same variations in

measurements as would be expected from random sampling including

mill tolerances on thickness of various members.

(b) Measurements of specimens. Thickness measurements, when required,

shall be made with a suitable micrometer. All other dimensions

shall be made with a commercially obtainable measuring tape and

all dimensions reported to the nearest 1/16 inch.

C5. Procedure of Test

(a) The scaffold tower or shore to be tested shall be erected in

such a manner as to simulate field conditions and aligned ver- - 44
tically so that it is not out of plumb more than 1/8" in three

feet. No greater attempt should be made to adjust the components

concentrically than would be expected in actual use.

(b) The load shall be applied directly on the load bearing member or members by use of load transfer beams or cross head of testing

machine; or directly by hydraulic jacks in an approved testing

apparatus or fixture.

C6. Duration of Test

The scaffolding tower or shore shall be subject to increasing loads

until the ultimate load is reached.

C7. Speed of Testing

(a) The allowable limits for rate of loading on scaffolding towers

shall be not less than 5,000 lbs. per minute not more than 10,000 lbs.

per minute.

(b) The allowable limits for rate of loading on Post Shore shall be

not less than 1,000 lbs. per minute nor more than 2,000 lbs. per minute.

(c) The rate of loading in each test shall remain constant.

C8. Types of Tests

(a) Scaffolding leg loading shall consist of frames erected into

towers composed of four (4) vertical legs with normal bracing,

base plates, and/or adjustment screws. When adjustment screws

are used they shall be extended equally on the top and equally

on the bottom, but top and bottom extensions need not be the

same. All four load bearing legs shall be subjected to simultaneous loading until the ultimate load is reached by the

weakest leg. Tests according to this method are "A" series tests.

(b) Scaffolding horizontal member loading. The scaffolding components

shall be erected into towers composed of four (4) vertical legs

with normal bracing, base plates, or adjustment screws on the

bottom. The load shall be applied to the horizontal members of

the panels to simulate uniform loading. Loading shall be continued until the ultimate load is reached. Tests performed

according to this method are "B" series tests.

45
(c) Post Shores shall be tested individually at their minimum and

maximum heights and at every foot throughout their operating

range. The shores may be tested in both a braced condition and

an unbraced condition with the individual test data displayed

as a graph showing allowable load versus overall height with the manner of bracing, if any, clearly indicated. Loading shall be

continued until ultimate load is reached. Test performed ac- cording to this method are "C" series tests.

(d) Extension devices - shall be positioned in or upon the legs of

the scaffold and tested in the manner outlined in paragraph

8 (a) . The extension devices may be tested extended from the

top, bottom or both ends of the legs, and also in a braced or unbraced condition. The devices shall be tested at their maxi- mum and minimum height and at every foot through their operating

range with the individual test data displayed as a graph showing

allowable load versus extended length and the manner of bracing,

if any, clearly displayed. Tests performed according to this

method are "D" series tests.

(e) Scaffolding leg loading shall consist of frames erected into

towers composed of four (4) vertical legs with normal bracing,

base plates and/or adjustment screws. When adjustment screws

are used, they need not have the same adjustment for any of

eight adjustment screws in any 4-legged tower. All four load

bearing legs shall be subject to simultaneous and/or variable

loading until the ultimate load is reached for the weakest leg.

Tests performed according to this method are "E" series tests.

C9. Witness of Test

All tests must be witnessed by a reputable independent testing laboratory or University, who must attest that the test was performed in accordance

with applicable provisions of this standard.

CIO. Report of Test Results

Test results shall be reported on Form A (see attached) including

drawing of test setup with the following:

46
(a) Ultimate Total Test Load

(b) Ultimate Leg Load

(c) Type of Test

(d) Laboratory;

(e) Witness
Cll. Test Reporting Form A

TEST REPORTING FORM A

SCAFFOLDING and SHORING INSTITUTE

1. Submitted by: Date of Test

Test Number

2. Type of Test (Machine or Appartus )

a

.

Series (A), (B), (C), (D), (E)

b. Number of Tiers 1, 2, 3, 4, 5, 6, 7, , (Circle One)

c . Ultimate Total Test Load

d.

e

.

f

gUltimate Leg Load Each 5

Ultimate Shore Load

Ultimate Ledger Load

Total Extension Beyond Leg; Top ", Bottom

3. Witness to Test

a . Company Representative

b . Independent Laboratory

c . Other Witness

d . Other Witness

4. Certification:

I certify that the above descirbed test was performed in accordance with the

applicable provisions of the Procedure for Conpression Testing of Scaffolds

and Shores as published by the Scaffolding and Shoring Institute.

(Laboratory)

By

5. Attach Sketch of Test

48
NB5.114A IREV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. IMMil.K A I l<)\ OK Kl.r(1|< 1 \(>. NBS BSS-80

2. Ciov 't A( ccssiott 3. Ki (

1 1. H III ' . Ai cfs.sion No.

4. Tl ill ,\Nn SI M i l 1 i.i: Safety During Construction of Concrete Buildings--

A Status Report

5. I'liM i< at ion D.iic January 1976

6. Pcrforniin^ Or^an i /a( loii ' oi!<- 46L 03

H. S. Lew

8. Pcrforniin^; C^rgan. Kcpori No.

9. PI- KFORMINC, 0RC.ANI7.A riON NA.MI- AND ADDRI SS NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Pro|( cr/Ta.sk/W<,rk (Inn .No. 4615200

11. ( .ontraci /(ir.mi No,

12. Spun.soring Organization Nami- and ( ompleic AJdrcs.'i (Street, City, State, ZIP)

Same as No. 9

13. Type- of Kcport & Pc-riod

14. Spon.soring Agency Code

15. SUFPI,i;.MHN l AKY NOI HS

i_,ibrary of Congress Catalog Card Number: 75- 619389

16. ABSTRAC 1 (A 200-word or Jess (actual siimmory of most signilicant information. If document includes a significant

bibliograptiy or literature survey, mention it here.) The current state-of-the-art of safety in concrete building construction is reviewed

and summarized. The material presented considers only the technical aspects of

the construction safety. Safety of the individual in using equipment and in following

construction procedures is not included in this study.

The report presents comparative accident frequencies in concrete construction.

Based on reported construction failures, the relative vulnerability of various

categories of concrete construction is estimated. The report examines causes of

construction failures and reviews major regulatory standards at the federal, state,

city and industry level affecting safety in concrete construction.

The factors which affect safety in concrete construction are examined relative to the state- of- the- knowledge and, where appropriate, recommendations are made for areas needing improved standards.

17. KEY WORDS ("six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Building; codes; concrete; construction; falsework; formwork; loads; reshoring;

safety; shoring; standards.

18. AVAILABILITY jx Unlimited

1

' For Official Distribution. Do Not Release to N71S

[xH Order From Sup. of Doc, U.S. Government Printing Office Washington. D.C. 20402. SD Cat. No. C;i3. 29/ 2; 80

1 Order From National Technical Information Service (N'l IS) Springfield, Virginia 221^1

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

21. NO. OF PAGES

56

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

22. Price

USCOMM-DC 29042-P74

ANNOUNCEMENT OF NEW PUBLICATIONS IN

BUILDING SCIENCE SERIES

Superintendent of Documents,

Government Printing Office,

Washington, D.C., 20402

Dear Sir:

Please add my name to the announcement list of new publications to be

issued in t,he series: National Bureau pf Standards Building Science Series.

Name

Company

Address

City State Zip Code

(Notification key N-339)



NBS TECHNICAL PUBLICATIONS

PERIODICALS JOURNAL OF RESEARCH reports National Bureau

of Standards research and development in physics,

mathematics, and chemistry. It is published in two sections, available separately:

• Physics and Chemistry (Section A)

Papers of interest primarily to scientists working in these fields. This section covers a broad range of physical and chemical research, with major emphasis on

standards of physical measurement, fundamental constants, and properties of matter. Issued six times a

year. Annual subscription: Domestic, $17.00; Foreign,

$21.25. • Mathematical Sciences (Section B)

Studies and compilations designed mainly for the mathematician and theoretical physicist. Topics in mathematical statistics, theory of experiment design, numerical analysis, theoretical physics and chemistry, logical

design and programming of computers and computer

systems. Short numerical tables. Issued quarterly. Annual subscription: Domestic, $9.00; Foreign, $11.25. DIMENSIONS/NBS (formerly Technical News Bulletin) —This monthly magazine is published to inform

scientists, engineers, businessmen, industry, teachers,

students, and consumers of the latest advances in science and technology, with primary emphasis on the work at NBS. The magazine highlights and reviews such

issues as energy research, fire protection, building tech- nology, metric conversion, pollution abatement, health and safety, and consumer product performance. In addition, it reports the results of Bureau programs in measurement standards and techniques, properties of matter and materials, engineering standards and services, instrumentation, and automatic data processing.

Annual subscription: Domestic, $9.45; Foreign, $11.85.

NONPERIODICALS Monographs—Major contributions to the technical liter- ature on various subjects related to the Bureau's scientific and technical activities. Handbooks—Recommended codes of engineering and

industrial practice (including safety codes) developed

in cooperation with interested industries, professional

organizations, and regulatory bodies.

Special Publications—Include proceedings of confer- ences sponsored by NBS, NBS annual reports, and other

special publications appropriate to this grouping such

as wall charts, pocket cards, and bibliographies.

Applied Mathematics Series—Mathematical tables, manuals, and studies of special interest to physicists,

engineers, chemists, biologists, mathematicians, computer programmers, and others engaged in scientific and technical work.

National Standard Reference Data Series—Provides

quantitative data on the physical and chemical properties of materials, compiled from the world's literature and critically evaluated. Developed under a world-wide

program coordinated by NBS. Program under authority

of National Standard Data Act (Public Law 90-396). NOTE: At present the principal publication outlet for these data is the Journal of Physical and Chemical

Reference Data (JPCRD) published quarterly for NBS

by the American Chemical Society (ACS) and the American Institute of Physics (AIP). Subscriptions, reprints, and supplements available from ACS, 1155 Sixteenth

St. N. W., Wash. D. C. 20056. Building Science Series—Disseminates technical infor- mation developed at the Bureau on building materials,

components, systems, and whole structures. The series presents research results, test methods, and performance criteria related to the structural and environmental functions and the durability and safety characteristics of building elements and systems. Technical Notes—Studies or reports which are complete

in themselves but restrictive in their treatment of a

subject. Analogous to monogrraphs but not so comprehensive in scope or definitive in treatment of the subject area. Often serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under pro- cedures published by the Department of Commerce in Part 10, Title 15, of the Code of Federal Regulations.

The purpose of the standards is to establish nationally

recognized requirements for products, and to provide

all concerned interests with a basis for common understanding of the characteristics of the products. "NBS

administers this program as a supplement to the activities of the private sector standardizing organizations. Federal Information Processing Standards Publications (FIPS PUBS)—Publications in this series collectively constitute the Federal Information Processing Standards Register. Register serves as the official source of information in the Federal Government regarding stand- ards issued by NBS pursuant to the Federal Property and Administrative Services Act of 1949 as amended,

Public Law 89-306 (79 Stat. 1127), and as implemented

by Executive Order 11717 (38 FR 12315, dated May 11, 1973) and Part 6 of Title 15 CFR (Code of Federal

Regulations). Consumer Information Series—Practical information, based on NBS research and experience, covering areas

of interest to the consumer. Easily understandable language and illustrations provide useful background

knowledge for shopping in today's technological

marketplace.

NBS Interagency Reports (NBSIR)—A special series of interim or final reports on work performed by NBS for outside sponsors (both government and non-govern- ment). In general, initial distribution is handled by the

sponsor; public distribution is by the National Technical Information Service (Springfield, Va. 22161) in paper copy or microfiche form.

Order NBS publications (except NBSIR's and Biblio- graphic Subscription Services) from: Superintendent of Documents, Government Printing Office, Washington,

D.C. 20402. BIBLIOGRAPHIC SUBSCRIPTION SERVICES

The following current-awareness and literature-survey

bibliographies are issued periodically by the Bureau:

Cryogenic Data Center Current Awareness Service A literature survey issued biweekly. Annual subscription: Domestic, $20.00; foreign, $25.00.

Liquefied Natural Gas. A literature survey issued quarterly. Annual subscription : $20.00. Superconducting Devices and Materials. A literature

survey issued quarterly. Annual subscription: $20.00.

Send subscription orders and remittances for the pre- ceding bibliographic services to National Technical Information Service, Springfield, Va. 22161.

Electromagnetic Metrology Current Awareness Service Issued monthly. Annual subscription: $24.00. Send

subscription order and remittance to Electromagnetics

Division, National Bureau of Standards, Boulder,

Colo. 80302.
U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington, D.C. 2Q234

OFFICIAL BUSINESS

Penalty for Private Use, $300

POSTAGE AND FEES PAID

U.S. DEPARTMENT OF COMMERCE COM-2 1 5

SPECIAL FOURTH-CLASS RATE BOOK