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Punching Shear Resistance of

Lightweight Concrete Offshore

Structures for the Arctic:

Planning of Experimental Study

Long T. Phan

H.S. Lew

David I. McLean

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Building Technology

Gaithersburg, MD 20899

July 1986

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

•QC

100

.U56

86-3440

1986

#</*CAU o*

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Research Information Center

National Bureau of Standards

Gaithersburg, Maryland 20899

NBSIR 86-3440

PUNCHING SHEAR RESISTANCE OF

LIGHTWEIGHT CONCRETE OFFSHORE

STRUCTURES FOR THE ARCTIC:

PLANNING OF EXPERIMENTAL STUDY

AJBSB

Cicfcd

AO. 2k-

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C{ $

Long T. Phan

H.S. Lew

David I. McLean

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Building Technology

Gaithersburg, MD 20899

July 1986

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS. Ernest Ambler. Director

ABSTRACT

This report outlines the planning for the experimental phase of

an investigation currently being conducted at the National Bureau

of Standards to study the punching shear behavior of heavily

reinforced, thick, lightweight concrete plates and shells. The

test specimens are representative of structural elements used in

Arctic offshore structures. The planning includes selecting

dimensions and boundary conditions, materials, reinforcing ratios

and patterns, and loading procedures for the test specimens such

that behavioral similarity in punching shear is achieved between

representative prototype Arctic structures and the scaled models.

Several factors which are considered to affect the punching shear

behavior of the prototype structures will be studied.

Key words: Arctic structures, experimental study, lightweight

concrete, prestressing, punching shear, reinforced concrete,

scale models.

ii

PREFACE

A project to study the performance of offshore concrete

structures in the Arctic environment was initiated at the

National Bureau of Standards. An initial experimental program to

study the punching shear behavior of lightweight concrete

offshore structures was developed with the support of the

following organizations:

- The Minerals Management Service, Department of the Interior;

- Chevron Corporation;

- Exxon Production Research Company;

- Mobil Research and Development Corporation;

- Sohio Petroleum Company; and

- Shell Oil Company.

The authors gratefully acknowledge the cooperation, support, and

guidance provided by an advisory group formed by the project

sponsors.

Any opinions, findings, and conclusions or recommendations

expressed in this report are those of the authors and do not

necessarily reflect the views of the sponsors of this project.

in
TABLE OF CONTENTS

Page

ABSTRACT ii

PREFACE iii

LIST OF TABLES vi

LIST OF FIGURES vii

1.0 INTRODUCTION 1

1.1 OBJECTIVE 1

1.2 SCOPE OF REPORT 2

2.0 DESIGN OF FULL-SCALE SPECIMENS 3

2 . 1 INTRODUCTION 3

2.2 GEOMETRY AND DIMENSIONS OF FULL-SCALE SPECIMENS 3

2.2.1 GEOMETRY 3

2.2.2 DIMENSIONS 4 2. 2. 2.1 Criteria For Selecting Specimen

Dimensions 4

2. 2. 2.

2

Boundary Conditions 6

2.2.3 SUMMARY 8

2.3 LOADED AREA 8

2.3.1 GEOMETRY 8

2.3.2 SIZE OF LOADED AREA 8

2.4 REINFORCEMENT AND PRESTRESSING 10

2.4.1 REINFORCING RATIOS 11

2.4. 1.1 Flexural Reinforcement 11

2.4. 1.2 Shear Reinforcement 12

2.4.2 PRESTRESSING 13

2 . 5 SUMMARY '. 13

IV
TABLE OF CONTENTS (Continued)

3.0 MODEL 14

3.1 INTRODUCTION 14

3.2 SCALE FACTOR FOR MODEL ...14

3.3 MODEL MATERIAL... 15

3.3.1 CONCRETE 15

3.3.2 REINFORCEMENT 16

3. 3. 2.1 Flexural Reinforcement 16

3. 3. 2.

2

Shear Reinforcement 17

3.3.3 PRESTRESSING 17

4.0 TEST SETUP 19

4.1 LOADING SYSTEM 19

4.2 TEST SETUP .' 19

4.2.1 FLAT PLATE MODEL ’ 19

4.2.2 ARCHED SHELL MODEL 2 0

5.0 INSTRUMENTATION 21

5.1 INTRODUCTION 21

5.2 DETAILS ON INSTRUMENTATION 21

6.0 PROPOSED TESTING PROGRAM 2 4

6.1 SUMMARY 2 4

6.2 PROPOSED TESTING PROGRAM 2 5

7.0 REFERENCES 2 6

v
LIST OF TABLES

TABLE Page

1 Predicted Flexural and Punching Shear

Capacities of Plate and Shell Specimens 28

2 Scale Factor For Modeling Reinforced

Concrete Structures 2 9

3 Proposed Testing Program 3 0

vi
LIST OF FIGURES

FIGURE Page

1 Specimen Geometries 31

2 Modeling of Transverse Dimension of

Flat Plate Specimens 32

3 Modeling of Transverse Dimension of

Arched Shell Specimens 3 3

4 Modeling of Supporting Bulkheads for Flat Plate

Ice Walls 34

5 Span Moment in Three and Five-Span Flat Plates 3 5

6 Ice Contact Pressure vs. Loaded Area 3 6

7 Flexural Capacity of Flat Plate Model 37

8 Flexural Capacity of Arched Shell Model...* 3 8

9 Dimensions of 1/6-Scale Models

' 39

10 T-Headed Stirrup 4 0

11 Loading System 41

12 Flat Plate Test Setup 42

13 Arched Shell Test Setup 4 3

Vll

1.0 INTRODUCTION

1.1 OBJECTIVE

This report is the second in a series of progress reports on the

punching shear study being conducted at the National Bureau of

Standards. This study, which includes both analytical and

experimental work, is aimed at developing more rational punching

shear design criteria for lightweight concrete offshore structures

for Arctic environments.

The main objective of this report is to describe the planning of

an experimental program to study punching shear behavior in heavily

reinforced, thick, lightweight concrete plates and shells subjected

to high intensity concentrated loads. The experimental program will

include testing a number of reduced-scale models representative of

portions of ice walls of Arctic offshore structures.

The selection of the following experimental factors will be

discussed:

a) Geometry, dimensions, and boundary conditions of the fullscale specimens.

b) Scale factor and materials used for models.

c) Reinforcement ratios and prestress force to be used in the

models

.

d) Area and method of loading.

e) Instrumentation, and data acquisition.

Variables to be examined in this study will include:

- Area of loading

- Radius-to-thickness ratio (R/t)

- Shear reinforcement ratio

- Effect of multiple span

- Effects of prestressing

1
1.2 SCOPE OF REPORT

Chapter 2 outlines the criteria for selecting the full-scale

specimens

.

Chapter 3 discusses the selection of the scale factor and materials

used for the models.

Chapter 4 describes the loading system and the test setup for model

testing.

Chapter 5 describes the instrumentation that will be used to collect

data.

Chapter 6 summarizes the experimental program. A testing program

for the models is proposed.

2
2.0

DESIGN OF FULL-SCALE SPECIMENS

2 .

1

INTRODUCTION

The selection of the cross sectional shapes of the full-scale

specimens, their dimensions and boundary conditions, and the area

of loading are discussed in this chapter. The term full-scale

specimen will be used to refer to the portion of a prototype

Arctic structure that will be isolated and scaled down for

testing. These selections were made so that the punching shear

behavior between the full-scale specimens and the Arctic offshore

structures would be the same for a given loading condition.

Linear elastic finite element analyses were used in this

selection process. Therefore, the results of the analyses would

be valid only in the elastic range.

2.2

GEOMETRY AND DIMENSIONS OF FULL-SCALE SPECIMENS

2.2.1 GEOMETRY

The test specimens geometries were selected based on typical

configurations (see Fig. 1) of existing and proposed designs of

concrete offshore structures [2,7,10,11,12,13,14] having exterior

walls or ice walls.

A typical ice wall is generally supported by a combination of

bulkheads and thrust beams to enhance its resistance to ice

pressures. The concrete segments between the two supporting

bulkheads are flat plates or arched shells. Two geometries- a flat plate and an open cylindrical shell with two radius-tothickness ratios (R/t) of 6 and 12- were selected for this study.

Arch action induces compression in shells and thereby enhances

its punching shear strength by minimizing principal tensile

stresses. The contribution of arch action to punching shear

resistance in shells is dependent on the R/t ratio of the shells.

3
2.2.2 DIMENSIONS

2. 2. 2.1 Criteria For Selecting Specimen Dimensions

The criteria for selecting the thickness, span length, and

transverse length of the full-scale specimen are discussed in

this section.

a) THICKNESS: The thickness is uniform for all specimens and was

selected based on typical thicknesses of existing and proposed

designs of Arctic ice walls. A review of the literature on

Arctic offshore structures [10] indicates that the ranges of

typical thicknesses are on the order of 2 to 5 ft, with the

larger thicknesses in flat plate ice walls.

Based on this information, a uniform thickness of 42 in (1 m) was

selected for the flat plate specimens and a thickness of 3 0 in

(0.8 m) was selected for the arched shell specimens.

b) SPAN LENGTH: The span length of a specimen is defined as the

spacing between two supporting bulkheads. Typical span lengths

of existing and proposed designs of Arctic offshore structures

range from 15 to 25 ft (4.6 to 7.6 m) . For this study, a span

length of 20 ft (6 m) was selected as the span length for all

full-scale specimens.

c) TRANSVERSE LENGTH: In this report, the term transverse length

denotes the vertical dimension of the ice walls. A transverse

length for the test specimens must be selected so that the

behavior of the test specimen in the region of loading will be

similar to the behavior of an actual Arctic ice wall, which

typically has vertical dimensions of 40 ft (12 m) or larger.

This vertical dimension is the distance between thrust beams or

the distance from the top to the bottom of a wall. The

4
transverse length of the full-scale specimens is determined so

that when structures with different transverse dimensions are

subjected to a concentrated load, the bending moments in the

region around the loaded area are the same. The smallest

transverse length for which the bending moments in the test

specimen converge to those calculated for an actual structure was

determined by the following procedure:

A series of three-span plates having different

transverse lengths were analyzed. The results are

shown in figure 2 . It can be seen that a three-span

plate having a transverse length (2L) of 28 ft (8.5 m)

revealed only about a 3% difference in bending moments

in the localized region around the loaded area as

compared to plates having transverse lengths- of 40 ft

or larger. This insignificant difference in moments

indicates that a three-span plate with a transverse

length of 28 ft (8.5 m) can be considered sufficient to

represent the very large vertical dimension of a typical ice wall. The 28 feet (8.5 m) transverse

length was therefore selected as the transverse length

for all the flat plate test specimens.

The same criterion and procedure used in selecting the transverse

length of the flat plate specimens were used to select the

transverse length of the shell specimens. A series of single

span arched shells with different transverse lengths were

analyzed and the results are shown in figure 3. About 3%

difference in bending moments in the vicinity of the loaded area

can be observed for the shell of 28 ft (8.5 m) transverse length

and the shell with infinite transverse length. The transverse

dimension of 28 ft (8.5 m) was therefore selected as the

transverse length of all arched shell specimen.

5
2. 2. 2.

2

Boundary Conditions

FLAT PLATE SPECIMENS

:

For the flat plate test specimens, two

boundary conditions are considered:

(1) Boundary conditions to simulate the supporting

bulkheads, and

(2) The number of spans needed to sufficiently represent a multi-span structure in the horizontal direction of a flat plate

ice wall.

- Boundary Condition for Bulkheads:

A unit width of a five-span section of a flat plate ice wall with

equal span lengths of 20 ft (6 m) was analyzed to study the

differences in bending moments for the following two cases:

- Case 1: Supporting bulkheads are represented as actual

structural members with realistic dimensions.

- Case 2: Supporting bulkheads are idealized as pin

supports.

A concentrated load was applied at the center of the middle span

and distributions of bending moments along the five-span beams

were plotted for both cases as shown in figure 4. The comparison

shows that the differences in moments between the two cases are

insignificant (less than 3%)

,

indicating that modeling the

supporting bulkheads as pin supports for the flat plate test

specimen is reasonable.

- Number of Spans:

In order to select the number of continuous spans which would

represent the behavior of the actual structure, a five-span

6
simply supported flat plate was analyzed to examine the localized

behavior. It was assumed that a five-span plate would reasonably

represent continuous multi-span behavior. A uniform pressure was

applied over a small area at the center of the middle span of the

five-span plate.

The analysis showed that lines of zero moment exist in the two

spans adjacent to the middle span. These lines are located at 16

feet (5 m) , or 0.8 times the span length, away from the supports

of the middle span. Based on this observation, a simply

supported three-span plate was analyzed for the same loading

condition. The center span was 2 0 ft (6 m) long and the two

outer spans were 16 ft (5 m) long. The distribution of bending

moments obtained from the three-span plate analysis were compared

with that obtained from the five-span plate analysis. This

comparison, shown in figure 5, indicates that there is less than

one percent difference in the distribution of bending moments for

the two cases. Thus, a simply-supported, three-span flat plate

of 20-ft (6-m) central span and 16-ft (5-m) adjacent spans was

selected.

ARCHED SHELL SPECIMENS: The selection of boundary conditions to

model supporting bulkheads and continuity in the horizontal

direction for the arched shell specimens are discussed in this

section.

Analyses similar to those performed for the flat plate were also

carried out for the arched shell. In the analyses of the shells,

the lateral stiffness provided by the supporting bulkheads had a

significant influence on the distributions of moments and inplane thrusts in the loaded span of the arch. It is difficult to

duplicate this influence in the laboratory unless supporting

bulkheads with realistic dimensions are modeled. In addition,

unlike the flat plate, no simple set of boundary conditions could

be prescribed for the arched shell specimen which properly

7
accounted for the multi-span nature in the horizontal direction

of arched shell ice walls.

For these reasons, it was decided to study the arched shell

specimens using simple boundary conditions that could easily be

established both experimentally and analytically. A pinned

support, single span with a 20-ft (6-m) span length was selected

for this study.

2.2.3 SUMMARY

A three-span continuous, simply-supported flat plate and a single-span, arched shell were selected to represent typical

Arctic ice walls. The plate has a 20-ft (6-m) central span and

two 16-ft (5-m) adjacent spans and an uniform thickness of 42 in

(1 m) . The shell is pin supported along the straight edges

(transverse direction) and has an uniform thickness of 30 in (0.9

m) and a span length of 20 ft (6m). Both the flat plate and the

arched shell specimens have transverse lengths of 28 ft (8.5 m)

.

2.3 LOADED AREA

2.3.1 GEOMETRY

In this study, the geometry of the loaded area was selected so

that a uniform state of stress would be achieved over the contact

area of test specimens. Since a uniform state of stress is more

easily achieved experimentally with a circular loaded area, a circular load was selected.

2.3.2 SIZE OF LOADED AREA

The size of the loaded area was selected based on the following

criteria:

8
(1) The contact pressure for punching shear failure to

occur in the test specimens should be within the range of

realistic local ice pressures.

(2) The size of the loaded area should be selected so that

a punching shear failure will occur prior to flexural failure.

A relationship between the applied punching shear pressure and

the loaded area can be obtained for any assumed value of nominal

ultimate shear stress. This ultimate stress is assumed to act on

a critical section defined by the ACI code. For a circular

loaded area, as shown in figure 6, the critical section is a circle that has its perimeter located a distance equal to half of

the effective depth away from the perimeter of the loaded area

[3]. Figure 6 shows that the size of the loaded area is

inversely proportional to the applied pressure, i.e. if the area

of loading becomes smaller, the required pressure for a punching

shear failure will increase. If the area of loading is too small,

the pressure at punching shear failure will become

unrealistically large. However, if the loaded area is made too

large, flexural failure of the specimen will occur instead of a punching shear failure.

In order to select the size of a loaded area that would satisfy

the stated criteria, the following two assumptions were made:

(1) The upper-limit of a realistic value for localized ice

pressure acting on an Arctic structure is approximately 3500 psi

(24 MPa)

.

(2) The mean value of nominal ultimate shear stress acting

on the critical section is as sumed to vary between 9\Jf

' c to

15\/f ' c. The upper value of 15\Jf ' c is lower than the upper limit

obtained from another experimental program [1] which found that

9
the nominal ultimate shear stress of normal weight; reinforced

concrete plate and shell specimens ranged from llVf

'

c to 18^f' c

. However, this value of lsVf'c is reasonable for lightweight

concrete because the contribution of aggregate interlock to the

punching shear strength of a lightweight concrete structure will

be less than that of a normal weight aggregate concrete

structure

.

Based on these two assumptions, loaded areas ranging from 12 to

25 ft2 (1.1 to 2.3 m

2

) are obtained using figure 6. Any loaded

area within this range satisfies the stated criteria. For this

experimental program, a circular loaded area of 12.5 ft 2 (1.2 m

2

) was selected for the majority of the specimens, although two

specimens will be tested with a circular loaded area of 25 ft 2 (2.3m2 ).

2.4 REINFORCEMENT AND PRESTRESSING

Most Arctic offshore structures, due to their large dimensions,

are built segmentally. Post-tensioning is used to connect

segments of the ice wall together and to enhance punching shear

strength by creating a state of triaxial compression within the

wall. Also, high percentages of flexural and shear reinforcement

are used in Arctic ice walls because of the high-intensity loads

they are designed to withstand. In determining the reinforcement

ratios and the prestressing forces for the specimens, the

following two criteria were used:

(1) The reinforcement ratios and prestress force should be

selected so that they are within the typical ranges used in real

Arctic offshore structures.

(2) The flexural and shear reinforcement ratios should be

selected so that a punching shear failure will occur prior to a flexural failure in the specimens.

10
2.4.1 REINFORCING RATIOS

2. 4. 1.1 Flexural Reinforcement

Available literature on the design of Arctic offshore structures

indicates that flexural reinforcement ratios range from 1.5 to

2.5 percent [10]. This ratio represents the amount of steel in

one direction and on one face of the cross-sectional area of the

ice wall. A previous experimental investigation on punching

shear [1] concluded that differences in punching shear strength

due to varying the amount of reinforcement in structures of this

type is insignificant (an increase of 43% in flexural

reinforcement caused only a 2% increase in punching shear

strength). Therefore, flexural reinforcement was 'not considered

as a variable in this study.

To select the ratios of flexural reinforcement for the specimen

that would satisfy the stated criteria, a method developed and

used succesfully in another punching shear study [1] was used in

this study. The method involved obtaining design moments by

averaging the peak values of elastic moments over an appropriate

width [1]. The calculated design moments obtained by this

averaging method were then used to establish applied loadcalculated design moment relationships for both the flat plate

and arched shell specimens.

The flexural capacity curves for both the plate and the shell

specimens were obtained. The shell sections were treated as

columns subjected to eccentric loading. Two ratios of flexural

reinforcement, 1.75 and 2.5 percent, were chosen in the specimen.

These values are within the range typically used in Arctic

offshore structures. The predicted flexural capacities of the

specimens were obtained by superimposing the applied loadcalculated design moment relationship on the flexural capacity

11
curves as shown in figures 7 and 8. The points of intersection

of these curves represent the predicted flexural strengths of the

specimens. The flexural capacities of the specimens

corresponding to the two chosen ratios of flexural reinforcement

are summarized in table 1. However, the 2.5 percent ratio will

be used in only one specimen to examine the effect of a high

flexural reinforcement ratio on the mode of failure.

2. 4. 1.2 Shear Reinforcement

The contribution of shear reinforcement to punching shear

resistance in offshore concrete structures is one of the

variables examined in this study. No rational basis currently

exists for the determination of shear reinforcement ratios in

Arctic offshore structures. Shear reinforcement -ratios as high

as 1 percent have been suggested [10]. To study the contribution

of shear reinforcement to punching shear resistance, three ratios

of shear reinforcement were selected: 0, 0.22, and 0.44 percent.

/

The punching shear strengths of the specimens were also predicted

by summing the nominal shear stress acting on the critical

section defined by the ACI code. The nominal shea r stress acting

on the critical section was assumed to be llVf ' c for the plate

specimens and 15\Jf 1 c for the shell specimens (these assumptions

were made based on the results of the experimental study

conducted by BWA [1]). The results of the comparison are given

in table 1. A comparison of the predicted flexural and punching

shear strengths of both the plate and the shell specimens is

presented in table 1; this comparison indicates that punching

shear failure would occur prior to flexural failure in all

specimens

.

12
2.4.2 PRESTRESSING

Prestressing is used in the majority of Arctic offshore

structures. Prestressing values ranging from 300 to 1000 psi (2

to 7 MPa) [10] have been reported in the design of Arctic

structures. The prestressing force, by creating a state of

triaxial compression in the structures, is expected to have an

effect on the failure load and the failure mechanism. The effect

caused by prestressing specimens in one and two directions with a prestressing force of 500 psi (3.5 MPa), will be studied in this

experimental program.

2.5 SUMMARY

This chapter has presented the rationale behind the selection of

the geometry and boundary conditions for flat plate and arched

shell specimens to be studied in this experimental program. The

selection of flexural and shear reinforcement ratios and the use

of prestressing was also discussed.

13
3.0

MODEL

3.1

INTRODUCTION

It is a common engineering practice to use a reduced-scale model

to predict the behavior of the larger prototype structure. A

reduced-scale model has the advantages of being less costly to

fabricate and test, and it does not require a large testing

facility. The reduced scale model is related to the larger scale

specimen by a set of similitude requirements. These

requirements, if satisfied by the reduced-scale model, will allow

a direct extrapolation of the behavior of the model to the larger

prototype structure. The selection of the scale factor for the

model specimens and the model materials will be discussed in this

section.

3.2

SCALE FACTOR FOR MODEL

Reinforced concrete has been established as the most suitable

material to be used in modeling reinforced concrete structures

[4,6] and therefore was selected as the model material in this

study. The set of similitude requirements governing the modeling

of a reinforced concrete structure using a reinforced concrete

model is given in References 4 and 6 and reproduced in this

report in table 2. Table 2 shows the scale factors for the

different quantities for a reinforced concrete model.

It may be seen from table 2 that the similitude requirements for

a reinforced concrete model are specified solely by the geometric

scale factor S]_ if the stress-strain relationship of the model

concrete and its elastic modulus are identical with those of the

prototype. The choice of this geometric scale factor is governed

by such factors as the capacity of the testing facilities, the

practical dimensional limitations of the model and the cost to

14
build and test the models. Taking the above factors into

account, a scale factor, S^, of 6 was selected for use in this

study.

Having selected the scale factor, the overall dimensions of the

models can be obtained by directly scaling down the dimensions of

the full-scale specimens by a factor of 6. Figure 9 shows the

dimensions and boundary conditions of the 1/6 scale models in

this study.

3.3 MODEL MATERIAL

In general, a model material is required to have well defined

properties that are similar to those of the structure being

modeled. In this study, the concrete and the reinforcements used

for the 1/6-scale models were selected so that certain properties

possessed by the materials used for Arctic structures will be

duplicated.

3.3.1 CONCRETE

Theoretically, a model concrete may be produced by scaling down

the sizes of all materials which compose concrete, including

cement, aggregate, etc., using the geometric scale factor Sj.

However, this is impractical since a finer fine aggregate has

excessive water demand and more finely ground cement is not

available. As a result, model concrete is proportioned as it

would be for the structure being modeled. Information obtained

from literature on Arctic offshore structures shows that highstrength lightweight concrete has been used extensively as the

principle building material for these structures. Lightweight

concrete is required for economic and practical reasons, and is

achieved by using lightweight coarse aggregate with a nominal

maximum aggregate size typically being 3/4 in (19 mm) . The use

of many additives, such as condensed silica fume, high-range

15
water reducing agents or "superplasticizers" , and air-entraining

agents, is also reported. These additives are used to improve

workability of the concrete mix and strength and durability of

Arctic offshore structures which are constantly exposed to severe

environmental conditions [8,9]. Based on this information, the

following material properties were selected for the model

concrete in this experimental program:

(1) A mean ultimate compressive strength f' c of at least 7,000

psi (48 MPa) is required for the model concrete. This

compressive strength will be determined by testing 4x8 in

(101x203 mm) cylinders according to recommendations of the ACI

Committee 444 [5].

(2) North Carolina Solite aggregate with a nominal maximum

aggregate size of 1/2 in (13 mm) will be used as coarse

aggregate. This size of aggregate was selected to satisfy the

practical dimensional requirements.

(3) Acceptable proportions of condensed silica fume (10 percent

replacement of cement) , superplasticizer, and air-entraining

agent will be used to achieve similarity and to improve

workability of the model concrete.

3.3.2 REINFORCEMENT

The 1/6-scale models will have the same ratios of flexural and

shear reinforcements selected in Chapter 2 for the full-scale

specimens. Conventional reinforcing steel will be used for model

reinforcing. The size and type of reinforcing bars are discussed

in this section.

16
3. 3. 2.1 Flexural Reinforcement

The same two ratios selected for the full-scale specimens will be

used for the 1/6-scale models, 1.75 and 2.5 percent. The high

percentage of flexural reinforcement and the dimensions of the

1/6-scale models dictated the use of #4 (1/2 in diameter)

deformed reinforcing bars for the flat plate models and #3 (3/8

in diameter) deformed bars for the shells. A mean yield strength

of 60 ksi (414 MPa) was also specified for these reinforcing

bars

.

3. 3. 2.

2

Shear Reinforcement

The same three ratios of shear reinforcement, 0,- 0.22 and 0.4 4

percent, selected for the full-scale specimens will be used in

the 1/6-scale models. There is a trend in industry toward using

mechanically headed stirrups as shear reinforcement in Arctic

offshore structures due to the congestion of reinforcement

created by high reinforcing ratios. The shear reinforcement is

fabricated so that it has a T-shaped head at both ends that may

be interlocked into the flexural reinforcing bars (see Fig. 10)

.

Facility of construction is the main advantage of using T-headed

bars. To achieve similarity, T-headed bars, stamped out from

steel plate, with a mean measured yield stress of 52 ksi (359

MPa) were selected for use as shear reinforcement. Typical

dimensions of the T-headed bars are given in figure 10.

3.3.3 PRESTRESSING

The tendons were selected to be finely-threaded bars, 7/8 in (22

mm) in diameter, with a yield strength of 125 ksi (862 MPa) . The

tendons will be equally spaced across the concrete section to

provide uniform distribution of 500 psi (3.5 MPa) compressive

stress in the models. The use of tendons with a fine pitch of

17
thread will help to minimize prestressing losses due to end

anchorage slip, which otherwise can be significant due to the

short length of the prestressing tendons.

Grouting of the post-tensioning ducts is usually performed in

real Arctic structures to protect the post-tensioned tendons from

being damaged by corrosion. However, it is believed that the

absence of grout will not affect the punching shear strength of

the post-tensioned structures. Therefore, the tendon ducts were

not grouted in this study to simplify the construction of the

test specimens.

Prestress force will be applied sequentially and monitored using

a strain gage mounted on each of the tendons. It is anticipated

that sequential tensioning of the tendons will result in loss of

prestress due to elastic shortening of concrete, and retensioning will therefore be necessary to account for this loss.

Other causes of losses of prestress, such as friction loss and

time-dependent losses (creep and shrinkage of concrete,

relaxation of post-tensioned tendons) , are considered

insignificant since the tendons are straight and short and the

post-tensioned specimens are to be tested shortly after being

prestressed.

18
4.0

TEST SETUP

4.1

LOADING SYSTEM

A loading system designed to provide a uniformly distributed

pressure on a circular area was selected for use in this testing

program (Fig. 11) .

Load produced by the NBS 12,000 kip (6,000 tons) universal

testing machine is transferred to a solid steel piston contained

in a steel cylinder. The cylinder also contains layers of

natural rubber and leather pads. The rubber is used to simulate

fluid behavior and will impart a uniform pressure on the loaded

area of the model. The leather pads are used to contain the

rubber within the cylinder.

Molybdenum grease was used on the inner surface of the steel

cylinder during calibration tests on the loading system. It was

shown that loss of applied load transmitted through the loading

system due to friction was approximately 1 percent and therefore

was considered negligible.

4.2

TEST SETUP

4.2.1 FLAT PLATE MODEL

The test setup for the three-span flat plate model and its

boundary conditions are shown in figure 12. The flat plate model

will be set on top of two interior supports spaced 40 in (1 m) on

center. Each of these supports consists of a 4x1 in steel

bearing plate, a 3 in diameter half-round steel bar, and a W8x67

beam. At one of the two interior supports, two layers of Teflon

will be placed between the bearing plate and the half-round bar

to provide a friction-free surface. Similarly, the assembly of

the exterior supports also consists of a 4x1 in bearing plate,

19
Teflon layers, a 3 in diameter half-round bar, and a W12x53 beam.

These exterior supports will be placed 32 in (0.8 m) away from

the interior supports and on top of the specimen. They will be

tied down to the test floor to prevent vertical displacements of

the specimen at the exterior supports. Half-round steel bars and

Teflon are used to allow rotational and horizontal displacements

at the supports, simulating roller supports. For the interior

support in which no Teflon is used, friction is assumed to

prevent horizontal displacement and allow only rotational

displacement, simulating a pin support.

4.2.2 ARCHED SHELL MODEL

A steel test bed has been designed for testing arched shell

specimens. A schematic of the test setup is shown' in figure 13.

The test bed consists of three wide flange beams (Wl0x60) , 78 in

long, placed parallel to the span direction of the shell

specimens. Two cut-off beams (W8x58) , cut with two different

angles to accomodate two R/t ratios of the shell specimens, will

be bolted onto the top flange of each of the wide flange beams to

accomodate the thrust forces. Eight 7/8 in-diameter (22 mm)

bolts will be used for each connection. The wide flange beams

will be tied down to floor at the center to prevent upward

displacement of the test bed. A 2x12 in plate (5x30 cm) , 56 in

long (142 cm) , will be placed on each side of the test bed,

connecting the three cut-off beams to provide a continuous

support.

20
5.0

5.1 INSTRUMENTATION

INTRODUCTION

Monitoring of the specimens during and after testing will include

making the following observations and measurements:

(1) The ultimate failure load and the mode of failure.

(2) Strain in the shear and flexural reinforcement and in the

prestressing tendons in the vicinity of the loaded area.

(3) Crack development and crack patterns.

(4) Deflections under the load and at points along the major

axes of the test specimens.

(5) Support rotation in the shells.

5.2

DETAILS ON INSTRUMENTATION

The test load will be applied to the specimens incrementally

using the NBS 12,000 kip capacity universal testing machine.

Applied load will be monitored by the testing machine and by a 400 kip-capacity load cell. Readings of strains and

displacements will be recorded at each increment of load using an

electronic data acquisition system, which is capable of scanning

and recording 128 data channels in 1/100 of a second.

Electrical resistance foil strain gages will be used to measure

strain in the shear and flexural reinforcement and in the

prestressing tendons. All gages are temperature-compensating

gages. The gages will be mounted on the reinforcement using an

M-bond 200 adhesive and protected from moisture with layers of

protective coating.

21
Strain in the shear reinforcing steel will be measured using

small strain gages so that they can fit onto the stem section of

the T-headed bars. The gages will be placed at mid-height of the

T-headed bars. Four T-headed bars with gages will be placed

along the two major axes of the specimens at a distance of half

of the effective depth away from the edge of the loaded area, and

an additional four bars with gages will be placed at a distance

equal to the effective depth. The T-headed bars with gages will

be greased after they are in place so that bond stresses on these

bars will be minimized. Signals received from these strain gages

on the T-headed bars and flexural reinforcement will be used to

determine the initiation of cracking.

During tensioning of the post-tensioning tendons; the force in

the tendon will be measured using a strain gage mounted on each

tendon. These gages will be the same type of gages used on the

flexural reinforcement. The gages will indicate tension force in

the tendon and will serve as an indicator of loss of prestress in

the post-tensioning process. The gages will also provide a measure of any stress changes in the tendon during testing,

although it is expected that little change will occur.

Deflections of the test specimens will be measured using linear

variable differential transformers (LVDTs) . Measurements will be

taken on the underside of the test specimens at distances of 0

and 12 in (30 cm) away from the center of the specimens and at

the supports

.

Externally-mounted dial" gages will be used to monitor the support

rotations of the shell specimens during testing. Monitoring of

the support rotations will provide information on the behavior of

the test setup.

22
Cracks and crack patterns of the model specimens will be visuallymonitored during testing. After testing to failure, the

specimens will be removed and sectioned in both directions, and

the cracks highlighted and recorded.

23
6.0 PROPOSED TESTING PROGRAM

6.1 SUMMARY

The selection of full-scale specimens and the design of 1/6-scale

model specimens of Arctic ice walls have been outlined in this

report. Test specimens were selected so that the behavioral

similarity between the specimens and the prototype structures was

maintained. The scale factor of 6 was selected based on

practical and economical reasons.

Reinforced concrete was selected as the model material. North

Carolina Solite with a nominal maximum size of 1/2 in (13 mm)

will be used as the coarse aggregate. Condensed silica fume,

air-entraining agent, and superplasticizer will also be used.

Reinforcing steel, with 3/8 and 1/2 in diameters (#3 and #4) and

a specified yield stress of 60 ksi (414 MPa) were selected for

use as flexural reinforcement in the models. Two flexural

reinforcement ratios of 1.75 and 2.5 percent were selected.

Three ratios of shear reinforcement, 0, 0.22, and 0.44 percent,

were selected to study the contribution of the amount of shear

reinforcement to the punching shear strength of the models. Theaded bars, stamped from steel plate, with a measured yield

strength of 52 ksi (359 MPa) will be used as shear reinforcement.

Two circular loaded areas of 50 and 100 square inches (300 and

600 square centimeter) for the 1/6-scale models were selected.

Approximate failure loads and modes of failure of the 1/6-scale

models were predicted based on finite element analyses. The

results of the analyses indicate that all models will fail in

punching shear.

24
6.2 PROPOSED TESTING PROGRAM

The proposed test program is presented in table 3 . The program

consists of a series of 1/6-scale model tests. A total of 15

specimens are proposed. Of these, six are open cylindrical

shells with two radius-to-thickness ratios (R/t of 6 and 12) . The other specimens include eight three-span continuous flat

plates and one single span flat plate (model FP1) . Two of the

three-span continuous flat plate models (FP2-1 and FP2-3) are

duplicated to check the variability of test results. The singlespan flat plate specimen will be used to study the effect of

multiple span on the punching shear strength. The models are

different either in configuration or properties and are designed

such that the variables mentioned in Chapter 1 can be examined.

Following is a list of variables to be studied - and the test

specimens designed to study each variable.

a) Area of loading: Models FP2-1 and FP2-2 vs. FP2-3 and FP2-4

b) Shell radius-to-thickness ratio: Models FP1 vs. AS 7 vs. AS 8

,

Models AS 10 vs. AS12

c) Shear reinforcement ratio: Models FP2s vs. FP3 vs. FP4

,

Models AS 9 vs. AS 7 vs. ASH

d) Effect of multiple span: Models FP1 vs. FP2-3 and FP2-4

e) Effects of prestressing:

1 . One way prestress

:

Models FP3 vs. FP5

Models AS 7 vs. AS 10

Models AS 8 vs. AS 12

2 . Two way prestress

:

Models FP3 vs. FP6

FP5 vs. FP6

25
7.0 REFERENCES

1.

"Experimental Work on Punching Shear Resistance of Concrete

Structures for the Arctic", Brian Watt Associates, Inc. AOGA

project no. 152.

2.

Buslov, V.M . and Krahl, N.W. (Brown and Root Inc .), "Fifty-one

New Concepts for Arctic Drilling and Production". Part 1,

Ocean Engineering, 1983.

3.

American Concrete Institute, "Building Code Requirements For

Reinforced Concrete", ACI Standard 318-83.

4.

Sabnis, G.M., Harris, H.G., White, R.N. and Mirza, M.S.,

"Structural Modeling and Experimental Techniques", PrenticeHall, 1983.

5.

American Concrete Institute Committee 444, "Models of

Concrete Structure", SP-24, ACI, Detroit, Mich., 1970.

6.

Harris, H.G., "Use of Structural Models as an Alternative to

Full Scale Testing", Full-scale Load Testing of Structures,

ASTM STP 702, Shriever, W.R., Ed., American Society for

Testing and Materials, 1980, pp. 25-44.

7.

Schlecten, J.R., Fernandes, R.L., Dolan, D.K. and Bivens,

H.R.," Analysis and Design of an Ice Wall Framing System For

an Arctic Drilling Structure". Proceedings 1984 Offshore

Technology Conference, OTC 4691, Houston, Texas.

8.

Gjorv, O.E., "Durability of Concrete Containing Condensed

Silica Fume", Fly Ash, Silica Fume, Slag and Other Mineral

By-Products in Concrete, ACI, SP 79-36, pp. 695-702.

9.

Burge, T.A., "High-strength Lightweight Concrete with

Condensed Silica Fume", Fly Ash, Silica Fume, Slag and Other

Mineral By-Products in Concrete, ACI, SP 79-39, pp. 731-745.

10.

McLean, D.I., Lew, H.S., Phan, L.T. and Sansalone, M .

,

"Punching Shear Resistance of Lightweight Concrete Offshore

Structures for the Arctic: Literature Review", The National

Bureau of Standards, 1986.

26
11. Byrd, R.C., Coleman, R.K., Weiss, R.T., Boaz, I.B., Sauve,

E.R. and White, R.M.,"The Arctic Cone Exploration Structure:

A Mobile Offshore Drilling Unit For Heavy Ice Cover".

Proceedings 1984 Offshore Technology Conference, OTC 4800,

12. Houston, Texas.

Wetmore, S.B., "The Concrete Island Drilling System: Super

Series (Super CIDS)". proceedings 1984 Offshore Technology

Conference, OTC 4801, Houston, Texas.

13.

Gerwick, B.C., Potter, R.E. and Rojansky, M., "Development

of a Structural Concept to Resist Impacts From Multiyear Ice

Floes, Ridges and Icebergs". Proceedings 1984 Offshore

Technology Conference, OTC 4799, Houston, Texas.

14.

Bhula, D.N., Birdy, J.N. and Bruen, F.J., "Design of Concrete

Gravity Structures to Withstand Concentrated Ridge and Flow

Impact Loads". Proceedings 1984 Offshore Technology

Conference, OTC 4708, Houston, Texas.

27
XXQO£X

i-3 XXw

PREDICTED

FLEXURAL

CAPACITY (K)

OPm

r~• <—i

a/

478 340

aX

03 z X M >H

cj

X EH

X CJ

M 1 CJ < 2 X - X < ijw X X3 —* m

m

m

Q

XX

H »H rH

X

w co co

Eh

Xu

X < M x Q X M) X X XX

4J 04 s lO pH Xa OP

C5 in

m

X • m X CM co X 1 X fH

X

X Eh a/

X

M

Q pu« X < X £ Eh X ~

CJ 3 OP

X

w CJ in 04

Eh

Q r~ in < X • <N X X X X n

E-i 3

PSCu

f°T , w

x

x z o M >H X X EH z X M 1 o M Z CJ - X < Uh4 z X X o 3 —

cjQ CJ

X *-h rH

X co

Eh

X

u 04

CJ < M X Q X

k9

X

X *—p

XX • X c < Z -H U Ch H X O Q • O < cr m o m x —

28

TABLE

1.

PREDICTED

FLEXURAL

AND

PUNCHING

SHEAR

CAPACITIES

OF

PLATE

AND

SHELL

SPECIMENS.
QUANTITY DIMENSION SCALE FACTOR FOR

PRACTICAL

TRUE MODEL

Concrete stress FL"

2

1

Concrete strain — 1

Modulus of concrete FL"

2

1

Poisson's ratio — 1

Mass density FL'3 l/st Reinforcing stress FL~2

1

Reinforcing strain — . 1

1 Modulus of reinforcing FL

-2

' 1

Bond stress FL"2

1

1

Linear dimension L

!

Sc ! Displacement L * ! Angular displacement —

• 1 i

i Area of reinforcement L

2

S?

Concentrated load F Sc

2

Line load FL"

1 Sc Pressure *3

KJ 1

Moment FL S?

TABLE 2. SCALE FACTORS FOR MODELING REINFORCED CONCRETE

STRUCTURES [14,16]

29
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CQ •H

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Eli • OS Eli o < Z M ~ CN CN hT CN CN CN CN CN N* CN

EdH Eh dP CM

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T CN

X Ed < • • • • • • • • • • CO K K O O O O O O O O O o O

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05 «£ CN CO CN CN CN CO 2 1 1 1 1 ‘ ‘ 1 rH rH rH rH

Eh

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m in

mm

m

X

O to rH < J

co

CO

iJ Ed »* P Z • < XS c r~ r~ r- r- t" r~ in

mmm inm S <J -H WX

Eh CN HI* 1 1 r~ Z CN CN o rH X X rH W • X rH

EH

O <u ex < z P TJ 'O p 2 to c c to o rH CN p p p rH to (0 CO *r in CO 00 O'* rH rH rH

CJ Z Ed ex

XX

X

X TJ to CO 10 to to

l-H < p rH

m

X u»X

X a> < < < < <

El,

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p i 1 p z z <tj CN CN o o rH X X U u Cl, X <

30

TABLE

3.

PROPOSED

TESTING

PROGRAM
COco3a

<*-coO

*5£©o

"5o

'EL >*o©>c©

31

FIGURE

1.

SPECIMEN

GEOMETRIES
CENTER

SPAN

MOMENT,

M

x

(in-kip/in)

FIGURE 2. MODELING OF TRANSVERSE DIMENSION OF FLAT PLATE SPEC LI NS

32
SPAN

MOMENT,

M

x

(in-kip/in)

FIGURE 3. MODELING OF TRANSVERSE DIMENSION OF ARCHED SHELL SPECIMENS

33
o

o

in

cm ~

o

o

<fr o

o

CO

o

o

o

o

o

o

CM T- o0

CM

1

o

0

CO

1

o

o

o

0

in

1

3

e

c

(diii-u.i) X|A| ‘1N3INOW NVdS d3±N30

34

DISTANCE

FROM

CENTER

LINE
500

o

lO

tO

O

lOCO

o

03

to

CM

o

CM

to

to

O

(djX-uj) 1N3NON NVdS

35

DISTANCE

FROM

CENTER

LINE

IN

THE

SPAN

DIRECTION,

X

(ft)
REQUIRED

PRESSURE

FOR

PUNCHING

SHEAR

FAILURE

(psi)

0 10 20 30 40 50

LOADED AREA (sq. ft.)

FIGURE 6. ICE CONTACT PRESSURE vs. LOADED AREA

36
700

600

500

400

300

200

100

0

10 20 30 40 50 60

MOMENT (in-kip/in)

FIGURE 7. FLEXURAL CAPACITY OF FLAT PLATE FODEL

37
FICURE 8. FLEXURAL CAPACITY OF ARCHED SHELL MODEL

38
FIGURE 9. DIMENSIONS OF 1/6-SCALE MODELS

39
1.25

CO

40

FIGURE

10.

T-UEADED

SHEAR

REINFORCEMENT
NBS

testing

machine

head o co4-< ©

Q.

O) o

CN4 £ © « C©Eo©a©

-4-> ©©

©©©

CO 0

41

FIGURE

11.

LOADING

SYSTEM
Box

beams

8x8 ]

CMCO

GO

If)

42

FIGURE

12.

FLAT

PLATE

TEST

SETUP
All

dimensions

in

inches

o

co

r- .a

43

FIGURE

13.

ARCIIED

SHELL

TEST

SETUP
NBS-114A (REV. 2-80

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 86-3440

2. Performing Organ. Report NoJ 3. Publication Date

SEPTEMBER 1986

4. TITLE AND SUBTITLE

PUNCHING SHEAR RESISTANCE OF LIGHTWEIGHT CONCRETE OFFSHORE STRUCTURES FOR THE

ARCTIC: Planning of Experimental Study

5. AUTHOR(S)

phan t H.S. Lew, David I. McLean

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions) 7. Contract/Grant No. national bureau of standards

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

Technology Assessment and Research Branch

Minerals Management Service

U.S. Department of the Interior

Reston, VA 22091

10. SUPPLEMENTARY NOTES

| | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bl iography or literature survey, mention it here)

This report outlines the planning for the experimental phase of an investigation

currently being conducted at the National Bureau of Standards to study the punching

shear behavior of heavily reinforced, thick, lightweight concrete plates and shells.

The test specimens are representative of structural elements used in Arctic offshore

structures. The planning includes selecting dimensions and boundary conditions,

materials, reinforcing ratios and patterns, and loading procedures for the test

specimens such that behavioral similarity in punching shear is achieved between

representative prototype Arctic structures and the scaled models. Several factors

vdiich are considered to affect the punching shear behavior of the prototype structures

will be studied.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s Arctic structures; experimental study; lightweight concrete; prestressing;

punching shear; reinforced concrete; scale models

13. AVAILABILITY

[ ]] Unlimited

|y | For Official Distribution. Do Not Release to NTIS

H Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

Order From National Technical Information Service (NTIS). Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

IS. Price

jlCOMM.OC 404 M' I