NBSIR 73-148

Evaluation of the Column Connections

Used in a Precast Concrete Modular

Housing System

Felix Y. Yokel and Thomas W. Reichard

Center for Building Technology

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

March 26, 1973

Final Report

Prepared for Office of Research and Technology

Department of Housing and Urban Development

Washington, D. C. 20410

NBSIR 73=148

EVALUATION OF THE COLUMN CONNECTIONS

USED IN A PRECAST CONCRETE MODULAR

HOUSING SYSTEM

Felix Y. Yokel and Thomas W. Reichard

Center for Building Technology

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

March 26, 1973

Final Report

Prepared for Office of Research and Technology

Department of Housing and Urban Development

Washington, D. C. 20410

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

Evaluation of the Column Connections

Used in a Precast Concrete Modular Mousing System

by

F. Y. Yokel and T. W. Reichard

Abstract

The column connections used in a housing system employing

stacked precast concrete box modules were tested to evaluate

their structural performance. The system was proposed for

construction in Operation BREAKTHROUGH, a research and

demonstration program sponsored by the Department of Fiousing

and Urban Development. The system uses innovative structural

design concepts, which include: confinement of the concrete

in the vicinity of the column bearings by reinforcing ties

in order to increase concrete compressive strength; neonrcne

pads between column bearings in the upper stories, stories;

steel -neoprene -s teel sandwich in the lower stories; and a grouted

dowel through the center of the columns to provide resistance

to tension and shear.

Tlie test program included the following: tests to determine

the effect of various bearing pads on the load capacity of

the connection; tests to determine the load-deformation

characteristics of the neoprene pads; a test to determine

tlie performance of a lower-story connection using a steel

neoprene-s teel sandwich and a grouted dowel; and tests to

evaluate the strength and ductility of the connections when

subjected to a shear force. Tlie test results are presented

and interpreted and the findings are summarized.

i
Building system, column connection, concrete

triaxial strength, ductility, neoprene bearing

pad, performance test, precast concrete,

structural design, Operation BREAKTHROUGH.

ii
SI Conversion Units

In view of the present accepted practice in tliis country for

building technology, common US units of measurement liave been

used throughout this paper. In recognition of the position

of the United States as a signatory to the Heneral Confereiice

on Weights and Measures, which gave official status to the

metric SI system of units in 1960, assistance is given to the

reader interested in making use of the coherent system of SI

units by giving conversion factors applicable to US units

used in this paper.

Lengtii

1 in = 0.0254 meter (exactly)

1 ft = 0.3048 meter (exactly)

Force

1 lb (Ibf) = 4.448 Newton (N)

1 kip = 4.448 Newton

Pressure

1 psi = 6895 N/m^

1 ksi = 6.895 x 10^ N/m^

iii
Table of Contents

Evaluation of the Column Connections

Used in a Precast Concrete Modular Housing System

Page

1. Introduction 1

2. The Building System 1

2.1 Description of System 1 — 2.2 System Features That Required Performance 4

Evaluation

3. Scope 7 4. Tests 9

4.1 Bearing Capacity of Column Connections Using 9

Various Types of Bearing Pads

4.1.1 Objective 9

4.1.2 Test Specimens 10

4.1.3 Test Procedure 11

4.1.4 Test Results 12

4.1.5 Discussion of Test Results 12

4.2 Tests to Determine the Load-Deformation 16

Characteristics of the Neoprene Pads

4.2.1 Obj ective 16

4.2.2 Test Specimens 16

4.2.3 Test Procedure 16

4.2.4 Test Results and Findings 17

4.3 Tests to Evaluate the Bearing Capacity of 18

Connections Using a St eel-Neoprene-St eel Sandwich

as a Bearing Pad and a Grouted Dowel

4.3.1 Objective 18

4.3.2 Test Specimen 19

iv
4.3.3 Test Procedure

4.3.4 Test Results

4.3.5 Discussion of Test Results

4.4 Test to Evaluate the Shear Capacity of Column

Connections

4.4.1 Objective

4.4.2 Test Specimen

4.4.3 Test Procedure

4.4.4 Test Results

4.4.5 Discussion of Test Results

Summary of Findings

Acknowledgement

References

Tables

Figures

V

Used

Evaluation of the Column Connections

in a Precast Concrete Modular Mousing System

by

F. Y. Yokel and T. W. Reichard

1. Introduction

The introduction of industrialized building systems may

lead to the development of untried structural details which

cannot be evaluated by analysis based on our present design

standards. Whenever deviations from design standards are

proposed, the structural adequacy of the system must be determined

by performance evaluation.

This report presents the results of the performance

evaluation of innovative column connection details in a multistory housing system employing stacked precast reinforced

concrete box modules which was one of the systems proposed

for construction under the Operation Breakthrough program,

sponsored by the Department of Housing and Urban Development.

2. The Building System

2.1 Description of System

The proposed system consisted of precast reinforced

light-weight concrete box modules, stacked in a checkerboard

pattern. Figure 2.1 shows one of the buildings proposed

for construction, a 19-story structure. Figure 2.2 shows

the model of a complex of buildings which is under construction

using the structural concepts and connection details evaluated

in this report, however, the modules differ in size and conf iguratior

from those proposed for the structure shown in figure 2.1.

1
A sketch of a proposed module is shown in figure 2.3.

The outside dimensions of the module are 44 ft x 14 ft x 8

ft-11 in. The concrete used in the modules consists of a regularweight sand and light-weight coarse aggregate mix with a unit

weight of 120 lb/ft and a 28-day strength ranging from 4000

2 psi (lb per in ) to 5000 psi as specified in the design. An

individual module of this type weighs 81.3 kip (kilopounds)

Some of the boxes in the proposed system have balconies cantilevering

from one, or both ends. A box with balconies on both ends,

which is the largest module used in the system, is 52 ft long

and weighs 92.3 kip.

The structural skeleton of the modules is shown schematically in figure 2.4. A module contains four bents (ribs) which

1/

are rigid frames consisting of 12 x 15 1/4-in— columns and

12 X 12- in—''' beams near the module ends, and 12 x 12-ini'' columns,

with a 8- in thick slab section providing a common beam to both

ribs at the center of the module. The bents are connected

by a 4-in thick top slab, 3-in thick sidewalls, and a 4 in

thick bottom slab. The bents, top slab, and sidewalls are

cast monolithically in a single operation using a steel form

creating a "hull". The bottom slab, which is cast in a second

casting operation is attached to the hull by bent dowels protruding

from the bottom of the sidewalls and by special steel inserts

welded to the four center columns. The modules are erected

in a checkerboard pattern as shown in figure 2.5. Part of

the living space is contained within the modules and part

is space created between adjacent modules. The space within

modules provides a clear height of 8 ft-3 in and a clear width

between concrete wall surfaces of 12 ft-6 1/2 in. The created

space has the same clear height and a clear width of 12 ft11 1/2 in between the concrete surfaces of the sidewalls.

The structure is completed as shown in figure 2.6 by inserting

special end wall, roof, and floor panels to complete enclosure

of the created spaces.

— Since column and beam sections have flared sides (they are not

exactly rectangular or square), average dimensions are given

which represent equivalent rectangles or squares.

2
The method of connecting modules is illustrated schematically in figure 2.7. The entire load is transmitted through

the columns. Transmission of load between the sidewalls

in consecutive modules is prevented by suitable structural

detailing. Anchorage through the column connections is provided

by grouted dowels consisting of consecutive lengths of #8

(1 in diameter) deformed steel reinforcing bars, with a 60

ksi minimum yield strength, extending from midheight of one

module to the midheight of the module above.

Details of the proposed column to column connection

are shown in figures 2.8 and 2.9. Figure 2.8 shows the detail

of an interior column joint. The heavy lines show the column

and part of the floor slab of Module A, and column and part

of the 8-in thick slab connecting the interior column bent

in Module B. The dashed lines show the location of the roof

and wall slabs. Note that the bottom of the floor slab extends

2 3/8 in below the bottom of the column and that the column

connection is located in a 2 3/8-in deep recessed pocket.

The top of the ceiling in Module B is thereby located approximately

at the same elevation as the top of the slab in Module A,

thus eliminating the need for a stepped transition. It can

also be seen that the wall of Module A does not bear on the

wall of Module B. Thus all compressive and shear forces

are transmitted through the bearing pad in the column connection.

The joint between the boxes at locations other than the column

connection contains a compressible filler material. Proposed

connection details are shown in figure 2.9. Figure 2.9(a)

shows the reinforcement near the column end where additional

confinement ties are provided in the vicinity of the joint.

Figure 2.9(b) shows a section through a connection.

3
Erection of the system proceeds in the following manner.

Modules are precast in a plant, not too distant from the erection

site, and transported to the erection site on specially-designed

trailers. A 50-ton crane unloads the modules and lifts them

to their desired position. Tolerances in fit to insure proper

load distribution between the eight bearings are within limits

that can be accommodated by the compressible bearing pads. Leveling

during erection is achieved by inserting steel shims to bring

all bearing pads to a level position before a module is placed.

At the end wall of the building grouting proceeds continuously

during construction to prevent instability of the sidewall panels

enclosing the created spaces. All interior joints are grouted

only after completion of the erection sequence.

2.2 System Features That Required Performance Evaluation

The proposed system differs in several ways from conventional

cast- in-place reinforced concrete structures: (1) Reinforcement

details differ from those normally used in column-beam connections.

The unconventional details were required because of discontinuities

between modules in the structure, the thin concrete sections

and the rather heavy reinforcement, together with the need for

an arrangement of reinforcing bars that would not impede efficiency

in the mass production of modules, (2) All the vertical and

horizontal forces are transmitted across the column connections

which were planned to be "dry" connections to facilitate coldweather construction and which include compressible bearing

pads to accommodate construction tolerances.

The specific objective of this evaluation was to determine

the load capacity of the column connection which had several

4
unique details. The various parameters considered in the

evaluation of these details are discussed below:

1. Except for the No. 8 dowel, vertical column reinforcement

is interrupted at the connection. Thus, the forces resisted

by columns containing up to 3 percent vertical reinforcement,

are transmitted through a plain concrete bearing with a single

grouted dowel in its center. The maximum load permitted in

such a bearing by ACI-318.71 [1]^ would be exceeded in the

bearing. It has been shown [2,3,4,5] that the compressive

strength of concrete under triaxial confinement is greater

than the unconfined compressive strength and the design therefore

provides for additional ties in the vicinity of the joint

as shown in figure 2.9(a). To permit a bearing stress in excess

of that permitted by ACI-318 the load capacity of bearings

was evaluated by testing.

2. In order to accommodate erection and fabrication tolerances

a 1/4- in thick neoprene pad is inserted between the columns.

Available data on neoprene bridge bearings [6] are limited

to stress levels less than 1000 psi, which are substantially

lower than the 6000 psi stress developed in the proposed

bearing at the failure load and are based on tests of neoprene

pads bonded to steel plates. At the stress levels expected

to occur when the required load capacity is reached the modulus

of elasticity of neoprene is lower than that of concrete,

and Poisson's Ratio is higher. The neoprene, therefore,

exerts a splitting force on the concrete column faces, reducing

the load capacity of the bearing [7]. Thus while the concrete

capacity is increased by the confinement ties, as previously

discussed, the beneficial effect of the confinement may be

offset by the splitting force exerted at the concrete-neoprene

2/ - Numbers in brackets designate literature references

5
interface. This effect also had to be evaluated by testing.

3. Since it was recognized that the neoprene would reduce

the capacity of the bearing, another type of bearing pad was

proposed for the more heavily loaded bearings. This pad consists

of a steel-neoprene-steel sandwich, using + 1/8-in thick (11

ga.) stainless steel plates and a 1/4-in thick neoprene pad.

In this case it was assumed that the steel plate would act

to resist the radially tangential shear exerted by the neoprene

and to provide additional triaxial confinement at the steelneoprene interface. This type of bearing also had to be evaluated

by testing.

4. In addition to the information on the load capacity

of the connection in compression, the deformation characteristics

of the connection under compressive load had to be determined

in order to compute the added horizontal drift under wind

load caused by the compressible bearings.

5. The load capacity of the joints containing the grouted

dowel had to be determined. The sequence of grouting in the

construction had to be considered in this determination. As

previously noted, the dowels in the columns near the side

(gable) walls of the building are continuously grouted during

construction (whenever a story is complete, the dowel is grouted).

This procedure differs from the grouting procedure proposed

for the other columns which is to grout in a single lift after

completion of the erection. As a result of the continuous

grouting, most of the load in the sidewall connections would

be transfered to the dowel and surrounding grout, rather than

to the compressible pad. This may cause crushing of the grout

and yielding of the dowel in the lower stories of the building.

These effects were not judged to be detrimental, since they

would be confined to the vicinity of the compressible pad

which is only 1/4 in thick in the uncompressed state.

6
However, this loading condition had to be simulated in the

performance evaluation.

6. In addition to the load capacity in compression,

the shear capacity of the connection had to be evaluated.

The shear capacity is critical in the resistance of the building

to wind and earthquake load and for the specific buildings

under consideration, shear capacity was critical in the prevention of progressive collapse under abnormal loads. In the

determination of shear capacity the previously-discussed effect

of continuous grouting had to be simulated. In view of the

use of compressible pads, displacements across the joint (slippage)

had to be evaluated in addition to shear capacity. Not only

the capacity of the joint in shear, but also the ductility

of the joint was evaluated. Ductile behavior would prevent

the premature failure of a single joint in a group of joints,

thus increasing the load capacity of the building, and would

also be an important consideration in determining the resistance

to seismic loading. Special consideration was given to load

reversals that may be caused by seismic events.

3. Scope

The results and conclusions from four different test

programs are presented in this report. A slightly scaleddown column cross section (10 in x 12 in) was used in order

not to exceed the capacity of the testing equipment.

(1) Tests to evaluate the bearing capacity of column

connections using various types of bearing pads

(specimens did not have grouted dowels).

This program had two phases: In Phase I, two specimens

consisting of two 10 x 12 x 31 1/2-in long column stubs

with 1/4- in thick neoprene pads were tested in compression.

7
Subsequently, one of these stubs which did not fail in the

initial test was bedded in high- strength plaster on a steel

bearing plate and retested in compression.

In Phase II nine specimens, consisting of two 10 x 12

X 51 1/2-in long columns, were tested in compression. Eight

specimens with various proposed types of bearing material

inserted between the two stub columns and one specimen

using a steel-neoprene- steel sandwich with low-friction

membranes inserted between the neoprene and the steel

plates were also tested.

Subsequently, one of the stub columns which did not fail in

the initial test was tested bearing on a steel plate with

a low-friction membrane inserted between the column

face and the steel plate.

(2) Tests to determine the stress - strain characteristics

of neoprene pads.

In this test three specimens consisting of two 1/4-in thick

neoprenfe pads each were tested in compression between steel

plates and their load- deformation characteristics were

recorded up to a stress level of 4000 psi.

(3) Tests to evaluate the bearing capacity of connections using a steel-neoprene-steel sandwich as

a bearing pad and a grouted dowel.

In this test one pair of 10 x 12 x 51 1/2-in long

column stubs, using the connection details planned for

the lower stories, was tested in compression using the most

critical preloading condition before grouting of the

dowel (simulating continuous grouting).

(4) Tests to evaluate the shear capacity of the column

connection

8
Under this program three tests were conducted on threesegment specimens, using 10 x 12 x 51 1/2-in long columns

segments with the proposed connection detail between them.

Specimens were tested by applying a shear force to push

out the middle segment.

Two specimens were tested in shear while subjected to

low axial load, simulating upper-story conditions.

One of these was tested in one direction while the other

was subjected to several reverse cycles of loading.

The third specimen was tested in shear while subjected

to high axial load, simulating lower-story conditions.

This specimen was subjected to several reverse cycles of

loading.

4. Tests

4.1 Bearing Capacity of Column Connections Using

Various Types of Bearing Pads

4.1.1 Objective

The problems associated with the load transmission from

column to column are discussed in Section 2.2. The objective

of this particular test program was to determine the failure

load of bearings using additional column ties in the vicinity

of tlie joint in order to provide triaxial confinement for

the concrete and using various types of bearing pads proposed

or considered as alternates. This test was necessary, since:

(1) the design concrete stress at the bearing surface,

used in the lower-story joint, exceeded that permitted in

ACI-318;

(2) the effect of the radially tangential shear force

exerted by the neoprene on a neoprene -concrete interface

could not be determined by analysis.

9
(3) there was a need to substantiate the effect of the

steel plates in the proposed steel -neoprene-steel sandwich

in triaxially confining the concrete and resisting the radially

tangential shear force exerted by the neoprene.

The tests were conducted during the design stage of the

system and served as exploratory (pilot) tests. The grouted

dowel was omitted in these tests to avoid introduction of an

added variable. After sufficient preliminary information was

available from these tests, another test was conducted on a connection which included the grouted dowel.

4.1.2 Test Specimens

The typical specimen for these tests consisted of two

column stubs placed end to end with the joint materials separating

the two concrete bearing surfaces. For two of the tests single

column stubs were used as the specimen. A

Two types of stub columns were used. The first type, I 10 in X 12 in X 31 1/2 in high, was used in the series of tests

identified as Phase I tests. The second type, 10 in x 12 in

| X 51 1/2 in high, was used for the Phase II tests. The Phase

I stub column is shown in figure 4.1 and the Phase II column

in figure 4.2. The main reinforcement was welded to a 1 1/2-

in thick steel plate at one end. Load was applied through

the steel plate and the welded reinforcement connection insured

that the load would be transferred simultaneously to the concrete

and the main reinforcement. The Phase II columns exclusive i of the steel bearing plate were 1/2 the length of the column

in the proposed structure.

Typical details of the test specimens are shown in figure

4.3. Main vertical column reinforcement consisted of deformed

steel bars meeting ASTM designation A615-68, Grade 60 [8] and

had an average measured yield strength of 67.4 ksi (tensile

tests were conducted in the laboratory) . Ties were deformed

bars meeting ASTM designation A615-68, Grade 40 and had an

average measured yield strength of 51 ksi. I

i

10

:i

I
The welded wire mesh cage shown in the figure corresponds

to tlie detail in the system used to prevent concrete breakage

caused by form stripping. The welded wire mesh had an average

yielil strength of 63 ksi according to information obtained

from the manufacturer.

The concrete in all specimens was a 4000 psi mix using

sanded lightweight aggregate of 3/4 in maximum size. Figure

4.4 shows the age-strength relationship for tliese concretes.

The types of jointing materials used in these tests

are described in table 4.1.1.

4.1.3 Test Procedure

All specimens were loaded in a 600 kip hydraulic testing

machine. The specimens were preloaded to 200 kip (1670

psi) unloaded and subsequently loaded at 75 kip (62.5 psi)

per minute until failure. The failure was not sudden, in

that considerable splitting and spalling occurred prior

to the decrease in load resistance. Application of load

was continued past the point of maximum resistance until

the load fell off to 80 - 90 percent of the maximum.

The method of load application is shown in figure 4.5

The specimen was positioned concentrically with respect to

the applied load, with its base set on the testing machine

platen in a bed of high strength gypsum plaster. The load

was applied by the spherical ly- seated head of the testing

machine which was fixed against rotation after applying a compressive load of 1 kip. A 1/2-in thick fiberboard plate

was inserted between the steel plate at the surface of the

specimen and the head of the testing machine. This setup

resulted in approximately concentric loading.

In the Phase II tests the vertical deformation of the

joint material was monitored by means of a 0.001-in dial gage.

Lateral deformations were measured over a 9-in gage length

using linear variable differential transducers. These

11
deformations were monitored by X - Y recorders with a resolution

of 0.001 in to aid in the determination of the onset of

spalling (see figure 4.1).

4.1.4 Test Results

The results for all the tests are presented in tables

4.1.2, 4.1.3 and 4.1.4. Typical failure modes are illustrated

in figures 4.5 through 4.9.

4.1.5 Discussion of Test Results

In tables 4.1.2 and 4.1.3 the results are normalized

by dividing the ultimate load by 0.85 f^A, where:

f^ is the average concrete compressive strength of

6 X 12-in cylinders taken during the casting of the

specimens and tested at the time the specimens were

tested

A is the cross sectional area of the column.

0.85 f^A is the failure load of a plain concrete bearing

with no reinforcement ties to provide confinement, computed

in accordance with ACI 318-71. In the case of Test 6

2 of Phase II of this testing program, A was taken as 80 in

accounting for the 2-in eccentricity from the major axis.

By normalizing the loads in this manner a strength coefficient (k) is computed which represents the ratio of the

strength of the test specimens to the strength of a plain concrete bearing with no confinement and no splitting effects

exerted by neoprene pads. Examination of the strength coefficients in the last columns of Tables 4.1 and 4.2 leads to the

following observations:

12
(1) In spite of the confining reinforcement, the average

strength of the plain neoprene bearing tended to be less

than that estimated for an unconfined concrete bearing (Tests

1 and 2, Phase I; Test 1, Phase II).

(2) The strength of the connection using the steelneoprene-s teel sandwich with the 11 ga. (gage) steel plate

tended to exceed the strength estimated for a plain concrete

bearing by approximately 30 percent (Phase II, Tests 4, 5,

and 6). With the 16 ga. steel plates the strength was reduced

to 11 percent over that estimated for a plain concrete bearing

(Phase II, Test 8); however, when low-friction membranes

were inserted between the neoprene and the steel plates,

the strength increased to 20 percent overstrength (Phase

II, Test 9).

(3) The two grouted specimens exceeded the estimated

strength of an unconfined plain concrete bearing by 12 to

26 percent (Phase II, Tests 2 and 3).

(4) The column stub with the low-friction membrane

exceeded the estimated strength of an unconfined plain concrete bearing by 20 percent, while the stub set in highstrength plaster on a 1-in steel plate exceeded the estimated

strength by 56 percent.

Note that in Tests 5, 6, 8, and 9 the size of the neoprene

pad was smaller than that of the bearing area. This size

reduction was to prevent excessive extrusion of the neoprene

from the sides of the bearing. Since under pressure the

lateral dimensions of the neoprene pad increased and extended

over the entire bearing area, there was no apparent adverse

effect from the decrease in the size of the neoprene pad.

It can be concluded from these observations that the

radially tangential force exerted on the concrete bearing

surface has a major effect on the capacity of the joint.

The specimens ranged from those where the joint exerts a splitting force on the surface to those where the joint

is capable of developing a significant confining force.

13
At one extreme is the neoprene bearing pad which exerts

a splitting force on the concrete counteracting the confining

effects of the reinforcement ties.

The specimens bearing on low friction surfaces, as well

as the grouted specimens probably did not develop significant

radially tangential shear forces on the concrete bearing

surfaces (in the latter case Poisson's Ratio and the Modulus

of Elasticity were similar for concrete and grout). Thus,

the k- coeff icient for these specimens which averages approximately

1.2, probably represents the effect of the confinement provided

by the reinforcement ties.

The steel -neoprene-steel sandwich using the 11-ga.

steel plates, where the k-coeff icient averaged 1.32 exerted

a confining force which prevented concrete splitting and

was transmitted by the friction between the concrete and

the steel plate and resisted by the steel plate. The steel

plate also resisted the additional splitting force exerted

by the neoprene pad. The 16-ga, steel plates which yielded

during the test did not have sufficient strength to resist

the splitting force.

The greatest confining force was provided by the steel

plate set in the plaster where a confining force greater

than that provided by the frictional resistance of the steel/concrete

interface could be developed. This additional confining

force increased the k- coeff icient to 1.56. I The previous conclusions are based on both Phase I and

Phase II testing. Originally the longer column stubs used

in Phase II testing were used because there was concern that

tlie short length of the Phase I columns might be insufficient

to develop the bond between the concrete and the main column

reinforcement. The tests, however, do not provide evidence

that the change in column length had a significant effect

on the mode of failure or on the load capacity. The conclusions

are also based on the assumption that, except for Test 6

in Phase II, the applied load was approximately concentric.

14
The effect of the type of bearing pad on the mode of

failure can be observed in figures 4.5 to 4.9.

Figure 4.5 shows a typical failure of a connection

using a neoprene pad. The spalling between the core inside

the ties and the shell outside the ties is an indication

tliat at ultimate load the core supported most of the load.

The same observation can be made in figure 4.6 which shows

the typical failure of a grouted connection.

On the other hand, it can be seen from figure 4.7 that

in the specimens using the steel -neoprene-steel sandwich

there was no spalling of the shell, and tlius , the entire

cross -sect ional area contributed to the load resistance.

It can also be seen in this figure that the concrete bearing

surface is smooth and locally deformed with protruding

imprints of the main reinforcing bars. The deformation is

probably attributable to plastic flow of the concrete before

failure which is taken as an indication of triaxial confinement.

The imprint of the bars was caused by bond breakage near

the end of the bars where the strain in the concrete could

not be transmitted to the steel bars.

The failure of the column stub that was bedded in highstrength gypsum plaster, which is shown in figure 4.8, occurred

in the middle of the column above the region of the additional

confining ties. This is taken as an indication that in this

case the effect of the confinement was even greater than

that provided by the steel -neoprene-steel sandvvach. On tlie

other hand, the failure mode of the column stub which was

tested with a lov\r-fr iction membrane at the column face and

is shown in figure 4.9, gives clear evidence of spalling

of the shell.

Thus, in summary, it is concluded that tlie radially

tangential shear force exerted on the concrete face at the

interface with the bearing pad had a significant effect

on the bearing capacity of the column connection. The coefficients

shown in the last column of 4.1.2 and 4.1.3 give an indication

of the magnitude of this effect.

15
4.2 Tests to Determine the Load-Deformation Characteristics

of the Neoprene Pads

4.2.1 Objective

The modulus of elasticity of neoprene generally tends

to increase as compressive stresses in the neoprene are increased

[6] . The load- deformation characteristics of a neoprene bearing

pad depend on its shape and on the properties of interfacing

materials

The objective of these tests was to determine the loaddeformation characteristics of the proposed neoprene pads.

This information could be used in improved analytical models

for the prediction of joint deformation characteristics and

for the assessments of the effect of dimensional inaccuracies

on the distribution of loads between columns. A secondary

objective was to arrive at an estimate of the magnitude of

the lateral expansion of neoprene bearing pads when subjected

to compressive load.

4.2.2 Test Specimens

All specimens were 1/4 in thick, and made of 70 durometer

neoprene pad materials. Four 9 x 11 x 1/4 in specimens were

used in two tests. Each of these four full-sized specimens

was tested with a 2-in diameter hole in the center. In an

additional test two half-sized specimens (9x5 1/2 x 1/4 in)

were tested without center holes.

4.2.3 Test Proceudre

All tests were made in a hydraulic testing machine by

loading the specimens through a 2 1/2-in thick bearing block.

In each test, two pads were sandwiched between three 2 1/2-in

thick steel blocks as shown in figure 4.10. Thus,

16
two pads were tested in each test. The size of the steel

blocks used was as shown in figure 4.10. The bearing surfaces

of the steel blocks were ground flat in a tub-grinder. Deformation

measurements were made on the four sides of each pad using

dial gages with a 0.001-in resolution and were referenced

to the platen of the testing machine. Compressive deformations

were measured in Tests 1 and 2 and lateral expansion deformations

were measured in Test 3. Full-sized pads were used for Tests

1 and 3 and the half -sized pads were used for Test 2.

Prior to each test the specimens were preloaded to the

maximum test load for 10 minutes in order to reduce creep

effects during the test. The specimens were tested immediately

after removal of the preload and loaded at a slow average

rate because of the interruptions necessary for dial gage

reading. Gage readings were taken for each load increment

after a compression was reached that could be maintained

in a stable position while the readings were taken.

4.2.4 Test Results and Findings

Figure 4.11 presents the data from Tests 1 and 2. The

deformations are the average for the eight measurements on

the two specimens in each test. The effect of the shape

factor of the pad on the compressive deformation is quite

apparent from this figure. For load levels above 100 kip

the deformation of the full-sized pad is only 58 percent

of that for the half-sized pad. It is also apparent from

figure 4.11 that the modulus of elasticity for these pads

increases with increasing stress.

Figure 4.12 is a plot of compressive stress (based

on the 9 x 11-in area) versus tangent modulus of elasticity

for the full-sized pad. This plot shows that the tangent

modulus at 4,000 psi is over 6 times the initial tangent

modulus

17
In all of the tests the lateral expansion of the pad

was non-uniform; i.e. the pad was extruded from the steel

blocks significantly less at the corners than at mid-length

and width. At high stress levels the pad assumed a roughly

elliptical shape. In Test 3 an attempt was made to measure

the lateral extrusion of the full-sized pad using dial gages.

Because of the out of plane distortion of the pads the measurements

only approximately convey the magnitude of this effect.

Figure 4.13 shows the extrusions measured for various load

levels. The extrusions plotted are the arithmetic means

of 8 measurements made on 2 specimens. The measurements were

made at the center of each side so that the extrusions are

maximum values

.

As stated above, all test results were obtained after

applying a preloading cycle immediately before the load application

for which deformations are recorded. Since the visco -elastic

properties of the material may delay load recovery it is

reasonable to expect somewhat greater deformations in the

initial load cycle. Thus the data presented would more accurately

predict load-deformation relationships for interior column I connections which are preloaded before grouting of the dowel.

In Table 4.1.4 data obtained in this test are compared

^ with measurements of compressive deformations of pads in ^ Phase II of the bearing tests on column connections. Even

though in the Phase II tests preloading was only 200 kip,

there is reasonable agreement between the test results discussed

herein and the measurements in the Phase II tests.

4.3 Tests to Evaluate the Bearing Capacity of Connections , Using a Steel -Neoprene- St eel Sandwich as a Bearing ; Pad and a Grouted Dowel

4,3.1 Objective

1 The objective of this test was to evaluate the capacity

of the connection as proposed for the Iokot story containing

18
the grouted dowel and the steel -neoprene - steel sandwich.

The most critical condition with respect to the dowel was

simulated, namely, continuous grouting. This was achieved

by preloading with an axial load of 10 kip before grouting

which simulated the weight of a single story. This preloading

condition could cause the dowel to yield before occurrance

of a compression failure in the concrete. The effect of sucli

premature yielding, together with crushing of the grout and

bond breakage between the dowel and the grout in the vicinity

of the neoprene pad, could not be predicted.

4.3.2 Test Specimen

The test specimen shown in figure 4.14, consisted of

two 51 1/2-in high stub columns similar to those described

in section 4.1.1. However, these stub columns, as shown

in the figure, had five confinement ties instead of the three

used previously. This change was made because of a design

change in the proposed system. In addition, a 2-in round

flexible conduit was cast in the center of each stub. The

hole formed in the column by the conduit was for the #8 deformed

reinforcing bar (ASTM A615-68, Grade 60 Steel) used as the

dowel

.

The two stubs were placed end to end with the steelneoprene- steel sandwich joint separating the two concrete

bearing surfaces. The length of the dowel inserted through

the holes in the two stubs was slightly longer than the overall

length of the two stubs (103 in). The six externally threaded

1-in diameter steel bars, which can be seen in figure 4.14,

were used to apply a 10 kip axial preload to the specimen

before grouting and to sustain the preload until testing.

The grout was type III portland cement and water mixed

at a water-cement ratio of 0.4. The concrete in the column

stubs v/as as described in section 4.1.2.

19
4.3.3 Test Procedure

The test was performed in a 600-kip hydraulic testing

machine. Load was applied in 25-kip increments and maintained

after each increment for electronic recording of measurements.

Loading was continued to the 600 kip level. The 600-kip load

(maximum testing machine capacity) was maintained for 12

minutes. Then the specimen was unloaded, reloaded to 600 kip

in three 200-kip increments, maintained at 600 kip for two

minutes and unloaded.

The load was applied concentrically as shown in figure

4,14, The steel plates at the ends of the specimens as well

as the loading plates had a 2-in hole in their center to prevent

direct load transfer to the steel dowel. The preloading

bars were left on the specimen during the test. Displacement

transducers as shown in figure 4.15, were used to measure

the deformation of the joint during the test. Bonded-foil

strain gages were used to measure the strain on two opposite

sides of the dowel in the center of the joint and also the

strain in the concrete directly over each of the five confinement

ties on two adjacent faces of one column stub (see figure

4.15).

4.3.4 Test Results

The test results are given below:

Maximum Load:

Loading Condition:

Concrete Compressive Strength:

Grout Compressive Strength:

Condition After Testing:

600 kip

Concentric (figure 4.14)

(6 X 12 in cylinders) 3930 psi

(2 X 2 in cubes) 11,000 psi

Minor cracking

Joint deformation data are shown in figure 4.16. Strain data

for the dowel and typical strain data for the concrete over the

confinement ties are shown on figures 4.17, 4.18 and 4.19.

20
4.3.5 Discussion of Test Results

(1) Load CapacityIt is noted in section 4.3.4 that the test load was limited

by the 600-kip capacity of the testing machine. The maximum

test load was maintained for a 12-minute period and, after an

unloading and reloading cycle, for another 2 -minute period.

The specimen experienced some cracking as shown in figure 4.15,

but did not lose its ability to support the 600-kip load.

On the basis of this evidence it may be assumed that the ultimate

load would have exceeded 600 kip.

Strain data from the gages on the column faces are

plotted in figures 4.18 and 4.19. On the basis of the

characteristics of the load-strain relationship and the magnitude

of strain it appears that significant cracking in the concrete

3 / occurred above the 350-kip load level. Very large apparentstrain developed at load levels above 500 kip. Three of

the gages failed at 475, 525 and 600 kip respectively, probably

as a result of severe cracking; gage 18, the closest to

the joint, survived the entire load cycle. While the 600-

kip load was sustained this gage showed a substantial increase

in apparent strain (from 0.0059 to 0.0107). After removal

of the load the gage returned to a residual strain of 0.0082.

A similar trend is shown by the strain gage data on the

other instrumented column face which are shown in figure

4.19. In this case all the gages failed between the 500

and 600 kip load levels. Thus, it can be concluded from

the strain gage readings that substantial cracking occurred

above the 500-kip load level and during the time the 600-

kip load was maintained.

- The term apparent is used since cracking was mostly

responsible for the observed deformation.

21
Figure 4.16 shows the length change in a 11-in vertical

column segment which includes the joint at its center. It

can be seen in this figure that substantial "creep" occurred

while the 600-kip load was maintained. As noted in the

subsequent discussion, much of this length change was attributable

to concrete deformation rather than joint deformation.

The previously discussed trends in measured deformations

are taken as an indication of incipient failure at the applied

test load of 600 kip. Thus while the load capacity of the

specimen exceeded the 600-kip test load, it probably did not

exceed that load by a substantial margin. Using the concrete

strength of 3930 psi, a dowel capacity at yield of 53 kip

(based on the 67.4 psi yield strength), and a k-coeff icient

of 1.32 for the confining effect of the s teel -neoprene- steel

sandwich (see section 4.1.4, Table 4.1.3, average k-coeff icient

for Tests 4, 5, and 6), a load capacity of 580 kip is predicted

for this specimen.

It can be concluded on the basis of the previous discussion

that the load capacity of the specimen exceeded the capacity

predicted on the basis of the confining effect of the steelneoprene- steel sandwich. It can also be concluded that yielding

of the dowel, bond slippage, and local grout crushing apparently

did not adversely affect the load capacity.

(2) Load Response Characteristics

Figure 4.16 shows a plot of load versus the compressive

deformation of an 11-in high segment of the specimen including

the joint at its center. The deformations were measured

at four column faces and it can be seen from the plot that

there is reasonable consistency in the trend of these four

measurements.

A definite break in the load-deformation curve occurred

between the loads of 100 and 150 kip. Since the specimen

was preloaded to 10 kip before grouting it is assumed that

the initial 10 kip load is supported by the neoprene

22
pad. Subsequently, however, because of the low neoprene stiffness,

most of tlie load was transferred to the steel dowel and the

2-in diameter grout column surrounding it. This mechanism

of load transfer had to eventually break down by breaking

of the bond between the dowel and the grout in the vicinity

of the joint and by crushing of the grout column surrounding

the dowel in the region of the 1/4-in neoprene pad. The

break in the load- deformat ion curves is probably attributable

to this change in the load transfer mechanism. Dowel yielding

is ruled out on the basis of the strain measurements in the

dowel at the 100-kip load level.

At the 500-kip load level there was substantial additional

deformation during the period when the 600 -kip load was

sustained. Some of this deformation may be attributable to

creep of the neoprene pad; hoivever , in view of the large

increase in transverse concrete strain shown in figures 4.18

and 4.19, it is reasonable to assume that most of this deformation

was attributable to cracking of the concrete. This contention

is supported by the large residual deformation after removal

of the load. This residual deformation was approximately

0.21 in. The total deformation of the neoprene pad, when

subjected to a 550-kip load, (50 kip would be carried by

the dowel) is not expected to exceed 0.1 in (extrapolation

from Table 4.19 and figure 4.11); and even though recovery

of some of this deformation may have been prevented by a permanent set in the dowel and by visco-elastic time lag,

most of the 0.21-in residual deflection should be attributable

to the concrete compressive deformation.

Figure 4.17 shows the strain measured in the dowel over

a 1/4-in length centered in the center of the joint. Very

little strain was recorded during the first 20 kip of loading.

This is in part attributable to the 10-kip preload which

prevented significant load transfer to the dowel during the

first 10-kip load increment. The subsequent relationship

23
between strain and compressive load leads to the conclusion

tliat up to the 100-kip load level the dowel transmitted

only part of the load. At the 100-kip load level the approximate

load transmitted by the various components of the system

is estimated as follows on the basis of the data in figures

4.17 through 4.19: The average strain in the steel was 0.009

which corresponds to a 22-kip load acting on the dowel. The

neoprene pad is estimated to have supported approximately

30 kip. This leaves a 48-kip load to be supported by tiie

1/2-in high grout column. Such a load would be associated

with a 20,000 psi stress in the grout which probably could

be sustained because of triaxial confinement (cube strength

of the grout was 11,000 psi). It is evident from figure 4.16

that some change occurred at the 100 kip level. Since dowel

yielding is ruled out on the basis of the average dowel

strain this change was probably caused by crushing of the

grout and load transfer to the neoprene pad.

Between the load levels of 170 and 270 kip the dowel

strain did not increase. A possible explanation for this

observation may be provided by breakage of bond between the

dowel and the surrounding grout in the vicinity of the connection

which caused any additional load to be transferred to the

neoprene pad. Dowel yielding may have contributed to this

process , however the strain level at which dowel yielding

is expected was reached at the slightly higher load of approximately

300 kip. No large vertical deformation was associated with

dowel yielding since at the 300-kip load level most of the

load v\?as carried by the neoprene. After unloading the permanent

set in the dowel corresponded to an average strain of 0.0048.

In the re-loading cycle the dowel yielded at approximately -1 400-kip and the final permanent set in the dowel corresponded I to an average strain of 0.0055. *

24
The conclusion that can be drawn from the preceding

discussion is that load transfer to the neoprene pads occurs

at a load level of approximately 100 kip and is probably

attributable to crushing of the grout column and limited

breakage of bond between the dowel and the grout; and that

dowel yielding occurs at approximately 300 kip and was not

associated -with a significant decrease in the vertical stiffness

of the joint.

4.4 Test to Evaluate the Shear Capacity of Column

Connections

4.4.1 Objective

Since all loads are transmitted through the column

connection the shear capacity of the connection may be critical

with respect to the resistance of the system to horizontal wind

or seismic load. Because of the complex nature of the connection,

the relatively low resistance of the neoprene pad to distortion

under in-plane shear, and the effect of dowel yielding during

construction, testing was used to determine load -deformation

characteristics, load capacity and ductility of the connection.

The load- deformation chart eristics were to provide information

that could be used to evaluate the contribution of the joints

to drift of the structure under wind and seismic load. Determinatio

of ductility, including application of several cycles of

reversed loading, was to provide important information with

respect to required safety margins (a higher margin would

be required if a single joint could fail and lose its load

resistance before the other joints in the same story level

are loaded to capacity) and with respect to structural response

to seismic loading which is related to the ability of the

joints to absorb energy.

25
4.4.2 Test Specimen

The test specimen for these tests consisted of three

1/2-story high (51 1/2 in long) stub columns wliich were connected

end to end by a continuous grouted dowel using connection

details previously described. The stub columns were similar

to those described in section 4.3.2 except that there were

5 confinement ties at both ends of each stub column replacing

the heavy steel plate used at one end of the stubs in the

compressive tests. Figure 4.20 shows stub reinforcement

and several column segments under construction.

A specimen arranged for preloading and grouting of the

do\\rel is shown in figure 4.21. The three stub columns shown

aligned in the figure were preloaded to 10 kip using an external

bar system similar to that described in Section 4.3 before

grouting a full-length dowel. This 10 kip preload was sustained

until after the test for Tests 1 and 2. For Test 3 the external

preload was increased to 90 kip after the grout had attained

the specified compressive strength (5000 psi).

The joint for Tests 1 and 2 was a 1/4-in thick neoprene

pad. For Test 3 the 11 ga. stainless steel neoprene sandwich

was used.

The concrete and grout were similar to those described

in section 4.3.2

4.4.3 Test Procedure

The test setup is shown in figures 4.22 and 4.2 3 for

Test 1, and Tests 2 and 3, respectively. Shear load was

applied simultaneously to both connections by applying lateral

load to the center stub while the two end stubs were restrained

from translation and rotation. Load was applied in the

direction of the narrow dimensions of the columns. The tests

were performed in a 600-kip hydraulic testing machine.

26
Joint displacement (slip) was measured electronically

by vertical displacement transducers with a resolution 0.001

in and referenced to the laboratory floor. The displacement

of the center segment was measured in Test 1 by two transducers

mounted near each joint on either side of the segment and in

Tests 2 and 3 by three tranducers mounted in its centerline,

two near the joints, and one in the center of the segment.

In all three tests the two end segments were each instrumented

by two transducers on either side of the segment near the joint

and by one transducer in the centerline of the segment near

the end of the specimen. Even though an attempt was made

to keep the end segments in a fixed position, displacements

up to 0.03 in were measured near the joints. Thus net joint

slip had to be computed by compensating for these displacements.

In Test 3, axial do\-/el strain was measured by bonded-foil

strain gages on two sides of the dowel in the center of each

joint. The gages were arranged on the two sides of the

dowel in a horizontal plane normal to the direction of loading

to eliminate the effect of dowel bending. Dowel strain was

also monitored during axial preloading. Loads were monitored

electronically by pressure transducers connected to the

hydraulic systems of the rams and the testing machine.

All data were electronically recorded after load increments

varying from 5 to 20 kip. The displacements at one of the

transducers of the center segment was also graphically monitored

by an X-Y recorder to provide information on the behavior

of the specimen and to aid in the determination of load increments.

In Tests 2 and 3 the information from the X-Y plotter was

used to determine the magnitude of the displacement excursion

in each load cycle.

In Test 1 the lateral load was applied as shown in figure

4.22 in only one direction by the testing machine. In Tests 2 and 3 the lateral load was applied alternately in two directions

by using the testing machine in one direction and hydraulic

rams in the opposite direction. This second test set-up

is illustrated in figure 4.23.

27
In Test 1 the center segment was loaded in increments i until the +_ 1 - in clearance prevented continuation of the test.

In Tests 2 and 3 reverse cycles of load were applied as

shown in the following schedule.

»i Loading Schedule of Tests 2 and 3:

a/ Cycle 1: 35— kip total lateral load in both directions

(down and up)

b / Cycle 2: Excursion of 2Ay— in both direction (magnitude

of load 50 kip maximum was determined by the i requirement to produce the desired excursion).

Cycles 3 through 7: Excursions of 5Ay in both directions

(maximum load approximately 70 kip)

Cycles 8 and 9: 35 kip load in both directions

Cycles 10 through 14: Excursions of 5Ay in both directions

Cycles 15: Downward load to failure

If

The 90 kip axial preload in Test 3 was monitored and adjusted

at the beginning of each load cycle.

4.4.4 Test Results

Figure 4.24 shows the Test 3 specimen after failure. Note

the large slip at the joints, the extrusion of the neoprene and

the cracked concrete near the upper left corner of the center

segment. The two inset photographs show the appearance of the

fractured dowel. The low-cycle fatigue fracture of the dowel

originated at a defect (see inset in lower left corner) and

was similar in appearance in both Test 2 and Test 3.

— 35 kip (17.5 kip per connection) was considered the highest

load level at which the load-displacement relationship

was still linear.

h / — Ay is the deflection at yield as determined from the X-Y plot.

Each excursion was measured from the unloaded position at

the completion of the previous load cycle.

28
Test results in the form of load versus displacement at

one side of the center segment relative to the laboratory floor

are shown in figures 4.25, 4.26 and 4.27. "Load" is the load

on each connection which is 1/2 of the total applied load.

Dowel strain caused by the axial loading of the Test 3 specimen

is shown in figure 4.28. Strain gage 8 did not function.

Figure 4.29 shows the dowel strain during Test 3 (including

that caused by the preloading to 90 kip). Figures 4.30,

4.31, and 4.32 show the net joint displacement (slip) in

Test 1 and the first half of the second cycle in Tests 2 and 3, corrected for the measured displacements of tlie end

segments near the joints.

4.4.5 Discussion of Test Results

(1) Load Capacity

Test results for the connections subjected to low axial

load are shown in figures 4.2 5 and 4.26. In figure 4.2 5 (Test 1) a change in stiffness similar to yield occurred

at approximately 19 kip at a joint displacement of approximately

0.05 in. The test was discontinued at a 40-kip load when

the joint displacement reached approximately 1 in. In Test

2 the load was applied in reverse cycles. In the first

reverse load cycle the load was carried to 17.5 kip in either

direction and there was substantial deflection recovery

(over 80 percent). In the subsequent cycle yield occurred

at approximately 2 5 kip. The envelope of the reverse load

cycles in the direction of initial displacement (downward)

is similar to the load-displacement curve in figure 4.25.

At the failure load of +_ 37 kip the displacement was approximate!

1.2 in. Thus the joint was able to resist 10 reversed load

cycles with excursions equal to 5 times the initial yield

deflection (initial yield deflection was +_ 0.1 in in Test 2)

and failed at a final joint displacement equal to approximately

12 times the yield deflection.

29
Since Tests 1 and 2 were upper story simulations plain

neoprene pads without steel plates were used. Figure 4.26

shows the result of shear tests on a connection using the

steel-neoprene-steel sandwich and subjected to a 90-kip axial

load. In this case the preload before grouting was 10 kip

and the total axial load was increased to 90 kip before the

shear force was applied. Even though it was anticipated

that frictional resistance would affect the strength of this

connection, yield occurred at approximately 20 kip and thus the

yield strength did not differ from that of the specimens in

the previous ly-disucssed tests. Dowel fracture, however,

occurred at a higher load level (62 kip) than in Test 2. As in

Test 2 the response in the initial load cycle, carried to

17.5 kip in both directions, was substantially elastic (80

percent displacement recovery).

In both Test 2 and Test 3 the initial loading was in

a downward direction. After the second load cycle a residual

deflection in the downward direction remained after unloading.

This downward drift increased progressively as additional

load cycles were applied.

From the previous discussion the conclusion is drawn

that the axial load applied during the shear test had no

significant effect on the yield load in shear which was approximately 20 kip. The compressive strain in the dowel caused

by the 90-kip axial load applied in Test 3 was approximately

0.0011 as shown in figure 4.28 which is within the range of

strains recorded for a 90-kip axial load in figure 4.16 for

the compressive test on the column connection with the

grouted dowel. It has been noted in Section 4.3.5 that, in

the compressive load test, load transfer to the neoprene

occurred at compressive load levels above 100 kip. Thus

it is reasonable to assume that the applied axial load was

supported by the dowel and the surrounding grout column.

Frictional resistance to sliding was, therefore, probably

30
mobilized only after the axial load was transferred to the

neoprene, and did not affect the load resistance of the specimen

up to the yield point. Since in the proposed structure

only columns in the side walls are continuously grouted,

and since these side-wall columns support only 1/2 the

load acting on interior columns, it is anticipated that all

joints in side-wall columns would develop a shear capacity

of approximately 20 kip. Interior lower-story column joints,

where the entire dead load is transferred to the neoprene

before grouting, could be expected to develop greater yield

strength when subjected to lateral load. The loading condition

of interior joints was not simulated since it was not considered

critical

.

In summary it is concluded that the lateral load capacity

at yield of upper-story joints and lower story side-wall

joints is approximately 20 kip and that all joints failed

in a ductile manner when subjected to reverse cycles of lateral

load. The load capactiy of interior joints is probably higher.

(2) Response to Lateral Load

Load-displacement characteristics are shown in figures

4.30, 4.31 and 4.32. Displacement at each joint was computed

by compensating for the translation of the exterior segments

of the specimen near the joint. Figure 4.30 shows the data

for Test 1. Since there was considerable rotation in the

north joint the measurement at the south joint is considered

more reliable. The test data for Test 2 are shown in figure

4.31. In this case the transducer measuring the north joint

displacement may have been obstructed since south joint

displacements agree reasonably with the displacements at the

center of the center section. Thus, in this case, the south

joint measurements are taken as representative. In Test 3,

shown in figure 4.32, there is good agreement between the

displacement in both joints. The broken lines sho^m in Tests 2

and 3 approximately follow the envelope connecting the

peak loads in the various cycles of downward load. In

31
figure 4.33 the three load-displacement curves are superimposed.

Test specimens 1 and 3 had a similar load-displacement relationship

up to the 20-kip load level. The Test 2 specimen had lesser

stiffness, probably because it was subjected to a previous

load cycle. Thus there is no indication that the magnitude

of the axial load had any effect on the stiffness in the

elastic range. After yielding occurred the Test 3 specimen

developed greater stiffness than those in Tests 1 and 2.

Figure 4.29 shows the average axial dowel strain measured

in Test 3. Only strain gage 7 is shown, since the data

from the other strain gages were erratic. The erratic readings

were probably caused by dowel bending and other disturbances

in the connection. It appears that average axial yield strain

was reached during the unloading cycle of cycle 2, which

is after the completion of an excursion equal to twice the

yield displacement. Thus, "yielding" in the load-displacement

curve may be attributable to crushing of the grout column

in the joint, and not necessarily dowel-yielding. This observation

however, should be qualified since the strain gage was located

in the neutral axis of the dowel with respect to bending

and, therefore, approximately measures net axial tensile strain

rather than maximum strain, while initial dowel yielding

may have been caused by bending rather than tension.

It can be concluded from the previous discussion that no

correlation between axial load and stiffness in shear was

observed prior to yielding. After yielding occurred the

specimen subjected to the high axial load developed greater

stiffness and strength. This observation is attributed to the

effect of frictional shear which was not mobilized prior to

compress ive- load transfer to the neoprene.

32
5. Summary

(1) The compressive-load capacity of the column connection

is affected by the radially tangential shear exerted by

the bearing pad on the column face. The effect of the

various types of bearing pads on the joint capacity is computed

in the last column of Tables 4.1.2 and 4.1.3.

(2) A connection using a steel-neoprene-s teel sandwich

and a grouted dowel similar to that proposed to be used

in the lower stories of the building systems had a load capacity

which exceeded that predicted using the strength coefficient

computed on the basis of column connection tests and tabulated

in the last column of table 4.1.3.

(3) In a test specimen simulating the continuously-grouted

column used in the plane of the side wall:

1. the load transfer from the dowel and the surrounding

grout column to the neoprene pad occurred at the load

level of approximately 100 kip. The load transfer

mechanism was probably associated xvith crushing of

the grout column in the connection space together

with bond breakage between the dowel and the grout

in the vicinity of the connection;

2. dowel yielding occurred at the approximate load level

of 300 kip;

3. neither the initial load transfer to the neoprene nor

the dowel yielding caused a sudden large increment

of compressive deformation (a deformation which is

not associated with an increase in load)

;

(4) The load-deformation characteristics observed in the

tests on the neoprene pads presented in figure 4.11 are

reasonably consistent with joint compression measured in the

bearing tests on column connections and presented in Table 4.1.4.

33
(5) For three pairs of column joints subjected to combined

axial compression and shear:

1. Yield in shear occurred at a lateral load level

of approximately 20 kip per connection and the

yield load was independent of the applied axial

compression

.

2. Up to the yield load in shear no correlation was

observed between the applied axial load and the

stiffness in shear (magnitude of slip displacement)

.

After yielding, the connections subjected to

high axial load developed greater stiffness in

, shear and their ultimate load capacity was greater

than that of the connections subjected to low

axial load

,

3. The connections failed in a ductile manner under

monotonic loading as well as under reversed

cycles of loading.

The findings under 1 " and 2, as related to connections subjected

to high axial load, apply to the continuously-grouted columns in

the plane of the side wall. Interior column connections where

the entire dead load is supported by the neoprene pads could

develop greater stiffness up to the yield load and yield at a higher shear load.

34
6. Acknowledgement

Th.e authors wish to acknowledge the cooperation of the

following persons:

Dr. Norman F. Somes; Mr. Frank A. Rankin; Mr. James F. N.

Seller; Mr. Charles Bulik; Mr. Randolph Williams;

Mr. Thomas E. Ruschell; and Miss Cathy M. Glaze, Structure

Section, Center for Building Technology, National Bureau

of Standards.

35
Rcf erenc es

American Concrete Institute, Buildincr Co-io Reciui remen ts

for Reinforced Concrete (ACI 31?^ -71) Ainericm Concrete

Institute, (Detroit iMich. , 1070).

Richart, F. E. , Brantzaeg, A., and Proim, P.. L., A ^tndv

of the Failure of Concrete Under Combine''' Co'-'-nress ive

Stresses, P.ulletin 18 5, University o^ Illinois Fnf^ i ne'^r ing

Experimental Station, (Urbana, 111., 1028).

Richart, F. F. , Brantzaeg, A., and Provrn , L., The

Failure of Plain and Spirally Reinforced Columns in

Compression Bulletin 190, University of Illinois LxperimenLal

Station, C^rbana 111., 1929).

Blumc, J. A., Newmark, IJ. M. , and Corning, L. 11., Design

of Multistory Reinforced Concrete Buildings for Earthquake

^lotion, Ch. 5, Portland Cement Associatioa, (Chicago, 111.,

1961)

.

Somes, N. F., Compression Tests on Hoop-Rein !^crced Concrete,

Journal of the Structural Division Proceedings of tiie

American Society of Civil Engineers, (Nev/ York, N. Y. , July 1970) . -

National Cooperative Highway Researcli Programs, 'Tlastonoric

Bearing Research," Report No. 109 Highv/ay Research Bonrd I (h'asliington, D. C. 197 0).

36
[7] .Niewiiian, K. , and Lachajice, L., "The Testing of Brittle

'latorinls Under Uniform Uniaxial Compressive Stress,''

AvSTM Proceedings Vol. 64, American Society for Testing and

;iatcrials, (Phila. Pa., 1964).

[S] ASTM A615-68, Deformed Billet Steel Bars for ConcreteReinforcement, ASTM Standards, Part 4, American Society

for Testing and Materials, (Phila, Pa., 1^)53).

37
•HO

no0 mH

CO

CO

<D

J-i 43

bO 14-4u c o a u q;

4-1 cu 01 CO !3 1 CO 4-1 CO .c 0) Q> iH (iO u 43 o

IW •H 43 o

.12 •^3 iH

CO c: (U 43

M-4 o O

O

O •H CO 50 P 4J iH C o •H

cu •HU

(U >^

g CO (X CO 14-1 iH (1) Cu t-i •H 43

CO CO iw o C rH

•H c O 01

d) o •H QJ o 4-1 4-) +J

CO 4J CO CO

14-1 CO cu CO

»-l CO rH CO 3 si Pu

CA 4-> •H e! iH

d

CO •H CO

C e

•H CU 0) U o cu o

CO CO 4J CO

OJ IW CO XI !-l CO o M 3 iH

(U CO D- CO CO

4-) e <u T)

4-t (U (U

a

O cn a. o CO cu o >. CO

CO 60 CM o

CX)

38
Table 4,1.2 Phase I Test Results on Bearing Capacity of Column Connections

r

^ Stub Column Size Test

Ntimber

f

,'cpsi

Type of Joint Max, Load

kip

a/

... Pu7

0.85f' A

c

10 X 12 X 31% in 1 3660 % X 10 X 12 in neoprene 350 0.94

II 2 3660 % X 10 X 12 in neoprene 354 0.95

It 3 3660 Single stub set in plaster on 1 in thick plate

585 1,56

S3mibols: ^ Concrete cylinder (6"x 12") strength at time of test,

P = Maximum load

A = Bearing area= 120 sq, in.

39
Table 4,1.3 Phase II Test Results on Bearing Capacity of Column Connections

1

1 Column Stub Size Test fi a./ Type of Joint

Max.

Load

a/

Number c

.

psi Kip 0.85 A

I 10 X 12 X 51% in 1 4340 ^ X 10 X 12 in neoprene 477 1.08

i

i " 2 4160

b/

3/4 thick grout" 535 1,26

i

i 3 4510

c/

3/4 thick grout"" 519 1.12

i 4 4155 2-St.St. plates and k

X 10 X 12 in neoprene

580 1.37

\

' ' -'

' 5

- 4342 2-St St nl ;9te<3 and k: X 9 X 11 in neoprene

586 1.32

i "

' n '' \- :

1

• .

: 4881 2-St,St. plates and %

e/ 420^ e/ 1.26-

1

:

. X 9 X 11 in neoprene

1

-

' -

II y-'' 493^/

.

7 4590

i

;

^ X 10 X 12 in reinforced

rubber

1.05

1 - .; ,, , -

J

8 4881 2-St St nlateq and h: X 9' x 11 in neoprene556 1 11

II

1

' 9 4881 2-St.St. plates, wax paper 598 1.20

i

: and k X 9 x 11 in neoprene

600^''

i

t

1

II 10

1

4881 1 «5tub RPt in nlastei" 4- 2

sheets of plastic film be-

;tween plaster and steel bear-

, ing plate.

1 .20

_ . . t _

1 Symbols: f = Concrete cylinder strength at time of test. u A - Maximum load

Bearing area= 120 in

b_/ Grout compressive strength (2 in, cube) at time of test=5210 psi )

c/ " " " " " 6200 psi !

d/ Smaller size (9 x 11 in) neoprene pad filled bearing area at feailure. Smaller size was selected to prevent unsightly extrusion from joint,

|

e_/ Loaded with 2-in^ major-axis eccentricity at the joint. Bearing area. A, was computed using the Whitney stress block concept. j

r

f./ Rubber pad ruptured before maximum load was reached,

S./ Held 600 kips for 5 min. before failure initiated.

40
a/

Table 4ol.4 Measured Compressive Deformations of Phase 1T~

Rubber Pad Joints at Various Compressive Loads

Test Number Type of Joint

100 Kip

Deformation at Stated Load

200 Kip 300 Kip 400 Kip 500 K]

in in in in in

1 Neoprene 0 0J4 .037 .041

4 11 gao plate and neoprene o034 ,042 .049 .057 .068

5

II .029 o036 .041 .049 .062

7 Reinf» rubber o026 o033 .041 .045

8 16 gao plate and neoprene c022

,01^/

.038 .062 .088 .118

9 16 gao plate, wax paper &

neoprene

.024 .046 .068 .086

Test Results

Section 4o3

Neoprene and steel o027 o040 o052 .060

a/~ All specimens were preloaded

Deformations do not include

to about 200 kip prior to test run. "permanent set" from preload. ~ During preloading to 200 kip pad was compressed approximately

0.10 in. Recovered only to 0.09 in. Initial compression was significantly lower and recovery greater for others.

41
Figure 2.1 Proposed Building

42
FIGURE 2.2 BUILDING COMPLEX UNDER CONSTRUCTION

43
a:

a

o

on

44

iI
47

TOP SLAB

WALL MODULE A

COLUMN (MODULE A)

BEARING PAD

BOTTOM SLAB

r S

4"

-•^ COLUMN (MODULE 8)

Figure 2.8 Detail of Interior Column Jo INT

49
COLUMN TIES

MAIN REINFORCEMENT

(a) COLUMN REINFORCEMENT AT

CONNECTION

GROUT

^^^^^

1^:

A

STEEL DOWEL

(b) CONNECTION DETAIL

GROUT CHANNEL

NEOPRENE PAD OR

NEOPRENE STEEL SANDWICH

Figure 2.9 Proposed Connection Detail

50


SECTION AA

h- 10

Notes

Diameter of ^3 Bar is 0.375 in. Dlometer of^S Bar is 1.125 in. Diameter of WO. 5 wire is 0.080 in.

3 TIES

SECTION BB

3/4 COVER

CP Q

L^) Q dj

6-^*3 BARS

Figure 4.3 Typical Details of Test Specimens

S3
1





CVJ

UJ>

UJQ

UJ>Q

UJ

LU

cmoLl_

(_) 1—1 I-

»—I

q:

UJI-o<o

LUI

UJ

Ul

hUJ

hLU

CD

UJ

C/)

UJ

cr

CL

CO o LLi LLl

CD
0 .02 .04 .06 .08 .10 .12 AVERAGE COMPRESSIVE DEFORMATION (!N 1/4 in), in

Figure ^.11 Relationship Between Compressive Load aind Average Compressive reformation

OF [Ieoprene Pads

6/
5000

4000

CL

in

[2 3000

I-z

E 2000

<

9"Xll"x 1/4" in neoprene pad'

Test No. 1

\\\\\

rri

(iiiiii/in

/////// J

^ r

1000

0

iOOOO 20000 30000

TANGENT MODULUS OF ELASTICITY, psi

Figure ^.12 Relationship Between Compressive Stress and TaiNgent i'duulus of

Eu\sTicm FOR the Full-Sized iJeoprene Pads
AVERAGE LATERAL EXPANSION (2H),in

Figure '1. 13 Relationship Between Compressive Load atnid I'Iaximum Extrusion for the

Full-Sized liEOPRENE Pads

^3


COMPRESSIVE DEFORMATION, in

4.16 Specimen with Grouted Dowel Vertical Deformations of Ar>i U-in Segment

Including the Joint

ii
COMPRESSIVE STRAIN

4.17 Specimen with Grouted Dowel, Longitudinal Dowel Strain in -mE Joint

^7
oo 1\\ \ \ w

-\ \ \ ^ T \ ^ ^ \ \ \ \ \ \ \ oUJce

IQ.OC

hiO<< oo

ID oo

5^

OO

ro d!M 'avon oo

roOOO <

IT

OO

OO

CO oo

uu

(_> <P

c5?

ixi u

GO

_J<oaa

\^Oq; CD

a. COoo
o

o

11

\ \

\ \ \

1 \\

\\ \

W\\\

u

o

CO

UJ fOo

Q

C\J o

o

<

I-cn

O

liJ oc

ID¥-

a.3

cc


 <

<

a:\-

o

o

-X:

o

o

o

o

in o

o

r.

o

o

to o

o

o

o

d!>i 'avoi

67

Figure ^.21 Specimen ArratsIged for Pre-Loading and Grouting of tvie Dowel

71

13

0 0.2 0.4 0 6 0 8 1.0

DISPL_ACEMENT , in.

Figure ^,25 Vertical DISPLACE^ENTS of South End of Center Semjt in Shear Test 1

75
1.0 12 1 .4 IS

DISPLACEMENT m

Figure ^.26 Vertical DISPLACE^Er^lT of South Bnid of Center Segment in Shear Test I

lie
77

CL

(E1 COoo

UJ

>-

LlI _J

CO<mo

<ra.

tn

CVJ OO

CMoO

•Jt:

IJJ <: U3

I-

>-

inoo ou oomooo <

q:

— CVJ ro

UJ UJ UJ o o > > o o <aix: CO

CO

_J

LU i 5oor

7?
i
Figure 4.31 ,"Jet Joint Dispu\cbients in Shear Test 2
•1
TEST 3 ENVELOPE

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 DISPLACEMENT, in

Figure ^.33 Comparison of IJet Joint Dispucements in Shear Tests 1, 2, md '5.

FORM NBS-IUA (1-71) U.S. DEPT. OF COMM. 1. PUBLICATION OR REPORT NO, 2. Gov't Accesaion

BIBLIOGRAPHIC DATA

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Evaluation of the Column Connections Used in a Precast

Concrete Modular Housing System

5. Publication Date

6. Performing Organization Code

7. AUTHOR(S)

Felix Y. Yokel; Thomas W. Reichard

8. Performing Orgaaization

NBSIR 73-148

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Taslc/Work Unit No. 4600431

11. Contract/Grant No. IM-H-16-70

12. Sponsoring Organization Name and Address

Office of Research and Technology

Department of Housing and Urban Development

wasnington, u. l. zu4iu

13. Type of Report & Period

Covered

Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

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

bibliography or literature survey, mention it here.) The column connections used in a housing system employing stacked precast concrete

box modules were tested to evaluate their structural performance. The system was

proposed for construction in Operation BREAKTHROUGH, a research and demonstration

program sponsored by the Department of HDusing and Urban Development. The system

uses innovative structural design concepts, v4iich include: confinement of the

concrete in the vicinity of the column bearings by reinforcing ties in order to

increase concrete con^ressive strength; neoprene pads between column bearings in

the upper stories, stories; steel -neoprene -steel sandwich in the lower stories;

and a grouted dowel through the center of the columns to provide resistance to

tension and shear.

The test program included the following: tests to determine the effect of various

bearing pads on the load capacity of the connection; tests to determine the

load-deformation characteristics of the neoprene pads; a test to determine the

performance of a lower-story connection using a steel -neoprene -steel sandwich and

a grouted dowel; and tests to evaluate the strength and ductility of the connections

when subjected to a shear force. The test results are presented and interpreted

and the findings are summarized.

7. KEY WORDS (Alphabetical order, separated by semicolons) Building SyStCm; COlumn COnneCtioni

concrete triaxial strength; ductility; neoprene bearing pad; Operation BREAKTHROUGH;

performance tests; precast concrete; structural design.

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