NATIONAL BUREAU OF STANDARDS REPORT

4286

TEST? OF PRECAST CONTINUOUS SPLICED GIRDERS

by

L. P. Skoda and J. 0. Bryson

Report to

Bureau of Yards and Docks

Department of tlie Navy

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

IOOI-IO-I1.8II August 15, 1955 4286

TESTS OP PRECAST CONTIMJOUS SPLICED GIRDERS

by

L. P, Skoda and J, 0, Bryson

To

Bureau of Yards and Docks

Department of the Navy

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publlcatloi

unless permissi 25,D.C. Sue!

tally prepared

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015.

e or In part, is prohibited

jf Standards, Washington

a report has been specifi*

ar report for its own use.
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TESTS OP PRECAST CONTINUOUS SPLICED GIRDERS

by

L. P. Skoda and J, 0. Bryson

Abstract

The rigidity and strength of two precast

girders with welded splices of two different designs were determined in tests of

the girders as beams continuous over three

supports. The girders consisted of three

reinforced concrete box sections welded

and grouted together to form a beam 1- by

2-ft in cross section and 2$ ft long.

The splices located at points of inflection

were formed by lap welding suitable lengths

and amounts of positive and negative reinforcement projecting from the ends of adjoining sections and then grouting the

pockets

.

The designs of the splices of the two girders were identical, except that girder No.

2

contained an additional amount of reinforcement consisting of three inclined stirrups

of No, I4. bars on each side of the splice.

The addition of the inclined stirrups resulted in an Increase of 6? percent in load

carrying capacity of girder No. 2 as compared with girder No. 1,

1. INTRODUCTION

At the request of the Bureau of Yards and Docks, two

precast continuous spliced girders were tested to evaluate

the structural strength of proposed welded splices.
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The following report presents observations and data

obtained during the construction, fabrication and testing of two such girders.

2. PREPARATION OF THE SPECIMENS

2,1 Description of the specimens

Each girder consisted of three sections approximately

8 ft long joined together to form a girder 24.5 ft long.

The sections were joined by lap welding suitable lengths

and ariiounts of positive and negative reinforcement projecting from the ends of adjoining sections and then

grouting the pockets. Each girder section was fabricated

by welding together two channels which formed a box girder having a 1- by 2-ft cross section. The flange of the

channel was 6 in. in breadth and its thickness varied from

6 in. at the base to 4 3/4 where the channels joined.

The web thickness was 2 l/2 in. Each channel had three

stiffening ribs, one at each end and one at the center.

Steel plates, welded to bent No. 5 bars for anchorage,

were embedded in the concrete at the time of casting.

They were placed at the edge of the flanges, both on top

and bottom, 2 ft 4 from the ends and 2 ft 4 from

the center line of the joints. This provided each pair

of channels with four points of attachment. Details of

the girder and the reinforcement are sho^^Tn in figure 1.

2 .

2.2 Forms

The symmetrical design of the girders made it possible to construct forms for only one-half of the specimen.

To keep the cost of casting at a minimum, each girder was

made from two casting operations.

The forms, made at the National Bureau of Standards

carpenter shop, had a base of 3/lj. in. plywood braced with

2- by timbers. The pans were made of 3/4- in. plywood, The sides of the forms were made of 2- by 6-in.

white pine and were removable. The heavy side pieces

prevented warping. This resulted in an excellent duplication of the channels and a precise fit. The forms received three coats of waterproof spar varnish and, prior

to each casting, oil and asbestine powder was apj3ied as a

separator. The forms with reinforcement in place are

shoxfjn in figures 2 and 3.

2.3 Concrete

The proportions of the concrete were 1:2.48^2,02,

by weight. High- early- strength cement was used to expedite-curing, The aggregates were sand and pea gravel,

with a maximum size of 3/8 in,, from White Marsh, Maryland,

Three batches of concrete, with slump from 3 to 4 in.,

were mixed for each casting operation.

3 .

Two 6- by 12-in, control cylinders were cast from

each batch and gave the follotd.ng average results:

Girder No. 1

Girder No. 2

Stress- strain curves for the concrete in each specimen are

shown in figures 4 and 5.

Both specimens were kept under damp burlap for two

days and then transferred to a curing chamber where they

were moist- cured for periods ranging from 1 to months.

The concrete was then air-dried until tested at an age

of about 8 months.

2.4 Reinforcement

Reinforcement in the upper flanges of the end sections

of each girder consisted of four No, 8 deformed bars. The

two inner bars were completely embedded while the outer

bars extended 5 in. beyond the joint end. The upper

flange of the middle section was reinforced, with two No.

4

deformed bars. These bars extended 5 in. from the middle

section to lap the No, 8 bars from the end sections. The

lower flanges were reinforced conversely: four No. 8 bars

in the middle section and two No, 4 bars in the end section.

4 .

Compressive

strength

psi

7,560

8,130

Tangent modulus

psl

4.5 by 10

4.4 by 10^

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The web reinforcement in the portion near the joint

was different for the two girders. In girder No. 1 there

was a single, hooked No. 1]. bar while girder No. 2 had a bent No. bar tied to three inclined stirrups of No. 4

bars on each side of the welded splice, as shown in

figure 1.

All of the deformed r^nforcing bars were of intermediate grade steel. The girders were further reinforced

throughout their webs and flanges with U- t>y welded

wire fabric. No. 6 gage,

2,5 Assembly, welding, and grouting

After curing, the channels were dried for two days

before being assembled.

Each pair of channels was welded together with 1- by

3/8- by 2- in. connector plates, centered over the embedded

steel plates (2.1). Small cracks, 1-in. to 2-in. long, adjacent to and radiating from the embedded plates, resulted

from the heat produced by welding. These cracks were

negligible in width, however,. Another crack appeared in

girder No. 1 parallel to the embedded No. 5 anchorage bar,

biit it seemed to have no effect on the results of the test.

The joined channels were then put in position for

welding with the protruding reinforcement overlapping ij. in,,

« (see figure 1). To reduce heat transfer to the concrete

5 .

as the lap welding was done, each weld was allowed to cool

after only half of the bead was complete. All welding

was done by a qualified Navy welder.

After the welding was completed, forms were constructed around the pocket at each Joint and filled with concrete having the same proportions as the concrete used in

the specimen. The average compressive strength of 6- by

12-in, control cylinders was 8,i|.00 psi,

3, TESTING PROCEDURE

3,1 Test setup

To simplify the testing procedure, the girders were

tested in an inverted position, A mechanical testing

machine with a 600,000 lb capacity was used.

The girder was supported on three rockers, one of

which was at the center and one at each end 12 ft from

the center. The load was applied to the girder through a

loading beam at two load points 16 ft apart on top of the

Inverted girder. The machine load was applied to the loading beam through a spherically seated compression block.

The load was to be so distributed that the sum of the

end reactions equaled the center reaction. To insure such

a distribution, it was necessary to adjust the end reactions as each increment of load was applied by the machine.

This was accomplished by means of hydraulic Jacks which

6 .

served as end reactions for the girder. Calibrated load

cells were placed between the jacks and the girder. As each

increment of load was applied, the end reactions were adjusted to equal one-fourth of the load. A diagram of the

loading arrangement is shown in figure 6. Figure 7 shows

a close-up of the end reaction and figure 8 shows girder

No. 1 in the testing machine.

3.2 Instrumentation

The instruments employed to measure the strains of the

girders were 0,001-in, dial gages and SR-lj. electrical resistance strain gages.

The dial gages were placed opposite the three reactions

and the two points of load application, and measured the displacements of these points relative to the platen. It was

necessary, therefore, to measure also any possible deflection in the platen of the testing machine in respect to the

laboratory floor. The SR-4 gages were placed on all of the

reinforcing bars at the points of maximum bending moments

(see figure 6). A complete set of readings was taken at

each machine load Increment of 20,000 lb.

7 .

li, RESULTS

The girders were tested as beams continuous over three

supports with the loads applied at points 4 ft from each

end of the girder. This arrangement of supports and loads,

shown in figures 1 and 6, was devised in order to have the

welded splices at the third points coincide with points of

inflection in the continuous girder.

Both girders failed by diagonal tension. Girder No.l

failed at a total applied load of 151,000 lb and girder

No. 2 at 253,500 lb. Thus, the addition of six Inclined

stirrups of No, I4

. bars on each side of the welded splices

resulted in an increase of 67 percent in resistance to

shear of the girder No. 2 as compared with girder No. 1.

The loads given here are the applied machine loads and do

not Include the weights of the specimen and the loading

fixtures

,

The load deflection diagrams for the two girders are

given in figures 9 and 10. The values of the deflections

in the diagrams are ’the displacements of the center and the

ends of the girders with respect to a straight line passing

through the points of application of load. Negative values

of deflection indicate downward displacements of the girder

in its actual position in the structure.

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Girder No. 1 reached a center deflection of 0.314 In,

at a load of 139 kips, at which load the last deflection

reading was obtained prior to failure. The end deflections

at this load averaged 0.135 in.

Girder No. 2 reached a center deflection of 0.395 in.

at 220 kips, the last observed deflection prior to failure.

The end deflections at this load averaged 0.242 in.

The relation between the observed strain in the reinforcement and applied load is Illustrated in figures 11

and 12, The strains were observed at the center of each

girder and the points of application of load in both compressive and tensile reinforcement.

The strain in the reinforcement was also plotted in

figures 13 and l4 to show the distribution of strain along

the length of the girder and to check the theoretical location of the points of inflection. It can be seen that as

the loads increased and cracks in the concrete becarae wider

and more closely spaced, the sections of zero bending

moments shifted somewhat toward the center of the girder.

This was indicated, in general, by both the tensile and com- pressive strain measurements in the reinforcement.

Close-up views of the sections of the girders where

failure occurred are shown in figures l5» 16, and 17, and

the general crack pattern is shown in figure l8. It can

9 .

be seen that the shear failure in girder No. 1 was sudden

and complete, as is usually the case with shear failures

in reinforced concrete beams containing little or no web

reinforcement. The major diagonal tension crack in

girder No. 1 which ran from the point of application of

load to the mid- section of the welded splice was wide

enough to cause tensile failure in the 4- t>y welded

wire fabric of No. 6 gage which was present in the webs of

the girder.

The diagonal tension crack at which failure developed

in girder No, 2 was considerably steeper than that in

girder No. 1. As can be seen in figures 16 and 17, the

diagonal tension crack ran from the point of application

of load to a section about midway between the splice and

the load point. The splice Itself, as seen in the closeup of figure 17, appeared to be substantially intact after

the test, even though crossed by several cracks.

The distribution of cracks Illustrated in figure l8

shows substantially the same crack pattern in both girders

in the vicinity of supports and applied loads. However, on

account of its greater load carrying capacity, girder No. 2

developed extensive tensile cracking in the vicinity of the

splices which was absent in girder No. 1.

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DATE. 5-13-53 SKETCH B-3

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FIG. I





STRESS,

KIPS

CONTINUOUS GIRDER NO. I STRESS-STRAIN CURVE OF 6'‘XI2" CONCRETE CONTROL CYLINDER

STRAIN, MICRO IN. /IN.

STRESS,

KIPS

CONTINUOUS GIRDER NO. 2

STRESS-STRAIN CURVE OF 6"XI2" CONCRETE CONTROL CYLINDER

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3, 1901, as amended by Congress in Public Law 619, 1950. These include the development and

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