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Loading Test on Prestressed Concrete Bridge Beam on Sussex Drive

(2)

CANADA

DIVISION OF BUILDING RESEARCH

LOADING TEST ON

PRESTRESSED CONCRETE BRIDGE BEAM ON SUSSEX DRIVE

by

L. J. Marcon

ANAL YZEO

Prepared for submission to the City of Ottawa

Report Noo 39

of the

Division of Building Research

OTTAWA

(3)

The City of Ottawa, through its Department of Works and Planning and the Waterworks Department, has co-operated with the Division of Building Research in a number of research investigations, it is believed to mutual benefit, certainly to the advantage of the Division.

The opportunity of working with the City on the testing of a prestressed beam for the new Bytown Bridges

along the Sussex Drive was therefore welcomed. The new

bridges are adjacent to the Sussex Street headquarters building of the Council, giving access to Green Island on which one of the original temporary offices of the Division was located.

This report is a factual record of the test

procedure and the results obtained. A comparison of the

measured deflection and the ultimate load with the figures

obtained from the designer is also included. The report

was circulated in draft form to those directly concerned for comment, and in its present revised form, incorporates most

of the suggestions received. This assistance in the

preparation of the report is gratefully acknowledged. The report has been written by L. J. Marcon, Junior Research Officer in the Building Design Section of D.B.R., who was in immediate charge of the test beam work, under the supervision of W. R. Schriever, head of the Building Design Section.

Robert F. Legget, Director.

Ottawa,

(4)

by L. J. Marcon

Two prestressed concrete highway bridges are at present under construction for the City of Ottawa on Sussex

Street over the Rideau River. They are the largest prestressed

concrete bridges so far built in Canada. General contractor

for the construction of the bridges is Key Construction

Limited of Montreal, Quebec. The beams were designed and are

being built under the supervision of the Pre-Compressed Concrete Engineering Company Limited of Montreal, Quebec.

Each bridge consists of three equal spans.

Three spans 75 feet 6 inches long will form the crossing over

the channel south of Green Island and three spans 89 feet 3

inches long, the crossing over the north channel. In each

span there will be 18 beams which, placed side by side, will

form a bridge deck 65 feet wide. The beams will be

trans-versely prestressed to insure that each span acts as a unit. All tensioning is done by the Magnel-Blaton system.

The Division of Building Research of the

National Research Council is interested in prestressed concrete construction and has already carried out a loading test on a lOO-foot two-span roof beam (Cobourg Army Ordnance Depot). When it was learned that the City of Ottawa was going to use prestressed concrete in the Sussex Street Bridges, the Division approached the City of Ottawa and offered their services in connection with the loading test which was required by the

contract specifications. This was done because of the growing

interest in this new type of construction and in view of the research value of the information to be gained from the

loading test.

The co-operation of the following persons and the staffs of their respective organizations is gratefully

acknowledged: Mr. J. H. Irvine, of the City of Ottawa;

Colonel V.S. Thompson and Mr. J. W. Lucas of the Department

of Public Works; Mr. G. Lapointe, President and Mr. H.P. K?egi, construction superintendent, of the Key Construction Limited (general contractor); and Mr. P. A. Benn, executive vice-president and Mr. V. Sibly, resident engineer of the Pre-Compressed Concrete Engineering Company Limited.

(5)

II. DESCRIPTION OF BEAM

A. Design and Test Specifications

The bridge was designed for H20 - s16 truck loads as shown in Appendix B of Specification S6-1952 of the

Canadian Standards Association (CSA). Each individual T-beam

was designed to support one half of the above axle loads with an impact allowance of 30 per cent, taking into account a reduction due to lateral distribution as stated in the above CSA Specification, page 13.

The general contract for the construction of

the bridges called for a test of a typical beam. The

performance requirements for the test beam were as follows: "If the selected beam fails to support one-half of

the design axle loads with corresponding impact before cracking and if the deflection of the beam does not show a recovery of at least 75% after removal of twice the specified design load, the beam shall be discarded and another submitted to

the same test. Repeated failure to supply beams

meeting the above requirements will result in the whole batch being discarded and fabrication

stopped until the cause of such failure can be determined".

The contract also stated that the contractor was responsible for providing the test beam and all labour and handling

equipment required to carry out the testing of the beam to destruction, whereas engineering supervision and direction of the testing operation would be provided by the City of Ottawa.

The Division of Building Research of the National Research Council offered to assist the City of Ottawa during

the test by measuring the deflections of the beam during the various loadings.

A beam 75 feet 6 inches long from the south

bridge was used for the test. Typical cross- and

longitudinal-sections of the test beam are shown in Fig. 1.

B. Construction Procedure

A typical prestressed beam was fabricated as

follows. One side of the metal lined form was erected for

its whole length. Reinforcing steel was' then placed and

secured in position. The reinforcing steel consisted mainly

of stirrups, transverse bars in セィ・ top flange and special reinforcing in the diaphragms and end blocks to resist and

(6)

cores for the formation of the longitudinal and transverse cable ducts were then installed and secured in place and then the remaining side of the framework installed.

The concrete was mixed at the site and carried from the mixer to the forms in a concrete bucket equipped

with a bottom 、ゥウ」ィセイァ・ gate. Internal and external

vibrators were used in the placing of all the concrete

in beams. High early strength cement was used in all

concrete for prestressed beams.

The forms were removed after about 18 hours and

the rubber cores pulled out. The cables of prestressing

wire were inserted into the ducts by a special winching

machine. The beams were prestressed when the concrete

cylinders reached a strength of 4,000 p.s.i., which usually

required 3 days. The wires were tensioned in pairs by

means of a hydraulic jack and anchored by Magnel-Blaton

sandwich plates. To obtain the necessary prestress of

458,000 lb., 64 high tensile steel wires each of 0.276 in. in diameter were used and stressed to approximately 130,000

p.s.i. each. The elongation produced in the wires was 4.25

in. Tensioning of a 75-foot 6-inch beam resulted in a

camber of about 7/8 in. at the centre of the beam span. The last operation before moving the beams into position was the pressure grouting of the longitudinal cable ducts in order to establish bond and to protect the wires from corrosion and the encasement of the end

anchorages in concrete. For the test beam the pressure

grouting equipment was not available, leading to incomplete

grouting. The beams were then moved to the bridge site by

means of two trailers designed and built for this purpose. The concrete for the test beam was placed on May 25, the forms were stripped on May 26, and the

tension-ing of the prestresstension-ing wires took place on June 3. The

longitudinal ducts were grouted on June 8. The beam was

loaded to destruction on June 29, at an age of 36 days. Concrete and Steel Data.- The owners' specifications

required a minimum ultimate compressive concrete strength

of 5,000 p.s.i. at 28 days. Three test cylinders were

taken during the placing of concrete for each beam. The

average 28-day cylinder strength obtained from the first

19 cylinders was 6,500 p.s.i. Figure 2 shows the

relation-ship of compressive strength of the concrete with time

based on the six cylinders taken from the test beam concrete. The concrete mix placed in the beam had a weight ratio of

1 : 2.13 : 2.81. The average slump was 1.5 in. with a water

content of 4.3 imperial gallons per sack of high early strength cement or a water cement ratio of 0.49 by weight.

(7)

The requirements for the prestressing wire were as follows:

Minimum yield strength (0.2% offset) Minimum ultimate tensile strength Maximum working stress

160,000 p.s.i. 217,500 p.s.i. 130,000 p.s.i. The wire actually used for construction exceeded these minimum requirements as can be noted from stress strain

information on Fig. 2A. The actual 0.2 per cent yield

strength is in the neighbourhood of 199,100 p.s.i. and the ultimate strength is approximately 226,300 p.s.i.

The Testing Laboratories of the Public Works

Department designed the concrete mix ヲッセ the beams and also

did all the concrete cylinder testing. The prestressing

wires were also tested in their laboratories.

III. LOADING TEST

A. Types of Loading and Loading Procedure

The test beam was subjected to the following series of loads:

(A) June 23, Dead Load

+

セ Axle Load of H20 - s16 loading

+

30% impact.

The specifications required that there be no visible cracks under this load, which is larger than CSA Live Load.

(B) June 23, Dead Load

+

2 CSA Live Load

The specifications required a minimum deflection recovery of 75 per cent.

( C) June 29, Dead Load

+

3 CSA Live Load

To demonstrate the deflection recovery and crack-closing properties of prestressed

concrete, load C was applied until considerable cracking had developed and then the load was removed.

(D) June 29, Test to Destruction

In order to simplify the loading arrangement, two equal and symmetrically placed loads were used instead of the three loads specified in the H20-S16 loading

representing an actual truck load. The magnitude of the two

loads was computed as to produce the same maximum bending

moments as the three-point truck loading. The beam was

(8)

Total Load (two jacks) (1 ) Dead Load + Load to producg an additional

bending moment of 2.5 x 10 in. lb. at

midspan. 23,400 lb.

(2 ) Dead Load

+

1 GSA Live Load 41,600 lb.

(3) Dead Load

+1-

2 H20-Sl6 Live Load

+

30;&

impact 54,200 lb.

(4 ) Dead Load

+

セ H20-Sl6 Live Load

+

Load to producg an additional bending moment of

2 x 10 in. lb. at midspan. 65,600 lb.

(5) Dead Load + 2 GSA Live Load 74,400 lb.

(6 ) Dead Load

+

3 GSA Live Load 107,000 lb.

(7) Dead Load + 4 GSA Live Load 139,800 lb.

(8) Dead Load

+

5 GSA Live Load 172,000 lb.

The load was applied by means of two 50-ton precalibrated hydraulic jacks, acting against a reaction weight of about 120 tons of large stone blocks piled on a

platform straddling the beam (Fig. 8). Each jack had its

own pump and pressure gauge, the latter being used to

determine the load applied. Figure 8 shows the general test

set up. The jacks were rented to the contractor by the

Division of Building Research, but at the request of the contractor, operated by the staff of the Division.

B. Deflections and Measurements

Three methods were used to measure vertical deflections of the beam:

(1) dial gauge indicators;

(2) optical level and level rod; and

(3) wire and pulley system.

Deflections were measured by methods (1) and (2) at points

shown in Fig. 1. Some of the deflection apparatus is

shown in Fig. 9. At both ends and at midspan dial gauge

indicators were placed at the outer edges of the top flange

to indicate any rotation of the beam during loading. To

provide lateral stability during the test, and to prevent torsional stresses, vertical greased timber gUides were erected on both sides of the beam, at the ends, and at the

two centre diaphragms. In the actual completed bridge the

beams are restrained from rotating by the adjacent beams and by transverse prestressing.

(9)

Deflections up to approximately 1.75 in. were measured at all points with dial indicators and ッセエゥ」。ャ

level. During load series D (test to destruction) the dial

indicators were removed after load 6 to save them from

being damaged. The wire and pUlley system was used at

midspan only, its main purpose being to give the spectators an easy way of observing the amount of maximum deflection. Deflection readings were taken after each load increment was applied.

The Division of BUilding Research supplied

most of the equipment to measure deflections. Personnel

assisting in the test were members of the staff of the Public Works Department (Structures Division and Testing Laboratories) and of the Division of Building Research.

C. Deflection Results

All deflection values are shown after corrections have been made for vertical movement of the end supports

during the test. At the ends and midspan the average

of the two dial gauge readings were always used. Deflection

readings up to and including load number 5 were derived

from dial gauges which can be read to 0.001 in. For

deflections at larger loads the optical level and rod

readings were used with readings to 0.001 ft. Final

corrected deflection results are tabulated in Table I. Figures 3 and 3A show the deflection curves

for load fGr series Band D. Figure 3 also shows the

figures calculated by the designer for loads two, three,

and five. An average of the four deflections recorded for

the same load increment was not used, since deflections recorded after the beam had been cracked were larger than the results obtained before initial cracking had occurred, as illustrated by Fig.

4.

Load series number B included a sustained

application of load 5, Dead Load

+

2 CSA Live Load, for

a period Qf 21 hours. Deflection readings were taken at

regular time intervals during and after load application

and are shown in Fig. 5 for the midspan location. During

the 21 hours, the midspan deflection increased- from 1.165

in. to 1.641 in., or by 40.8 per cent. On removal of

the load the midspan deflection decreased from 1.641 in. to 0.308 in., giving an immediate deflection recovery of 81.3% and a recovery of 90 per cent after 6.25 hours. The minimum recovery stipulated in the specifications

was 75 per cent. Readings taken the following morning

showed that the deflections had slightly increased during the night, probably due to temperature and humidity

(10)

TABLE I

CORRECTED DEFLECTIONS IN INCHES Load Series A

LOAD GAUGE NUMBER

3

4

5

& 6

7

H

1

0.170

0.267

0.294

0.272

0.174

2

0.357

0.555

0.555

0.560

0.358

3

0.485

0.756

0.821

0.759

0.487

Load Series B

LOAD GAUGE NUMBER

3

4

5

& b

7

ts

1

0.207

0.323

0.355

0.327

0.211

2

0.377

0.585

0.636

0.586

0.375

3

0.488

0.757

0.824

0.761

0.488

I

4

0.615

0.836

1.037

0.951

0.613

5

0.721

0.825

1.165

1.115

0.719

!

I

Load Series C

I

I

LOAD GAUGE NUMBER

3

4

5

& 6

7

H

1

0.217

0.340

0.371

0.338

0.217

2

0.408

0.635

0.692

0.633

0.409

3

0.540

0.836

0.910

0.835

0.539

4

0.670

1.041

1.135

1.037

0.670

5

0.778

1.209

1.321

1.213

0.784

6

1.92

3.24

3.72

3.19

1.92

Load Series D

LOAD GAUGE NUMBER

3

4

5

& 6

7

ts

1

0.215

0.335

0.365

0.335

0.214

2

0.392

0.608

0.663

0.608

0.389

3

0.530

0.825

0.900

0.823

0.528

4

0.683·

1.068

1.169

1.067

0.679

5

0.828

1.352

1.492

1.356

0.816

6

1.96

3.24

3.73

3.30

1.95

7

4.76

8.18

9.33

8.20

4.70

7 1/3 -

11.70

. 13.86

-

(11)

-A sound which was probably due to a "stress redistribution" in the beam was heard by one of the testing staff approximately 3 hours after load number 5 was applied. This redistribution of stress is probably responsible for the jog in the deflection curve at the 3.25 hour mark.

Figures 3A and 6 show the load deflection

curves for load series D to destruction. The maximum

deflection immediately before ultimate failure was 13.86 in. The greatest twist of the beam, which was very

small, occurred at midspan. The relative vertical movement

of the two opposite edges was 0.015 in. or an angular twist of 13 minutes.

Cracks and General Observations.- Several shrinkage cracks had been noted before the test in the top flange of the beam, mostly occurring near the location of the transverse

diaphragms. The beam had been cured by placing wet burlap

on the top flange. The beams for the actual bridge were

sprayed with a surface film curing compound solution

immediately after the forms wereremoved. No shrinkage

cracks have been noticed using this method.

In the first phase of the loading test (load series A) the beam fulfilled the first requirement of the test beam specifications by not showing any visible cracks under half of the H20-S16 axle load plus 30 per cent impact.

The first crack in the lower flange during

testing was noticed during load series B, after load 5 had been

applied for 20 hours. The crack occurred at the 31-ft. mark

shown in Fig. 7. On removal of the load the crack closed

up completely •.

During load series C, the crack at the 31-ft.

mark reopened after load 5'was applied. Cracks observed

after the application of load six are shown as a solid line

in Fig. 7. On unloading all the cracks closed up; some of

them, however, remained visible as fine hairlines. There

was some crushing of concrete apparent at the bottom of the main crack.

During load series D, the main crack reopened

after the application of load 3. All other cracks were

still only partly visible as fine hairlines. Load

4

opened

the main crack to approximately 1 mm. with no apparent

change in the size of the other cracks. Load 5 opened up

the main crack to 2 mm., all other cracks to the centre

of the 27-ft. mark began to open up. The degree of cracking

(12)

series C), but increased considerably with the application of load 7, the main crack opening up to slightly over 1/2

in. The extended crack pattern due to load 7 is shown in

Fig. 7 by dotted lines. Figure 10 shows the main crack

at load 7.

On reaching a load of approximately 149.000 lb. equivalent to Dead Load + 4.28 CSA Live Load, a wire was heard to snap, which resulted in a drop in jack pressure

corresponding to approximately QTLセPP lb. With continued

pumping up to 147,000 lb. another wire was heard to snap, which again resulted in drop of load of approximately

14,000 lb. The complete collapse of the beam occurred

at approximately 139,000 lb. or Dead Load

+

4.0 CSA Live

Load. The final failure was due to crushing of the concrete

in the top flange at the 31-ft. mark (as shown in Figs. 7,

11, and 12). Consequently, the maximum load carried by the

beam was 149,000 lb. (Dead Load

+

4.28 CSA Live Load). For possible future tests on concrete core samples, a 5-ft. section of the beam adjacent to the point of failure was removed and taken to the Division of

Building Research. An air drill and cutting torch were

used to free the required section. It was later noticed

that the ends of three prestressing wires in the removed section were not cut by a cutting torch, but resembled

the usual cup shape of a tension break. The snapping heard

at the end of the test was probably due to the failure of these wires, with the remote possibility that these wires failed when the beam was tumbled on its side after the test was completed.

E. Grouting

During the loading test it was observed that water from the interior of the beam was dripping from the

main crack at the 31-ft. mark. This started at the time

the crack appeared (load 5 of series B) and continued during load series C and D, but ceased during the intervening

periods of unloading. The amount of water was estimated to

be about half a mortar pail full.

After the beam had been loaded to failure the lower flange was broken up to expose the prestressing wires

and cable ducts. Examination showed that about a 12-ft.

length of the duct was incompletely grouted. At the location

of failure there was only a depth of about 3/4 in. of grout

in the 7.5-in. cable duct. It was from this void that the

water dripped during the test.

In view of the danger of corrosion of the wires and of freezing of the er.closed water during the wintertime the complete grouting of the wire ducts is extremely

(13)

important, especially in a cold-climate country. The presence of water in the duct of the test beam, which was the first beam made on this construction site, was,

however, due to special circumstances. The test beam had

been water cured (whereas subsequent beams were cured by spraying them with a surface film curing compound solution) and some of the curing water used for the test beam and also some rain water probably got into the ducts and flowed to its

lowest part at midspan. At the time of grouting of the

test beam the grout mixer and the pressure grouting pump had not yet arrived on the job so that only gravity grouting

could be done. Thus it was not possible to force the grout

through the duct from one end until it came out at the

other end. The grout had to be poured in at both ends until

it flowed out through the centre inspection hole. Although

it then seemed that the duct was completely filled, the cracking of the test beam revealed that some water and possibly air had been trapped, leading to approximately

12 feet of wtres not being covered by grout.

After the grouting of the test beam all beams

were grouted with the pressure grouting equipment. At the

request and in the presence of the owner a number of beams were drilled at different locations to check on the grouting

of the ducts. No voids were detected.

It is interesting to note that the large main crack at the 3l-ft. mark occurred in the zone of incomplete

grouting. The size of this crack and its distance from

adjacent cracks were larger than in the other zones, thus illustrating the effect of the bond between wires and concrete produced by the grout.

IV. Cor·1PARISON WITH DESIGN FIGURES AND CONCLUDING RErvlARKS

A comparison of the measured deflection and ultimate load with the design figures was possible through the kindness of Mr. P.A. Benn, Pre-Compressed Concrete Engineering Co. Ltd., who made the theoretical figures available for inclusion in this report.

The ultimate moment and the ultimate load have been determined according to Professor Magnel1 s theory as

outlined in his book "Prestressed Concrete", Concrete Publications Ltd., London,

1954,

page

85.

(14)

r.1 ult

=

=

214,000 ?\ b d 2 At

bd

b

=

width of top flange (inches)

d

=

distance between top fibre and centroid of wires (inches) At

=

Area of tension member (wires) (square inches)

A

=

0.2762164

=

3.84 in. 2

4

1\= 3.84 42 x 38 r.1ul t

=

214,000 x 3.84 x 42 x 38 2

=

31,200,000 in. lb. 42

x

38

The corresponding force at each jack is: MUI t - MD.\'1.

P ;

a

=

セQLRPPLPPPセ 29.25 x 12- 5,000,000-- -

=

74 500 Ib

,

.

The total load (two jacks) is:

2P

=

149,000 lb.

This represents Dead Load

+

4.29 CSA Live Load. This

theoretical figure, derived from Professor r.1agnel's empirical formula, is very close to the test results

(Dead Load

+

4.28 CSA Live Load). This formula has been

developed from the result of tests on twenty-two beams,

differing widely in size and percentage of steel. In

Professor Magnel's paper in the Proceedings of the Canadian Conference on Prestressed Concrete, Toronto, 1954, page 8, Figure 9, the results of these tests have been plotted

together with the function Mult

I

bd 2, showing that many results fall close to the line of the function.

(15)

This report, after a brief description of the design and construction of the 75-foot prestressed concrete beams used in one of the two Sussex Street bridges, covers mainly the loadings applied to the beam, the resulting deflections, the cracks observed, and some general

observations.

It is hoped that this "case record" will be of value to those interested in prestressed concrete.

(16)

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(18)

0 0-100 0·'200 0·300 (p W :r 0·400 u z 0 ..50 0 z

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(19)

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(20)

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(21)

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(22)

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(23)

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(24)

(Photograph courtesy City of ottawa)

(25)

pump (on the left)

Fig. 10 Main crack in beam at Load No.7

(west side)

(Photograph courtesy City of Ottawa)

(26)

Fig. 12 flange.

Beam after compression failure in top Note base of jack, hinge, and bearing

plate

(PhotographR courtesy City of Ottawa)

Figure

Fig. 10 Main crack in beam at Load No.7 (west side)
Fig. 12 flange.

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