Cobourg Beam Test: Third Progress Report

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Technical Note (National Research Council of Canada. Division of Building Research), 1953-07-08

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Cobourg Beam Test: Third Progress Report

Schriever, W. R.

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DIVISION OF BUILDING RESEARCH

NATIONAL RESEARCH COUNCIL OF CANADA

r

TEClHIN lCAL

NOT FOR PUBLICATION

NOTE

No.

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15

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FOR INTERNAL USE

PREPARED BY

W.R. Schriever CHECKED BY APPROVED BY R.F.L.

PREPARED FOR

SUBJECT

Cobourg Beam Test Third Progress Report

Jul y 8, 1953.

As the initial loading phase of the prestressed concrete beam test in Cobourg, Ontario, 1s now completed and the beam is now under the I'sustained load"; it seems appropriate to peview briefly what has been done so far and present the revised loading 'schedule and the proposed dates for the remainder of the test

phases.

After a great deal of time and effort had been spent on the preparation of the test equipment, the first load was applied on'the beam on Tuesday, June 30, 1953. For this, a crew of eight members of the staff of the Division of Building Research were brought to Cobourg and were assisted for one and a half days by Mr. R.B. Martin of the Ontario Hydro. All equipment performed

satisfactorily, so that all loadings prior to and including the application of sustained load were completed by the end of the week. Slight difficulties were encountered with some of the pumps operating the hydraulic jacks, because of some leakage, requiring almost constant attention when holding a load. For the 24-hour test and the sustained load test the lock nuts pro-vided on the pistons of the jacks are used to overcome this

difficulty. Then every half hour the pressure was brought up to the required value, the lock nuts tightened.and deflections and strain gauge readings taken.

Under dead load plus I 1/2 live load the maximum deflec-tions found were approximately 0.25 inch in one span and 0.3

inch in the other. No cracks became apparent. The complete re-sults of the measurements made will be presented, after, they have been analyzed, in a report to ,C.M.H.C.

After sending out the second Technical Note (No. ISO) on the test, a number of comments were received on the proposed loading schedule, notably from C.C. Parker and Associates,

P.A. Benn, ｐｲ･ｾｃｯｭｰｲ･ｳｳ･､ Concrete Engineering·Company and the staff of the OntaPio Hydro. All comments 'expressed the view that there was some danger of damaging the beam prior to the sustained lOad test by loading it to dead load plus 2 1/2 live load

as proposed for balanced and unbalanced loading. In ｣ｯｮｳｵｬｴ｡ｴｩｯｾ

with R.T, Sheppard of C, IVi .. H.Cq R.B. Martin cf the Ontario

..

..

-

.

- 2

-and P .A. Benn, it was therefore .decided to change the originan..y

proposed loading schedule to the following:

1. Dead load of roof (approx. 7780 lb/purlin).

2; Dead load + live load (8320 lb/purlin, total l6l00lb/purlin). Recovery period.

3. Dead load + live load {repetition).

4. Dead load + 1 1/2 live load (National Building Code requirement). Held for 24 hours. Recovery period.

5. Repeat 24-hour test if recovery less than 75% in 24-hour recovery period. This proved unneces-sary as immediate recovery was approximately 80%, after 3 hours apprOXimately 95%.

6. Span A: dead load + live load. Span B: dead load. (unbalanced ｬｯ｡､ｩｮｧＩｾ

7. Span A: dead load. Span B: dead load + live load. (unbalanced loading).

8.

( Proposal( only ( ( ( ( (

9. Dead load + live load (as check on previous same loading) •

10. Span A: dead load. Span B: dead load + 1 1/2 live load (unbalanced loading).

11. Span A: Dead load + 1 1/2 live load. Span B: de ad load (unbalanced loading).

12. Dead load + 2 1/2 live load (for observation of self-closing of cracks after unloading).

13. Loading to failure.

14. Testing of purlin seats.

Mr. Benn had pointed out that the beam had only been designed for 20 lb/sq. ft. unbalanced loading and that therefore the originally propose d loading steps 7, 8 and 9 were "very severe and sufficiently high to possibly develop some cracking which might cause misleading results in the latest stages of the test". It was therefore decider} to carry the unbalanced loading only to dead load + live load on one span and to dead load on the other. It is proposed to carry out an unbalanced loading test to 1 1/2 live load following the sustained load test"

3

-.

Regarding the loading to be used in the sustaineq load test, there was a general feeling that dead load + 2 1/2 ｾｬｶ･

load might be too high and a value of dead load + 2 live load was first suggested. Then Mr. Benn pointed out that even this load, which is unlikely to occur for 30 days in practice, might cause permanent elongation of the prestressed wires and cracking and in this way reduce the precompression of the concrete and the ultimate load at failure. Furthermore a loading to dead load + 1 1/2 live 10a0 would have the advantage of providing a direct comparison with the previous 24-hour test with the same loading. This was therefore·" adopte d.

In the last Technical Note it had been proposed to hold the sustained load for 30 days. In order to be able to complete the test to failure before a weeKend &to allow a recovery period of

5

July 3 - sustained load applied.

July 31 - sustained load removed.

July 31 to August

5 -

August

_{5 -}

schedule. August 6 - loading to failure.

August ₇ to 29 - tests of purlin seats and clean-up. It is proposed to allow a limited number of visitors for the final test to failure on August 6. Central Mortgage and Housing Corporation will inform a number of universities, Govern-ment agencies, etc., of the test but must reserve the right to