final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 94
in calculations
There was produced in evidence a design sheet which was made by the Assistant
Engineer and checked by the Field Engineer. This sheet contained the calculations
for the design of false bent N4 and two errors appeared on it.
Errors
The sheet bore the title "design of caps and distributing beams using 36WF160
beams between pairs of bents". Under the heading "CHECK SHEAR", it
appeared that in determining the shear stress the area had been taken as the
gross area of the whole beam including flanges and webs (47·09 sq in) instead
of the gross area of the webs (23·5 sq in) as is required by accepted elastic
theory and all design specifications. The shear stress had therefore been wrongly
calculated to be 6 ksi instead of 12 ksi. If 6 ksi had actually been the shear
stress, it might have justified the use of the adopted beams without stiffening,
as was the conclusion recorded on the calculation sheet; but this would not
have been justified if the correct shear stress had been calculated. Stiffeners
and diaphragms would then have been provided, and the accident would not
have occurred. There is a second error on the same sheet, under the heading
"CHECK FOR WEB STIFFENERS", in which the flange thickness (1 in)
has been used instead of the web thickness (0·653 in). A correct calculation
would have called attention to the high general stress level in these beams even
though they would still have been permissible in this case.
The first of these two errors was not detected by either the Field Engineer who
checked the sheet, or his Assistant who prepared it. The second was detected
and a pencilled note was made crossing out the figure "1" and substituting the
figure "653". Nobody knew definitely who made that note but whoever did so
did not make a recalculation on the basis of the corrected figure.
It appears that the decision to use the four WF160 stringers in the upper grillage
without stiffening, as shown on the sheet, was not reviewed or checked by anyone
except the Field Engineer.
The upper grillage was subsequently placed in bent N4 without stiffening.
Comment on the use of plywood
If the upper tier had been fitted with stiffeners and effective diaphragms the
plywood would not have interfered with the intended performance of the
grillage.
However, the arrangement of the grillage was such that the pressure on the
plywood was about 1340 lb per square inch under the west leg of bent N4. This
pressure was beyond the elastic range of the material. If plywood were to be
used as soft packing the grillage should have been arranged so as to keep the
pressure on the plywood well within its elastic range. This would have avoided
the "creep" in the plywood and would have mitigated the undesirably high
lateral bending stress in the flanges of the upper grillage beams.
Other major points extracted from the inquiry documents
(a) The Consulting Engineers had a reputation of being "a firm of first class
calibre in the matter of building major bridges". The Contractors had a
high reputation, had been carrying out business throughout Canada for
75 years, employed highly qualified engineers and had built many large
and difficult bridges.
(b) The Engineers prepared design drawings and specification which were
examined generally by the Bridge Division of the Department of Highways
of the Province of British Columbia and found to be satisfactory.
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