final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 109
heated and straigh tened previously in the same location where the buckling
had occ urred.
The investigation team believes the weakened diagonal brace, in not preventing
the tower collapse, contributed to the complete failure.
(f) Inadequate foundations
(g) Inadequate cross bracing between the steel girders
The 70-foot span of the steel girders was so great that they would have buckled
laterally if they had been unbraced. Because of this, 4 in x 4 in wood cross
bracing had been installed which would transmit compressive forces but not
tension forces. This bracing, by itself, would permit all the girders to buckle
northward simultaneously were it not for the resistance provided by the wood
decking which lay across the tops of the girders, but was not fastened to them.
This bracing system, because of the lack of positive connections, would not be
very effective after collapse had progressed to a certain point and would
not have provided much resistance against complete collapse. Inadequacies
of the bracing systems, however, did not cause the collapse.
Possible factors initiating collapse
Snap-ties
Two types of ties were used on this bridge, both having equal ultimate strengths.
It was discovered, however, that the factor of safety between the prescribed
working load of 5000 pounds and the failure load was very small; The average
failure strength was 6400 pounds and the lowest of 11 tests was 5240 pounds.
A hazard of working form ties at so near their ultimate strength is that if one
tie should fail, for whatever reason, the adjacent ties do not have the reserve
capacity to prevent the failure from propagating for the full length along which
fresh concrete is exerting fluid pressure.
It was believed that this low factor of safety could have been a factor in initiating
the collapse.
Addition of two unrelated forces
One member of the investigating team believed that the total evidence available
showed that at least two unrelated forces were being added together to cause
the initial failure. The lateral force produced by the wet concrete in the forms
which had not failed and the bowing and leaning of the beams with 48 inch
webs both exerted a force to push the 36 inch beams off their 15 inch stack of
blocks.
On other parts of the bridge falsework the cross bracing to the girders was
placed further apart than called for in the plans. The 4 in x 4 in cross braces
were usually loose - they probably had not been cut to the proper length- and
often were not very effective.
These unrelated effects appear first to have caused form separation and then
to have pushed the 36 inch beams off the blocks, so leading to the total collapse
of these spans.
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