final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 108
At this point the outermost 36-inch steel girder must have slipped off the wood
block which was supporting the east end and dropped the 15 inches to the
steel cap.
These events would have resulted in a large outward thrusting force being
applied to the cap beam of bent number 10 at the girder's eastern end.
The tower bent was on a 45 degree skew to the stringer girder so that the outward thrusting force coming into it would tend to rotate· the bent clockwise.
There is some flexibility in the sixty-foot high towers and it is probable, that
as the rotation started, the joint in the steel cap of bent 10 (which had no
provision to resist tensile forces) started to open. At the same time the lowest
diagonal brace in the outer tower frame buckled in compression and the outermost tower leg folded under.
Evidence indicated that continued rotation caused complete collapse of the two
outer towers.
It is possible that a few workmen noticed the original failure but were not
greatly concerned because stem form failures are not uncommon in the USA
and in fact minor ones had occurred during pouring earlier in the day.
Other possible causes investigated but not found to be responsible
(a) Earth tremors
(b) Wrong location of shoring members
(c) Sabotage
(d) Excess stresses in structural steel
(e) Weakness in steel towers
A portion of the collapsed member was removed and tested. On cutting open
the member, which is a hollow rectangular tube, it was apparent that it had been
'·
,•:
Loosely laid planks to
the working platform
Timber blocks-supporting steel
stringers
Heavy duty steel towers -
Steel cap beam -
-Stringer girders
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