final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 101
General desc ription of the falsework
The falsework for spans PT3S was a nailed and bolted timber structure resting
on concrete footings. At the top it supported steel beams, which in tum supported
the formwork for the permanent concrete bridge. The timber structure consisted essentially of vertical posts and longitudinal and transverse horizontal
bracing, designed to sustain the vertical and horizontal loads imposed on the
structure during the construction of the permanent works. The general arrangement of the falsework is shown on page 101.
The failure
The failure occurred while concrete was being placed oh section PT3S on 10
August 1966. Workmen were killed and injured as a result of the collapse.
At the time of collapse, the bottom slab had been in place over the entire area
of PT3N and PT3S for about a month. Concrete was being placed for the webs,
diaphragms and deck slab on the easterly half of PT3S. The pour had been
started at midspan at approximately 7 .30 a.m. and concreting had progressed
to a point some 15 to 20 feet east of pier S5.
The cause of the failure
The investigating engineers concluded that the prime cause of the failure was
an error in the design of the falsework, resulting in a lack of adequate bracing.
The basic cause of the collapse of the structure was a buckling failure of the
falsework which was insufficiently braced in the longitudinal direction. The
occurrence of the failure at that particular time may have been influenced by
one or a combination of the following secondary factors:
differential settlement of footings,
temporary overloading of posts of transverse bent 23,
a possible material weakness of some highly loaded member of the falsework.
However, had the falsework been sufficiently braced, these secondary factors
would not have caused failure of the structure.
The design adopted for the temporary falseworks was not adequate and not
suitable for the project, since it was erroneously based on the assumption that
the permanent piers would give adequate longitudinal support to all portions
of the falsework structure.
The nature, extent and standard of supervision and inspection of the construction
of the temporary falsework was inadequate.
The investigating team reporting on the falsework considered that the timber
posts, transverse bracing and some of the footings did not provide adequate
factors of safety.
Other areas of investigation
The following parts were investigated and found to be covered adequately:
(a) the terms and conditions upon which the engineering services were
obtained
(b) the design, specification and drawings of the bridge project (the design
of the bridge piers had not included the horizontal loads applied to them
by the falsework). The specification stated that "the Contractor shall
submit detailed plans of the proposed falsework to the Engineer", and
that "notwithstanding approval, falsework remains the responsibility of
the Contractor". The job records show that this procedure was adhered
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