final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 123
The forkheads, which carried the timber bearers for the formwork, were extended to 18 in in many cases and were· unbraced. Adjustable centres spanned
on to the timber bearers to give support for the curved plywood lining.
Owing to the large areas involved the concreting was done in segments working
from the centre downwards towards the edges of the circular tank. Two segments had been successfully poured and a third was two-thirds complete when
the falsework moved sideways and then collapsed.
The cause of collapse was attributed to the instability of the towers which were
30 ft high, unbraced and untied. They probably moved sideways under horizontal loading, thereby releasing the adjustable centres from their 2 in seatings
on the timber bearers. This falsework was very poorly constructed, despite the
fact that a drawing had been produced which showed the towers tied to the
concrete columns and properly braced and that forkhead bracing was detailed. In
addition the timber was of poor quality and some bearers were eccentric in
the forkheads.
Case 12
The first floor of a multi-storey hotel was being concreted when the accident
occurred.
The falsework consisted of plywood lining carried on 9 in x 3 in timber bearers
at 16 in centres, the bearers in turn being supported by timber beams. These
beams were supported on adjustable props on a grid 22 ft x 8 ft 6 in.
One bay of the floor had been successfully poured and another was almost
completed when it collapsed.
It was not certain what had caused the collapse, but the load on each prop was
nearly 3 tons and because of slenderness they were probably grossly overloaded.
The spacing of the props was certainly excessive.
Case 13
The first floor of a multi-storey supermarket in a shopping centre was being
built. It was of 'Kaiser' construction. This consisted of 2 ft deep ribs with
'honeycomb' pattern slabs between, the floor slab being 4 in thick and the
pattern effect being formed by the use of domed fibreglass formers. Each panel
of flooring was approximately 35 ft square.
Timber bearers spanning two ways supported the ply lining. These bearers
were generally supported on adjustable props in a diamond pattern in plan, but
in some areas the props were arranged in a conventional manner i.e, in rectangular panels. The props were generally on a 4 ft grid and were laced together,
but were neither tied back to a firm anchorage nor diagonally braced. They were
fitted with flat ends top and bottom.
After an area of floor 50 ft x 30 ft had been cast, it folded in the middle and
collapsed.
The failure occurred where the diamond pattern and rectangular panels were
adjacent but not connected. The absence of bracing and tying, in conjunction
with the heavy loading in this area from the 2 ft deep ribs, were thought to be
main cause of collapse.
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