final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 120
Case 3
The first floor of a building on a power station site under construction. The
floor was 18 in thick and was about 10 ft above ground level.
The formwork for the floor slab was supported on lightweight adjustable
centres 12 in apart, which in turn were supported on 5 in x 2½ in rolled steel
joists. The latter were carried on shoreload frames spaced 6 ft apart, by means
of cross head 5 in x 2½ in RSJ bearers which sat in the forkheads of the shoreload frames. Very little bracing was provided between the frames and the
forkheads which extended 18 in were not braced. In addition the 5 in X 2½ in
RSJ main bearers were eccentric in the forkheads.
As the pouring of the floor was being completed the decking caved in and the
floor collapsed, as the frames distorted.
The cause of the collapse was thought to be overloading of the shoreload
frames, as a result of eccentricity at the forkheads and the absence of bracings
or ties.
Case 4
The concrete roof of an assembly hall 102 ft long x 76 ft wide, for a new
school, was under construction. The roof was 17 in thick, of cellular construction and supported on brick walls.
Single adjustable props on a 6 ft x 4 ft grid supported the form work for the
roof deck. Since the height of the strutting required exceeded 16 ft, the flat
heads of the props were connected by rolled steel joists which formed the base
for another storey of props. Rolled steel joists also provided support for the
roof deck. Scaffold tubes at three levels connected the props together and there
was also some diagonal bracing. The props were fitted with base plates which
rested on the concrete ground slab.
Just after about-half of the roof slab had been concreted, the falsework moved
sideways and then collapsed.
It was thought that instability of the props was the prime cause of the accident.
These were inordinately long, split at the middle, and out of plumb. Furthermore there was an absence of diagonal bracing in two directions to compensate
for the break in the props.
Case 5
The roof of a two storey reinforced concrete factory building, 40 ft x 32 ft
in plan was under construction. The roof was 11 in thick, the two bay sizes
being 40 ft x 15 ft, and the height from first floor to roof was 12 ft.
The timber formwork for the concrete beams composed of three layers of
bearers was supported on pairs of adjustable props 4 ft apart. Adjustable
centres to support the ply formwork at 16 in spacing were carried on the sides
of the timber boxes for the roof beams. The bases of the props were founded
on the concrete first floor slab.
During the pouring of the second bay, the roof collapsed.
The cause of the collapse was not certain as the props were not overloaded
under direct loads, but the following items of poor construction were considered
to have contributed to the accident:
(a) Defective timber in the lower layer of the formwork
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