final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 122
Adjustable steel centres 12 in apart spanned on to the centre formwork boxes
and the outer bearers to carry the ply lining. Only a few bracing members for
the whole of the falsework were found in the wreckage.
When approximately 75 % of the area had been cast the roof collapsed.
No calculations were done for the falsework by the contractors, but a check
made later revealed that the standards would have been 25 % overloaded even
if properly braced. The lack of bracing however meant that their slenderness
was increased and that there was a 60 % overload. In addition some of the timber
bearers were overstressed.
Case 9
The roof slab of a single storey concrete factory building 60 ft O in x 30 ft O in
x 12 ft O in high was under construction. It was 6 in thick.
The falsework consisted of timber runners 16 in apart supported on beams at
their ends. These beams were carried in metal hanging brackets and were also
supported by props at their intermediate positions.
When most of the roof was poured it collapsed.
The hangers opened out under load allowing the end beams to loose thei
bearing and thus fall.
Case 10
The construction of a multi-storey car park was a mixture of precast and
in-situ reinforced concrete. The columns and edge beams were cast in-situ and
the main span beams, at 5 ft centres were precast. Between the main beams
were laid short precast in-fill panels to support the in-situ floor deck.
The in-situ and precast beams were supported on twin adjustable props 2 ft
apart and spaced at intervals ranging from 3 ft and 5 ft. Along two sides of
the building the main precast beams rested on the timber formwork for the
in-situ beams. The props had flat heads and the timber bearers sat eccentrically
on these. Furthermore the distribution of loading from the heavy pre-cast
beams was unbalanced due to the way the falsework was arranged. In addition
there was a lack of bracing and generally the falsework was of poor construction,
evidenced by the number of makeshift pins and split timbers that were used.
The accident occurred when one of the side shutters to the in-situ concrete
edge beam overturned causing a progressive collapse of four of the main precast beams.
The collapse can be attributed to eccentric loading on the formwork to the edge
beam coupled with a generally very poor standard of construction.
Case 11
The domed roof of a concrete reservoir 240 ft diameter x 30 ft high was under
construction. It was 9 in thick and supported on concrete columns.
The falsework was composed of towers made up from scaffold tubes in 16 ft x
4 ft units. These towers were fitted with base plates and adjustable forkheads,
and were about 30 ft high. They were not tied back to the concrete columns,
nor were they braced in any way, but merely connected together by ledgers and
transoms.
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