final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 121
(b) Insufficient bearing of the spade ends of the adjustable centres on the
formwork
(c) Absence of bracing or ties between the props
(d) Out of plumbness of some of the props.
Case 6
A single storey telephone exchange, 75 ftx46 ftx 17 ft high was being built of
in-situ concrete. The roof slab was 12 in thick, the whole being composed of
hollow blocks and in-situ topping, which were supported on in-situ concrete
beams.
The timber boxes to form the beams were supported on a double line of
adjustable props fitted with adjustable forkheads, and base plates. Adjustable
centres 12 in apart spanned on to the sides of the timber boxes, and supported
the plywood lining for the deck slab. The falsework was founded on timber sole
plates which rested on the 12 in thick ground floor slab. The props were laced
together and tied back to the concrete building columns. No drawings or
calculations were prepared for the falsework.
The whole of the concrete roof was to be poured by pumping. A bay about 24 ft
square had been completed and collapsed as it was being screeded level.
The cause of the collapse was the result of inadequate bearing of the adjustable
centres onto the timber formwork. The spade ends of the centres at one side
of the roof rested on two thicknesses of timber, each 1 ½ in thick, which were
inadequately nailed to the timber boxes. One of these timbers was displaced
thereby resulting in the spade ends bearing· on a single 1¼ in thickness. This
timber crushed under the load and released the centres at one end of the span.
Case 7
The opening in a factory floor, measuring 28 ftx 14 ft was being filled in with
in-situ concrete 9 in thick.
The plywood lining was supported on adjustable centres 1 ft 6 in apart, which
in turn rested at one side of the opening on a 5 in x 2 in timber bearer which
spanned 8 ft between the existing rolled floor joists.
When most of the concrete floor had been cast, one of the 5 in X 2 in timber
bearers broke in the middle and the floor collapsed.
No calculations or drawings had been done for this work. The general foreman
decided on the size of the falsework members. Check calculations showed that
the 5 in x 2 in timber bearers were grossly overloaded.
Case 8
The roof of a two storey factory building of in-situ concrete, measuring 30 ft
x 25 ft was under construction. It was 15 in thick.
The falsework consisted of three lines of scaffold tubes, the outer lines being
of single tubes spaced 4 feet apart and the centre line being of twin tubes also
spaced 4 feet apart. All the scaffold tubes were fitted with adjustable forkheads
and base plates. Timber bearers rested in the forkheads to support the timber
boxes for the centre roof beam. At the two sides of the roof, the timber bearers
sat in the forkheads but were also bolted to the concrete walls of the building.
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