final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 127
joists supported in the forkheads to carry the timber formwork.
Indumat trestles were used on the second bridge, with similar supports for
the timber formwork.
Defects
Structure Absence of plan bracing in the trestlex and indumat trestles.
Forkheads were extended above 15 in in some cases but no bracing was provided. It was claimed that reduction in the safe load of the trestles allowed for
this.
Trestlex trestles were not adequately braced together.
Trestles were free standing, i.e. no tying back to a strong point to provide
stability.
Case 3
A reinforced concrete arched bridge with clear span of 130 ft and a height of
40 ft above the river, was being built to carry a single carriageway on a deck
46 ft wide.
The falsework consisted of Mills V 800 type trussed beams of 40 ft span, which
were supported on a centrally placed millshore tower and radial raking shores
at the abutments thereby creating a two span structure. Curved timber formwork panels rested on the falsework structure.
Defects
(a) Foundations This supported the raking shores and the birdcage scaffolding
which formed the end support of the falsework. It was heavily fissured and
weathered. Upstream of the central tower, a temporary breakwater had been
constructed but no protection had been provided for the tipped rock base
support for the north side raking shores.
(b) Structure The intention was to commence concrete pouring operations with
only one set of trussed beams erected between the central tower and one
abutment. This was not as the designer intended and might have had serious
consequences. Many loose couplers were found and there was a lack of
bracing in the raking shores.
The central towers were not diagonally braced together.
Case 4
The first-floor beams, 23 in x 21 in· for a multi-storey concrete car park with
clear span of 50 ft were under construction.
The falsework consisted of timber boxes carried on a two tier system of
6 in X 3 in timber bearers which in turn were carried on adjustable steel props
spaced on a 4 ft grid. The beams in the lower tier were nailed to the flat heads
of the props.
Defects
(a) Structure Most of the props were out of plumb and were not in line under
the beams. No ties or bracing between the props were provided.
(b) Formwork In some cases the nails were only partly driven into the timber
bearers. Generally the 6 in X 3 in bearers butted together over each prop
with a total of four nails provided, but there were instances where the
bearers lapped and thus it was not possible to nail them correctly to the
props.
Case 5
The first floor of a multi-storey concrete library was under construction.
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