final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 124
Case 14
A motorway bridge over a river was under construction. The bridge was of
in-situ concrete spanning 100 ft on to concrete abutments.
Lattice type girders carried on steel trestles in groups of four, which were founded
in the river, provided temporary support for the wet concrete. The foundations
for the trestles consisted of precast concrete rings filled with mass concrete.
This was only a partial failure in that some of the concrete rings had been
washed away or moved by flood water. Remedial work had been completed
by bracing groups of trestles together, sand-bagging scoured areas, and providing temporary foundations to replace those that had disappeared.
The cause of this partial failure was attributed to the designers underestimating
the effect of flood water, in an area where it was likely to occur. The remedial
work was not satisfactory and the reconstruction of the foundations entailed
removing part of the falsework, concreting over the river bed to prevent further
scouring and rebuilding the foundations.
Case 15
In an in-situ concrete sub-station, 60 ft long x 35 ft wide, the 6 in second floor
slab was under construction.
Adjustable steel centres 2 ft apart, supported on a centre row of props and on
the concrete walls of the sub-station at their ends, carried the ply lining for the
floor slab. The centres spanned 10 ft between the end wall and the props. They
rested on timber bearers at the prop positions, the bearers being spiked in places
to the flat heads of the props. The props were founded on the first floor of the
building some 16 ft below the second floor, and were laced together in the
longitudinal direction.
The area of floor which collapsed was 23 ft X 20 ft.
An accumulation of defects was thought to be the cause of this collapse. These
included: props not vertical; timber bearers not fixed to flat heads of props in
many cases; and absence of diagonal bracing. In addition there were large
bundles of reinforcement placed on the formwork during concreting operations,
imposing heavy loading on the falsework.
Case 16
The first floor of a water treatment plant building was under construction. It
consisted of 9 in thick slabs in bays 16 ft x 14 ft.
The ply lining to the formwork was carried on adjustable centres 2 ft apart
and supported on adjustable props, These centres spanned 14 ft and were
supported on timber bearers which rested on the flat heads of the props. The
height of the props was 13 ft 6 in. There was no bracing or ties between the
props.
One bay had been completed and two-thirds of the second when the whole area
of floor collapsed.
It is most probable that the centre row of props was overloaded. This was the
result of mis-alignment of the props, and lack of diagonal bracing. Another
factor may have been the low resistance of the timber bearers to tipping.
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