final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 126
Wh en about half the slab had been poured the entire support system collapsed.
It was noted at the time that concrete had broken away from the edge of the
beam for a distance of 1 ½ in. Also the toe of the bracket which had rested on
the concrete wall was bent upwards at an angle of 60° to the horizontal.
This collapse was attributed to the failure of the concrete under the bracket
seating. It had failed under a load much lower than the safe working load of
the bracket as given in the manufacturer's trade literature. Clearly the strength
of wall brackets of this type is dependent upon the bearing surfaces by which it
is supported. The manufacturers have since co-operated by drawing attention
to this fact and have also de-rated the safe working load.
Case 20
A three storey hospital, 130 ft long X 56 ft wide was being built. The structure
consisted of steel columns with precast concrete floor and roof beams running
only in the logitudinal direction. Steel tie beams were provided in the transverse
direction as temporary supports.
This building relied on the completed floors for its stability and thus while the
frame was being erected falsework was necessary to provide temporary support
until the permanent structure was self-supporting. This falsework should have
been in the form of guy ropes or temporary bracing but none was provided and
only a few of the tie beams were used.
Collapse occurred when all the framework had been erected and a start had
been made on laying the precast concrete floor units. The wind at the time was
moderate.
Collapse was the result of overall instability because of the lack of temporary
bracing or guys. The only resistance to sway forces was provided by the few
ties connected to the beams, and this was inadequate.
Details of some inspections carried out by HMFI following
the Loddon falsework collapse
Case 1
A road bridge was being built to span a river. It had a prestressed concrete deck
slab 4 ft deep, supported on in-situ concrete columns on dry land.
The falsework on dry land consisted of millshore frames supporting conventional timber formwork, that over the river consisted of auto-fab girders which
supported transverse universal beams to carry the timber formwork.
Defects
(a) Foundations Base of millshore supports placed on edge of timber sleepers.
Timber sleepers not placed horizontally in some cases.
(b) Structure Some millshore support frames were tilted. Diagonal braces on
frames were not properly connected.. Adjustable forkheads not properly
used in some cases.
(c) Formwork Timbers were not properly secured together.
Case 2
Two motorway bridges were being built with prestressed concrete deck slabs
2 ft 9 in deep supported on concrete piers.
The falsework on one bridge consisted of trestlex frames with rolled steel
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