final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 115
No calculations were available from the contractor in respect of the buckling
and twisting effect of the thin webs of the grillage beams.
Although the bracing was typical of the standard found generally in falsework
it was not sufficient to ensure the high standard of stability necessary to cater
for the horizontal and dynamic forces likely to occur. It was not however
considered that this caused the collapse.
Although the trusses buckled and twisted, they had withstood the effects of
the collapse very well except for the weld failure in one of the top boom connections and the detachment of one of the diagonal tubular members.
Conclusions (All opinions are those of HM Factory Inspectorate)
It may never be possible to establish the precise order of events in the collapse.
The fact that only part of the final load was sufficient to cause the collapse is
significant and indicates defective construction or inadequate strength. On the
basis of the evidence so far established, defects in the grillage and its immediate
supports probably led to the successive failure of parts of the grillage as the
load was applied. If the grillage had started to collapse, the trusses would have
bowed and buckled and become displaced. The complete collapse of the
structure would then have been inevitable. The factor of safety of 1 ·3 in the
grillage, which was revealed by tests carried out after the accident, was too low
when the possiblity of horizontal forces being applied under site conditions is
taken into account.
The sliding of the falsework from under the completed southern carriageway
to the site of the northern carriageway was not undertaken with sufficient
care and weakened the structure. The final examination of the falsework, before
pouring commenced, was also open to criticism.
The deficiencies enumerated below indicate that insufficient consideration had
been given to the design and construction of this falsework and that the combined effect of these deficiencies reduced the theoretical overall factor of safety
below that acceptable for such temporary structures in which there are many
unknown factors relating to design and construction:
(a) The clearance between the trestles and the adjacent concrete piers was
appreciable. It is considered that the trestles on one side of the span
should have been positively anchored to the adjacent pier and movement
at the opposite side restricted.
(b) The damaged legs of the trestles were straightened and re-used instead of
being replaced.
(c) Many of the bolts connecting the component parts of the trestles were
missing and others were not sufficiently long to accommodate a full depth
of nut.
(d) There were no stiffeners fitted to the thin webs of the 10 in x 10 in x 49 lb
universal coloumns and 12 in x 6½ in x 31 lb universal beams of the
mild steel grillage assemblies supporting the trusses. These were subject
to considerable buckling and twisting loads.
It is considered significant that in the only place where a web stiffener
was fitted (because slight damage had occurred before use to one of the
10 in X 10 in x 49 lb universal columns) no distortion of the adjacent
web or flanges occurred.
(e) The ends of some of the 12 in x 6½ inx 31 lb universal beams had been
tapered by flame cutting, so reducing the effective web areas by approximately 30 per cent at points of appreciable loading.
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