final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 31
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Example of insufficient attention paid to stability during intermediate stages of erection of propped shuttering. There is a risk of
collapse of the incomplete and unloaded falsework by the failure to provide enough well-lined, plumbed and laced props from
the outset
from their correct position. Our own observations
and the reports of others confirm that these figures
are not exaggerated. These deviations could result in
a significant reduction in load carrying capacity: for
example the capacity of a prop 2·59 metres long
inclined at 1 ½ 0 to the vertical may in certain circumstances be reduced by 27 %We were also shown pictures of bearers placed
eccentrically in forkheads, and of base plates eccentrically positioned on sleeper supports. In one extreme
case, illustrated in our interim report, a vertical
support was mounted on the cantilevered end of a
timber beam some four feet away from the lower
standard to which its load was to be transferred. The
overturning moment exerted by the imposed load
would have been sufficient to cause the collapse of a
whole line of falsework had not the defect been
corrected under threat of legal sanctions.
Similarly the inevitable variability of large trusses
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and the difficulty of positioning supporting trestles in
exactly the right place lead to slight discrepancies
and the need for local matching arrangements.
All these effects lead to eccentricity of loading which
could have critical results on the strength of struts
or grillages. The large number of points at which the
load is applied multiplies the possible sources of
failure. We found no evidence, however, that designers
made any specific allowance for possible inaccuracies
in construction other than that incorporated in their
standard loading tables. They have in fact generally
and implicitly assumed that all loads would be applied
concentrically or used factors of safety which presumed deviations less than those which might actually
occur in practice.
Nor were we told that construction drawings always
specified realistic tolerances or drew attention to
areas where accuracy was essential.
