final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 23
Progressive collapse
Consideration of interconnection leads to the associated problem of progressive collapse. United States
authorities responsible for bridge and road construction were emphatic about the risks of collapses
seriatim of the elements in long falsework structures.
In the United Kingdom we have examples of progressive collapse initiated by the failure of an intermediate
section due in one case to river scour, in another to
the giving way of a bricked culvert used as a foundation, in others to road traffic damage or failure of an
arch upon which reliance had been placed. This
disastrous form of collapse may be averted by designing the several sections of the falsework to be
independent self-supporting structures, using lacing,
bracing and diagonal support in longitudinal and
transverse planes. A similar philosophy was applied
in the case of the Tasman Bridge at Hobart so that it
did not collapse completely when two piers were
knocked down by a ship in January 1975.
emptying of a skip of concrete in one place causes an
impact load and a high local static load until the concrete is spread. If placing is started at one end of a
bridge the deflections and possibly the torsions
applied to the falsework are quite different from those
obtaining when placing is started in the centre.
The designer may have assumed that loading would
produce progressive stressing whereas the method
actually employed may be conditioned by the jib
length of the crane handling the skips of concrete, so
producing maximum deflection at an early stage.
In one instance we suspected that the deflection of a
supporting beam in the early stages of a pour produced
a buckling torsion on a supporting trestle leading to
a serious collapse. There are many ways in which the
method of applying the load imposes a temporary
local overload which cannot immediately be shared
by other elements. The same effect may be produced
by local displacement of base supports during loading.
Overloads
We can distinguish three specific categories of over-
stress. In the first category the falsework is inadequate
for the design load. This seems the commonest type
of failure in small jobs. Often no proper design has
been done. For example we mentioned in our interim
report a case where a Sin x 2in timber beam, part of
the falsework enabling an opening in a factory floor
to be filled, failed and a man underneath was killed.
Once again we must emphasize that many accidents
occur on small sites on apparently simple jobs where
the standard of skill of those involved may be insufficient to enable them to produce an adequate design.
We have also seen instances where a genuine mistake
was made in calculation or where no check was made
on the ability of a particular member to withstand a
particular stress. In both cases the design was inadequate for the loads specified in the design brief.
In the second category the design is adequate to
support the loads assumed, but the loads actually
applied are different. For example, the plant in use
may be heavier than was catered for. Another example
concerns the support work for materials of unknown
density such as rubble-filled walls, brickwork or
masonry, where special allowances should be made for
possible inaccuracies of load assessment.
In the third category the way the load is actually
applied may not have been properly allowed for in
the design, or alternatively the load was not applied
in the way the designer specified. For example the
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Finally, local overloading can occur because the
designer, perhaps ignorant of actual site practice,
assumed a perfection in construction which was
impossible to realise. This whole subject of tolerances
and eccentricities is, however contained in a separate
section in our report.
Inadequate foundations
The evidence we received indicated that it was very
important to establish beforehand the true nature of
the foundations on which the falsew~rk is to be
constructed. The client does not always pass on full
details of the nature of the ground on which the falsework is to be founded, or even the soil analyses on
which the permanent works were based. We heard of
a case where a falsework designer was not told that
there was to be a deep excavation for a project carried
out by another contractor immediately alongside the
foundation of his falsework: when the information
was belatedly passed on a complete redesign was
required. It is not always appreciated that the soil
sampling and testing which were carried out for the
supports of the permanent structure may not be
reliable guides to the nature of the ground which will
support the falsework. Empirical tests may be quite
inadequate for falsework foundations which may be
on or near the surface of disturbed or prepared ground.
Practical load bearing test may be required.
We were also shown cases where conditions had
changed after the design had been done, due to diver-
