final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 19
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Technical Reasons for Collapse
An analysis of all the falsework failures brought to
our notice indicates that there is no single cause of
collapse. Indeed the examples already given show that
the causes are varied and widespread. In each case,
however, one may discern the direct technical reasons
for failure and also the contributory procedural
failures which allowed the technical faults to go
undetected. In this section we will discuss what
appears to be the commonest technical faults.
In providing this catalogue of errors we must make it
clear that it has been derived from a selective study.
The great majority of falseworks do stand up to the
loads imposed on them. In relatively few will any of
the errors occur and remain uncorrected to an extent
which would lead to failure.
In surprisingly few cases have we found weaknesses
which could not have been recognised and dealt with
by well established procedures. However, there are
evidently areas of particular difficulty, where special
attention is required, guidance is offered these in Part 4.
In general one may divide technical failures into three
categories:
(1) where the loads on which the design was based
are different from those actually applied
(2) where the design itself was inadequate for the
. specified loads
(3) where the works were not constructed according
to the design.
Thus faults of understanding, interpretation and
execution all occur the design and construction stages.
For example, a lack of sufficient stiffness in a pinjointed structure may result from a failure by the designer to recognise the need for it or from the absence
of clear detailing in a drawing or from the omission of
bracing due to carelessness or shortage of materials.
The principal technical causes known to us will now
be considered in detail.
Stability in the horizontal plane
The primary purpose of most falsework is to support
loads or to take reactions which are vertical. In
contrast the forces in the horizontal plane are relatively
small and difficult to quantify so they tend to be
inadequately allowed for. Several of the investigation
reports which we have studied and much of the evidence before us both from the United Kingdom and
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overseas emphasised lateral instability, and to a lesser
extent longitudinal instability, as one of the prime
causes of failure. In fact the most commonly occurring
fault in falsework design seems to be a neglect of
horizontal forces and a failure to make provision for
adequate lateral and longitudinal stability. Horizontal
forces may be introduced by wind, vehicle braking,
impact of skips or vehicles, attempts to free stressing
cables, guys, concrete pouring methods, vibrators and
a multitude of other causes. In at least two collapses
of bridge falsework shutter vibrators were being used;
these transmitted compressive waves to the outermost
members of the falsework. Poker vibrators, which
have a direct effect on the concrete impose fewer
stresses on the sub-structure.
It is by no means always possible to anticipate the
commencement or duration of every component of
the horizontal force which has to be catered for as in
the case of forces induced by accident. Adequate
measures need to be taken to forestall the effects of
such occurrences. Numerous accidents have been
recorded where there was no efficienesignalling system
between the person controlling the po~ring operations
and such operatives as crane drivers and the drivers
of concrete delivery lorries.
Local horizontal forces at interconnecting points in
the structure can also be introduced by such factors as
the non-verticality of members, hydrostatic pressure
on formwork, inclined soffit members or inclination
or uneven deflection of the foundation surface.
The effects of cross fall of the foundations are particularly important. The downwards slope causes
drainage towards the lower level with consequent
ground weakening and differential settlement. Taller
struts must be used on the lower side, and these, when
weaker, deflect more causing relative movement.
There may be a deeper filling of foundation material
which allows more movement of the sole plate if it is
inadequately compacted.
The probability of local horizontal forces makes it
essential to consider interconnection in the horizontal
plane. It frequently happens in falsework that the
assembly of separate timbers is not bolted or nailed
through joining plates, nor spliced, joined or otherwise
formed into a coherent structure. The reason is that
the members have to be easily dismantled and are
usually required for re-use.
This lack of structural connection prevents one portion of the whole helping another to withstand lateral
forces. For example we studied a case where the
