final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 46
and the shutter movement which may occur can be
both sudden and disastrous.
became temporarily airborne and was not relocated
upon its supports.
If the horizontal forces from the lateral pressures of
concrete are transferred by raking supports to the
uprights, the additional vertical forces in the uprights
should be taken into account.
Allowances for impact
(i) Shock loading In considering the loads applied to
falsework an allowance should be made for shock
loading: some shocks can be anticipated and evaluated
but others are accidental and unpredictable in size or
in location.
A vertical shutter resisting horizontal pressure tends
to rise despite the fact that superficially there is
apparently no vertical force to cause this. Vertical
shutters which are not firmly anchored down may
rise and permit the sudden escape of the concrete
which when once started may become dangerous on
account of horizontal impact and vertical load
redistribution. The shutters should be effectively
anchored down.
Wind forces
In general wind forces may be calculated by the
method described in BSCP 3, Chapter V, part 2.
Although this document is not strictly applicable to
temporary works, correction factors can be applied to
take account of a number of special factors.
For example the effect of wind on the falsework support for an in situ concrete structure before pouring
may be of greater consequence during construction
than after the concrete has been placed.
It is obviously difficult to cater for wholly exceptional
wind conditions nor would it be economic to design
for them in normal circumstances. However, designers
should pay special attention to two factors. One is
the geographical location of the site, which may be in
a specially windswept area or which may have a
topographical configuration, which funnels the wind
to produce particularly high velocities. The collapse
of the cooling towers at Ferrybridge shows how
adjacent structures can produce a devastating pattern
of wind flow. The seasonal variation in wind should
also be considered in relation to the likely life cycle
of the falsework.
The cross section of the falsework plus formwork is
also important in determining wind resistance.
Clearly the denser the supporting falsework, the less
reamlined the component, and the greater the area
of formwork, the higher the resistance. The relationship of the whole assembly to the direction of the
revailing or funnelled wind is another special con. leration to be taken into account. Wind forces can
act in an upward direction and we have on record
zstances of collapse where the support shuttering
In the first category is the shock loading due to the
normal delivery of concrete or loading of beams, the
use of vibrators (particularly shutter vibrators), the
braking of vehicles and concrete transportation
systems." It is important to differentiate the above
from shock loading resulting from accident, such as
the dropping of the contents of a bucket of concrete
or the collision of a vehicle with the falsework members. Incidents of this nature can clearly introduce
deformation of part of the structure and it is important that an inspection is carried out at the earliest
possible moment particularly if the accident occurs
while concrete is being poured.
Accidental damage should always be reported immediately in order that the full corrective procedures can
be agreed by all parties concerned.
(ii) Impacts during the application of the load The
generally accepted impact allowance of 25 % and 10 %
for mechanical and manual operation respectively are
satisfactory for ordinary crane loads. However, the
weights of precast units may be considerable and the
difficulty in handling them may necessitate greater
allowances. In working in exposed positions the wind
forces and swinging of the crane rope may cause
serious lateral forces at the moment of landing the
beams so that a horizontal resistance of, say, 10 %
of beam weight should also be built into the system.
For in situ concrete work a value of 240 Kg/m2 is
frequently used as an allowance for shutter weight
and impact of concrete placing. Of this figure about
70 Kg/m11 is the allowance for the shutter. For thin
slabs every underestimation of the live load is of much
greater significance as it comprises so much greater
a proportion of the total load.
In fact we are generally doubtful about the value of
expressing impact loads as a proportion of the static
load. Whilst the impact from a precast beam is
*Machinery driving conveyors delivering concrete should be
insulated to prevent the transmission of vibrations to the
falsework structure.
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