final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 45
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Technical Recommendations
In this section we make a number of suggestions
aimed at avoiding the faults discussed in Part 2. We
should make it clear that we are not here concerned
with laying down detailed codes of practice: that is
the responsibility of the British Standards Institution.
The Falsework Report of the Concrete Society and
the Institution of Structural Engineers has already laid
a most useful foundation for the work of the BSI
Committee and has been widely used prior to the
publication of the British Standard Code of Practice.
We are putting forward what we believe to be the
proper philosphy and the guide lines within which
detailed codes should be formulated. While many of
the points we make have derived from the evidence
presented to us we have also drawn liberally from the
experience of individual members in compiling this
section. Although we have not always been able to
consider every point exhaustively in committee we
consider it important that all the information we have
received or discussed should be included.
Estimation of loads
When the load applied to a falsework structure cannot
be calculated accurately - for example when a
relieving structure is being built under an older
structure or where a masonry wall has to be supported - the designer should consider all possible
modes of failure of the structure being supported and
anticipate the consequential loads rather than assume
empirical rules such as 45° cracking in panels.
When concrete is cast in situ concrete densities of 2·7
tonnes per cubic metre are sometimes achieved in
situations where the designer has assumed the conventional 2·4 tonnes per cubic metre. In heavily
reinforced structures an even distribution of the concrete may be temporarily prevented by the reinforcement producing concentrated loads where such were
not expected.
The effect of sloping soffit formwork in arches is
particularly important as the load is first concentrated
in the lower sections · and horizontal forces may be
considerable.
The process of building the permanent structure may
prevent an even load distribution. The deformation of
structural members as they are stressed or loaded
during construction may radically alter the distribution
of load on the supports. A typical case is the effect of
the change in shape of a concrete bridge deck during
stressing while it is still supported by the formwork.
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The effects of snowfall, drifting, and of ice formation
should also be anticipated and catered for where
necessary.
An extensive list of potential errors in load assessment
is given in the Falsework Report of the Joint Committee of the Concrete Society and the Institution of
Structural Engineers.
Loading procedure
The critical loads on falsework occur while the concrete is being placed and it is essential that the
procedure should be carefully controlled. Concrete
can now be delivered at rates and in quantities that
were unthinkable ten years ago. As a result the load
on the falsework reaches its design valueveryquickly.
The concrete which is placed first does not have time
to set and provide some support for the later batches
as was the case in limited batch and strip pouring. It
is common practice in the United States and Canada
and on some sites in the United Kingdom for the
bottom slab of a box section in such structures as
elevated roadways to be cast first. A period for curing
is then allowed during which further form work and
the additional reinforcement is added. Concrete for
the webs of the beams and the top slab can then be
poured upon a stiffened and strengthened base which
takes some of the total load.
It is important to adhere to the loading programme
both in place, time and rate of loading. Any proposed
alteration of the loading programme may change the
loads on the falsework and should be referred back
to the designer so that the design or the programme
can be modified.
Identifiable horizontal forces
We have already discussed the contribution of horizontal forces .to falsework failures. In this section we
consider some specific sources of sideways force
which must be allowed for in the design. They include
hydrostatic pressure from concrete and wind loads.
Lateral forces due to concrete pressures
Single sided shutters, i.e, those which are not bolted
together in pairs, exert unbalanced horizontal forces
which have to be resisted by the beams of a falsework
structure passing at right angles under the concrete
being placed. If however these beams are discontinuous and insecurely fixed the horizontal forces
may not be satisfactorily transferred to a firm poi_nt
