final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 55
Permission to box tie round the columns and piers
and to butt up against abutments should not be unreasonably withheld by the designer of the permanent
structure. With fixed end columns they are usually
designed to resist greater lateral forces from the completed permanent structure than will be applied to
them by the falsework during construction.
Of course there is a limit to the horizontal force which
may be applied to a part of the permanent structure,
particularly when this has not yet been stabilised by
the soffit under construction.
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It is preferable to design and construct so that adequate lateral stability can be provided entirely through
one route - that is entirely by bracing or entirely by
tying to the existing structure. However even if the
design has been made on the basis of bracing alone it
is sensible to take the opportunity to butt the lacing
up against the permanent 'structure.
Selection of materials and equipment
All falsework designs are influenced by the materials
available to the contractor. He may have certain
items in stock and may have arrangements for hiring
proprietary equipment from a particular supplier. The
choice of material may also be affected by the experience of the workforce in the erection of particular
falsework systems. Such limitations are normally acceptable but must not be allowed to be paramount if
particular features of a project in hand-for example
a very large unsupported span or an exceptionally
heavy loading imposed by the permanent works make the use of the available materials inappropriate.
The selection of appropriate timber, tube and fittings,
scaffolding and proprietary systems of steel beam
assemblies is discussed in the various draft standards
which we have seen. These include the Canadian
Standards Association 4th Draft entitled "Falsework
for Construction Purposes" and the Code published
by the State of California Bridge Department, entitled
"Bridge Falsework". We support the general lines
followed in these documents and would expect them
to be followed by the British Standards Institution
where they are relevant to British practice.
cross and diagonal grain, warping, wane and decay.
This detailed consideration emphasises the greater
use and importance of this material in a heavily
forested continent but the conclusions drawn are
equally valid for the United Kingdom.
The Joint Committee recommended that "grade
stresses listed for 50 grade material should be used
for the calculation of basic permissible stresses ... "
and that certain commercial gradings will consist
principally of timber as good as this (para 5.4.1).
Where stress graded timber is used, CP 112 provides
the necessary guidance. In both cases the appropriate
factors given in the relevant Code should be applied.
We would like, however, to emphasise that a competent person should inspect the timber on the job
and reach a decision on its acceptance or rejection.
When timber members are used side by side and are
intended to share the load, it is particularly important
that the qualities and dimensions are the same to
ensure that the load is shared.
Allowable bearing stresses require careful consideration. We agree with the Joint report that a 33 %
increase can be allowed if there is no wane, but we
draw attention to the fact that if several timber
members are laid one upon the other - e.g, wedges,
followed by primary beams, followed by secondary
beams, followed by packing or firing, the compression
of all the pieces together is very much greater than
that resulting from the load on one single piece. In
such circumstances consideration should be given to
a reduction in the allowable bearing stress to reduce
the disruptive effects resulting from deformation of
bearing areas.
Falsework is sometimes constructed in such a way
that timber beams do not fully cover or overlap their
supports but are butted with other timbers on the
same support. This reduces the end bearing area,
which is already small, and some modification in
allowable stress may be needed if this arrangement
cannot be avoided. The effects are more severe if the
beams have been used before, and the ends are
damaged.
Timber
North American standards deal extensively with timber. This includes consideration of compressive,
shearing and flexural strength and the effect of
duration of load. The moisture content is important
and so are the defects typically found in timber. These
include knots, "checks and shakes" (i.e, radial
separation of wood fibres) annual growth, splits,
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Discontinuity of primary and secondary shutter beams
It is essential that the end to end joints in shutter
beams are placed where the designer intended. If
they occur at a place where he intended continuity, the
full applied load may pass straight through the beam
system into one strut without distribution and cause
a failure.
