final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 57
It must never be forgotten that members of a grillage
5 The most concentrated loads are most likely to
of beams may be subjected to an overturning effect
which will itself produce a moment in a prop and
increase its tendency to buckle unless adequately
braced. Similar effects occur with sloping soffits or
foundations which tilt. Unfortunately there is very
little information available on the moments which
may be transmitted and their effect on jack strength.
occur at the supports. The end battens and braces
must be capable of supporting the end reaction. The
extensions of top and bottom chords beyond the last
node point must not be overloaded.
Composite beams
We have already made several points concerning the
stability of beams such as the importance of allowing
for the defelection of beam ends in the horizontal
plane.
It is also essential to ensure that the main beams are
tied together in plan which extends from one top
chord to the next top chord as well as by bracing which
connects the lower chords of the beams. In addition
there should be ample transverse cross bracing holding
the beams upright and preventing their rotation. We
were particularly impressed that special attention was
paid to these aspects of beam bracing on very large
bridge and motorway projects in the United States
and Canada. Jn addition to welded plan bracing in
the form of flats fixed to the beam chords, purposemade rectangular frames with two diagonal braces
were used and welded to adjacent beams, not only at
the end positions, but also at regular intervals along
their length.
We consider this subject to be of such importance that
it is worth recapitulating the main precepts which
must be taken into account in the design and erection
-of falsework using composite beams.
1 When first positioned a beam must be restrained
from rolling over on its supports, particularly if these
are below the neutral axis.
2 Beams tend to twist under load for the reasons
already discussed apropos of standard sections. They
must therefore be restrained all along their length
against these tendencies for the beam to roll over, and
for the lower chord to move sideways and rise.
3 Beams may be hogged so that the soffit surface
becomes level when the beam deflects under load.
This hogging may produce an additional over-rolling
effect.
4 The top chords may be adequately stiffened so as
not to buckle under compression.
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6 The supports must be capable of accommodating
the longitudinal deflections due to loading or to
thermal effects. This is specially important in skew
spans, and where joints are made to the permanent
structure.
7 It is always important to ensure that falsework materials have not suffered from damage or deterioration.
Welds in beam assemblies must be carefully inspected.
It is also important to check that the way the beams
are handled during transport, assembly and dismantling will not cause damage. For example, a
spanning Warren type girder should not be lifted by
attachments which could damage either chord of the
beams under its self-load.
8 Beams which overlap and are coupled should be
joined in a way that ensures that the compression,
tension and torsion forces and moments are properly
transferred where such transference is intended. Fishplates and bolts must be of the size and quality specified
by the designer, particularly if high tensile material
is called for.
9 Everything that has been said about single beams
applies to the individual units in a multiple assembly
of side by side beams. Particular attention must be
paid to skew spans where it is sometimes difficult to
stiffen the end lengths. Different types of beams which
have different characteristics should not be mixed.
10 Beams are heavy and the self weight of the beam
or assembly must be added to the design load. The
designer must also check that the load is not increased
by any redistribution of load resulting for example
from the deflection of the beam. The deflection of a
continuous beam affects the load distribution on its
supports and this must be allowed for.
11 Where beams are not symmetrical about both main
axes, care must be taken to ensure that they are used
the intended way up.
12 When the design of the beam permits multiple
chords to be attached for the purpose of increasing
strength, the attachment of the extra chords must be
such that they come fully into play. The improved
load-carrying capacity of the beam with multiple
chords must not overload the battens and braces.
13 When used on mobile towers or on end trolleys
or skids, beams must be battened together in such a
way that they will not be disrupted during movement.
