final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 50
If a single prop is used it is inherently laterally un-
stable, so is a small group of such props even when
some formwork is being supported on top of them.
We now consider some of the cases of support systems
proceeding from the simpler to the more elaborate.
Let us take as a starting point a flat and horizontal
bridge soffit to be built on a flat and horizontal firm
foundation roughly square in area. By exaggerating
successive modifications to this ideal the rules for
dealing with lateral instability will become apparent.
Firstly by modifying this ideal square falsework into
a long thin structure supporting a bridge span we
produce a structure which unbraced is weaker transversely than longitudinally. Such a structure must be
given more transverse stability than total longitudinal
stability because there is less chance of compensation
for weaknesses of one section by another.
Secondly if we assume the ends of this long temporary structure to be locked into the columns of the
permanent structure, there is a decreasing lateral
stability towards the centre. The transverse stability
must therefore be increased towards .the middle of
the structure. Stability at all transverse sections must
be adequate. If no permanent columns are there to
provide this, other measures will be required.
Thirdly if we now leave an access way through the
long thin temporary structure we have a discontinuity
at this point and the two "free" ends are in need of
further stabilising. The vertically loaded units on
each side will also need appropriate stabilisation.
As a fourth modification to the structure, imposed
after its shape change, assume that the foundation is
lower at one side and on weaker ground. Some rotation towards the lower side may take place. The
lateral effects referred to above instead of being selfcompensating become influenced in one direction.
TU.ey may become further enlarged by moments induced in each foundation if they rotate separately as
well as collectively. Consequent inaccuracies in verticality enhance the trend. This effect is due almost
exclusively to the weaker ground.
A fifth modification would be to raise the soffit at
one side, the higher side being supported on taller
and hence weaker struts. Increased stability is then
needed at that side, so that the strut strength is
restored to the general level.
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As a sixth modification the inclination of the soffit
may cause moments in the struts. (Pin joints in laced
truts do not ensure axial loading but only the absence
of applied moment at the ends.) Rocking fork heads
or their equivalent may be used to ensure axial loading
with no applied moment.
Seventh, the raised side of the soffit will probably be
on the outer side of a curved bridge which being
slightly longer than the inner side may be less frequently supported and have greater applied loads in
the struts. The design must be carefully checked for
the wider spaced and taller support work, and this is
likely to be the critical section controlling the design.
The possibility of the cumulative effect has to be
considered. In addition, the superimposition of the
other known lateral forces such as wind and plant
surge, if acting in similar directions, can produce a
disruptive and sometimes calamitous effect.
In two falseworks constructed from the same materials
it must be acknowledged that the smaller will be less
stable against disruptive forces and so account must
be taken of the overall size of the job.
For all these reasons particular attention must be paid
at the design stage to the provision of sufficient
bracing in plan, transversely and longitudinally.
Bracing between tower units, foot ties and head lacing
are specially important. Buttresses, rakers and anchors
may be all needed.
Bracing and lacing
In this section "lacing" refers to connections between
one strut and another in the horizontal plane and
"bracing" refers to diagonal members transferring the
forces in the lacing from one level to another.
Small sofflt supports
In the case of small soffits when there is not a multiplicity of supports the need for bracing and lacing is
actually greater than when there are many interlaced
supports and must be treated at least as seriously.
For example a telescopic prop without any lateral
support will cater for a certain safe working load. If
it is effectively held in position by lacing, this load can
be increased. Stabilisation at two levels will increase
it further (the limiting item may be the pin) but this
is only true if the lacing is effective. Correspondingly
if the lacing assumed by the designer is omitted the
strength required will not be obtained.
It is usually easy to secure a line of bracing in one
direction but frequently it is difficult in the direction
at right angles. This difficulty must be overcome if a
potential hazard is to be avoided.
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