final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 54
When a single level guyed system is used it must be
realised that the structure is not stabilised until it
reaches the height where the guy is attached. Similarly
on dismantling, the structure may not be stable when
the guy is removed and may require intermediate
stabilisation.
Longitudinal stability
In the longitudinal direction there is opportunity for
more braces per line of struts. Errors in verticality
may cancel one another out to a greater extent and
gradients of the soffits may be less than in the transverse direction. However longitudinal ground gradients may be more severe and require special attention.
In bridges there may be sloping columns within the
falsework which apply severe longitudinal forces.
These must be dealt with separately. The known
horizontal forces plus 1 % of the vertical loads should
be added algebraically and for the worst condition
of time and place. If this sum is less than 3 % of the
vertical load then at least 3 % must be assumed.
The designer should allow for the possibility of the
3 % minimum lateral force being transferred from the
top of the falsework to the ground directly by the
braces and also for the possibility that it is transferred
down the vertical struts to lower node points and
thence down the diagonal bracing.
Butting on to and tying back to the permanent
structure
It will have been apparent from the previous section
that lateral stability in a falsework structure is
achieved by the combination of a lacing system joining
the load bearing struts and by the bracing. system
which carries the forces in the lacing to the ground
or to an alternative firm point.
In certain circumstances the lateral forces in the
lacing can be transferred directly to firm points on the
permanent structure. For instance in the case of a
concrete slab being cast on four walls of a building
the soffit shutter is usually erected tightly up against
the four walls and thus prevented from any lateral
sway. The joinery work should be butted on to the
walls and not left clear with a gap taped up to prevent
leakage of concrete. This butting ensures the lateral
stability of the top of the falsework but it does not
assist the strength of the columns in the falsework by
tiffening them at lower levels.
The stiffening of the falsework columns should be
achieved by butting the horizontal lacing members
up against the walls - if necessary with a protective
cap. If scaffold tubes are used the butting end should
be the start of the lace and at the other end a separate
tube should be used to make up the gap. This procedure should be carried out on every lace on every
level in both directions.
Where one of the walls has not been constructed the
laces have nothing to butt up against at one end.
Two methods are available to overcome this:
(i) Scaffold tie fittings may be bolted on to the
existing wall to provide resistance to tension as well
as thrust in the lace
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(ii) Diagonal bracing can be used coupled to a foot
tie to provide lateral stability in the direction away
from the single wall.
If neither wall is complete then diagonal bracing is
the only solution to the stability problem and it
should be inserted according to the rules given earlier.
Bridge soffits
Bridge piers or abutments may provide stability in
the longitudinal direction but give little support to
transverse forces elsewhere.
Bridge falsework is long and narrow in plan which
means that a high degree of transverse stability is
required. The lacing in such falsework should always
butt up against the abutments and if possible be tied
to them. It should also be locked round the ends of
the abutment so that transverse stability is achieved
at the end of the falsework.
Lacing should butt up against intermediate piers and
be locked round the sides of them or, if the piers are
formed of separate columns, the falsework laces
should be box tied round each column.
It must, however, be remembered that when falsework
is properly anchored in this way it will not be free to
accommodate any longitudinal movement introduced
by the deflection of long loaded beams or trusses.
This must be allowed for separately.
In certain circumstances the design of the piers is
such that no lateral forces can be applied to them.
For example they may be pin jointed and maintained
in position by their own falsework. In this case the
soffit falsework should be made self supporting against
lateral forces by bracing to foot ties. These braces
should preferably be locked into the angle between
the columns and their foundations so that a firm point
is achieved for the bottom end of the brace.
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