final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 52
To stabilise the top of an isolated strut, the two main
directions should be considered and the 3 % rule
applied in both directions. The lacing and bracing
members themselves should be stiff enough to carry
this load. The classic example is the support of the
mast of a scotch derrick by two inclined struts at rightangles which also act as ties.
To resist buckling, at intermediate points up the
strut, a load capacity of 3 % of the thrust should be
provided unless the known external forces exceed the
3 % value.
Isolated towers and lines of towers in larger structures
Towers with three or four legs present some difficulties in construction work because, being founded
separately on each of their legs, they are not pin
jointed at the bottom. They may be loaded at the
top on individual legs or centrally through a grillage.
Where there is considerable internal bracing in support towers this can carry a certain fraction of the
horizontal forces to the ground. The amount of this
internal horizontal strength should be calculated for
the system used and checked by tests wherever
possible.
be added together and the total of them brought to
the firm point. In collecting the forces together any
one of the levels of lacing must be capable of transferring the 3 % to the firm point. But it need not
normally be assumed that the summation of all the
forces in all of the levels is ever more than 3 % unless
there are special horizontal loads which must be
catered for. In this way if the lacing forces are brought
to the ground at the firm points the bracing need only
be designed to transfer a total lacing force of 3 %,
provided that it can collect this force from any one
of the lacing levels or components of it from several.
When isolated towers or lines of towers are surmounted by a grillage it must not be assumed that
the grillage will apply only a vertical load to the
towers. Whilst the grillage may be centred over the
legs of the towers and thus apply its reactions at this
centre, the load may not be vertical. It may have
considerable inclination if there are substantial
horizontal forces at the top. If the grillage is made of
three levels of joists the reactions applied to the top
of the grillage may be a metre above the top of the
tower and may exert a considerable moment on the
whole tower.
The inclination from the vertical of the force applied
to the tower, or alternatively the inclination of the
tower if the force results in a lateral movement at the
In many systems, however, this internal bracing does
top of the tower, may be sufficient to throw the line
not transfer the forces directly to the ground because·
of the load towards, or even outside, the base of the
the adjustable legs at the bottom have much lower
tower. The loads in some of the legs of the tower will
resistance to horizontal forces than the bracing within
then be increased. The effect of lacing is to reduce the
the tower. This must be taken into account by, for
effective length of the tower, but it will not alter the
example, bracing from the top of the legs to a firm
inclination of the applied force, and thus the strength
point.
of the foundations and lower lifts of laced towers
must be carefully assessed.
The distance between the levels of lacing in either
direction should generally not be spaced more than
three times the dimension of the tower in the same The lower end of bracing members
direction. Thus a 2 m x 1 m tower should be laced Generally the upper end or the intermediate conat 6 metre levels in the long direction and 3 metre nections to the brace are uncomplicated but the lower
levels in the other. The forces in the lacing must be connection is sometimes difficult. It has to act in
resisted by attaching the members to a firm point.
compression and tension and the brace force must be
transferred to the ground.
It is not usually possible to estimate the force which
might come into the lacing system at any level and This does not mean that braces must have separate
therefore empirical rules are used. The total lateral foundations as in the case of the scotch derrick. But
resistance, i.e, internal plus external, should be at it does require that there is a foot tie joining the load
least 3 % of the load in the strut. From this value the bearing struts together near the ground level, and
internal resistance of the built-in bracing in the tower certainly at a level below which there are no adjustable
may be subtracted and the residual amount resisted or other units which have low lateral stability for the
by attachment to a firm point, provided the lateral attachment of the bracing. The vertical weight of the
resistance of the lowest leg element is adequate.
structure being supported must normally provide the
resistance against uplift of a brace under tension and
Where there is more than one level of lacing it need the foundation of the load bearing struts will suffice
not be assumed that the several three per cents are to for the thrusts of the braces under compression.
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