final-report-of-the-advisory-committee-on-falsework-bragg-report - Flipbook - Page 28
was inclined to the beam so that there was a high
local pressure and indentation. In others, perhaps due
to previous misuse, the ends of timber bearers were so
crushed or split or rotten that failure in shear was almost inevitable. Similarly there were cases where
insufficient extension of telescopic beams had reduced
the length of the beam bearing on the end supports.
An other comm on fault was the disregard of the effects
of deflection. One effect is to redistribute the load.
If, for example a relatively rigid beam is supported on
three relatively elastic supports the two end columns
will each carry one quarter the weight, and the centre
one half. But if the supports are rigid the end columns
only carry 3/16 of the load, and the centre one 5/8.
Thus if the designer makes unjustified assumptions the
end or centre columns can be under designed by 25 %.
If beams are crossed as in a grillage this increase in
reaction may be compounded.
Doubling of timbers to distribute load rendered ineffective by
their unequal size
Another effect of deflection is to introduce axial forces
unless the supports are free to move horizontally.
This effect may be most important in skew spans and
the differential movement in any beam arrangement
may have serious consequences.
Differential movement may produce a twist on the
supports and in one instance this may have contributed critically to the overload.
Good use of heavy duty proprietary system false~k supporting
decking of a bridge spanning a deep canyon in California, USA
The third common fault is failure to restrain beams
against twisting. Any beam under load will tend to
twist, the tendency increasing with the difference
between the section moduli about vertical and horizontal axes. We studied one case where the only
restraint was in the form of wooden cross struts
between neighbouring beams: these .fell out when the
beams moved away from each other, allowing overturning. The effect could be particularly important
on cambered spans where the loads produce twisting
moments if the beams are not exactly in the vertical
plane.
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