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Steel Designers' Manual - 6th Edition (2003)
The problem is compounded by the variability of the direction and speed of a
wind, and by the variability of the aerodynamic shape of the structure as each new
piece is added. Care must be taken to ensure that these issues have been properly
addressed at each stage of the erection of potentially problematic structures, e.g.
bridges erected by cantilevering. Bracings, guy ropes and damping weights may all
have to be considered as methods of changing critical frequencies of vibration and
of limiting movements as the job progresses.
33.9.6 Temporary supports and temporary conditions
Safety 1009
Much time and effort is invested in the design of the structure. However, the design
of the temporary works on which that structure may have to depend while it is being
built may not have been given adequate attention. The number of recorded collapses
that take place after an initial failure in the temporary supports bears testimony
to this omission. For example, a temporary support may be designed only to
take a vertical load. In practice, the structure it is intended to support may move
due to changes in temperature and wind loading, thereby imposing significant additional
horizontal loads.
Sufficient consideration should be given to the foundations. Settlement in a trestle
foundation can profoundly affect the stress distribution in the girder work that it
supports. Settlement under a crane outrigger from a load applied only momentarily
can lead to the collapse of the crane and its load. The Code of Practice BS 5975 12
for falsework (which includes all temporary works, trestling, guy wires, etc., as well
as temporary works associated with earthworks) deals with a wide range of falsework
types and should be carefully read and observed. Particular attention should
be paid to the paragraphs dealing with communication, co-ordination and supervision
since failure in any of these areas can lead to a failure of the falsework itself.
Re-used steelwork showing signs of severe corrosion must not be used for temporary
falsework carrying critical loading. In other situations re-used steel should
be measured to ensure adequate performance.
During construction a structure will move as its parts take up their design
load. Connections to temporary supports have to be capable of absorbing these
movements.
Unless the design allows for these movements, eccentricities can result which may
trigger a collapse. The cross-heads at the tops of bridge trestles have been known
to fail from this cause since they are often called upon to resist wind-induced loads,
vibration and temperature-induced movements in the structure, in addition to their
more obvious direct loading burden. For these reasons they must receive a special
design study.
Very tall buildings and chimneys as well as bridges can be affected by windinduced
vibrations, as can working platforms and those who have to work with them.
The force of the wind can make welding impossible without adequate shelter: therefore
the fixings for a working platform must be able to take the load of the wind
blowing on shelter area.