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Steel Designers' Manual - 6th Edition (2003)
118 Industrial steelwork
imposed on to the structure do not have a secondary adverse effect of inducing
resonant vibrations into it.
3.3.3 Lateral loadings from plant
Lateral loadings imposed by plant on the structure derive from three sources. These
are considered separately although there is a common theme throughout that the
operating process undergoes a change in regime which is the cause of loading. Many
of the actions which give rise to horizontal loads also cause vertical loads or at least
vertical loading components which must be incorporated into the design. However,
whereas vertical loadings are usually readily understood and allowed for in loadings
provided by plant designers, one feature of the horizontal components of such
loadings that causes confusion is the fundamental concept of equilibrium. Notwithstanding
exotic situations where masses (projectiles) leave or impinge on a structure,
or where motion energy is dissipated as heat of friction, equilibrium
considerations dictate that lateral components of forces are in balance and consequentially
they are often ignored. This is not satisfactory as balancing components
may act a considerable distance apart (the load path must be examined in detail)
or indeed the balancing components may act at different levels, leading to a more
conventionally understood requirement to transfer lateral loading.
The three causes are as follows:
(1) temperature-induced restraints
(2) restraint against rotational or (more rarely) linear motion
(3) restraint against hydraulic or gaseous pressures.
Where plant undergoes a significant change in temperature, plant designers will
typically assume that the structure is fully rigid and so can absorb the forces generated
by application of restraints at the structure connection points. They will then
design the plant itself for the additional stresses that are caused by preventing free
thermal expansion or contraction. This is a safe upper bound procedure since the
forces generated in both structure and plant represent maxima, with any deflection
at the support reducing forces in both elements. Naturally the structural steelwork
designer must be made aware both of the assumption of zero deformation so that
the support can be made as stiff as possible in the required direction, and of the
forces that are thus imposed.
In spite of the apparent complications of this approach, it is frequently adopted
by plant designers for convenience on small items, and to avoid complexities in
interconnection between plant items and with piped and ducted services on larger
items.
The alternative approach, common on major plant subjected to significant
thermal variation such as boilers and ovens, is to assume completely free supports
with zero restraint against expansion or contraction. This is a lower bound solution