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Steel Designers' Manual - 6th Edition (2003)
6 Single-storey buildings
they can support local loads and satisfy drainage requirements. The regime is therefore
for the loads to be transferred from the sheeting on to the purlins and rails,
which in turn must be supported on a primary structure. Figure 1.4 shows the simplest
possible type of structure with vertical columns and a horizontal spanning
beam. There is a need for a fall in the roof finish to provide drainage, but for small
spans the beam can be effectively horizontal with the fall being created in the
finishes or by a nominal slope in the beam. The minimum slope is also a function
of weatherproofing requirements of the roof material.
The simple form shown would be a mechanism unless restraint to horizontal
forces is provided. This is achieved either by the addition of bracing in both plan
and vertical planes or by the provision of redundancies in the form of momentresisting
joints.The important point is that all loads must be transmitted to the foundations
in a coherent fashion even in the simplest of buildings, whatever their size.
The range of frame forms is discussed in more detail in later sections but Fig. 1.5
shows the structural solutions commonly used.The most common is the portal shape
with pinned bases, although this gives a slightly heavier frame than the fixed-base
option. The overall economy, including foundations, is favourable. The portal form
is both functional and economic with overall stability being derived from the
provision of moment-resisting connections at eaves and apex.
The falls required to the roof are provided naturally with the cladding being
carried on purlins, which in turn are supported by the main frame members.
Architectural pressures have led to the use of flatter slopes compatible with
weathertightness; the most common is around 6°, but slopes as low as 1° are used,
which means deflection control is increasingly important.
Traditionally, portal frames have been fabricated from compact rolled sections
and designed plastically. More recently the adoption of automated welding techniques
has led to the introduction of welded tapered frames, which have been extensively
used for many years in the USA. For economy these frames have deep slender
sections and are designed elastically. In addition to material economies, the benefit
is in the additional stiffness and reduced deflections.
Although the portal form is inherently pleasing to the eye, given a well-proportioned
and detailed design, the industrial connotation, together with increased
Fig. 1.4 Simplest single-storey structure