Primary Frames - Steel Structures.

The frame supports the cladding but, with increasing architectural and service demands, other factors are important. The basic structural form has developed against the background of achieving the lowest cost envelope by enclosing the minimum volume. Plastic design of portal frames brings limitations on the spacing of restraints of around 1.8–2m. The cladding profiles are economic in this range: 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.

Simplest single-storey structure
Fig. 1.4 Simplest single-storey structure

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 moment- resisting 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.

A range of structural forms
Fig. 1.5 A range of structural forms

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 exten- sively 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 service requirements, has encouraged the use of lattice trusses  for the roof structure. They are used both in the simple forms with fixed column bases and as portal frames with moment-resisting connections between the tops of the columns for long-span structures such as aircraft hangars, exhibition halls and enclosed sports facilities.

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