Atria - Structural Form - Steel Structures.

For a new building there will be significant choice in the types of structural systems that can be adopted, as the perimeter supports can be designed accordingly.Where an enclosure is being added to an existing building, available support will depend on the capabilities of the existing structure and may determine the appropriate options.

The structures in Figs 5.36 to 5.40 illustrate a series of solutions in which there is a generally decreasing reliance on the perimeter structure.

Fig. 5.36 Arch
Beam or truss
Fig. 5.37 Beam or truss

Half portal
Fig. 5.38 Half portal

As a group the arch, the beam and the dome are potentially the most structurally efficient solutions but rely totally on the support provided by the perimeter structure. In the case of an existing building, the following should be considered:

• The strength and integrity of the existing structure if this is to be used for support.
• The percentage increase in vertical load imposed on the existing structure by the new.This will be smaller where new loads are introduced towards the lower levels of existing walls and columns than at the top.
• The horizontal forces imposed at the supports. Horizontal thrusts introduced  at lower levels will have smaller resultant overturning effect  than at a higher level.

Failure to investigate the implications of the choice of structural form could result in a costly and clumsy solution.

As town centres and streets are redeveloped to compete with out-of-town shopping, the need for enclosure is a common solution, often considered. In these cases, the adjacent buildings are typically of a wide variety of constructions of various heights and condition.The solutions defined in Fig. 5.39 may be appropriate to avoid the complexities of resolving the various boundary conditions.The perimeter structure is required to support a small proportion of the vertical  load and relatively small horizontal loads from wind and stabilizing forces for the new columns, which does not generally pose a problem since there is usually some redundant horizontal capacity due to the wind shielding effect of the new enclosure. The portal style solution (Fig. 5.40) is substantially self-supporting.

Temperature effects are often a governing design consideration  for an atrium structure due to exposure to heating from sunlight. Temperatures in steelwork painted black can be 30° higher compared with white where directly exposed to the sun’s radiation. It is important to allow for movement of the structure, to prevent locked-in stresses from building up.Releases of this sort are often incompatible with providing horizontal stability. The choice of structural form needs to reflect these opposing requirements. For example, a statically determinate structure such as a three pinned arch can accommodate temperature movements and differential  settlements without generating secondary internal forces (see Fig. 5.41).

Beam and column
Fig. 5.39 Beam and column

Portal frame
Fig. 5.40 Portal frame

Movements caused by temperature changes
Fig. 5.41 Movements caused by temperature changes

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