Environmental Factor - Multi-Storey Building Form.

There are a number of factors which influence the choice of structural form that are particular to the site location. These can have a dominant effect on the framing arrangement for the structure.

The most obvious site-dependent factors are related to the ground conditions.

A steel-framed building is likely to be about 60% of the weight of a comparable reinforced concrete building.

This difference will result in smaller foundations with a consequent reduction on costs. In some cases this difference in weight enables simple pad foundations to be used for the steel frame where the equivalent  reinforced concrete building would require a more complex and expensive solution.

For non-uniformly loaded structures it will also reduce the magnitude of differential settlements and for heavily loaded structures may make possible the use of a simple raft foundation in preference to a large capacity piled solution (Fig. 2.2).

Foundation savings
Fig. 2.2 Foundation savings

Difficult ground conditions may dictate the column grid. Long spans may be required to bridge obstructions in the ground. Such obstructions could include, for example, buried services, underground railways or archaeological remains. Generally, a widely spaced column grid is desirable since it reduces the number of foundations and increases the simplicity of construction in the ground.

Other site-dependent constraints are more subtle. In urban areas they relate to the physical constraints offered by the surrounding street plan, and the rights of light of adjoining owners. They also relate to the planning and architectural objectives for specific sites. The rights of light issues or planning  considerations may dictate that upper floors are set back from the perimeter resulting in stepped construction of the upper levels. Invariably the resulting framing plan is not rectilinear and may have skew grids, cantilevers and re-entrant corners.

These constraints need to be identified early in the design in order that they are accommodated efficiently into the framing. For example, wherever possible, stepped-back façades should be arranged so that steps take place on the column grid and hence avoid the need for heavy bridging structures. In other situations the designer should always investigate ways in which the impact of  lack of uniformity in building form can be contained within a simple structural framing system which generates a minimum of element variations and produces simple detailing.

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