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The Influence of Design on Building Cost - Shape on Plan
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SHAPE ON PLAN 

The shape of a building has an important effect on cost. As a general rule the simpler the shape of a building, the lower will be its unit cost. Let us illustrate this as follows:

IRREGULAR SHAPES

 

BUILDING A

 

BUILDING B

We note that both plans have exactly the same floor areas, yet building B is far more expensive due to the shape of its design. Reasons for the increase in costs are:

  1. Building B has a higher perimeter/floor area ratio (to be discussed in more detail later) and requires 6% more external walling to enclose the same floor area than A.
  2. Setting out costs will be increased by as much as 50%.
  3. Excavations will cost between 6% and 20% more.
  4. Drainage costs will increase by approximately 25% due to the extra manholes and extra length of piping needed.
  5. Additional costs will also result from other elements of the building such as the walling and roofing due to the work being complicated by the shape.

It can therefore be concluded that the irregular shapes of buildings add to their overall cost.

REGULAR SHAPES

Regular shapes in contrast, become more expensive the longer and narrower they are planned. This can be illustrated by the following examples of a square and rectangular building with the same floor area.

 

BUILDING A         BUILDING B

CASE A

Area on plan = 400m²
Length of walling = 80m
Assume 3m high walling @ R200.00/m²

Then cost/m² = 80 x 3 x 200 ÷ 400
R 120.00/m²

CASE B

Area on plan = 400m²
Length of walling = 208m
Assume 3m high walling @ R200.00/m²

Then cost/m² = 208 x 3 x 200 ÷ 400
R 312.00/m²

The length of building B thus resulted in an additional expense of R 76,800.00 over that of Building A.

CONCLUSION

It is important that both architect and client are fully aware of the additional costs (or savings) that probably will arise from even small changes in the shape of the building.

They can then adopt a rudimentary cost benefit approach in considering the advantages of different shapes in seeking a suitable balance between cost, aesthetics and functional aspects.

Although the square building is the simplest plan in shape, one must remember that the shape of a building is dictated by:

Function
i.e. Factory shapes depend on the form of machines used and the production layout. Schools and hospitals rely on natural lighting and therefor tend to be rectangular in shape. Hotels are orientated towards the best view.

Site
The slope and shape of a site might dictate the shape of the building especially where the plan to plot ratio approaches one.

Economics
Office buildings with depths up to 18m lease more readily to smaller businesses as areas can be easily split to accommodate different concerns. Buildings often become a function of the owners financial success and tend to reflect this in the use of materials and design detail.

   Introduction                       Size of the Building