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A guide to concrete foor slab design

Loading & span

No specific type of slab can be seen as the best for a specific application, one will have to consider all the factors below and then decide. If more than one floor system can be used for the same span, the next factors that might strongly guide you in a direction is the cost of erection and speed of erection.

Cost (Economic considerations of spanning)

In general spans of less than 6m is most economically done with reinforced concrete. When the span is between 6m and 9m, various other concrete slab systems can be used and be just as economical or even more economical that reinforced concrete. When the span is more that 9m, normal reinforced concrete is not economical anymore and one will have to look at pre-stressed or other special beam systems.

Pre-cast members can also be considered for economical reasons in large building construction for there is a lot of repetition of standard components and can reduce costs.

Speed of erection

Nowadays speed of building construction plays an all the more important role, especially for the money saving factor and this has to be looked at intensively when a project is started.

Depending on the application, in-situ cast reinforced concrete can be quite economical, but can take longer because of the temporary support required until the concrete has set and is not suitable for large multi-storey buildings. This is best for large residential buildings & small office blocks. With precast members the shuttering is reduced or eliminated totally and thus results in shorter construction times and can be very effective for specific applications but one has to look at the other factors before a decision is made.


Post-tensioning instead of pre-tensioned members is recommended where the elements require excessive transportation because pre-stressed members, is not cast on site and the sizes of the members might have an increase in transportation cost resulting because of the larger transport vehicles necessary to move them. The accessibility of the specific site also influences this factor, for some larger vehicles may not be capable of accessing the site.


This is especially important when one makes use of tensioned eg: (pre-stresses or post stressed concrete members) where openings in slab is to be provided one must make sure not to damage the tensioned cables as this may greatly reduce their strength and structural failure might occur. Any services protruding the slab at any point is to be designed beforehand to prevent such failures