A guide to concrete floor slab
design
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A guide to concrete floor 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 – 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.
Logistics
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.
Services
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.
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