The simplest approach to the design of concrete
members is to use prescriptive tables which provide minimum geometric
dimensions and cover to reinforcement. Read more. (link to prescriptive/concrete
structures)
Simple design methods are available for concrete members based
on the fundamental engineering principles used for cold design.
Reduction in strength of the concrete and reinforcing bars is
included, together with partial safety factors that apply at
the fire limit state.
For simply supported horizontal members the calculation of the
design resistance is simply calculated using the normal stress-blocks
adopted in normal cold design except the tensile strength of
the reinforcement is reduced based on its temperature. In addition
the partial material safety factors for both concrete and steel
reinforcement are taken as unity. Provided the insulation criterion
is met, the concrete in compression will remain at a low temperature
and is therefore assumed to retain its full strength.
For continuous horizontal members plastic design and normal
redistribution can be used. In hogging regions the concrete strength
in compression is reduced due to the effects of fire.
For concrete columns, the simplest design method is to ignore
the strength of concrete above 500°C and define the actual
temperature of the reinforcement. The load-carrying capacity
is calculated based on the reduced area of concrete and reduced
strength of reinforcing bars using the same design approach for
cold design but with material safety factors for the fire limit
state. Alternately, the column can be divided into a number of
zones and the actual strength of the concrete, based on the temperature,
together with the reduced strength of the reinforcing bars is
used to calculate the columns capacity. Both these methods are
given in BSEN1992-1-2. The main limitation of the simple design
methods for column members is that the temperature distribution
across the member cross sections given in the codes is limited
and the methods ignore the redistribution of moments within the
structure that occurs during a fire and also ignore transient
creep.
< Back to Simple Calculation Methods