Civil Engineering Reference
In-Depth Information
Stresses cannot be calculated with the same confidence as resistances of
members, and high values may or may not be significant.
An apparent disadvantage of limit states design is that several sets of
design calculations may be needed whereas, with some older methods,
one was sufficient. This is only partly true, for it has been found possible
to identify many situations in which design for, say, an ultimate limit state
will ensure that certain types of unserviceability will not occur, and vice
versa. In the rules of EN 1994 for buildings it has generally been possible
to avoid specifying limiting stresses for serviceability limit states by using
other methods to check deflections and crack widths.
1.4
Properties of materials
Information on the properties of structural steel, profiled sheeting, concrete
and reinforcement is readily available. Only that which has particular
relevance to composite structures is given here.
In the determination of the bending moments and shear forces in a
beam or framed structure (known as 'global analysis'), all the materials
can be assumed to behave in a linear-elastic manner, though an effective
modulus is used for the concrete to allow for its creep under sustained
compressive stress. Its tensile strength need not be taken as zero, provided
account is taken of reductions of stiffness caused by cracking. The effects
of its shrinkage are rarely significant in buildings.
Rigid-plastic global analysis can sometimes be used (Section 4.3.3)
despite the profound difference between a typical stress-strain curve for
concrete in compression and those for structural steel or reinforcement, in
tension or compression, that is illustrated in Fig. 1.1.
Figure 1.1
Stress-strain curves for concrete and structural steel
