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Structural Steel Frames 279
The limit state method of design has not been accepted wholeheartedly by structural
engineers because, they say, it is academic, highly mathematical, increases design time and
does not lead to economic structures. There is often little reward in employing other than
the permissible stress method of design for the majority of buildings so that the use of the
limit state method is confined in the main to larger and more complex structures where
the additional design time is justified by more adventurous and economic design.
5.3 Steel sections
Mild steel is the material generally used for constructional steelwork. It is produced in
several basic strength grades of which those designated as 43, 50 and 55 are most commonly
used. The strength grades 43, 50 and 55 indicate minimum ultimate tensile strengths of 430,
500 and 550 N/mm 2 , respectively. Each strength grade has several sub-grades indicated by
a letter between A and E; the grades that are normally available are 43A, 43B, 43C, 43D,
43E, 50A, 50B, 50C, 50D and 55C. In each strength grade, the sub-grades have similar
ultimate tensile strengths, and as the sub-grades change from A to E, the specification
becomes more stringent, the chemical composition changes and the notch ductility
improves. The improvement in notch ductility (reduction in brittleness), particularly at low
temperatures, assists in the design of welded connections and reduces the risk of brittle and
fatigue failure, which is of particular concern in structures subject to low temperatures.
Properties of mild steel
Strength
Steel is strong in both tension and compression with permitted working stresses of 165,
230 and 280 N/mm 2 for grades 43, 50 and 55, respectively. The strength-to-weight ratio of
mild steel is good so that mild steel is able to sustain heavy loads with comparatively small
self-weight.
Elasticity
Under stress induced by loads, a structural material will stretch or contract by elastic
deformation and return to its former state once the load is removed. The ratio of stress to
strain, which is known as Young’s modulus (the modulus of elasticity), gives an indication
of the resistance of the material to elastic deformation. If the modulus of elasticity is
high, the deformation under stress will be low. Steel has a high modulus of elasticity,
200 kN/mm 2 , and is therefore a comparatively stiff material, which will suffer less elastic
deformation than aluminium, which has a modulus of elasticity of 69 kN/mm 2 . Under stress
induced by loads, beams bend or deflect, and in practice this deflection under load is limited
to avoid cracking of materials fixed to beams. The sectional area of a mild steel beam can be
less than that of other structural materials for a given load, span and limit of deflection.
Ductility
Mild steel is a ductile material which is not brittle and can suffer strain beyond the elastic
limit through what is known as plastic flow, which transfers stress to surrounding material
so that at no point will stress failure in the material be reached. Because of the ductility of
steel, the plastic method of analysis can be used for structures with rigid connections, which