There are two design considerations: strength and stiffness. The structure must be strong enough to resist the worst loading conditions without collapse and be stiff enough to resist normal working conditions without excessive defl ection or deformation. Typically the requirements for strength and stiffness are split between the following ‘ limit states ’ :

Ultimate limit state (ULS) – Strength (including yielding, rupture, buckling and forming a mechanism), stability against overturning and swaying, fracture due to fatigue and brittle fracture.