BS5400 is a limit state Code, but it still specifies maximum Stresses under service load. This amounts to specifuing maximum allowable working stresses. The need for this restriction has been questioned on the grounds that stresses are not fundamental design criteria. The limits do, however, serve a useful function within the Code. The new revision to the Code allows the stress check to be avoided, provided certain rules are complied with. These rules have a logical bask and produce designs for simple beams which do not depart significantly from the stress limits themselves. One effect of the stress limits that has not been widely appreciated is that they produce a severe restriction on the economic advantage that can be obtained from the use of inelastic analysis at the ultimate limit state. This is because the service stresses, rather than ultimate strength, are critical in design, a fact that should be reflected in the attention that design service loads receive when loading is reviewed.
P.A. Jackson