4.1.1 Ultimate Limit State

Whatever the method used to determine the internal forces, the actions applied to the structure must be factored using those values pertaining to ULS, whether for single or multiple variable loads. Full plastic analysis may not be used for frames unless the plastic hinges, other than the last one to form, occur in the beams. 

This is because columns with high axial loads will not have any ductile capacity (and will not therefore satisfy the maximum limits for neutral axis depth to effective depth ratios (x/d).

This therefore means that frames are normally analysed elastically and the moment field then adjusted using redistribution to simulate plastic behaviour.

For slabs, plastic analysis using the Johansen Yield Line methods may be used, but there are severe restrictions on the x/d ratio to ensure ductility.

In practice, however, due to the relatively low levels of reinforcement, the x/d ratio, is rarely critical. 

It must be noted the maximum amount of redistribution is reduced and plastic analysis prohibited if Class A reinforcement is used.

In elastic analysis, the section properties are generally calculated on the gross concrete section (with no allowance for the reinforcement), and Young's modulus taken as the secant modulus, Ecm (in GPa), 

where fck (MPa) is the concrete grade strength.

Serviceability Analysis

This must be performed using an elastic analysis and contain all actions applied to the structure  including any deformations or settlement effects. 

It is not generally necessary to consider time-dependant effects such as creep, except where they will cause substantial changes in internal forces.

This last will occur in hyperstatic prestressed concrete structures such as continuous bridge decks. 

Although normal reinforced concrete undergoes creep, the effect is less as the stress levels are  potentially lower and thus is effectively ignored.

Generally a full serviceability analysis (and design) will be unnecessary. For most structures, it is sufficient to use "deemed to satisfy" approaches for, say, deflection or cracking.
Exceptions to this approach are where there is an explicit requirement to determine crack widths in liquid-retaining structures or bridgeworks, or a design which fails the "deemed to satisfy" clauses, but needs to be shown to be adequate.
It is accepted that "deemed to satisfy" clauses are conservative.