When widening a bridge, it is sometimes appears that the existing superstructure needs to be strengthened, especially if the analysis uses elastic methods. This can be expensive, but may not be necessary if it is realised that "elastic" models do not account for the true post-elastic behaviour of materials at the ultimate limit state.

SAM-LEAP5 is able to use its "plastic member limit" analysis feature, which involves setting "plastic member limits" on the existing weaker members equal to their ultimate flexural strength. When using this analysis technique, if the specified member limit is exceeded the load effect is forced to remain constant while the deflection increases by iteration until structural equilibrium is achieved for the applied load condition.

It can be demonstrated that loads are shed from the weaker overloaded members on the existing deck to the new stronger girders (and to lesser loaded existing girders). SAM-LEAP5's code checking facilities can be used for accurate calculation of the member limiting capacities, and for determination of concrete and reinforcement stresses.

As a consequence, strengthening techniques such as (say) carbon fibre reinforced polymer laminates can be reduced or even avoided altogether. It is important for engineers to recognize that bridge decks to not behave elastically at the ultimate limit state and there are significant savings to be achieved by adopting plastic methods of analysis where appropriate.

Member stress and strain diagrams