One of the most dangerous events that can occur on an offshore topside structure is an explosion. If blast analyses is performed without taking plasticity into account, then it may lead to very conservative design. On the other hand monitoring the weight by optimising the structural design is an important design aspect of such structures. Considering the time schedule restraints engineers face during the early phases of project, performing advanced non-linear blast analysis is not a practical approach in FEED phase engineering. CB&I, as a leading designer of topsides and processing equipment for offshore oil and gas production, plans to include the effect of advanced blast analysis in the early design phase by including plasticity effect in linear dynamic calculations and define a maximum unity check exceedance ratio which can be proved safe if the plasticity was taken into account. To this end, a study has been performed of the acceptable unity check exceedance for the preliminary linear dynamic calculations. Furthermore, the influence of the strain rate effect has been investigated. The acceptable unity check exceedance was analysed in SACS (FEM software package) by comparing the results of the elastic dynamic and plastic dynamic analysis of multiple frame structures. Analytic calculations were performed to verify the results. The influence of the strain rate effect is studied in SACS by using the strain progression over time of a simply supported beam exposed to various blast loads and the corresponding strain rate is determined. This was subsequently used in the constitutive equation of Cowper and Symonds to obtain the dynamic flow stress. The inclusion of the material sensitivity in linear calculations is achieved by comparing the increase of the dynamic flow stress to the static flow stress. It was found that a generic rule for the exceedance of the unity check was not possible to obtain with the used approach. However, due to the rapid exertion of the blast load a temporary increase of material strength of approximately 20%-27% can be achieved. This can be utilised in blast analysis by increasing the material strength by this percentage, both for linear and non linear dynamic analysis.