A 4-storey reinforced concrete framed buildings was modelled in 3-dimensional analysis with the same sections and loadings for both rigid and weak foundations respectively using ETABS and FORM5 Softwares in accordance with Euro code provisions. The weak foundation was initially analysed and designed as a fixed column-foundation joint and later re-analysed and redesigned as a hinged column-foundation joint. The ETABS software was used to obtain the most critical component member forces and bending moments while FORM5 software was used to obtain the reliability indexes. The results revealed that due to the effect of weak soil safe bearing capacities, allowable maximum displacement was exceeded resulting in lower predicted reliability indexes and higher probability of failures that enhanced progressive failure. The reliability index recommended by Euro code was not achieved due to the effect of weak soil-structure interactions which showed that it will be very disastrous if rigid soil-structure interactions were assumed for a weak soil safe bearing capacities. The use of a hinge column-foundation joint for structural analysis will produce increased sections and reinforcement areas in reinforced-concrete frames. These would consequently improve the reliability indices of structures built on weak soils and reduces its probability to fail. Therefore, it was concluded that, a hinged joint should be adopted as column-foundation connection when the soil is generally weak. The findings of this study would be a useful guide and reference materials for structural safety and reliability analysis with regards to variation of soil type.
Key words: Soil structure interaction, soil bearing capacity, reliability index, probability of failure.
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