Effect of weak foundation on the seismic response of core wall type buildings

This paper investigates the seismic behaviour of a typical wall-type reinforced concrete building with a footing that is unable to develop the flexural wall capacity. Nonlinear dynamic analysis is used to determine the response of the structure under historical earthquakes representing design conditions for a seismic zone 4 in Canada. The analysis incorporates the nonlinear behaviour of the core, footing and soil, and also the uplift of the footing from the soil. Three different structural models are considered: (i) the core on a rigid foundation, (ii) the core on a flexible (rocking) foundation, and (iii) the core on a flexible foundation with the two lower levels connected to a parking structure. The results show that the weak footing does not have a great influence on the performance of the building considered. The parking structure and the rocking foundation cause a reversal and increase of the shear forces in the lower storeys. Also, the reduction of bending moments due to the core yielding is not proportional to the reduction of shear forces. This result suggests a need for different force modification factors for shear and bending. Key words: dynamics, earthquakes, reinforced concrete, building codes, foundations, footings.