Dynamic Analysis of Three-Dimensional Frames due to Earthquake and Wind Load

Md. Monjurul Islam


In this work, three-dimensional RCC frames have been analyzed for earthquake and wind loads using the concepts of Structural Dynamics, with the purpose of observing the qualitative and quantitative differences between their effects on the structural response of 2, 3, 4 and 5-storied buildings. Both the maximum bending moments and maximum torsional moments are compared for the ground motion of El Centro earthquake (1940) and a sustained wind speed of 100 mph.

The results show that the internal forces (moments and torsions) due to the earthquake motion are much greater than the maximum forces due to wind loads for all the cases under study. In fact, the wind forces will remain smaller even if the wind velocity is doubled. Moreover, for the buildings under study (symmetric about one axis), the flexural moments are found to be much greater than the torsional moments induced by these loads. However, the torsional moments due to earthquake motion are also found to be significant from design point of view and should not be neglected for the design of the RCC sections under earthquake.

The study shows the qualitative differences of the effects of wind and earthquake loads on structures. Whereas the forces due to earthquake tend to diminish totally within 30-40 seconds, the wind forces tend to reach steady values (corresponding to the static forces) after a while. Moreover, whereas the forces due to wind load tend to increase with the height of the building, the variation of the seismic forces depend on resonance and continue to increase only up to a structural height when the structure’s natural frequency matches with the frequency of the major energy content of the earthquake. Therefore, although the ‘equivalent’ static analysis method is not appropriate for earthquakes, it is suitable for wind load analysis if an appropriate dynamic magnification factor is also used.

A study on the variation of torsional moments over the building heights show that these moments tend to decrease along the height of the structures, and this decrease is almost linear for wind loads