09-20-2009, 02:11 AM
I am a practicing structural and geotechnical engineer but I am no longer using ETABS as I am more focus now on bridges. Looking at all the threads of this topic, it looks like you are modelling incorrectly and may not necessarily conservative. The pile should have both the lateral and vertical springs. The lateral springs control the lateral displacement that includes pile displacement (so with moment) and lateral building drift. Without these springs, you will have large bending moment in the piles and displacements unless you use the basement walls to carry all the lateral forces. But even then, when the basement walls move laterally it also moves the piles. In this case, without the lateral springs included in the piles it will just move freely which gives a higher moment and displacement.
An if you have large drift because of no lateral springs or fixities at the base, you will have to increase the stiffness (EI) of the building. When you increase the stiffness, you are also increasing the mass and then increase the earthquake forces.
If it can not be done in SAFE/ETABS, use another software to model correctly the interaction of the foundation and the soil. Assume your building structure to be fixed at the base when using ETABS, and transfer all your forces to another software modelling correctly the soil and the basement or raft.
An if you have large drift because of no lateral springs or fixities at the base, you will have to increase the stiffness (EI) of the building. When you increase the stiffness, you are also increasing the mass and then increase the earthquake forces.
If it can not be done in SAFE/ETABS, use another software to model correctly the interaction of the foundation and the soil. Assume your building structure to be fixed at the base when using ETABS, and transfer all your forces to another software modelling correctly the soil and the basement or raft.