Determining the Speed Spectrum for Soils of Type I and II of Iran

In destructive earthquakes lots of break down in structures such as vital lifelines and buried pipelines also destruction of massive buildings have always been observed. These destructions can sometimes cause more serious injuries. The absorbed force and destruction caused by the earthquake in surface structures are proportional to their mass and inertia, so that with increasing acceleration, the amount of forces is higher. This is despite the fact that the behavior of buried structures actually influenced by the behavior of the surrounding soil and the mass of structure in comparison with their environmental soil is very small and negligible. Therefore, using the methods based on earthquake acceleration parameter can’t be sufficient in designing of such structures and existing of the speed spectrum designing along with the acceleration spectrum available in 2800 building code is also necessary, because the seismic design of such structures is based on the displacement response method. That is, first the displacement of the earth in location of buried structures using velocity response spectrum is calculated and the interaction between earth and underground structure is analyzed with quasi- static method. Velocity response spectrum for seismic designing of buried structures based on the maximum response of the earth surface layer due to the desired acceleration entering the bottom layer is obtained from a soil profile analytical model. In this study, we have tried to determine the curve of the velocity spectrum factor for soil type 1 and 2 according to the accelerograms data obtained from different earthquakes in Iran For this purpose, among the received accelerograms data from the Building and Housing Research Center, we selected 306 horizontal accelerograms of soil type I and 323 horizontal accelerogram of soil type 2.The normalized velocity spectrum of each accelerogram were calculated separately based on the maximum values of the velocity domain and acceleration. Accordingly, for each of these two types of soil we obtained two sets of normalized velocity response spectrum. Then, for each set, mid and 84% - design spectrum were determined. Finally, mathematical model of velocity spectrum factor curves was determined and presented in the formula for soil conditions of types 1 and 2 of Iran.