Fall 2001

Seismic Analysis of Three-Dimensional Offshore Structure and Electric Tower

Muhammod Delwar Hossain


There is no established building code in Bangladesh for the seismic (static or dynamic) analyses of trusses, and in most cases trusses are not designed against earthquakes. With this view, the primary objective of this study is to analyze the behavior of different trusses under seismic vibrations.

Three offshore structures (200, 400 and 600) proposed for construction in the Gulf of Mexico and also an existing electric tower are considered in this study. These structures are subjected to ground motions from two major earthquakes of the recent past; i.e., the Kobe earthquake in Japan (1995) and the Northridge earthquake in USA (1994). The earthquake data are used with a computer program written for the linear dynamic analysis of 3-dimensional trusses to analyze the structures and perform parametric studies.

Different structural behaviors are comprehensively analyzed showing time series results of various member forces of the trusses as well as the Fourier spectra of the maximum forces. A study is performed on the behavior of common steel structures of different heights in these earthquakes. It is found that the maximum axial forces in the offshore structures are almost proportional to their heights.

The stresses for the Kobe data are found to be greater than the Northridge stresses in most of the cases.

The allowable tensile stress is assumed to be 18 ksi for all the members. From dynamic analyses, the maximum tensile stresses in all the members are found to be below 18 ksi. However, the compressive stresses often exceed the allowable compressive stresses in the members, which are lower than allowable tensile stresses due to geometric nonlinearity. This indicates that earthquake may present a critical design condition for trusses.