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