Brasil R., Corbani S.

We present preliminary results of a numerical investigation of the inelastic behavior of steel structures under random seismic excitation. Although seismic problems are not important in Brazil, there is a strong market for Brazilian Engineering in the Pacific Coast Latin-American countries and in Portugal. Earthquake strong motions are known to lead steel structures into their plastic range. We simulate that effect in our portal frame models by the so-called plastic hinge concept. If a certain section reaches its full plastic bending moment, this top value is maintained and we introduce in the model a hinge in that section, with zero stiffness, allowing for free finite rotations. If the rotation in a certain hinged section is reversed, elastic behavior is restored by elimination of the hinge from the model. Random seismic loading is modeled by superposition of series of harmonic loads with their amplitude given by Power Spectra provided by the several Building Codes and randomly set phase angles. If a large number of those combinations are applied to our model we can get enough response data to allow for statistically based sound Engineering conclusions. This is a Monte Carlo type numerical simulation.