Proposing 3-DoF Model to Study of Mass Parameter Effects on Longitudinal Period in Base Isolated Bridges

The effect of mass parameters on the natural vibration period of isolated bridges is discussable, especially in case of substructure mass components, because of the disagreement of different researchers about it. This is while some bridge design codes such as AASHTO guide specification, ignores substructure mass in simplified method of analysis. Current paper investigates the effects of substructure mass components on the longitudinal period of base isolated bridges. These components include: pier mass, cap beam mass, pier rotational moments of inertia, and cap beam rotational moments of inertia. In this paper, a three degree of freedom analytical model of isolated bridge in longitudinal direction is presented and the natural vibration period of this model is calculated in four cases. These cases are: (1) exact solution, (2) by ignoring the rotational moments of inertia, (3) by ignoring the cap beam mass, and (4) by ignoring the pier and cap beam mass (AASHTO model). The periods obtained from exact analytical model were compared with those obtained from finite element model and the results showed the good conformity. The results also showed that the substructure rotational moments of inertia isn’t a determinative parameter in the calculation of the first longitudinal period; while the cap beam mass have a significant impact on the longitudinal period of base isolated bridges with tall piers or stiff isolators. The ignorance of cap beam mass caused 8% error in the period of such bridges. The results showed that the simplified method of AASHTO guide specification isn't applicable in some bridges and may cause more than 10% error. By increasing the ratio of isolator stiffness to substructure stiffness, the error of period which is calculated by AASHTO model, increases.