To auto generate earthquake loads, Substructure uses the single mode spectral method. Bridge deck and columns are modeled as horizontal and vertical members respectively. All columns (vertical members) are assumed to be fixed at the base. At the abutments, the bridge is assumed to be hinged. You must define the cross-section area and moment of inertia of the superstructure. For columns, Substructure uses the total moment of inertia and area for all columns, as default values. However, you can manually modify these values. The pier height is the average height of all columns. Column modulus of elasticity is used for the whole structure.

In Substructure v1.3 and later, in bridge longitudinal direction, column member stiffness is computed as a cantilever member fixed at base (3EI/L3). The transverse direction column stiffness is computed as a frame member (12EI/L3). The program automatically computes self-weight of the superstructure, which is based on the first generated dead load case. If no such load case is present, then the program computes the dead load on the pier to middle of span on each side and load of cap and load of column to mid-height is added to it. This load is then divided by the average of two span lengths to arrive at the computed value of the superstructure unit weight. However, if the user cans this value manually, that value will be used for the generation of EQ loads.

An assumed uniform longitudinal load is applied to the bridge deck and static displacements, vs(x) are calculated, assuming only that the columns resist the load. Then, factors a, b, and g are computed. These factors are used to determine the period, T. The seismic response coefficient, Cs, is computed using the calculated period. Next, the intensity of longitudinal seismic loading is computed. The calculated force is evenly distributed among all piers (with no contribution to abutments). The pier force is then equally distributed among all the bearings for the pier under consideration.

For transverse loading, a similar approach is used as for the longitudinal direction. However, transverse displacements are computed with the bridge modeled as a three dimensional frame with line elements. Once the displacements are known, factors a, b, and g is computed. These factors are used to determine the period, T. Next, seismic response coefficient, Cs, is computed using the calculated period and intensity of parabolic transverse seismic loading. This loading is converted to concentrated loads applied at quarter points of each span and the transverse reaction on the pier is computed. This reaction is then distributed equally among all bearings for the pier.