Approximate Method for Frame Subjected to Lateral Load

Forces developed in statically indeterminate frame subjected to lateral load can be calculated using approximate method. There are two approaches to do so: portal method and cantilever method. Both methods made the same assumption, where the hinge indicating the point of zero moment is located at the middle of frame members. These points are also the point of inflexion for the frame elastic deformation curve. Before conducting analysis, the frame needs to be dismantled at the hinges.

In portal method, the dismantled frame is divided into subframes, and the shear force developed in the hinge of each column is calculated one by one. After column shear forces are known, remaining forces in the hinges of both columns and girders are calculated solely using equilibrium equations.

In cantilever method, the centroidal of columns is first determined. Then, by taking moment about the axis, resisting moment induced by column axial force is determined. The applied moment is simply the product of lateral load and lever arm. After that, equilibrium equations are good to use to identify the remaining hinge forces.

Portal method is applicable for frame with low elevation and uniform column sections. When neither of the condition is fulfilled, cantilever method is recommended.

The following introduces the calculation of member forces for frame subjected to lateral loads using approximate method. Watch the video above for full details.