Question

I have a technical question regarding the correct use of fiber hinge elements in the SAP2000 program. I believe that other hinges (for example moment hinges or shear hinges) have a rigid-plastic behavior. Placing these hinges in a model composed of frame elements should not alter the elastic stiffness of the model. However, it is my understanding that the SAP fiber hinge has an elastic stiffness, based on the material properties. Since it occurs at a point, but computes hinge demands based on a hinge length provided by the user, I find that it introduces "artificial" flexibility to my model. By this I mean that if I defined a single degree of freedom cantilever column and computed its stiffness without any hinges, the period would be shorter than an identical model with a fiber hinge with identical material properties.

Can you help me to understand how to implement the use of the SAP2000 fiber hinge without introducing additional flexibility into my model?

Answer

The reason you are noticing this softening is because the fiber hinges response curve is calculated directly from the response of individual section fibers and the elasticity in the linear range of the hinge is added to the elasticity of the element. To avoid this softening you should discretize the member at hinge locations (divide the frames to create a small element with a length equals to the hinge length) and assign a large area property modifier to the small sections by using the Assign>Frame>Property modifiers menu command.