Problem Description
This example uses a single-bay, single-story rigid frame to test the gap link element. This link element carries compression loads only; it has zero stiffness when subjected to tension. The gap element is paced at the bottom of the righthand column in the frame. The frame is then loaded with a gravity load P (10 kips) at the center of the beam. Once the full load P is applied, a lateral load V (20 kips) is applied, pushing the frame from right to left. The compression load in the gap element after the full load P has been applied and the uplift at the gap after the full load V has been applied are compared with independent hand calculated results. The model is created in the XZ plane. Only the Ux, Uz and Ry degrees of freedom are active for the analysis. The gap element is modeled as a single-joint link element at joint 2. This means that one end of the gap element is connected to the ground and the other end is connected to joint 2. The gap element is oriented such that its positive local 1 axis is parallel to the positive global Z axis. This is the default orientation of single joint link elements. Only U1 degree of freedom properties are defined for the gap element. All three frame elements have identical properties. The compression stiffness of the gap element is chosen to be approximately 100 times the axial stiffness of the frame element directly above it, frame element 2. The initial gap element opening is zero inches. The loading is applied as follows. First the full load P is applied. Then, while the load P is maintained, the full load V is applied. The analysis is run several times using different types of load cases. Nonlinear static, nonlinear modal time history and nonlinear direct time history load cases are used. See the subsequent section titled Summary of Load Cases for more information. Two different models are used in this example. In Model A the linear effective stiffness of the gap element is set equal to zero. In Model B the linear effective stiffness of the gap element is set equal to the nonlinear stiffness of the gap element.
The gap linear effective stiffness is only used for the linear load cases, which in this example are load cases P, V and MODAL. The gap linear effective stiffness is not used in the other load cases and thus has no direct effect on them; however, it does indirectly affect the nonlinear modal time history cases named NLMHIST1 and NLMHIST2 because those cases are solved using the modes from the load case named MODAL.
The lumped joint masses shown in the table to the right are used for the modal and modal time history analyses.
Tested Features
- Gap element links
- Force-controlled nonlinear static analysis
- Nonlinear modal time history analysis
- Nonlinear direct time history analysis
- Frame point loads
- Joint force loads
- Joint mass assignments
- Ramp loading for time histories
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