G.188.8.131.52 Modeling Rules for Pushover Analysis in STAAD.Pro
It is better to start with simple analytical model. The analytical model of the building should represent all new and existing components that influence the mass, strength, stiffness, and deformability of the structure at or near performance point. Elements and components shown not to significantly influence the building assessment need not be modeled.
The model should perform as expected under linear static loads and modal analysis. The control node shall be located at the center of mass at the roof of a building. It should be selected in such a way that the displacement component in the direction of lateral load monotonically increases as push load increases. Separate mathematical models representing the framing along two orthogonal axes of the building shall be developed for two-dimensional analysis. A mathematical model representing the framing along two orthogonal axes of the building shall be developed for three-dimensional analysis. Independent analysis must be done while using different methods of lateral load distribution.
Since Pushover analysis depends very much on the incremental push load defined in the input file by using LDSTEP parameter, it is better to give a lower value of LDSTEP parameter (preferably within 25) to start with. If the analysis does not run successfully and indicate warning message to increase LDSTEP parameter, parameter value will have to be increased. However, it may happen that the analysis has ran successfully and achieved the convergence criteria, the program is still issuing warning message to increase the LDSTEP parameter to get more accurate results. The value of LDSTEP parameter can be increased to get more accurate results but note that by how much LDSTEP parameter will be increased and what the exact value it will take depends upon the engineer’s judgement. By looking at the Capacity Curve graph, hinge formation and the hinge status (IO, LS, CP or greater than CP) at different load steps, engineer has to judge whether to increase the LDSTEP parameter or not. The type of warning message only gives an initial indication to the engineer whether the value of LDSTEP parameter will be increased or not.
*** WARNING : ANALYSIS RESULT IS NOT CONVERGING. UNABLE TO FIND A SOLUTION. TRY TO REDUCE INCREMENTAL PUSH LOAD BY INCREASING LDSTEP PARAMETER.
If the warning message is issued as mentioned above, LDSTEP parameter has to be increased.
*** WARNING : STRUCTURE HAS REACHED AN UNSTABLE EQUILIBRIUM WHERE ALL OF A SUDDEN MEMBERS HAVE STARTED TO FAIL. MORE ACCURATE RESULT MAY BE OBTAINED BY REDUCING INCREMENTAL PUSH LOAD BY INCREASING LDSTEP >PARAMETER.
If the warning message is issued as mentioned above, LDSTEP parameter may or may not be increased depending upon engineer’s choice.