What is a Fixed End Member Load?

 Applies To 
 Environment: N/A
 Area: Modeling Solutions
 Subarea: Loading
 Original Author:Kris Sathia, Sanjib Das & Rajat Chakroborty

What is a Fixed End Member Load?

One of the steps involved in the stiffness analysis method is to convert a load acting on a member into FIXED END ACTIONs. That means, the member is treated as a fixed-fixed beam (called locking all degrees of freedom), the 6 reactions at start and 6 reactions are the end are calculated, and their algebraically opposite values are referred to as the 6 equivalent fixed end actions at start and 6 at end.

The success of doing this depends upon the variety of load types for which the program can calculate the corresponding FIXED END LOADs. STAAD for example is capable of handling concentrated loads within the member span, uniform distributed loads applied over a part of the length as well as over the full length of the member, triangular loads and trapezoidal loads. But what if the user wants to apply a parabolically varying member load? Or a load which is defined using a cubic function? There is no built-in facility to handle such loads because the program does not know how to convert them into their corresponding fixed end actions.

That is where the FIXED END LOAD can help. If the user knows how to calculate (by hand) the FIXED END actions for one of these unusual load types, he can apply the corresponding FIXED END actions in the STAAD input file using the FIXED END LOAD type. The advantage is that he will get the joint displacement.

However, there is a drawback to using this facility. While the joint displacements, support reactions, and member end forces for all the entities of the structure will be calculated accurately, the intermediate section forces will not be accurate for that specific member. That is because, the program does not know the loads acting on the span. One needs to know the variation of load along the span to calculate section forces. It has only been provided with an equivalent set of end actions for that load. So, the section forces and displacements cannot be calculated correctly for those members on which such loads have been applied. Instead of this, a better option would be to replace the load with an equivalent set of closely spaced concentrated forces.