FAQ1:
I have specified Rigid Diaphragms in my model. But when I run the STAAD file, it stops with the following warning.
** ERROR: NO REFERENCE LOAD CASE OF TYPE MASS/GRIVITY/DEAD/LIVE IS DEFINED FOR MASS MODELING.
CENTRE OF MASS CALCULATION IS NOT POSSIBLE.
Answer:
If you are specifying Rigid Diaphragms, it is a must that you specify the Mass reference Load Case. One of the important functions of specifying the Rigid Diaphragms is to calculate the torsion on the floor due to seismic loads and the program needs to identify the center of mass for the same. This is done with the help of the mass data defined in the mass reference load case.
FAQ2:
Which masses in the Mass reference Load Case is used to calculate the centre of mass of the diaphragm?
Answer:
By default, the masses specified in the Global Y directions are considered for the mass calculations. In absence of global masses in the Y directions, the masses along the Global X directions are considered for the same. In absence of the masses specified along the Global X and Global Y directions, the masses along Global Z directions are considered.
FAQ3:
If a Floor Diaphragm is modelled to represent a slab entity, is the weight of the slab automatically accounted for if we specify the SELFWEIGHT load item?
Answer:
No. Note that Floor Diaphragm is not an actual physical entity. The height at which the floor diaphragm is defined only helps in linking the master node with all other nodes at that level using SLAVE ZX entity. The Master node is located at the center of mass of that floor, calculated from the mass data in the reference load case. The Rigid Diaphragm only ensures the inplane rigidity of the slab to be accounted for in the analysis.
Thus, the selfweight of the slab needs to be separately specified as floor loads or equivalent member loads on the beams forming the slab panels.