There have been a few changes in soft storey checking in recent versions of STAAD.Pro. In order to perform soft storey checking, one of the following is needed in the model:
1. The FLOOR HEIGHT command
2. Rigid Floor Diaphragms
On finding those commands, STAAD.Pro identifies the position (Y coordinate) of the floors in your model. Soft Storey checks are done only if the CHECK SOFT STOREY command is included in the model. Also, the PRINT STOREY DRIFT command is needed to instruct STAAD to check for storey drift.
The FLOOR HEIGHT option is needed if there are no clearly identifiable floor levels in the model, or, if you want to define floor levels on your own instead of having the program scan for them on its own. These commands have to be specified before the IS:1893 seismic definition in the following format.
y1; y2; y3; ……….yn
The first of the floor heights, namely, y1, should be the fixity level (level where supports are specified). In case of supports that are scattered at different Y levels, use the Y value of the one that has the lowest Y coordinate. FLOOR HEIGHT commands should be specified before the DEFINE 1893 command block as well as before the primary Response Spectrum load cases. In the attached model, the following commands have been provided before the IS:1893 seismic definition.
0; 1.125; 2.25; 3.375; 4.5; 5.625; 6.75; 7.875; 9; 10.5; 12
The CHECK SOFT STOREY command should be specified immediately following the IS1893 seismic definition and before the start of the first load case.
In order to specify RIGID FLOOR DIAPHRAGMs, a MASS REFERENCE LOAD case needs to be specified. In such a reference case, the weights that go into the making of the mass matrix (often called mass modelling) are specified in exactly the same manner as one specifies weights in a dynamic load case such as response spectrum or time history. In the attached model the following commands are used:
DIAPHRAGM 1 TYPE RIGID HEIGHT 0.45
DIAPHRAGM 2 TYPE RIGID HEIGHT 3.8
DIAPHRAGM 3 TYPE RIGID HEIGHT 6.85
DIAPHRAGM 4 TYPE RIGID HEIGHT 9.9
DIAPHRAGM 5 TYPE RIGID HEIGHT 12.95
DIAPHRAGM 6 TYPE RIGID HEIGHT 16.0
DIAPHRAGM 7 TYPE RIGID HEIGHT 19.05
DIAPHRAGM 8 TYPE RIGID HEIGHT 22.1
DIAPHRAGM 9 TYPE RIGID HEIGHT 25.15
CHECK SOFT STOREY 1893
Results of soft storey checks can be found in the output file as shown in the next figure.