General error with models having ST 3 parameter in IS:1893 seismic definition

 Applies To 
 Environment: N/A
 Area: STAAD.Pro Analysis Solutions
 Subarea: Seismic Analysis
 Original Author:SANJIB DAS

Error in analysis as shown below:


In the IS:1893 seismic definition, you have used ST 3 parameter. This specifies that the type of structure is 3 which represents the moment resisting frame with brick infills. For this type of structures, approximate fundamental natural time period (Ta) is calculated as per cl-7.6.2 of IS:1893-2002.


Ta= (0.09 x h)/√d


Where, h= height of the frame

D= base dimension of the building at plinth level along X and Z direction (i.e, considered direction of lateral force)


The program fails to calculate base dimension in X and Z direction as at the lowest level there are no nodes created for any structural elements which happens if the supported nodes are at different Y co-ordinates. As a result of that time period on that direction can’t be calculated. To get rid of this problem, you can calculate the approximate fundamental natural time period (Ta) as per cl-7.6.2 in both the direction and provide the same in PX and PZ parameters. Here, you need to omit the ST 3 parameter.

Say, if you originally used the following commands:

ZONE 0.16 RF 5 I 1.5 SS 3 ST 3 DM 0.05

Change it to the following

ZONE 0.16 RF 5 I 1.5 SS 3 DM 0.05 PX T1 PZ T2

T1,T2: manually computed value of Ta from cl-7.6.2