change command in staad

dear all,

why we have to use the commands 'perform analysis' and 'change' commands in earth quake load cases.
please give exact reason for using those commands and what is the warning staad showing if we din't use those commands.

thank you all,
kommuri

If you are not using those commands its show

"**WARNING: IF THIS UBC/IBC ANALYSIS HAS TENSION/COMPRESSION OR REPEAT LOAD OR RE-ANALYSIS OR SELECT OPTIMIZE, THEN EACH UBC/IBC CASE SHOULD BE FOLLOWED BY PERFORM ANALYSIS & CHANGE."

Reason for this warning is :

In STAAD, when you analyse a model containing UBC, IBC, IS1893 e.t.c or any other seismic loads, the program (staad) has to first generate lateral loads as per the rules of that code.

if you are not using Perform analysis , change command .. the staad will simply skip the calculated lateral loads .. then there will be no seismic effect...

For Example :

LOAD 1
IBC LOAD X 1.0
LOAD 2
IBC LOAD Z 1.0
LOAD 3 GRAVITY
SELF Y -1.0
LOAD 4 LIVE
MEMBER LOAD
..
..

LOAD 21 DEAD + SEISMIC IN X
REPEAT LOAD
1 1.0 3 1.0

LOAD 22 DEAD + SEISMIC IN Z
REPEAT LOAD
2 1.0 3 1.0

then, load cases 21 and 22 must have access to the lateral load values generated in cases 1 and 2 respectively.

This means that STAAD must be told to retain those generated load values. If it is not told to do so, it will simply "forget" those load values once it has finished processing cases 1 and 2. Consequently, cases 21 and 22 will not reflect any effects of the seismic loads.

The warning message you saw is merely intended to remind you of these facts........

But praveen u no need to worry of these warnings.........

This is particularly important in situations where tension- or compression-only member specifications or compression-only support specifications have been used...

hope it will help you... have a nice day...

keep asking challenging Questions it will help all of us to get alert & learn something new...