BatMan
Structural
- Feb 23, 2014
- 41
Some advise needed here....
A braced-frame consists of tension-only element. Now, the analysis was done with Response spectrum method. Since the response spectrum method is linear analysis and the brace was modelled with beam-element having zero compression limit (requires non-linear analysis) and not a true cable, compression forces will be picked up in the brace during the analysis.
I'm trying to estimate the column forces, but a bit puzzled on this.
One suggests to double up on the brace tension loads (I sort of get this, since model picks up compression load), and use column dead load taken with modal mass ratio from z-acceleration (???), and take SRSS of each terms to estimate the column force. Now this, I'm a bit to hesitant to accept. I've attached a sketch as it'd be easier to see what I'm blabbering about...
Dead load on column is say 17 kips. Column load on SRSS from response spectrum analysis is say 84 kips (x, y and z-component accelerations). Dumping the doubled up braces forces under same SRSS combination is about 102 kips. The modal mass ratio from z-acceleration only is 0.24 (or so they claim). The resulting column load due to vertical acceleration by taking the modal mass ratio results in very small load (0.24 x 17 kips). However, specified vertical response spectrum was 70% of horizontal and I expected alot larger magnitude of column load due to vertical effect.
Without having an actual model to play around with, my confusion remains.. Is this something that's typically done for tension-only braces ran with response spectrum analysis? Any feedback is greatly appreciated..
A braced-frame consists of tension-only element. Now, the analysis was done with Response spectrum method. Since the response spectrum method is linear analysis and the brace was modelled with beam-element having zero compression limit (requires non-linear analysis) and not a true cable, compression forces will be picked up in the brace during the analysis.
I'm trying to estimate the column forces, but a bit puzzled on this.
One suggests to double up on the brace tension loads (I sort of get this, since model picks up compression load), and use column dead load taken with modal mass ratio from z-acceleration (???), and take SRSS of each terms to estimate the column force. Now this, I'm a bit to hesitant to accept. I've attached a sketch as it'd be easier to see what I'm blabbering about...
Dead load on column is say 17 kips. Column load on SRSS from response spectrum analysis is say 84 kips (x, y and z-component accelerations). Dumping the doubled up braces forces under same SRSS combination is about 102 kips. The modal mass ratio from z-acceleration only is 0.24 (or so they claim). The resulting column load due to vertical acceleration by taking the modal mass ratio results in very small load (0.24 x 17 kips). However, specified vertical response spectrum was 70% of horizontal and I expected alot larger magnitude of column load due to vertical effect.
Without having an actual model to play around with, my confusion remains.. Is this something that's typically done for tension-only braces ran with response spectrum analysis? Any feedback is greatly appreciated..