Cross Bracing in Staad

[cgstrucg]

Hello,

I have an ordinary frame which has some cross bracings in it. The unit is actually to support small HVAC equipment. I have created a basic model to see if my angles will be able to take that load or not. However, I am designing cross bracings in staad for the first time and instead of designing them as tension only members, I went with releasing My, Mz approach as I want to design them for compression too. Can someone please look at the code and tell me if anything is wrong in it. I am not sure I am getting such less unity ratios. I was expecting less but not to this extent.

Also, the reason to use cross bracings in this frame is to reduce the effective length of angles in both local y and z-direction. Can I just use single diagonal brace with My and Mz releases at the start and end to effectively use the angle capacities and have effective length more then recommended in codes in one direction only?

Thanks

Code -

STAAD SPACE
START JOB INFORMATION
ENGINEER DATE 17-Apr-19
END JOB INFORMATION
INPUT WIDTH 79
UNIT FEET KIP
JOINT COORDINATES
1 0 0 0; 2 0 16 0; 3 2 0 0; 4 2 16 0; 5 0 0 3; 6 0 16 3; 7 2 0 3; 8 2 16 3;
9 2 15 3; 10 0 15 3; 11 0 15 0; 12 2 15 0; 13 0 8 3; 14 2 8 3; 15 0 8 0;
16 2 8 0;
MEMBER INCIDENCES
1 1 15; 2 3 16; 3 5 13; 4 7 14; 5 2 6; 6 4 8; 7 10 9; 8 9 8; 9 10 6; 10 11 2;
11 12 4; 12 11 12; 13 13 10; 14 14 9; 15 15 11; 16 16 12; 17 13 11; 18 15 12;
19 16 9; 20 14 10; 21 16 11; 22 15 10; 23 13 9; 24 14 12;
DEFINE MATERIAL START
ISOTROPIC STEEL
E 4.176e+006
POISSON 0.3
DENSITY 0.489024
ALPHA 6e-006
DAMP 0.03
TYPE STEEL
STRENGTH FY 5184 FU 8352 RY 1.5 RT 1.2
END DEFINE MATERIAL
MEMBER PROPERTY AMERICAN
1 TO 24 TABLE ST L35358
CONSTANTS
BETA 45 MEMB 3 5 7 9 13
BETA 135 MEMB 4 8 14
BETA 225 MEMB 2 11 16
BETA 315 MEMB 1 6 10 12 15
MATERIAL STEEL ALL
SUPPORTS
1 3 5 7 PINNED
MEMBER RELEASE
5 TO 7 12 17 TO 24 START MY MZ
5 TO 7 12 17 TO 24 END MY MZ
LOAD 1 LOADTYPE Dead TITLE LOAD CASE 1
MEMBER LOAD
5 6 CON GY -0.15 0.1
5 6 CON GY -0.15 2.8
7 12 CON GY -0.3 1
PERFORM ANALYSIS
PARAMETER 1
CODE AISC UNIFIED 2010
LY 8 MEMB 1 TO 4 8 TO 11 13 TO 16
LZ 8 MEMB 1 TO 4 8 TO 11 13 TO 16
METHOD LRFD
RATIO 0.8 ALL
FU 7200 ALL
FYLD 5184 ALL
CHECK CODE ALL
FINISH

 

[WARose]

A couple of comments:

1. There is no lateral load in this thing, so the bracing won’t be “engaged” to really do anything. The unity ratio is likely based on unbraced length. (More on that in a moment.)

2. For a static analysis, I typically designate bracing as “truss” members rather than using releases. (There is a command for that in STAAD.)

3. There is an instability warning in the output file. But when I rotationally fixed the bases (i.e. about the Y-axis), that went away.

4. Your post alludes to the unity ratio in the angles. Be advised: the unity ratio STAAD will spit out at you will be meaningless for eccentrically loaded members like angles, WTs, etc. STAAD doesn’t know they are eccentrically loaded. You’d have to check those members manually. (AISC has tables for that.)

5. I don’t follow your question (at the end of the post).

 

[cgstrucg]

Thank You that makes sense and helped me understand the concept better.

Regarding Point 5 - I meant that if I use an angle long enough that it exceeds its effective unbraced length criteria then I have to reduce it's unbraced length by providing say diagonal bracing. In my model, I have provided cross bracings in both x and z-direction. So can I just provide bracing in one direction say x and let effective unbraced length be length full length for z-direction?