No changes in ratio

Question

Dear sir, 
 I designed one portal frame, even though i changed wind load from 2 kg/m to 5000kg/m. the utilization ratio does not change for rafter, but their is a changes in column. Please check the attached image and give the solution. Thanking you.

Sye · Accepted Answer

The ratios for member 1 and 2 were 0.025 when the applied wind load was 0.02 KN. The ratios changed for both columns to 0.918 when the wind load was increased to 50 KN. 
 For the member 3, the failure was due to slenderness as I already explained in my earlier post and so this ratio is independent of the load you apply. Hence ratio would not change even if you increase the load from 0.02 KN to 50 KN. You need to either bypass the slenderness check or use a bigger section to make it pass. 
 The 350 mm+ displacement I was referring to, was for the case when wind load was increased to 50 KN. The screenshot below shows that

Sye · Answer

From the screenshot I see that the rafter is failing in slenderness. For CODE AISC, slenderness is checked for any member prior to designing it for other cases and if the slenderness check fails, the Clause is reported as L/R-EXCEEDED. Since that condition remains same for both scenarios, your ratio is not changing. If you want to bypass the slenderness check for any particular member, you may assign the MAIN parameter to that member and set it to a value of 1.

Sye · Answer

To get rid of the instability warnings you may change the supports as noted below 
SUPPORTS 
*1 2 PINNED 
1 2 FIXED BUT MZ 
 
In design parameter, you do not need the following parameter as you do not have tapered section 
TAPER 0 ALL 
 
MAIN 0 ALL is not required as slenderness is checked by default anyway. 
 
Your beam member 3 is failing in slenderness. If you do not want to check slenderness for the beam, you may set MAIN 1 for that beam. However once I by passed the slenderness check I found that the member is failing in strength check as well. So seems like you need a bigger section than what you are using now. 
 
But even before you worry about the design part, there is another very important thing you need to check and that is the lateral displacement. You have high X displacement of 358 mm for lateral load cases 3 and 4. This is because you have pinned supports at the base. Check whether this amount of displacement is acceptable or not. If not, you may have to increase the structure's lateral stiffness. Using stronger columns, fixing the bases, adding bracing etc. are some of the options.