David,
I looked into the issues you reported and here are my observations
Regarding your query 1, this is how the ratio of 1.251 is being arrived at. The section is subjected to bending + compression and fa/Fa = 3/24.63 = 0.122 < 0.15. Hence the software is checking as per the equation H1-3. Now for checking the bending stress for the unsymmetrical section, it is conservatively considering the minimum section modulus which is calculated at the bottom ( = 2.81297x12^4/31.418) = 1518.29 in3. In this case, as you mentioned, the top flange is in compression and so it would have been appropriate to use the section modulus corresponding to the top flange. However for un-symmetric sections the software makes a conservative assumption and uses the minimum out of the two moduli. Hence bending stress is calculated as 3162.57x12/1518.29 =24.995 ~ 25 ksi. Allowable bending compression is calculated as 22.13 Ksi and hence the equation H1-3 results in a ratio of 0.122 + 25/22.13 = 1.251.
Regarding your query 2, the latest AISC codes like 360-05 or 10 should use a more exact approach compared to a conservative one but I will double check on that from our development desk and let you know.
Regarding your query 3, I agree that both should generate the same FA of 24.63 Ksi and I have referred the matter to our development desk for review.
Regarding your point 4, my hand calculation exactly match your’s and I agree that the allowable bending compression should be 24.69 Ksi based on equation F1-6. I have referred this one to our development desk for review too and will revert back.
Your patience in this matter is greatly appreciated.
Thanks,
Sye
I looked into the issues you reported and here are my observations
Regarding your query 1, this is how the ratio of 1.251 is being arrived at. The section is subjected to bending + compression and fa/Fa = 3/24.63 = 0.122 < 0.15. Hence the software is checking as per the equation H1-3. Now for checking the bending stress for the unsymmetrical section, it is conservatively considering the minimum section modulus which is calculated at the bottom ( = 2.81297x12^4/31.418) = 1518.29 in3. In this case, as you mentioned, the top flange is in compression and so it would have been appropriate to use the section modulus corresponding to the top flange. However for un-symmetric sections the software makes a conservative assumption and uses the minimum out of the two moduli. Hence bending stress is calculated as 3162.57x12/1518.29 =24.995 ~ 25 ksi. Allowable bending compression is calculated as 22.13 Ksi and hence the equation H1-3 results in a ratio of 0.122 + 25/22.13 = 1.251.
Regarding your query 2, the latest AISC codes like 360-05 or 10 should use a more exact approach compared to a conservative one but I will double check on that from our development desk and let you know.
Regarding your query 3, I agree that both should generate the same FA of 24.63 Ksi and I have referred the matter to our development desk for review.
Regarding your point 4, my hand calculation exactly match your’s and I agree that the allowable bending compression should be 24.69 Ksi based on equation F1-6. I have referred this one to our development desk for review too and will revert back.
Your patience in this matter is greatly appreciated.
Thanks,
Sye