The steel design checking according to AISC 360-10 reverse the sign of the axial load making compression as tension in the design stage leading to consider all columns (compression members) as tension members and vs. versa. Be aware of the this guys, and this is not acceptable from Bentley/STAAD to release updated version without checking for such big mistakes.
The new layout for the output of an AISC 360-10 providing more detailed information from the design process has indeed included a change in the sign being used to represent compression or tension forces in the heading. I agree that this should be reversed to maintain the sign convension used throughout the rest of the program. However, it is important to note that the design is correct in that compression and tension forces are used in the correct situations and with the correct sign. It is just the report that uses the opposite sign and this will be changed.
You are absolutely not right, please you need to verify that in deep, the design will not be correct as the load will be reversed and the stress ratio will consider the -ve sign. Here I am attaching a test file for you to see and investigate. As you will see there are some members failing since updated code to 360-10, while it was passing the 360-05, when i verify results, i got that the sign were changed. when you done the run, please click on the failed braces which are in compression and see what will design summary show you (also see columns will show tension/ there is small drop hanger with downward point load (actual in tension) but compression in result). Then see what you will see when you change the code in file to AISC 2005. see also what is the trend of stress ratios since there is no change in section E4 in 360-10 than it was in 360-05.
Regarding the file you sent structure1.std, Don't you see it shows the controlling load case 2 which is actually compression and section E4 of AISC 360-10 applied to it, while the checking was made according E1 (tension member) which means it is not only change the word of load case from Tension to compression but apply tension checking for compression member. Fortunately, customer still asking for compliance with 360-05, so there is time to test the issue and revised the software ASAP.
Regards
The sign is not the problem. I looked at member 167 which is simply a truss member so it's very straight forward yet, as you say, the results of 2005 and 2010 don't match. I looked up the available strength of 2L8x6x1/2 LLBB in Table 4-9 of AISC 2010. For L=12' the available strength is 194 kips and for L=14' the available stength is 183 kips (ASD). Your case is L=12.73' so the available strength should be between those two, yet STAAD got 123.9 kips for the 2010 code and 183 kips for the 2005 code. THIS is the problem. I don't have time to look into it further but this is the starting point for the investigation. In 2010 STAAD checked for flexural-torsional buckling which I'm not convinced is right. If it is right, why isn't this lower value given in the table? Anyway, this needs to be figured out.
OK, I've looked at this closer. As I guessed it appears that STAAD is not computing flexural-torsional buckling capacity correctly for double angles. To make things a little easier I used 2L6x6x1/2 so that we would not have to contend with slender elements. I used a length of 12 feet and a 3/8" seperation between the angles. Per section E4 of AISC 360-10 I got a capacity of 285 kips. In AISC Table 4-8 they got 286 kips so it looks like I did it correctly. In the STAAD report they got 203.681 kips. One possible difference could be in how many and what type of intermediate connectors are being used. I used the same criteria AISC used in coming up with the values in the table which in this case would be 2 pretensioned bolts. I thought perhaps STAAD just assumed one bolt in the middle but I still couldn't get their number. I see no means to tell STAAD how many and what type of intermediate fasteners will be use and I didn't see where their summary shows anything. This clearly needs to be improved.
In addition to this issue the results are a mess. I've attached a screen shot of the Steel Design tab. Note the following:
1) PNC is shown as 271.8 kips but that is not the controlling compression load. According to their own summary in the ANL file (below) it should be 203.681 kips (which as I said I couldn't get this value)
2) The thing that I think the OP is on about, next to FX it has a "T" for tension but this is a compression case! Yes, I understand that internally the program realizes it actually compression but I don't think the guys at Bentley understand how big a problem this is. When your looking at a model with thousands of members and hundreds of load cases you tend to trust what the program is telling you. This could lead to big problems in any subsequent hand checks you might do for a connection or whatever.
3) The ratio is listed as 0.3869496 (why do show so much precision, that's ridiculous but that's beside the point). The report in the ANL file also says the critical ratio is0.387 but the highest ratio I could find in the report was 0.245 for flexural torsional buckling. What am I missing?
4) Under Critical it says "SLENDERNESS". Right next to it, it says KLR = 77.38992 which is less than 200. What do you mean SLENDERNESS?
As you can see there are some serious problems with the code checking in AISC 360-10. I hope other users will join me in urging Bentley to correct the problems quickly and redeploy a patched version of STAAD.
STAAD.PRO CODE CHECKING - (AISC-360-10-LRFD) v1.0 ********************************************
ALL UNITS ARE - KIP INCH (UNLESS OTHERWISE NOTED)
-------------------------------------------------------------------------------- MEMBER NO: 1 CRITICAL RATIO: 0.387(PASS) CRITICAL LOAD CASE: 1 <---- Where does 0.387 come from the highest value below is 0.245? LOCATION(ft): 0.00 CRITICAL CONDITION: SLENDERNESS SECTION: L60608 (AISC SECTIONS) CB: 1.00
UNIT: KIP FEET
CRITICAL DESIGN FORCES: Fx: -50.00 Fy: 0.00 Fz: 0.00 Mx: 0.00E+00 My: 0.00E+00 Mz: 0.00E+00 SECTION PROPERTIES: AZZ: 11.500 AYY: 6.000 CW: 2.640 SZZ: 9.227 SYY: 12.950 IZZ: 39.816 IYY: 80.128 MATERIAL PROPERTIES: FYLD: 36.000 FU: 58.000 ACTUAL MEMBER LENGTH(ft): 12.000 PARAMETERS: KX: 1.000 KY: 1.000 ACTUAL SLENDERNESS RATIO: 77.390 ALLOWABLE SLENDERNESS RATIO: 200.000 SECTION CLASS: FLANGE:/ l: lp: lr: WEB: COMPRESSION: Non-Slender 12.000 12.772 N/A Non-Slender 12.000 12.772 N/A FLEXURE: Compact 12.000 15.326 25.828 Compact 12.000 15.326 25.828 TENSION: FORCE: CAPACITY: RATIO: CRITERIA: LOAD CASE: LOCATION(ft): 0.000 372.600 0.000 Eq. D2-1 1 0.000 COMPRESSION: FORCE: CAPACITY: RATIO: CRITERIA: LOAD CASE: LOCATION(ft): MAJOR: 50.000 271.838 0.184 Sec. E1 1 0.000 MINOR: 50.000 318.567 0.157 Sec. E1 1 0.000 INTERMEDIATE: Ag: KL/r: Fcr: Fe: Pn: MAJOR: 11.500 77.390 26.265 47.789 302.043 MINOR: 11.500 54.553 30.779 96.175 353.964 FLEX TOR BUCK: FORCE: CAPACITY: RATIO: CRITERIA: LOAD CASE: LOCATION(ft): 50.000 203.681 0.245 Sec. E4 1 0.000 <---- How do they get 203.681 kips? INTERMEDIATE: Ag: Fcr: Pn: 0.000 19.679 226.313 <---- Why is Ag = 0? SHEAR: FORCE: CAPACITY: RATIO: CRITERIA: LOAD CASE: LOCATION(ft): MAJOR: 0.000 116.640 0.000 Eq. G2-1 1 0.000 MINOR: 0.000 116.640 0.000 Eq. G2-1 1 0.000 INTERMEDIATE: Aw: Cv: Kv: h/tw: Vn: MAJOR: 6.000 1.000 1.200 12.000 129.600 MINOR: 6.000 1.000 1.200 12.000 129.600 STAAD SPACE -- PAGE NO. 5
YIELDING: FORCE: CAPACITY: RATIO: CRITERIA: LOAD CASE: LOCATION(ft): MAJOR: 0.00E+00 3.99E+01 0.000 Sec. F1 1 0.000 MINOR: 0.00E+00 5.59E+01 0.000 Sec. F1 1 0.000 INTERMEDIATE: Mnr: My: Cb: Lp: Lr: Lb: MAJOR: 4.43E+01 0.00E+00 1.000 0.000 0.000 12.000 MINOR: 6.22E+01 0.00E+00 1.000 0.000 0.000 12.000
LAT TOR BUCK: FORCE: CAPACITY: RATIO: CRITERIA: LOAD CASE: LOCATION(ft): MAJOR: 0.00E+00 5.69E+02 0.000 Sec. F1 1 0.000 INTERMEDIATE: Mn: Me: Cb: Lp: Lr: Lb: MAJOR: 6.32E+02 0.00E+00 1.000 0.000 0.000 12.000
INTERACTION: RATIO: CRITERIA: LOAD CASE: LOCATION(ft): FLEXURE COMP: 0.245 Eq. H1-1a 1 0.000 FLEXURE TENS: 0.000 Eq. H1-1b 1 0.000 INTERMEDIATE: Mcx: Mcy: Mrx: Mry: Pc: Pr: FLEXURE COMP: 3.99E+01 5.59E+01 0.00E+00 0.00E+00 203.681 50.000 FLEXURE TENS: 3.99E+01 5.59E+01 0.00E+00 0.00E+00 372.600 0.000
Hi Randy,
We are investigating these issues. We did observe few discrepancies in the design check as per AISC 360-10 and the matter is being looked into by our development team with topmost priority. We are going to get the issues addressed through a patch release within the next couple of weeks.
Thanks,
Sye
Sye,
Thanks for getting back. I hope that once your team has looked into this you will give us users some feedback as to what you fixed. Quite frankly I've noticed historically when there are several issues like this, one or two get fixed and the others languish.
I'm especially interested in why STAAD gets a different value for the angles in compression that I described from the AISC table. If you're making a different assumption than AISC did in creating the table we need to know what that assumtion is. If it's just a plain 'ole mistake then obviously that needs to be fixed.
What is the progress of this? When are we to expect the necessary patch?
This is a reply to what RKillian had found out which is i believe is quite very interesting.
Although the max ratio you are seeing based on demand vs capacity is 0.245, the most critical ratio is 0.387. That is calculated by dividing the actual slenderness by the allowable for compression which is 200. Therefore, 77.4 (actual slenderness) divided by 200 (allowable slenderness) will give you 0.387. However, the FX says it's in tension but this is actually in compression. If indeed it is in tension, then the allowable slenderness should be 300 and not 200.
When will this issue be closed down and solved? I am waiting...