Beam Design - Derivation of Bending and Axial Load Capacity

[Anonymous]

I am designing a beam that also has axial load applied to it and trying to understand how ASBD is checking its capacity.

Within the results calculation it states that the ultimate moment capacity is in conjunction with the axial load: 

Noting: this is  under the heading of a column buckling load check (AS5100-5 10.4.4). 

Please could you confirm how this Moment capacity is calculated? 

Initially i thought it was using some form of column interaction diagram, however, if I increase the axial load to an extreme amount, the beam still has some moment capacity and does not state the member is beyond its squash load limit:  

Overall I am wanting to know, do i need to complete a separate column check for buckling/column interaction or does ASBD have the check already nestled within the calculation? 

Thanks in advance for any assistance! 

[dave_geeves · Accepted solution]

Hi,

Although I know ASBD quite well, so I know the basic principles on how the software generally works, I am not an expert on AS5100 so I had to do some testing to see how the calculations came out for me.  I wasn't sure what version of AS5100 you were referring to so I tried both and found it essentially made no difference to these calculations.

I created a standard simply supported Super T girder and applied some standard loading - without any axial loads. I I extracted a design section at mid span from the beam to establish what the axial load capacity was (on its own), which turned out to be about 30000kN.

I then created further live load case identical to the first load case but with additional constant axial forces of 1000kN,  2000kN, 5000kN, 10000kN, 20000kN and 40000kN.

Looking at both graphical and printed results for ULS capacity seemed to give consistent and expected  results with the moment capacity reducing as the axial load increased - except with the last two load cases, where the section failed, and the calculations did not seem to make any sense except that it can be clearly seen that the applied moment exceeded the capacity.

I inspected the first 5 load case results and made checks on the values by referring back to the design section I created and checking the values here, which all seemed to make sense.  Below are the results for the 5000kN axial load case which I have commented on.

Design Beam Results for Ax=5000

Calculation of Mub

Verification of Besign Beam capacity using design section analysis

So, in conclusion, I believe that the design beam results are generally correct and when the calculations indicate that the moment of resistance is that in conjunction with a specific  axial force then this is correct.  I can also confirm that the value of Mub calculated as a buckling load parameter is also correct but that the program does not use this and it appears is just for reference.  In fact I'm not sure if this check is relevant to PS beams as this appears to be for the design of columns … although I suspect that the developers may have put it there for completeness as they were not sure either.  If I am wrong about this I'm sure they will tell us.

I'm not sure what is happening to the calculations when the axial force exceeds a certain high value but hopefully the developers will note this and make comment.

I hope this has been helpful and has answered your main questions, and if so please mark my reply as a solution so that others may benefit.  If there is still some doubt then please send me your data file by zipping it up and attaching it to a reply, together with some comments on what you actually expect.  Thanks

Kind regards

Dave Geeves

[Anonymous]

Hi Dave, 

Thanks for getting back to me so quickly and with such a comprehensive response! 

You answered the majority of my queries and, coincidently, I found another thread of yours that closed out any remaining questions. 

I've attached your other thread here for completeness so that others may benefit - https://forums.autodesk.com/t5/structural-bridge-design-forum/checking-of-interaction-surface/td-p/9...

Thanks again! 

Kind Regards, 

Thomas