single equal angle bending with tip in tension - ASD 1

[tweedledee]

Hi,
I need help understanding the specification for single angle bending in the ASD.

The ASD section 5.2.2 says that for an unbraced equal angle with the tip of the leg in tension, that "Fb is determined only by section 5.1.2".

5.1.2 indicates Fb=0.66Fy.

Does this mean that we do not need to check for lateral torsional buckling whenever the tip is in tension??

Is the reason for this the fact that the tip of the angle in tension will ALLWAYS reach 0.66Fy before buckling of the compression leg will occur??

 

[aggman]

tweedledee,
From my interpretation of the code, and from my experience I would say that you can't have lateral torsional buckling when the tip is in tension IF you are braced to resist lateral torsional buckling of the section itself. Note that the member must have lateral torsional restraint to use the geometric axis procedure. If you do not have restraint to resist lateral torsional buckling then you should design the member using the principle axis procedure. This accounts for the tendacy of the member to translate out of plane. See section 5.2.2 and 5.3. Section 5.2.1 defines the limitations which must be met for the procedure of 5.2.2 to be used.

 

[JAE]

I guess I don't totally agree with you aggman...

Here's my logic:

1. I agree that 5.2.1 sets the conditions for two situations:

[blue]Paragraph a) is where the angle has LTB restraint along its length. If it does, then it permits you to use geometric axis method and you can then follow 5.1.1 and 5.1.2 to get the allowable stress. [/blue]

[red]Paragraph b) is where equal leg angles have a LTB restraint at the point of maximum moment. If this is true, then it also allows geometric axis design AND requires you to use 5.2.2b. [/red]

2. Now if you have equal or unequal legs - and you do not meet 5.2.1 a or b, then you still can use 5.2.2 - this is seen by the phrase "Equal leg angle members WITHOUT lateral-torsional restraint subjected to flexure...". Here you do not have continuous LTB restraint OR LTB restraint at the point of max. moment...yet it includes this condition in 5.2.2...

3. With 5.2.2, then, you must increase the fb by 25% (which is not required by 5.2.1) and you must use 5.2.2b.

 

[aggman]

JAE,
I think after reading the section again you are right although I have never designed an angle this way. I agree with what you are saying, but I have always felt that if the angle did not at least have resistance to lateral buckling at the maximum moment point that you are better off to design it using the principle axis method. While this is more involved, it is more accurate. I must say though, that 90% of the single angles I design are braced so it generally isn't as much of a problem for me. I do have to say that the code seems to be a little fuzzy in this section about when to do what, etc. Note that 5.2.3 states that unequal leg angles without lateral restraint must be designed using the principle axis procedure.

 

[JAE]

Yep - I totally agree on the concern...or better, caution, over non-braced angles.

 

[tweedledee]

Jae & Aggman, thanks for your responses. But I am still confused.
The end sentance in 5.2.2b states that "When the leg tips are in tension, Fb is determined ONLY by sect. 5.1.2" i.e. Fb=0.66Fy. This suggests that without LT restraint, and for the tip in tension, the allowable stress is 0.66Fy with no regard to LTB???

 

[aggman]

5.2.2 states that you can consider only geometric bending provided that part a and part b are followed. So it would appear that you can use 5.1.2 if the angle legs are in tension if you increase the applied bending stress by 25%. The commentary seems to back this up. Personally I would still design the angle using the principle axes if you don't have lateral torsional restraint.

 

[JAE]

Agree with aggman -
and I'd add that yes, it says that Fb is ONLY determined by 5.1.2. That is correct - but - that is dealing ONLY with the allowable stress Fb....not fb which is the actual stress.

The paragraph above in 5.2.2.a tells you what to do with the actual stress fb. Keep them separate in your mind.

 

[UcfSE]

tweedledee

Read the whole 5.2 section. If you meet all the requirements then yes you use 0.66Fy as it states. It's not a matter of reading the one sentence that statement is in. You have to meet all the previously stated conditions in that paragraph as well.

 

[tweedledee]

Aggman,Jae, and Ucfse,..Thanks for all your help. I think I have a good understanding of it now

Tweedledee

 

[locksman]

I understand how to calculate fb for the major axis (X and Y) but how do go about finding your actual bending stress about the z axis,i.e. the fbz as mention in section 5.3.1. Also, the property tables only provide S for the x and y axis. Where can I find any info on how to calculate this?

 

[UcfSE]

You have to calculate it yourself. In the back of the AISC Manual there are charts on sections and their properties, circle, rectangle and so forth. It seems like somewhere around page 6-23 there's one for an "L" shape. I made a spreadsheet using those equations that can reproduce all the angle properties in the manual, so those equations are plenty accurate. An easier way would be to use the rz value in the beginning of the manual to find Iz (rz^2*A=Iz)and from there you can get Sz (I/z). Remember also that Iw+Iz=Ix+Iy. You can use this to get Iw as well and then Sw. If you want any shape that isn't in the manual then you need to use the equations in the back of the manual.