aashto weak axis bending

[bjb]

In the ASD 9th manual by AISC there is guidance for the weak axis allowable bending stress. I don't see anything similar in AASHTO.

Specifically I am interested in the allowable bending stress for a solid round bar according to AASHTO. If AASHTO doesn't give any guidance, is it reasonable to use AISC specs?

 

[PEinc]

Can a solid, round bar have a weak axis?????

 

[bjb]

PEinc:

To answer your question, obviously the answer is no.

However, in the '89 ASD steel manual the allowable bending stress of solid round bars is lumped in with the allowable weak axis bending stress for I shaped members (section F2). That's why I mentioned them both together. AISC probably lumped them together due to the fact that lateral-torsional buckling won't be a factor for solid rounds or I shapes bent about the weak axis, but we are digressing from the point.

In my first post I am asking about the allowable bending stress for solid round bars according to AASHTO.

 

[DRC1]

I dont have my green book, but I believe weak axis is in 9th editon with bending specs for channels. I believe weak axis is 65 or 75% Fy.

 

[DRC1]

I'm sorry, I misread the post. I don't recall ASSHTO weak axis bending.

 

[jimaitken]

bjb

I think it would be acceptable to use AISC if there is no guidence available in AASHTO. I have done this on several occasions when the AASHTO Steel Specs did not specifically address my problem, but AISC did, I just made sure that I referenced this somewhere in my design notes and drawings. AASHTO table 10.32.1A even makes reference to using the 9th edition of AISC in specific situations in the footnotes of the table.

That being said, AASHTO Section 10.32 specifically says to use table 10.32.1A to determine allowable stress of steel. This is where you need to use some engineering judgement and decide how to interpret the code, and satisfy the requirements of the client/owner.

 

[miecz]

Solid Pins up to 20 inches in diameter are covered by Table 10.32.4.3A.

 

[miecz]

If you don't fall under AASHTO 10.32.4, then I think you are stuck with Table 10.32.1A. I don't believe that it's okay to use AISC here, as, there is guidance in AASHTO. If you choose to ignore Table 10.32.1A, and use AISC allowables, then you're not meeting AASHTO.

 

[bjb]

jmiec,

It seems to me that using 0.55Fy for the allowable bending stress for a solid round bar is extremely conservative.

I think a strict reading of Table 10.32.1A does restrict you to 0.55Fy. That's unfortunate, as we know that the allowable bending stress for a solid round, or for a compact I shape bent the weak way can be much higher. I think that the intent of Table 10.32.1A is to address I shaped members bent the strong way. I'm glad I don't have to deal with AASHTO that frequently.

 

[miecz]

bjb,

I agree that 0.55Fy is extremely conservative. AASHTO is generally more conservative than AISC. However, I believe most solid rounds in bridge applications would qualify as pins and fall under 10.32.4.3A, in which case Fb=0.80Fy.