AASHTO - Steel Beam Flange Thickness

[SoCalPE]

So in the lateral exam, I came across a question where steel beam span was given along with lateral wind loads (I believe wind loads applied to the steel beam in the minor axis direction). The question ended up asking for the required flange thickness. I got fixated on the problem and ended up spending more than the 6 minutes on it and eventually wasn't able to find the correct AASHTO section. I am still looking for the section and can't find it.

To the bridge guys out there, does the data and what is asked above ring a bell? And what AASHTO section would cover the answer?

Thanks,

 

[thedaywa1ker]

Not a bridge guy, but I think the relevant section would be 4.6.2.7

 

[SoCalPE]

Thanks! I have looked at that section, that's the loading/structural analysis part of it. That load/moment was given in the exam problem. The actual thickness/member requirement is not covered in 4.6.2.7

 

[thedaywa1ker]

I gotcha...well if it is anything more complicated than straight M/S_flange for spans between cross bracing then I'd love to know as well

 

[SoCalPE]

Exactly my thought process. However, with so many adjustment factors involved, I wanted to throw the question out there to try and confirm. From what I remember M/S didn't land me close to any of the numbers or was just in the middle of two answers, can't exactly recall.

Edit: Or I think I may have used M/S, as I don't remember if the width of the flange was given or not.

 

[thedaywa1ker]

I poked around in AASHTO a bit again this evening...I'll take another stab at what section I think is relevant....I think section 6.10.8.1.2 would control.

'Discretely braces flanges in tension' and an interaction equation for stresses in the tension flange due to vertical loads and lateral loads.

Hopefully they'd tell us to neglect vertical loads or give them to us, because I've done a brain dump of HL93 etc etc, and I don't know how proficient I'd be 6+months removed from studying for vertical

 

[SoCalPE]

Unfortunately, Section 6.10.8.1.2 is an infinite loop of cross-references and parameters. One of the reasons i spent so much time on the problem was that i was going back and forth among those sections to no avail. It could be that I was simply overlooking an assumption in the problem that would cut the time or not require looking through the sections. It's frustrating to say the least...

Regarding the gravity loads, I think they will either be irrelevant or given as moment etc. I wouldn't worry about that.

 

[Titleistguy]

I remember that question.

 

[EBAT75]

lateral wind loads (I believe wind loads applied to the steel beam in the minor axis direction).).....”

Please clarify this for my understanding.

Is the beam web upright or is is flat? Is the wind load an uplift or is it horizontal?

Could not wind pressure applied in the direction of the minor axis be uplift in one case and horizontal in the other, each with a different flange thickness requirement?

 

[Br_Engr]

A lot of missing information here.

Is it a simply supported beam? With or without the deck?

If this is checking a tension flange, it more than likely will NOT control. Checking the lateral flange bending is usually performed for the top compression flange before the deck is cast/cured (a constructability check) or for the bottom flange in the negative moment region of a continuous structure.

Aside from the requirements of AASHTO 6.10.1.6 for the other lateral flange bending limits, simply convert the wind pressure to a uniform load along the bottom of the flange. M= w * l^2/10 (divide by 10 due to continuity) and divide by the section modulus of your flange to obtain lateral bending stress.

Add 1/3 of your lateral bending stress to your in-plane stress and compare to Eqn. 6.10.8.1.1-1.

If it does not satisfy this equation, you may have to bump up the flange thickness a bit.