2005 ASD - Weak Axis Bending

[ToadJones]

I am checking some calculations....
I'm coming across something that just doesn't seem right while checking combined stress on a very large wideflange shape...W40x397

in looking at Eqn. F6-1 (2005 ASD) Mn=FyZy < 1.6FySY

I get an available moment of (50ksi x 300 in3)/1.67 = 8,982 k-in.
This is less than (1.6FySy)/1.67 so it controls.

The reason it seems incorrect is this would equate to a bending stress of 8,982 k-in/ Sy=191 in3 = 47 ksi.

this seems unreasonably high; near yield.

What I am missing here?


I also am inclined to only use Zy for the top flange for the loading, but that is another matter I suppose.

 

[nutte]

You're not missing anything. If the stress at the tip did hit yield, there is still reserve capacity, up until the whole section (Zy) has yielded. The equation is calculating that moment, MyZy, then applying a factor of safety to it.

 

[ToadJones]

I guess I'm just one of those Greenie's used to seeing 0.75FY

Thanks for the sanity check Nutte

 

[ToadJones]

If I were to check the top flange alone do you think F.11 is the appropriate section?

 

[nutte]

I'm not sure I understand the question. Do you mean only counting on one flange for the bending resistance? If that's what you're doing, I'd use F6 and adjust Zy accordingly. Flange local buckling is contained in that section, which would be more appropriate than assuming the flange was just a plate.

 

[ToadJones]

Yes...the load is delivered directly to the top flange, so I am only going to use the top flange.
My original thought was to still use F6 with adjusted Zy...
Thanks Nutte.

 

[ToadJones]

chances are here that F6 and F11 in this case will be identical anyway.

 

[RFreund]

That is a huge beam, if you don't mind me asking, just out of curiosity, what is it for?

EIT

 

[ToadJones]

It's just a garage door header for a residential home....

Its a crane runway girder....not my design.

 

[ToadJones]

The 2005 & 2010 codes are very difficult to implement on crane runways with built up sections, by the way.

 

[RFreund]

huh, I believe it. I would be excited just be involved in a project with a beam that size.

yeah I'm a noob.

EIT

 

[ToadJones]

yea- I can remember getting excited/ scared when I saw beams that big.
But, after working for some time now in industrial applications, it's become almost routine. I have worked on buildings that had girders 14 ft deep and spanned 120 ft.
I have worked on Boiler buildings for power plants that had trusses 28ft deep supporting the hanging boiler (top steel as the industry calls it).

Its good to be excited about it.

On the other hand, I'd love to design a W10 for a distributed load at this point!!!