W shape loading? 1

[TonyES]

I have a large W shape as my beam with 6X14 timbers (rafters) every 2' on both sides of the beam, each timber has a 6k reaction. The owners want to place these rafters on the bottom flange of the beam. I've never done that, all I've ever seen is the rafters or joists bearing on top of the beam (top of the top flange). Are W shapes designed such that one can load it an any shape or form or do I need to do some other kind of check. the loading will be equal on both sides so the torsion will be small. Any advice would be great. Thanks

 

[Lion06]

Check the bottom flange for localized bending (as a short cantilever fixed at the web). Also, check the bearing on the joists since you will likely have less bearing distance now.

 

[UcfSE]

This may also interfere with your bracing assumptions. Look into these carefully.

 

[DWHA]

Actually your beam may be able to handle a larger load if loading is via the bottom flange. That is if localized flange bending does not control and the LTB bracing is the same.

 

[frv]

TonyES- Check out section J10.1 (This is what StructuralEIT was referring to)


DWHA- How do you figure that?

 

[nutte]

DWHA, how do you figure?

 

[nutte]

Dang, beat me by a second.

 

[frv]

funny

 

[DWHA]

AISC 13th ed Commentary F2.1 pg 16.1-271.
In any new design I ignor it. But if there is an existing structure that has bott flange loading, I will utilize it if needed.

 

[MiketheEngineer]

People like Simpson and UPS make special hangers for these types of applications. They stradle over the top of the beam and provide a hanger for the beam...

You might want to check them out

 

[frv]

DWHA-

This only applies if LTB controls. For a section adequately braced to develop phi Mn, it makes no difference where the load is. Additionally, you are ignoring that fact that by moving the joists and, consequently, the bracing to the bottom flange, you now have an unbraced compression flange (assuming a simply supported beam)

 

[nutte]

DWHA, from a theoretical standpoint, yes, there could be an increase based on where the load is applied relative to the shape's center of gravity. As far as the code goes, though, it wouldn't affect the member's allowable capacity.

How would you even utilize the increase? Would you dig into research papers to find a more exact buckling moment?

 

[nutte]

Three seconds that time.

 

[nutte]

Oops, three minutes. I'll be quiet now.

 

[DWHA]

frv
"For a section adequately braced to develop phi Mn, it makes no difference where the load is"-Correct

"Additionally, you are ignoring that fact that by moving the joists and, consequently, the bracing to the bottom flange, you now have an unbraced compression flange (assuming a simply supported beam)"- Incorrect
I stated that if LTB bracing was the same, and never said that LTB did not control.

You are assuming that the rafters are connected to the top flange. The OP did not state that therefore, it may (not definatly) give additional capacity. That is for TonyES to determine.

 

[DWHA]

Nutte,
What do you mean it will not affect the allowable capacity. Do you mean it will not affect the plastic capacity? I will agree with that.

Isn't 99% of what we do based on theory? Why are you arguing with it now.

When I used this, I prevented a factory from being shut down for period of time. The factory saved a lot of money due to the fact that they did not loose production during this time.

 

[nutte]

I mean "in theory" in that laboratory testing will show an increase. This is different from "in practice" in that the location of loading (relative up or down to the c.g.) does not affect the LTB capacity of a beam as defined by the AISC specification. That is why I asked the question of how you used it to get an increased capacity for your factory beams. The AISC specification doesn't give you different LTB capacities for top-loaded or bottom-loaded beams, so how did you determine the LTB capacity of your bottom-loaded beam?

 

[Hokie93]

For the 9th edition AISC steel manual, allowable compressive bending stresses may exceed the code-prescribed values for the case where the unbraced length exceeds Lc provided the increased values are "justified by a more precise analysis". See section F1.3.

The "Guide to Stability Design Criteria for Metal Structures", 5th edition, provides methods to account for the location of the load relative to the shear center for several common loading conditions. See pages 194 to 197. I have successfully used this method in the past when evaluating the capacity of existing members. The method contained in the Guide is a "more precise analysis" in my opinion.

Hokie93

 

[msquared48]

I would weld two 1/4" side plates for the timber beam to the top and bottom flanges and the web and thru-bolt the beam to the side plates. This should engage both the top and bottom flanges to resist the beam reaction. Plus, the side-plates serve as web stiffeners.

Mike McCann
McCann Engineering

 

[csd72]

I have done a similar detail many times, but I would recommend that you bolt a timber bearing plate to the bottom flange and then toe nail the bottom of the joists to that.

As noted above you will then have no lateral restraint to the top flange, so I usually call up say 2"x1/4"x1' long straps each side bolted to every 4th joist which ties the beam into the floor diaphragm (via the flooring nails). Then check the beam for that effective length.