Torsion on WF lintels

[kaisersoze]

As the topic suggests my question is in regards to eccentric loads on beams. Say I have a 12" cmu sitting on a W30. Technically if the lintel is 2" off of center line, I would consider e=2" on my beam but from a practical point of view, wouldn't the load coming down into the cmu re-align to some extent to be closer to the stiffest part of the beam i.e. beam center line?

Assume that the joists bearing elevation (on cmu) to be about 8' above the top of WF lintel.


Kaiser

 

[Lion06]

The load comes down where it physically comes down. I don't think you can ignore that fact.

You wouldn't neglect the eccentricity of the joists to the wall in the wall design, right?

With that being said, if you can show that the wall the lintel is supporting can take the moment (Pe) then you can neglect it in the lintel. This would be like providing a pin support for your wall that is 2" off of its own centerline, but in line with the web of the WF. This isn't something I would typically do, and you'll still get some torsion in the WF due to rotation of the wall taking the WF along for the ride.

 

[BAretired]

Is the beam offset to the inside or the outside of the wall? If the joist reactions are offset to the inside, the c.g. of load is not the c.g. of wall.

If the load has an eccentricity of e (inches), there is a moment of W*e where W is the total gravity load on the beam. This may be carried by a horizontal force on the beam of W*e/96 and an equal and opposite force on the joists 8' above plus a small variable moment in the wall between. Torsion in the beam will be very small because the wall does not permit it to rotate very much.

BA

 

[slickdeals]

@BA:
Would you design the W30 for weak axis based on the entire Sy or only Sy/2 of the W30 (i.e. top flange trying to span horizontally to carry the force)?

 

[BAretired]

@slick,
I would use the full Sy to span horizontally. If only the top flange resists the horizontal load, the beam is subjected to a torsion of opposite sign to the applied eccentric moment. Web stiffeners are needed at each end and perhaps at the third points (location depends on span).

BA

 

[slickdeals]

BA:
The reason I asked is because the wall is bearing on the top flange of the W30. How does the load get down to the centroid of the W30 for it to span horizontally as a beam? I am not able to picture it in my head. I see the top flange spanning between end stiffeners.

What effect will the intermediate stiffeners have? Will they make it span as a beam as opposed to only a flange?

 

[BAretired]

slick,
At first, I too was thinking that only the top flange would be engaged, but if that were true, the beam would need to rotate torsionally in the opposite direction to the applied moment W*e which doesn't seem to make sense. I think the stiffeners act as a continuation of the wall through the the beam. If the wall is fixed to the top flange of the beam, the stiffeners ensure that the bottom flange flexes at least as much as the top flange.

Don't know...would have to think more about it.

BA

 

[slickdeals]

@kaizer......sorry if the thread is going on a little tangent.

I was investigating LTB and torsionally loaded situations recently and I must admit I don't have a complete handle on it.

Salmon (on Page 559) has an example where he checks torsion with a flexural analogy and uses Sy.

If you use that logic, then a spandrel beam with curtain walls reaction at the bottom flange would have zero torsion and will only need to be checked for strong axis + weak axis bending. Similar to a crane girder?

 

[Lion06]

slick-
S&J does use the full Sy, but notice that the force is direct P + and additional P to account for the torsional loading, essentially increasing the lateral force, P, by 100%.

 

[slickdeals]

SEIT, you are right, I just noticed that........So it goes back to the top flange trying to span laterally...?

 

[Lot2Learn]

The thread going tangent in perfectly fine by me, I am learning more and more from these discussions.

I would have to explain the situation more once I get home but for now I am attaching the sketch of the situation and also some sheets of the info I use for torsional design.

 

[hokie66]

kaisersoze...Lot2Learn...are you the same person? The last post is similar to the OP, but confuses the issue.

 

[kaisersoze]

No, I guess someone else must have been logged from our office full internet access computer. But the last post with the sketch was supposed to be from me, Kaiser.

Kaiser

 

[kaisersoze]

The attached sketch shows the situation. Its for a proposed new opening. After running some load checks, the resultant e is about .6" to the right of centerline in the sketch, so I am not as worried as I was a day ago.

And I am glad to have posted this question. What BAretired suggested makes a lot of sense to me. I always had a hard time imagining a rigid wall just rotate along with a WF.

Kaiser

 

[BAretired]

A W30 is a pretty deep beam. The markup on your cross section seems to indicate a steel beam about 20" deep (2.5 block courses). What is the clear dimension of the opening?

You already have a 16" bond beam at the top of the masonry wall. From a construction standpoint, I would think it better to use a masonry beam above the opening with a steel angle to pick up the face brick. The masonry beam, according to the section, could be as much as 8 courses (80") high. This would have ample torsional resistance and would allow the masons to continue without waiting for a crane to install a hefty steel lintel. The brick angle can be attached to pre-set bolts at a later time.

BA

 

[Lion06]

BA-

That's a good thought. It didn't even really hit me until you just said it, but this situation smacks of wanting a CMU (or precast) lintel for the CMU wall and a loose angle lintel for the brick. We have a General Note that calls for loose angle lintels with a schedule of different sizes associated with different opening widths. We call for an inch of bearing for each foot of opening width.

 

[BAretired]

Lion,
At the moment, we don't know the span. If the beam is W20 or W30, it must be a fairly hefty span, maybe 10' or 15'. In that case, you probably can't use loose angle lintels but you can use angle lintels attached to the masonry beam with bolts or weld plates at, say 4' centers.

BA

 

[Lion06]

Agreed. A W21 is huge for a lintel.

 

[kaisersoze]

Actually the beam is W30x108 and the opening is large, 22'. So I have HSS column supports on each end, KL=2x10'.

It wasn't my design (now I guess it is), I am putting out fires for someone not working here any longer. When I saw this huge opening in the wall in the field, I thought I had to check the design of the lintel.

What I would have done is to take the columns all the way upto the roof to brace them. Now this is an existing condition, that makes it very very difficult to do that. And then I would have put some studs on top of the WF and provided a new bond beam right on top of the WF.

Kaiser

 

[RFreund]

I know this is a mute point know, but if you were to use a bond beam for a 10-15' span you would probably need a good size pilaster or something on each side of the opening to take the out of plane forces, right? Or would you use typically use a steel beam/column and brace the masonry with it? Depends?



EIT