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






RE: Torsion on WF lintels
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.
RE: Torsion on WF lintels
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
RE: Torsion on WF lintels
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)?
RE: Torsion on WF lintels
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
RE: Torsion on WF lintels
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?
RE: Torsion on WF lintels
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
RE: Torsion on WF lintels
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?
RE: Torsion on WF lintels
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%.
RE: Torsion on WF lintels
RE: Torsion on WF lintels
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.
RE: Torsion on WF lintels
RE: Torsion on WF lintels
Kaiser
RE: Torsion on WF lintels
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
RE: Torsion on WF lintels
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
RE: Torsion on WF lintels
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.
RE: Torsion on WF lintels
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
RE: Torsion on WF lintels
RE: Torsion on WF lintels
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
RE: Torsion on WF lintels
EIT
RE: Torsion on WF lintels
How much load do you have coming down on that wall that you need a W30 to span 22'?
RE: Torsion on WF lintels
BA
RE: Torsion on WF lintels
RE: Torsion on WF lintels
EIT
RE: Torsion on WF lintels
Load coming down on the beam is quite high = 5.1 klf
W30 was designed as unbraced entire length ti limit deflection to 0.3".
HSS columns are 7x7x5/8 and when I ran them in RISA with e=1", the deflection or drift @ top came out to be about 0.4", a little high for my taste but basically I am dealing with something thats already in the field.
The torsion and flexural check on the beam is less then unity.
I am going to ask for welded plates at T&B flanges to the column to create a torsional connection.
I did a quick check of anchor bolts (have to do it again to be sure) but basically there was no uplift or tension with e=1" just because there is like 56k of reaction coming down.
The existing structure under the base plate has already been checked with much higher loads by last engineer, so I am not worried about that.
So I guess now folks can see my point in asking the question. As one old bosses once told me "design the components but ignore the building envelope".....I am checking the beam as if the wall is free standing without some lateral support provided by the two roofs above, and I wanted to see peoples take on this approach.
Kaiser
RE: Torsion on WF lintels
Low roof trib is about 32'.
High roof trib is 12', but its a 8" conc slab.
Roof LL=30
Kaiser
RE: Torsion on WF lintels
ACI 530 took out the 0.3" limit on deflection and allows L/600 in 530-08
RE: Torsion on WF lintels
Check out pages 4,5 and figure 2.6 in the AISC Design Guide for Torsion. It concludes what BA was alluding to. The bending stiffness of the wall (if properly attached to the W beam) can take out the torsion.
http://cob