Steel lintel 2

[mike0123m]

I came across a design problem that I am not sure if I have enough information to solve. I have a 6x6 steel angle supporting 500 plf of brick. Deflection is limited to L/600. What is the minimum thickness of the angle? I am not sure how to go about this when the span of the lintel is unknown.

 

[JAE]

Keep in mind that with BRICK (see the original post above) you don't use control joints exactly - you use expansion joints. And I agree that the joints can and do mess up the arching assumptions.

Arching not only hates joints, it also requires adequate masonry width on either side of the opening to take the lateral thrust of the brick.

And I agree that the question the OP references is BS.

 

[paddingtongreen]

I think there is a misunderstanding of "arching" in this context, this is not a stone acrch with a keystone, there is no significant horizontal component at the ends of this type of arch. Consider; if you build your masonry on falsework there is no horizontal force on the falsework, and then remove some of the support; where the support is removed, a triangle of wall drops to the floor, no horizontal load exists there.

Even with joints at both sides, arching can be considered, if needed, because all of the load above the arch comes down as two, almost point loads adjacent to the supports. Having said that, I have to agree that if I cannot control the location of the joints, I use the uniform load.

I've been retired for a while, but I mainly used precast lintels or bond beams in the later work years.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[concretemasonry]

paddingtongreen -

I agree with your post since there is more to the structure than just the veneer. This was not posted as having a classic arch, but only a typical veneer on a structural back-up installed within the construction restraints and few details.

Since there is no information on the rest of the structure, the arch action may not be used because of detail restraints (horizontal or vertical joints), the use of the lintel to support the veneer is the only design resort or alternative.

It sounds like a probing classroom question with not enough information to give an intelligent answer, but they may grade on opinions and the "curve".

Dick

 

[apsix]

To me, arching by definition means the presence of a horizontal component of force.
The arch will act the same, whether it's a built classic arch, or an arch formed by the dropping away of the brick triangle above the lintel.

 

[hokie66]

Yep, apsix has it right. Any arch has both horizontal and vertical reactions, and continuity over the arch.

 

[paddingtongreen]

I did put "arching" in quotation marks, it is not a true arch. It is a series of cantilevers, supported on cantilevers, all locked in by the weight of the courses above them, each course cantilevering half a brick more towards the center until they meet. Above that point, the bond, combined with mortar's resistance to shear makes them act together like a beam.

With the help of some falsework, you can model this with children's wooden blocks and dry joints.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[JAE]

paddingtongreen - I think there is a horizontal component. In fact, I've attended seminars by the brick industry where they've shown many,many photos of residential "arch's", lintels over openings where there wasn't enough brick veneer on the sides and the cracks and failures that developed were illustrated.

I think what you are getting at is that there is a component of friction between the brick and the steel angle that takes this horizontal thrust and thus the weight can all come down as concentrated right at the ends of the angles. I don't like to count on friction for structural stability.

 

[paddingtongreen]

JAE, you are correct, of course, we generalize to keep things short and lose the nuances.

I'm nerdy enough that I did the experiment with wooden blocks when I was young and learning the business. I admit that I did nothing to lessen the friction though.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[dhengr]

I tend to agree with Michael’s thinking, it is certainly not a true arch, and I don’t doubt his dry blocks experiment. Maybe the word he and I want is ‘a long, high, corbel, and he said as much with his canti. on canti. thought. Certainly the corbel has some horiz. tension component at its top and some horiz. compression (thrust) component at its base. I wouldn’t completely ignore some thrust, since I suspect for a few courses at the top there will be a compressive reaction in those brick courses, at the meeting of tips of the facing corbels, but not to the extent of a keystone in a real arch. I would like to see some continuous, in plane, vert. & horiz. veneer at my lintel bearings, to take whatever the reactions are.

I have seen pictures and field examples of what JAE talks about. Obviously, a real arch over the opening, in a brick veneer, will have more horiz. thrust reaction than the opening with a structural lintel under the brick. The lintel is their, at least, to carry the triangle of brick that Michael says falls away (an unreliable function, in tension, of bond btwn. brick and mortar), when the false work is removed. I agree with JAE re: friction at the steel lintel, but that’s called bond btwn. the bricks as we talk about the corbel.

 

[apsix]

Friction on the lintel would classify it as a tied arch.
I wouldn't rely on it for desigh though, unless a positive stop is fixed to the lintel.

 

[BAretired]

A very unusual problem. To calculate the minimum thickness of a 6x6 steel angle which must carry a specified uniform load over an unknown span. I have no idea how this could be done but would be interested in seeing the solution if someone is able to provide it.

BA

 

[concretemasonry]

I think this original post may a an academic "red herring" for an exam since there was not any definitive information provided and let the posters/critics add information.

Dick

 

[cntw1953]

Agree. Learned a new phrase today.

 

[NITTANYRAY]

Since the span was not given, it must not be great enough to be a consideration for the solution. The problem also did not specify whether the angle was A36 or grade 50 steel so stress must not be a consideration.
The only possible criteria is availability so the answer must be 5/16" since that is the smallest size available.

 

[MultiVar]

The answer is a formula, for any arbitrary length, not a value. Possibly a simple linear formula, or at most a catenary?... not sure since I'm not in the field, and not clear what the "angle" in question looks like, and behaves like. Extruded? Cold formed? Any pics or data book links? Maybe I can derive the formula... or nip off to a Christmas party. ;')

Anyway, forget all these digressions about "arching", and how the supported load may or may not behave--they've TOLD you the load is exerted in a simple linear distribution across the lintel span. That's why I mentioned catenary. Pull out those undergrad texts on beam deflection.

Plug and chug. Enter a L-ength, solve for thickness. IIRC you'd need modulus of elasticity, and possibly other INtrinsic properties of the steel, plus EXtrinsic (handbook) properties of the angle stock.

Academic perhaps, but it seems like a good P.E. exam question, as it gets to FUNDAMENTALS, and knowledge thereof. Work-a-day solutions gleaned from handbooks lead us away from the analytic principles of how the values were arrived at. (Hopefully there's a large empirical component as well.)

I'm pretty sure in the real world there is not a continuous range of angle stock thicknesses, e.g. every 5 mils, so after you plug n' chug your MIN thickness, you'd select the next largest size, yes/no?

Anyway, what do I know?... I'm a Comp Sci/Math guy who used patching concrete to repair/reinforce/seal the severely rusted mild steel C-channel beams holding up his propane BBQ grill, LOL! (Working out great, BTW.)