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Steel lintel
2

Steel lintel

Steel lintel

(OP)
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.

RE: Steel lintel

Hi mike0123m

Yes I agree, I suppose you could select a length like say 10 metres or whatever and workout the thickness then pro-rata it if and when you do now the length.
Why haven't you got the length whats the reason for that?

desertfox

RE: Steel lintel

(OP)
I saw the question in a textbook and thought it was odd that the length was not given.  I thought there might be a way to solve it so I decided to post the question and see if there really is a way to solve it.

RE: Steel lintel

hi mike0123m

In that case I wonder whether you solve it for a minimum length of 1ft, then I would check the answer if you have it in the book.

desertfox

RE: Steel lintel

Well, if the angle is a ledge angle, bolted to the wall, the problem might be aimed at determining the angle thickness necessary to extend out the 6" and support the brick.  L/600 on that, thuogh, is questionable whether it applies.

 

RE: Steel lintel

Other typical case is when some header brickwork is hanged from a floor. Then typically spacing between hangers (they may or need to be braced) uses to be within 2 and 3 ft; this spacing is common with lesser size angles and so you might start to check on the upper range.

RE: Steel lintel

hi mike0123m

Even working a deflection for a beam only a foot long probably won't be very accurate because the length to depth ratio for beam formula is to small.
Anyway mike any chance you can upload the question and/or give the answer in the book?

desertfox

RE: Steel lintel

You can determine the required thickness based on the localized case of leg local bending and shear, but you need to know the span to fully design the lintel.  Even if it is attached to a wall using expansion anchors for instance, the span would be the distance between the anchors.

RE: Steel lintel

A lintel usually supports the masonry over an opening such as a door or window. I've never considered them loaded uniformly, I take account of the arching of the brick or block, up from the support points.

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

RE: Steel lintel

It is a little bit of a trick problem and paddingtongreen is on the right track. - The arch action is critical since it changed the dead load distribution and dead load forces. If you have enough height of bonded masonry above the opening, the the vertical load on the lintel is just the triangle of masonry bearing on the lintel, which can be much, much less than the weight if you look at the wall as being composed by vertical columns of dead load. If you have some architectural patterns (stack bond or soldier courses) this can have an effect on the arch stability and lateral thrust.

Since you have a weight of the brick veneer of 500 plf, that indicates a significant height of veneer above the opening, which indicates arch action is very possible. The arch action dramatically reduces and almost eliminates the deflection.

You are right that it all depends on the opening width, which does affect the load in the lintel, but if you have enough height over the lintel, the arch action will reduce the load on the lintel dramatically and the Romans recognized this principal very quickly.

Dick
 

RE: Steel lintel

So, can we estimate/guess the unit weight of the brick veneer, and assume the 500 plf is the peak load (tip of a 45 degree triangle). From there, h is known, and L = 2h, any comments?  

RE: Steel lintel

Due to the prolific inclusion of control joints in masonry veneers, I don't consider the arching action any longer.  I've seen large openings with a control joint at each side of an opening as well as smaller openings near a corner with a control joint on the opposite side.  In this case no arch can develop as there is not enough brick to resist the thrust on the corner side.

Patrick

RE: Steel lintel

The 500 plf is not the "peak" load, since the arch action reduces the maximum load on the lintel. The lintel just carries the weight of the triangular shape of veneer bearing on it and the amount above the peak of the triangle is distributed to the areas between the openings below. This is why the height of the masonry above the lintel must be high enough to create the arching and load distribution effects. The 45 degree assumption is common in masonry, although some engineers will use a slightly different assumption.

Since the compressive strength of the veneer is not a problem, it usually is a mute point.

Pay attention to any shelf angles and soft horizontal joints that will disrupt the arching action and then you have a simple vertical load bearing situation with none of the benefits or arching. Depending on the properties of the veneer (clay brick expands with time) and concrete masonry and concrete have some long term shrinkage/creep. Woof frame has serious shrinkage and stability issues and usually will have a horizontal shelf angle at 16'or so. I have seen 7 story buildings without a shelf angle and the top floor windows suffered because of the 2" different vertical movement between the vertical expansion of the brick and the relative stability of a concrete frame.

Since you have a 6x6 angle, you probably have a true masonry veneer with an air space or insulation between the veneer and the structural back-up. Wall ties provide the horizontal resistance/alighnment to transfer horizontal loads to the structure

Dick

RE: Steel lintel

The above is just trying to figure out the missing info in the "trick" problem. For real world project, do all suggestions by others. :)  

RE: Steel lintel

The answer to the OP and the first half of the thread is; you can't design a beam (lintel) without knowing its span length and bearing conditions.  And, otherwise; that you can guess all day at the intent of an incomplete question.

From paddingtongreen's post of 16DEC, 10:57 and down, there is much good, real world info. on lintel design, once you know the span length.  But, read and digest druminman's post before you apply this real world info.
 

RE: Steel lintel

Agreed with druminman.  A control joint negates arching action.  I assume a rectangular stress distribution acting on the lintel from the weight/load above.  If that is too much to get a reasonable-sized lintel to work, I use arching action but ensure that a CJ is not placed on either side of the opening.

RE: Steel lintel

steellion -

The vertical control joint also destroys the arch effect since the horizontal thrust cannot be recognized.

I agree since I neglected the vertical control joints that negate the advantages of the arch action that the ancient Romans did not have to deal with for permanent construction in total masonry construction with is historically proven for the total structural performance.

Dick

RE: Steel lintel

I think location of the control joint is key to whether arch action is possible or not.

In middle of the opening - possible
On sides beyond bearing areas - possible
On edges of the opening - not possible

The OP question wasn't a trick, it was seriously flawed. Where was that question came from?  

RE: Steel lintel

Not in the middle either.  I agree with druminman.  Ignore arching action unless you can't get it to work any other way.

RE: Steel lintel

Yes, I agree the above.
The "possibility" has no practical value.

RE: Steel lintel

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.

 

RE: Steel lintel

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.

RE: Steel lintel

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

RE: Steel lintel

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.

RE: Steel lintel

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

RE: Steel lintel

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.

RE: Steel lintel

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.

 

RE: Steel lintel

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.

RE: Steel lintel

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.  

RE: Steel lintel

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.

RE: Steel lintel

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

RE: Steel lintel

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

RE: Steel lintel

Agree. Learned a new phrase today.

RE: Steel lintel

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.

RE: Steel lintel

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.)

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