Fundamental Wall Arching Question

[Lion06]

I have a question about wall arching that's always kind of bugged me.

When you have a wall with no opening there is no arching, each foot of wall carries its own weight for the full height. Simple and it makes sense.

Every lintel example I've ever seen involves arching of the wall (provided there is enough wall left on both sides to resist the thrust), which dramatically reduces the loads for a tall wall.

To look at an extreme example, if a lintel were placed with infinite bending stiffness then it makes sense that no arching would take place.

It seems like the stiffness of the lintel is what does, or does not, allow the arching action to take place. I've never seen any literature on this, though. For almost all CMU or brick lintels, I use the L/600 deflection criteria per ACI 530. This is pretty stiff. Granted, it's not as stiff as the wall that was removed, but it's still pretty stiff. Is it so stiff that the classical arching (where you're designing for the triangular load that starts at 0 plf at the ends) will not take place?

I guess I'm trying to get a sense of how soft a lintel needs to be or how much deflection needs to occur to allow the arching to really start kicking in.

 

[ishvaaag]

Probably you are right and are in one of such cases where at service level you are having a non factored load higher than the "under the arch" loading yet works well, and an assumed situation where the full arching action develops at bigger deflection of the lintel-tie, even with the assumed formation of cracks, that, notwithstanding, are not expected to develop even with the imperfect check (due to a bad loading) at service level.

I will look for some lintel, arching literature to see. Other than that we can range the field with FEM models but still would be dependent on when cracks form, or notional strut and tie models etc all scarcely prone to exactitude. For ceramic brick masonry cracks may start as soon as 1/2000 distortion so not expecting hair cracks for attached masonries of such kind is somewhat adventurous.

 

[ishvaaag]

See p. of this reference

http://www.emcoblock.com/pdf/divisions/brick/contractors/tech-note-31b-structural-steel-lintels.pdf

Not very explicit on why but at least somewhat on how.

 

[ishvaaag]

More info on how to in

BS 5628-1:1992
Code of practice for use of masonry
Part 1: Structural Use of Unreinforced Masonry.
Section 34

Well, soon there is not going to be any unreinforced masonry in advanced countries, the earthquakes make the chunks fall and better we do something tying everything to something able to restrain such masonry rains.

 

[ishvaaag]

It is also worth to consider that even if some beam able to retain some load at some L/600 deflection, however the stiffness over the beam won't be but a fraction of the stiffness towards the supports, and hence such strut/funicular/arches will be the preferential system of discharge for the loads over such notional load path.

 

[BAretired]

Lion06,

I think the arching action starts as soon as there is any deflection of the lintel. The lintel cannot be as stiff as the wall in arching action because it does not have enough depth.

The main purpose of the lintel is to support the masonry in the small triangular area above. The lintel prevents loose bricks or blocks from separating from the wall and falling down, an unacceptable hazard in an occupied building.

BA

 

[dhengr]

Lion06:

I agree with BA, but would add... The arching action causes tension forces in the masonry, in the plane of the wall, in two directions. Vertically, under the arching masonry ( units in comp.) to hold up those lower masonry units, or not. And, horiz. btwn. the ends of the arch, (the arch thrusts) usually reacted by the extended wall structure you mentioned. You could fab. an abutment structure on the ends of your steel lintel, making it act in tension against these forces, improve the arching action and get buy with less extended wall beyond the opening. The old church builders used abutments and flying buttresses to take these forces.

I think you would find that the stiffer lintel will help limit the vert. tension and its cracking; and falling brick or at least a maintenance headache, and an aesthetic issue. Some mortars or masonry units will tolerate more of this tension or movement without cracking and that would set the max. tolerable lintel delta. Try looking in various masonry tech. notes, since that’s were the problem shows up, it’s really their issue. It’s not really a steel problem if that’s where you’ve been looking, but they obviously recognize the arching action when they show the triangular loading on design examples.

 

[csd72]

As BA has said, the arching will be the preferential system as long as you have any deflection of the lintel at all.

Look at the alternative load paths, either vertical load down to the lintel which will have movement from the compression of the brick plus the deflection of the lintel, or at a 30 degree line from vertical to the support which will have movement from brick compression only (plus about 30% for the angle). Since the brickwork in compression is much stiffer than the angle in bending then this would mean that the arching is much stiffer.

As for having a less stiff lintel, The L/600 is there to prevent tension cracking in the triangular area above the lintel and much more than that would likely result in too much cracking.