In educational construction material as well as engineering publications, the corbel angle for significant excess loads, and also for substantial openings, in an ordinary brick wall is usually stipulated as being taken as 60 degrees to the horizontal.

For example: enter image description here In the first illustration, the reduced load is spread out, in the second a distributed load is spread out, and in the third, a point load is spread out. There are no buttresses or other "out-of-plane" support. The wall can be imagined as an indefinite length ordinary 100mm thick brick wall.

I've tried to find the source of that 60 degree value, to check when it's applicable and how it's calculated, but I can find nothing whatsoever on the topic. The few mentions online of "corbel angle" are all of a form that just state/assume 60 degrees (with no reasoning). I can't find anything else.

Related to this, when dealing with an opening, which of these is the correct way to use the corbel angle, and why? They can't both be correct, as they contradict, but there doesn't seem to be an obvious reason why the answer should be one rather than the other.

enter image description here

In the first diagram, the reduced weight is treated as causing a reduced load in nearby masonry at 60 degrees outward; in the second the heavier surrounding weight is treated as causing an increased load at 60 degrees inward.

How does the corbel angle actually work in these typical situations, where there is an excess loads + openings at some point in an ordinary brick wall, and how universally appropriate is it to always assume it's 60 degrees in ordinary brick or concrete construction?

  • $\begingroup$ Didn’t they work this out when building cathedrals etc But you may find this interesting : scielo.br/… $\endgroup$ – Solar Mike Mar 23 '19 at 19:11
  • $\begingroup$ Thanks S.M. - an interesting source! So they've modelled a protruding corbel as a section of masonry in tension, (the "tie") supported by a triangular section of masonry in compression (the "strut")? I can use that. But it's not directly applicable as it stands, to calculations on corbel in-plane spreading within masonry wall, such as these examples. Is there anything more directly relevant on these, and on the 60 degrees usually quoted? Ideally Eurocode if available, but anything really. $\endgroup$ – Stilez Mar 23 '19 at 19:32
  • 2
    $\begingroup$ 3 quick comments: 1) Mike is linking to an article on corbels in reinforced concrete, so be very careful in applying it to masonry. 2) Why would you expect to be able to calculate the load distribution angle? It looks like a purely empirical value to me. 3) Regarding your question on window openings: This angle is for distributing a concentrated load on a limited length of wall until it becomes evenly distributed on the entire length of the wall (assuming the wall is tall enough). (See also the figures in EN1996-1-1 section 6.1.3.) $\endgroup$ – ingenørd Mar 23 '19 at 21:29
  • $\begingroup$ @ingenørd - Thank you! 1996-1-1 section 6.1.3 is perfect. Can you put this in an answer, so I can mark it? $\endgroup$ – Stilez Mar 23 '19 at 22:25
  • $\begingroup$ @Stilez, yes, sure, I've rewritten it in a slightly more answer-like format. $\endgroup$ – ingenørd Mar 24 '19 at 8:09

I recommended reading EN1996-1-1, and especially section 6.1.3, at least if you are in an area where eurocodes are applicable. This section contains the load distribution angle of 60 degrees. It has a few figures showing applications of the angle, but no formulas for calculating an alternative value.

I would not expect to be able to find a formula to calculate a more exact value of the angle. To me, it appears very much to be an empirical value, which is backed by experiments rather than theory.

As for the question on window openings: The angle of 60 degrees is for distributing concentrated over a higher length of wall. Therefore the figure on the right in your diagram is the correct one. Strictly speaking, you could use the figure on the left for distributing the weight of the window sill and the figure on the right for distributing the weight of the wall between and above the windows, but that will rarely be worth the bother. Just think of it as distributing a load, not distributing the absence of a load.

  • $\begingroup$ Isn’t most theory written to match (approximately) what happens in the real world? $\endgroup$ – Solar Mike Mar 24 '19 at 8:31
  • $\begingroup$ @SolarMike Yes, of course, but sometimes you've got both experimental data and a theoretical model that matches other data from a very different experiment, and sometimes you're basically just fitting a line to a set of data points. Compared to, say, steel, designing masonry is a highly empirical material in the sense that you're often using formulas that are only a fit of experimental data (it worked on the cathedrals…), whereas in steel, you might be using equations that in addition to matching the data, also can be derived from elasticity theory and the von Mises yield criterion or similar. $\endgroup$ – ingenørd Mar 24 '19 at 9:25
  • $\begingroup$ So theory only works due to assumptions or some "fiddling" constant... $\endgroup$ – Solar Mike Mar 24 '19 at 9:30
  • $\begingroup$ @SolarMike Right, and my claim is that the 60 degrees is a fiddling constant. $\endgroup$ – ingenørd Mar 24 '19 at 9:53
  • $\begingroup$ Quite a lot of research has been done on the mechanics of masonry structures, e.g., sciencedirect.com/science/article/pii/S0141029618315189. These findings take time to filter down to practice. $\endgroup$ – Biswajit Banerjee Mar 25 '19 at 0:29

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