why does the self-wt of a masonry wall flow triangularly

huangyhg

why does the self-wt of a masonry wall flow triangularly?
if you are designing a lintel or bond beam supporting a cmu wall above an opening, you can design for the self-weight of only the triangular area (striking an imaginary line at 45 degrees upward and inward from the ends of the beam)...of course unless there's a vertical control joint, in which case you have to design for the self-wt of the entire rectangular area above your beam.
anyway, my question is, why does the weight flow like this in a cmu wall?  is it the mortar?  what causes the arching behavior in cmu and brick, but not in concrete, etc.?
please don't tell me "cmu arches"   i can see that---i am wondering why it arches.
thanks!
it works this way if you have running bond but not stack bond!
imagine the effect on the wall with the lintel in its deflected shape.  visualize which bricks move and which ones stay put.  the ones that moved were supported by the lintel.
yes it is because of arching action.
basically there will be two compression struts just above each diagonal in the triangle and these will spread the load out to the supports.
it is a case of strain compatibility, the arching action is stiffer than the supporting beam so it will take the load above that point.
just beware when you either do not have rigid supports each end or you do not have a significant amount of brick either side of the opening. in both of these two cases the arching action may not work as assumed.
cd has it right on!!!
the classic arch action works even without mortar, but mortar makes it more reliable and predictable. the old arches have worked without mortar, but there may have been some settlement/adjustment, but that was accepable in the short early term, considering the actual long serviceable life. very accurate masonry units without mortar may not have required an adjustment period. - modern arches with construction shoring and mortar have well documented performance as reflected in the pricipals behind aci 530.
you can go crazy, with the amount of reseach on why the "arch action" works so well.
the key is to have enough height of masonry above the opening and enough masonry at each side of the arch area to resist the lateral thrust. the type and strength of the mortar is really immaterial since there is no way to relate mortar cube strengths to actual properties in place.
dick
i think it could be a very similar arching action as in a deep concrete beam design using a "strut and tie model". 45 degrees seems like the typical angle everybody uses but perhaps could be as low as 30 degrees. nice thread.
the mayans knew this eons ago, but their arch was more in the range of 60 to 70 degrees from the horizontal instead of 45 degrees.  they used it extensively in their stonework.  the same principle of no load on the stones in the triangle applies - arching action as previously mentioned- and that's why the interior stones under the arch can be removed without the arch collapsing.  we only have to support those stones that can collapse.   
mike mccann
mmc engineering