Mar2805:
In your last two attachments the entire brick wall load is on the spandrel beam below that particular wall, and the spandrels at each level should be designed accordingly. Keep in mind than concrete shrinks and clay masonry tends to expand with age, so you must leave a joint at the top of the brick, at each story, to tolerate this expansion/contraction activity.
Take a look at a few good masonry design texts, masonry construction handbooks or masonry association tech notes for some better understanding of the loading of lintels and the arching action of various types and ages of masonry. The 45̊ triangle is a practical representation of the masonry below an arch, which might/would fall out without support, or is loading the lintel. The exact angle is subject to a bunch of variables about the make-up of the wall, otherwise the arch is in compression and starts to support the brick. The 60̊ angle is a reasonable representation of a line where the masonry is being held up by a normal corbeling action of the wall system above and around it. And, you might think of the space btwn. the two angles almost as a real arch that has its two arcs separated by a height about twice the thickness of the wall. Both of these actions require some monolith of masonry wall beyond the extent (width) of the opening below. That is, the arch needs some structure beyond the opening to take the horiz. thrust induced by the arching action. If you cut these angled lines (the arch) by an opening above your lintel, then the arching action can’t really take place, and all bets are off. Thus, Tony’s first attachment 27FEB14, 16:16, is really only correct for a large expanse of wall with no openings or concentrated loadings in the region of the triangles. His attachment from is code book gives a pretty good general explanation of the way the loads are figured, note the “common sense...” admonition. But, in your earlier attachments I would apply the full slab above and the couple feet of masonry to the lintel, and quit the screwin around with the load, unless I needed to finesse the shear, reaction or bearing a bit at a jamb. In your sketch showing symmetrical windows above, you’ve cut into the loading triangles, so it is doubtful that arching action can take place. I would apply all the wall load btwn. the windows directly to the slab. I would allow that the inner window jamb load corbel out (45̊) down to the slab, and then apply that to the lintel. The outer window jambs pretty much get carried straight down and added to the jamb load at your large, lower opening. On your sketch with the door opening on the left, there is just no arching action possible because of the door opening. Again, if I needed to finesse things I would allow the 60̊ corbeling thinking on the right side, but all of this load must still be taken down to the lower, large opening jambs.
The thing that you have to be careful about, for all this chasing the loads around, is that you don’t lose or forget any loads in the process. That is to say... even if the lintel doesn’t carry all the loading from above it in bending, due to the arching action; the opening jambs and wall section immediately adjacent to the jambs must carry all the loading (total actual load) from above the opening, and concentrated loads above may concentrate these total loads more on one jamb than the other.
