Hollowcore vs. CMU Arching Action
Hollowcore vs. CMU Arching Action
(OP)
This question pertains to the design of lintels in CMU loadbearing walls that support precast hollowcore panels in multi-story buildings (i.e., “block & plank” buildings). It is common knowledge that lintels can be designed for only the loads within the triangular area below the apex so long as arching action can occur (block in running bond, a minimum of 8” of wall above apex, etc.). But what happens if the apex occurs above the level of the supported hollowcore? In many cases there is only 16”-24” of masonry between the opening and the bottom of the hollowcore above.
Option #1: Assume no arching action and design the lintel to support the full load of all levels above. This can result in a very heavy lintel and seems excessive.
Option #2: Design the lintel to support the full load of the level immediately above. Assume arching action occurs through the hollowcore interface so the levels further up can be ignored.
Option #2: Similar to Option #2 but the base of the triangular area is located at the top of the hollowcore. Arching of the masonry is assumed to occur above the hollowcore plane.
The fundamental question here is how does the hollowcore affect the development of arching action in the CMU? Which of the three design options is most appropriate (the attached PDF may be helpful)?
Many thanks.
Option #1: Assume no arching action and design the lintel to support the full load of all levels above. This can result in a very heavy lintel and seems excessive.
Option #2: Design the lintel to support the full load of the level immediately above. Assume arching action occurs through the hollowcore interface so the levels further up can be ignored.
Option #2: Similar to Option #2 but the base of the triangular area is located at the top of the hollowcore. Arching of the masonry is assumed to occur above the hollowcore plane.
The fundamental question here is how does the hollowcore affect the development of arching action in the CMU? Which of the three design options is most appropriate (the attached PDF may be helpful)?
Many thanks.






RE: Hollowcore vs. CMU Arching Action
I'm assuming there is no lintel in the walls above at this location?
RE: Hollowcore vs. CMU Arching Action
RE: Hollowcore vs. CMU Arching Action
That is the problem: The lintel will not work without relying on arching action. We are designing a 5-story and an 8-story block & plank building and the lintel will not support the full load. Intuitively Option #3 seems reasonable but we cannot find anything in the literature to support our gut feel.
RE: Hollowcore vs. CMU Arching Action
I would probably install a steel beam below the PC that can carry all loading above it. Then the lintel above the door is carrying minimal loads.
RE: Hollowcore vs. CMU Arching Action
RE: Hollowcore vs. CMU Arching Action
And the questions do not end there: be aware that arching action generates a lot of thrust that the adjacent walls have to take like a shear wall.
RE: Hollowcore vs. CMU Arching Action
1) Like Mike, I generally don't bother with arching. This is because I'm too lazy to evaluate the thrust capacity either side of the wall.
2) Like Jayrod, I feel that #3 is conservative and entirely defensible.
3) Were I a betting man, I would wager that the precast doesn't mess with the arching action at all. As you've detailed it, horizontal shear transfer through the precast is probably better than it is through the uninterrupted CMU.
4) I propose an option #4 in the sketch below. Basically, you would assume that the compression load turns vertical as it crosses the precast. Effectively, this shifts your load triangle up one block to create a two layer cake. Depending on how you look at things, this might also imply the need to deal with some thrust at the top of the precast but I don't see that as a deal breaker. If thrust works at the level of the lintel, it probably works at the top of the precast too. Also, if this were my baby, there would be a bond beam right above the precast and that would make the thrust moot point.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Hollowcore vs. CMU Arching Action
RE: Hollowcore vs. CMU Arching Action
RE: Hollowcore vs. CMU Arching Action
RE: Hollowcore vs. CMU Arching Action
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Hollowcore vs. CMU Arching Action
Which option is #3..., #2 or #2?
While the lintels are pretty much confined to carrying the loads up to the immediate fl. above them, when the arching triangle extends above that fl. level; that is, they are all designed essentially the same, without much arching action up to that fl. But don’t forget that the jamb loads are cumulative when the openings are stacked. Of course, these reactions/loads start corbelling out too, immediately below the lintel bearing. But, you might still end up needing more vert. rebar and a wider jamb as you move down, in the bldg. Also, for the arching action to really work, you need a significant run of wall on either side of the arch to take, and distribute, the trust from the arching action. So, your detail at stacked openings in a large expanse of wall will is o.k. But, if that opening occurs near the end of a wall run, within a couple opening widths? of the wall end, then the arching action is (may be) suspect, without some special detailing. And then, the jamb load distrib. comes into question too. And, another issue that can come into play in a taller bldg., with stacked openings, like corridor openings, is that a wall which runs from exterior wall to exterior wall, as a shear wall, really acts like two shorter separate wall elements coupled together at each fl. level by these lintels. Thus, those lintels can see some significant fixed end moments and added shears as they try to couple the two tall wall elements together. And, these moments and shears will reverse depending upon the direction of the lateral loading.
RE: Hollowcore vs. CMU Arching Action
Dave
Thaidavid