Arching action in CMU

[BAGW]

Hi,

Is there a minimum spacing the CJ has to be located from the opening to get arching action? Is there a rule of thumb no. somewhere?

Thanks

 

[MIStructE_IRE]

One of our old codes here looked for 600mm so that the remaining pier has sufficient beef to withstand the lateral thrust.

 

[WARose]

I that situation, I've normally sketched out the arch lines....and also figured how much masonry wall (adjacent to the opening) is needed to handle the thrust.

 

[BAGW]

@ WARose,

What do you mean by sketched out the arch lines?

 

[WARose]

Something like this:

https://www.google.com/imgres?imgurl=https://

http://www.researchgate.net/profile...hUKEwiDsPC08OP0AhUT8VMKHYpQDesQMygEegUIARCvAQ

I don't know if you have point loads or what.....but I'd keep the control/expansion joint outside of these lines.

EDIT: Actually beyond those lines.....it depends on how much masonry you need to transfer the bearing and the thrust (the 2 principal reactions at each end of the "arch").

 

[bones206]

I generally use judgment, but you can break it down into a simple overturning statics problem. Here is an excerpt from The Stone Skeleton by Jacques Heyman illustrating the concept:

 

[masonrygeek]

As with most issues, it depends. It depends on how the wall is reinforced. If there is a lot of vertical reinforcing, then you most likely have a vertical bar adjacent to both sides of the opening. In that case you are at least 8" from the opening and for a full block you are 16" away from the opening. That is usually enough to maintain bearing of the lintel (hopefully masonry and not steel). For most normal reinforced masonry walls, you don't have a lot of horizontal thrust like you would if you have an unreinforced mutli-wythe brick beating wall. So its mostly the bearing forces that you should be most concerned about. Look at NCMA TEK 10-2D for more info:

https://ncma.org/resource/control-joints-for-concrete-masonry-empirical-method/.