Bond breaker in masonry detail.

[CivilPipe]

I was reviewing some Plans and noticed a masonry detail for a block building with a flat concrete roof. The flat roof has a peripheral beam that rests on top of a bond beam. The peripheral beam has 2 no. 5s top, 2 no. 5s bottom and no. 3 stirrups at 12" o.c. The bond beam has 2 no. 5s. The block wall is reinforced with no. 5s at 48" o.c. There is a bond breaker shown between the bond beam and the peripheral beam and the vertical reinforcement in the wall does not extend into the peripheral beam. The detail calls for one no. 4 bar at each corner of the building connecting the bond beam to the peripheral beam. This basically separates the roof diaphragm from the walls of the building. Anyone want to speculate why its detailed this way? Why break the bond at this location? Does the roof slab expand and contract? Anyone have a better detail for this?

Thanks for your input.

 

[Teguci]

Is there a movement joint (typically 1/2-1" wide with sealant) between the corners of the CMU wall and the CMU wall under the peripheral beam?

I have disconnected the tops of CMU walls from the underside of the floor above for locations where I do not want the CMU wall attracting floor to floor lateral loads (I did brace the top of the CMU against the upper floor in the out of plane direction though).

 

[CivilPipe]

There are no movement or contraction joints in the walls.

There is no support provided at the top of the wall in the out of plane direction.

 

[TXStructural]

Is your floor sitting "loosely" on top of the bond beam? You would HAVE to connect a concrete roof to some lateral force-resisting system.

 

[JAE]

This seems really odd. I suppose there could be concern that there would be thermal movements over time and a desire to let the roof slide relative to the walls.

But cripes - the walls provide the ONLY lateral stability to the building correct? From your description, you say this is simply a four-walled box with a lid and no other structural entities attach to it? And it is exterior?

Somethings wrong here. The four #4 bars in the corners probably aren't adequate to drag the lateral forces into the shearwalls. But we are going by your description only.

 

[JAE]

....and I might add - if thermal expansion/contraction were a concern, then the four #4 bars wouldn't be placed in the corners but rather in the center, to let the corners of the roof expand in both directions from the center of the wall.

As it stands with corner bars tying in the lid to the walls, there is no expansion possible as the corner bars will fight each other.

 

[CivilPipe]

My mistake. The #4 bars are at the center of the wall.

"This seems really odd. I suppose there could be concern that there would be thermal movements over time and a desire to let the roof slide relative to the walls.

But cripes - the walls provide the ONLY lateral stability to the building correct? From your description, you say this is simply a four-walled box with a lid and no other structural entities attach to it? And it is exterior?"

My concerns exactly.

I think that the vertical wall reinforcement should extend into the peripheral beam on all the exterior walls. I will end up debating this with another engineer that has 40 plus years of experience, so I wanted to see what others had to say about it before.

 

[CivilPipe]

Either extend the vertical wall steel into the peripheral beam, or add a shear key of some type between the bond beam and the peripheral beam. The shear key could provide lateral support for the wall, but still allow some movement.

 

[hokie66]

How large is the building? If quite small, the single bar in each face could be enough.

 

[msquared48]

Although the use of friction is not allowed except for shear friction, is the friction generated between the peripheral beam and bond beam enough to transfer any lateral forces between them? In fact, could shear friction logically be applied here?

I agree that the situation is not good, but...

Mike McCann
MMC Engineering

 

[csd72]

This reminds me of a detail that I have seen used a lot in houses in Australia where a sliding joint was provided at the top of masonry walls that supported concrete slabs.

It was assumed that the sliding joint provided enough resistance for normal loads such as wind e.t.c. but would allow movement under the much higher stresses caused by shrinkage and temperature.

Sound like your designer may be using a similar philosophy.

I would definately not use it in a seismic area but for wind it may be okay (unless prohibited by code) as long as the weight of the slab exceeds the wind uplift.