Masonry Control Joints 4

[pelotoner]

I understand that typical masonry joints are at 24' on center in a wall. However, how does that impact the rest of the structure?

ie... A 500' long building that has 12' tall masonry exterior walls for a pre-engineered metal building. There is a tie beam at the top of the wall that is spanning between metal building columns. For joints at 24' o.c., how does that impact the concrete beam? Is all of the temp/shrinkage transferring to the tie-beam? What about the Z-purlin continuous along the top of the wall?

Aren't we just preventing the stair-step crack and placing more stress on another element?

 

[msquared48]

For the 24' span between CMU wall control joints, you are probably at or near the mainframe spacing of the structure. I would position the CMU wall control joints at the mainframes where possible.

Regarding the steel beam at the top of the CMU wall, this is a wind/seismic beam between the mainframes that is there to support the CMU wall, and is designed to take the wind/seismic thrust from the CMU wall to the mainframes. The Z roof purlins have nothing to do with this. There should be a CMU bond beam at the intersection of the steel beam with the CMU. Is this the concrete beam you are referring to?

Mike McCann
MMC Engineering

 

[pelotoner]

Typical to this area of Florida, we have a half-wall (cladding) of masonry that transitions to Z-purlins horizontally to make the remainder of the vertical wall surface. The Z-purlins span between columns and so does our concrete tie-beam, which in turn is transferring the wind from the cmu to the columns.
But more to the point, if I position the control joints at the columns (24' o.c. +/-), doesn't this just stress the concrete tie-beam as the masonry is free to expand and contract?

 

[hokie66]

I dont' know why you would use a concrete tie beam. It is better practice to use a steel beam bolted to the top of the wall and to the steel frames. Suggest a channel. As Mike said, put the wall vertical joints at the columns. The Z members on the wall are properly called girts rather than purlins, thus some of the confusion.

 

[pelotoner]

I don't think my intent is clear.
If we have any masonry structure: bearing walls with bar joists and non-composite concrete deck, bearing walls with metal roof deck or trusses, cladding masonry walls as called out above, etc...
When we create all of these control joints in the masonry, what is the effect on the surrounding elements? Is there undue tension stresses placed on the tie beam? Should we provide a control joint in the tie-beam also? If the Z-girt is continuous at the top of the wall as PEMB components usually are, but the masonry is discontinuous with control joints, is the girt in tension also?

 

[hokie66]

The control joints in the masonry provide relief for in plane masonry stress. The beam at the top of the wall has to support the wall and span between the columns, but if it is a steel beam, the slip in the end connections will be enough to prevent reapplying force longitudinally to the wall. If you use a concrete beam, providing similar flexibility is problematic. It can be done, but is more labor intensive and more can go wrong. The girts do not figure in, they can move quite freely because the holes are bigger than the bolts.

With your system, for global stability, there will need to be diagonal braces from the knee of the frame to the beam supporting the top of the wall

 

[pelotoner]

Please ignore the specific example stated at first and change focus to all masonry construction.

So the intent of masonry control joints is to relieve the temp stress in the masonry? And the idea is to have small 24' sections do this, whereby reducing the amount of tension stress on the connecting elements?

Is there any detrimental effects on, say, a concrete tie beam at the top of the wall in typical bearing wall construction?

 

[DaveAtkins]

The main reason for control joints in CMU is because CMU shrinks. In brick, you have expansion joints, because brick expands.

Theoretically, nothing should cross a control joint or an expansion joint, because it will restrain movement.

In practice, due to overriding structural concerns, sometimes a bond beam will continue across a control joint, or sometimes a deck edge angle will continue across a control joint.



DaveAtkins

 

[ron9876]

I also live in Florida. I rarely see any control joints in masonry unless it is designed by someone from the north. Always worried me when I was first starting out because everyone in other parts of the country use joints but I haven't ever seen a problem with it down here.

In response to the original question as recommended by hokie66 use a pre-engineered steel shape with a horizontal slot at expansion bolts to attach the masonry to the steel framing with a bond beam at the connection. If the block wall is full height we brace the building is the short direction by placing horizontal wind bracing in the end bays which attach to the rigid frames and then attach the rigid frame to the wall with plates and expanson bolts.

 

[pelotoner]

Thanks ron9876, we have typically only shown the locations of control joints on exceedingly long stretches of masonry (+100'), but our typical detail calls for 24'.

Anyone's idea for a more reasonable spacing? In a 500' wall, 20 control joints seems excessive.

 

[concretemasonry]

DaveAtkins has it right. The purpose of a control joint is to isolate one masonry panel from another. The bond beam is really integral to the panel it self.

Each panel should be isolated. The joint reinforcement should stop at the control joint. The steel in the bond beam should also stop at the control joint. To carry any shear across the control joint or to maintain alignment, use a steel bar enbedded in one panel and allow the end on the other side of the joint to "slip" by one of the common methods.

Florida has conditions that preclude many of the nationally used details. This is the reason why no problems are seen. Cracks in masonry walls are caused by the stresses induced by temperature CHANGE/DIFFERENCE, moisture CHANGE/DIFFERENCE and shrinkage combine to cause enough length change to cause a crack. Most buildings in Florida and constructed within a relatively narrow temperature change compared to other areas. The moisture/humidity is relatively uniform and the walls do not see the drastic drying that happens in colder climates where the dew points are much lower. Because of the relatively high humidity and temperature, the carbonization of the cement in the mortar and units proceeds much sooner, increasing many structural properties. - Just some un-scientific observations based on experience.

 

[pelotoner]

Thanks concretemasonry, I have seen you post on masonry before and was waiting for your input. Invaluable, as I expected. Cheers!

 

[miecz]

I've always continued bond beam reinforcing through the control joint.

 

[MikeE55]

Control joints at 24' are not excessive. I have inspected some buildings where the cj's were omitted and the brick pushed out the corners of the building. Also brick bowed outward from the cmu backup, especially noticeable at lintels. The expansive force can be tremendous and will find a way to relieve itself if you don't give it room to grow.

 

[hokie66]

In addition to the reasons for control joints listed above, they are useful for protecting walls from distress due to differential vertical foundation movement, whether settlement or volume change due to change in moisture content. On sand, as most of Florida is, you wouldn't have as many of these problems to deal with as most of us do. Still, 24 feet is not an excessive requirement. My company requires them at 8 metre centres and 4 metres from corners.

 

[OzEng80]

Hokie makes a good point. Almost all wall damage that i have seen can be attributed to settlement & ground issues. I believe reinforcing in the bondbeam and footing (restraint lines) typically prevents/minimizes shrinkage cracking.

I have (unintentionally) gone 20m in a basement wall, with wall returns (with N12-400 horizontally) without evidence of cracking.

My company provides galvanized, greased dowels to bondbeams at cj locations. Continuing the reinforcing defeats the purpose of the joint and subjects the reinforcing to the elements.

 

[TTUengr51]

One thought regarding the continuity of the bond beam...if the bond beam in question is at a structural diaphragm, the bond beam typically is treated as a diaphragm chord. If the chord is in tension, wouldn't it be necessary to have a continuous bond beam to develop the diaphragm chord?

Or is this issue alleviated by considering a continuous edge angle bolted to the wall and considering it as the chord?

Any thoughts...

 

[miecz]

Well, yeah, this is why I make mine continuous.

 

[OzEng80]

Continuous edge angle forms the chord. Similar to tilt-up/precast construction with slotted fixings to allow for shrinkage movements.

 

[haynewp]

I keep all bond beam reinf. continuous and all deck angles continuous and no horiz. slotted connections to shear walls. How do you transfer the diaphragm shear if the edge angle is connected with horizontal slots to the shear walls?