Shelf Angle Requirements for CMU Veneer (not Clay Brick) 1

[abusementpark]

Has anybody seen any literature on shelf angle requirements for CMU veneer? The TMS/ACI masonry code indicates a shelf angle is needed for height above 30' and every story above for all masonry veneer. However, I'm fairly certain this guidelines come from the Brick Institute of America and is intended to accommodate expansion of clay brick veneer.

Wouldn't CMU veneer have different temperature characteristics? Perhaps better than clay brick?

 

[concretemasonry]

CMUs have a long term shrinkage and clay brick has a long term expansion and that is the reason for that portion of the ACI/TMS/NCMA joint standards. The basis for this is the large number of old buildings with clay brick veneer stacked up to high elevations on buildings that had long term shrinkage, so there were alignment problems due to the height of the expansive clay veneer compared to the back-up materials.

The document you referenced is a masonry industry consensus standard similar to the ASTM standards for materials, testing methods and reporting.

Practically speaking, going up with veneer 3 stories at a time between the relief angles is not a hardship since the angle dimensions are more manageable and are not a cost or construction problem. The weights of the different veneer materials are variable depending on the materials and configuration of the units.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[abusementpark]

So is CMU veneer better, worse, or about the same as clay brick veneer in terms of temperature cracking?

Basically, I have a situation where I have 34' height of CMU wall veneer and I am trying to justify not putting in a shelf angle, even though the height exceeds 30' and technically violates code.

 

[concretemasonry]

If it a true, proper masonry veneer with rigid insulation and an air space, there would be no difference between CMUs and clay brick because they are both at the ambient exterior temperature.

If there is no insulation in the cavity, CMUs may be slightly better from a thermal standpoint.

It is not a structural question but a matter architectural detail for horizontal spacing of control or expansion joints.

If you can justify different spacing based on documentation, you can sign off on the extra 4' vertically.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[DST148]

Ref ACI 530-05: Coeff of thermal expansion for clay masonry is 4 x 10^-6 where as for concrete masonry it is 4.5 x 10^-6. The movements caused due to temperature changes are reversible. Therefore from temperature considerations, clay and concrete masonry veneer are comparable.
Fired clay products expand upon contact with moisture and do not return to original size upon drying. Moisture expansion is reversible in concrete masonry.
Providing for horizontal movement joint to accommodate vertical movement of clay and concrete masonry is crucial in masonry veneer construction. However, for low-rise buildings, the entire height of the masonry veneer can be supported on the foundation wall. For wood framed and cold formed steel backup walls, the code limits the height above the foundation to 30 feet except that 38 feet is permitted at a gable. No limit is specified for masonry back up walls. These are prescriptive requirements for anchored masonry veneer.
Given 34 feet height of concrete masonry veneer, you can choose to use alternative design for anchored veneer per Section 6.2.1 to justify omission of horizontal movement joint altogether.