How to Design Dry-stack Concrete Masonry Units

Brief 

David Biggs explores frequently asked questions about The Masonry Society’s latest update to guide the design and construction of dry-stack concrete masonry units, such as the need for movement joints. Biggs also covers where dry-stack units can be used and how to approach their seismic requirements.

Insight

The use of masonry began with unreinforced dry-stack (mortarless) construction. For millennia, stone has been laid dry without mortar. The. Great Pyramid of Giza (c. 2600 BC) is an example of limestone and granite ground smooth and fits tight without mortar (Figure 1).

There are numerous examples of residences, walls, fortresses, and towers constructed using dry-stack masonry throughout the world. Unreinforced, these structures rely on gravity and friction to maintain their stability.

The fascination with dry-stack masonry continues today. While stone retaining walls are popular, the emphasis in North America is now primarily on manufactured masonry units. In addition to building applications, dry-stack masonry for structures has been applied to segmental retaining walls using gravity or mechanically stabilized earth techniques.

Internationally, dry-stack construction is being used in modern residential and commercial construction. There are several features common to many contemporary dry-stack concrete masonry systems, including:

Most use a form of grouted reinforcement with proprietary unit configurations that interlock and have aligning cores to accommodate the grout.Prestressing may or may not be grouted in place.

Their use is often justified based on successful performance, experience, and available research.In the United States, dry-stack masonry has been in our building codes for decades. However, it utilizes a surface bonding coating on both sides of the wall to provide structural flexural capacity.

The current International Building Code (IBC), Section 2114 Dry-Stack Masonry, is based upon Allowable Stress Design procedures in Chapter 8 of TMS 402, Building Code Requirements of Masonry Structures. The materials and construction are based upon ASTM C946 – 18 Standard Practice for Construction of Dry-Stacked, Surface-Bonded Walls.