Masonry Is Strong in Compression but Needs Reinforcement for Tension
Masonry - including concrete masonry units (CMU), brick, stone, and glass block - is one of the oldest and most enduring building materials. Modern masonry construction is well-codified and tested on the ARE PPD division because architects regularly specify masonry for walls, veneer, and bearing systems. The fundamental concept to understand about masonry is that it performs excellently in compression (it can carry enormous vertical loads) but is weak in tension. Reinforcement and proper grouting transform masonry from a brittle, tension-weak material into a ductile, versatile structural system.
Concrete Masonry Units (CMU)
CMU is the standard structural masonry unit for most commercial applications. CMU is available in standard nominal widths of 4, 6, 8, 10, and 12 inches, with standard height of 8 inches and length of 16 inches. The actual dimensions are 3/8 inch smaller in each direction to account for the mortar joint (so an 8×8×16 nominal block is actually 7-5/8 × 7-5/8 × 15-5/8). CMU is used for: bearing walls, shear walls, retaining walls, backup walls behind brick veneer, and fire-rated partition assemblies.
Reinforced vs. Unreinforced Masonry
Unreinforced masonry relies solely on the mortar and masonry unit bonding for structural capacity. It is limited in height, span, and lateral resistance. Unreinforced masonry is generally not permitted in high seismic design categories and has limited application in modern construction. Reinforced masonry incorporates steel reinforcing bars in the vertical cells of CMU (which are then grouted solid) and horizontal joint reinforcement (wire mesh laid in mortar joints). Reinforcement dramatically increases the masonry's flexural strength, allowing it to resist lateral loads from wind and seismic forces.
Grouted Masonry
In partially grouted masonry, only the cells containing vertical reinforcement are filled with grout. In fully grouted masonry, all cells are filled. Full grouting significantly increases the structural capacity and fire resistance of the wall, and is required in high seismic areas. The grout must have adequate fluidity to flow into cells and around rebar during placement - a mix with high slump is required, which is different from standard concrete mix design.
Brick Veneer vs. Structural Brick
Brick is most commonly used as veneer - a non-structural cladding attached to a backup structure (wood frame, steel stud, or CMU) with metal ties. Brick veneer is not structurally connected to the backup in a way that transfers lateral loads - it simply hangs from the ties and transfers lateral loads through the ties to the backup. Structural brick (used in actual bearing wall systems) is different: the brick must have sufficient strength, and the wall must be designed for all applicable loads.
Wall Ties
Wall ties are metal connectors that attach brick veneer to the structural backup. They must be corrosion-resistant (stainless steel or hot-dipped galvanized) and spaced at maximum 24 inches o.c. vertically and 36 inches o.c. horizontally (or as specified by the structural engineer). Ties transfer lateral loads from the veneer to the backup but allow the veneer to move slightly relative to the backup (to accommodate differential movement) - this is why ties are typically bent or corrugated rather than rigid.
Key Exam Points
- Masonry strong in compression; weak in tension - reinforcement required for flexure and seismic.
- CMU nominal size: 8×8×16 (actual: 7-5/8 × 7-5/8 × 15-5/8).
- Reinforced masonry: rebar in grouted cells + horizontal joint reinforcement.
- Brick veneer: non-structural; ties connect to backup; veneer does not carry structural loads.
- Full grouting: all cells filled; required in high seismic design categories.
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