Masonry Structural Systems: Unreinforced, Reinforced, and Design Principles

Masonry is one of the oldest structural materials in continuous use. From Roman concrete-and-stone arches to twentieth-century concrete masonry unit (CMU) bearing walls, the fundamental principle has not changed: assemble small, strong-in-compression units with mortar into an assembly that behaves structurally as a whole. The challenges are also enduring. Masonry assemblages are strong in compression but weak in tension. They are heavy, which is an asset for stability and a liability for seismic design. They are brittle, which makes detailing in earthquake-prone regions demanding. Understanding these material properties is the starting point for masonry structural design.

Masonry Unit Types and Mortar

Three unit types dominate structural masonry in current practice. Concrete masonry units (CMU), commonly called concrete blocks, are the workhorses of modern structural masonry. Standard nominal dimensions are 8 x 8 x 16 inches (actual 7-5/8 x 7-5/8 x 15-5/8 inches to allow for mortar joints). CMU have cores that can be left open, filled with grout, or used to place vertical reinforcing bars. The net compressive strength of a CMU depends on unit strength and mortar type; typical values of f'm (masonry assembly compressive strength) used in design range from 1,500 to 3,000 psi (10 to 20 MPa) for standard CMU construction.

Clay brick offers higher unit strength, around 3,000 to 10,000 psi for face brick, and superior durability in exposed environments. Structural brick walls are typically wythe construction, with ties connecting inner and outer wythes in cavity wall systems. Stone masonry, now rarely used for new structural construction, appears primarily in restoration of historic buildings.

Mortar bonds the units and transfers stress between them. ASTM C270 classifies mortar into Types M, S, N, O, and K in decreasing order of compressive strength. Type S (minimum 1,800 psi compressive strength) is commonly specified for structural masonry below grade and for exterior applications. Type N (750 psi) suits above-grade exterior and interior load-bearing walls. Using the highest-strength mortar is not always optimal: stiffer mortar can cause more cracking by restraining the thermal and moisture movement of the units.

Unreinforced Masonry: Keeping Tension Out

Unreinforced masonry (URM) relies entirely on the compressive and limited flexural tensile capacity of the mortar-to-unit bond to carry loads. The critical design rule for URM is keeping the resultant compressive force within the kern of the wall section. For a rectangular wall cross-section, the kern is a zone extending one-sixth of the wall thickness on either side of the centroidal axis. When eccentricity of vertical load, lateral wind pressure, or both push the resultant outside this zone, net tension develops on one face of the wall. Since URM has very low tensile capacity (the modulus of rupture of masonry in tension ranges from about 20 to 68 psi depending on mortar type and orientation), cracking follows quickly.

The slenderness ratio of a URM wall, defined as the effective height divided by the nominal thickness, governs its allowable axial compressive stress. The Masonry Standards Joint Committee (MSJC) code, now incorporated into TMS 402, limits the allowable stress in URM to:

F_a = 0.25 f'm [1 − (h/140r)²] for h/r ≤ 99

where h is the effective height and r is the radius of gyration. This parabolic reduction penalizes tall or thin walls for their susceptibility to combined axial load and eccentricity buckling. URM walls are limited to low-seismic zones (Seismic Design Category A or B) in most current codes because of their brittle failure mode.

Reinforced Masonry: Adding Ductility and Tensile Capacity

Reinforced masonry places deformed steel bars in the cores of CMU walls (or in the collar joints of brick walls) and fills those cores with grout. The resulting composite behaves analogously to reinforced concrete: the masonry carries compression and the steel carries tension. This transforms masonry from a material that cracks and crumbles under tension to one capable of flexural and shear resistance comparable to concrete.

Vertical reinforcing bars resist out-of-plane bending from wind and seismic lateral loads. The design follows a procedure similar to reinforced concrete beam design: calculate the factored moment demand, locate the neutral axis depth from strain compatibility, and size the reinforcement to provide the required moment capacity with adequate ductility. The masonry compression stress block is approximated as rectangular with an equivalent depth of 0.80c for CMU construction (TMS 402).

Horizontal reinforcement serves two purposes. Joint reinforcement, typically small-diameter wire trusses or ladders placed in the bed joints every second or third course, controls shrinkage cracking and improves the composite action between mortar and units. Structural horizontal bars placed in bond beams at regular intervals (typically every 4 feet or at floor and roof levels) resist in-plane shear. A bond beam is a continuous CMU course with the webs knocked out to allow a continuous horizontal bar to be grouted in place.

Special reinforced masonry shear walls designed for seismic resistance follow prescriptive reinforcement ratios and spacing limits. TMS 402 requires that the maximum reinforcement spacing not exceed 48 inches horizontally or vertically, and that the total reinforcement ratio not exceed a maximum that depends on the ductility demand category. Boundary elements or end returns at wall edges confine the masonry in the compression zone and prevent premature crushing during large lateral deformations.

Wall System Configurations

Masonry walls appear in three structural roles. Bearing walls carry both gravity loads from floors and roofs and lateral loads from wind or seismic forces. They transfer floor loads directly downward without a separate beam, making them efficient for low-rise residential and commercial construction. The floor-to-wall connection must transfer both vertical reaction and lateral anchorage force.

Shear walls resist lateral forces in their plane and transmit them to the foundation. Masonry shear walls are rated by in-plane stiffness as well as strength. Because masonry is stiffer than wood framing, masonry walls in a mixed structure typically attract the majority of the lateral load in proportion to their rigidity. Rigidity of a masonry wall pier varies with the height-to-length ratio: a squat pier (low h/l ratio) behaves in cantilever-shear mode; a slender pier (high h/l ratio) behaves in flexural mode.

Veneer walls are non-structural cladding attached to a structural backup system. Brick veneer on steel stud backup is the most common form: the steel studs carry the gravity load of the floor above, and the brick veneer carries only its own weight between shelf angles at each floor level. The veneer-to-backup connection is through adjustable wall ties, which must accommodate differential movement between the stiff masonry and the more flexible steel frame while still transferring out-of-plane wind pressure.