Brick by Brick

To view the figures and tables associated with this article, please refer to the flipbook above.

Masonry construction is prevalent in buildings as a material that is straightforward to assemble and can be sourced locally to each region. Beyond its widespread use across various building types, masonry’s true marvel lies in its ability to meet diverse design requirements through a singular, composite system:

One technique used to achieve a unique appearance with masonry is varying the bond pattern. Most masonry walls use rectangular blocks, and these blocks are often built with each course offset one-half the width of the course below in a pattern called “running bond.” Refer to Figure 1 for an example of a running bond pattern as well as other types of patterns. The conventional running bond, with its staggered courses for a balance of strength and efficiency, contrasts with “stack bond” where vertical joints align or nearly align (Fig. 2).

This article delves into the implications of the bond pattern on the structural design of masonry. While a wide range of bond patterns can be used, for practical purposes the most commonly used are the running bond and horizontal stack bond patterns. Other patterns, such as diagonal basket bond, diagonal bond, and basket weave are beyond the scope of this article.

The term “stack bond” is not explicitly defined in the Building Code Requirements and Specification for Masonry Structures (TMS 402/602-22). Instead, it is referenced in the commentary as one type of bond pattern that is “not laid in running bond.” Running bond is defined to exist when the overlap of units is at least one-fourth of the unit length. In typical running bond construction, the units overlap by one-half of a unit length; however there may be situations where it is desired to use less of an overlap for aesthetic purposes. The code recognizes that stack bond, where the vertical joints of the blocks are in alignment, is one type of pattern that does not achieve the required one-quarter unit overlap; however, any bond pattern where the vertical joints overlap by less than one-quarter are subject to special provisions in the code for masonry “not laid in running bond” (Fig. 3).

The building code requirements for masonry were primarily based on structural testing and research performed on wall panels laid with running bond construction. The default assumption in the code is that masonry is laid in a running bond pattern, therefore, when other patterns are used there may be some effect on the strength of the wall.

The Portland Cement Association (PCA) performed structural tests in accordance with ASTM E72 (Standard Test Methods of Conducting Strength Tests of Panels for Building Construction) to determine the impact of varying bond patterns on the strength of masonry walls constructed with hollow units. The tests considered vertical loading with an eccentricity equal to one-sixth of the wall thickness to determine compressive strength, and a uniformly applied lateral load to determine bending strength (Portland Cement Association, 1961).

There was no decrease in the compressive strength for different bond patterns when blocks are laid in the horizontal running or stack bond position. However, walls tested with blocks laid in the diagonal or vertical position exhibited some loss of compressive strength due to the reduction in net area associated with those patterns (the webs of the vertical units do not align with respect to the direction of force, therefore do not contribute to the strength). The result of this outcome is that the equations for compression in the code do not require reductions in strength based on the pattern used.

In many patterns, including horizontal stack bond, the flexural strength of a wall spanning vertically was surprisingly determined to be higher when compared to running bond. Only three patterns (Vertical Stack, Basket Weave B, and Coursed Ashlar, refer to Figure 1) resulted in a reduction in vertical flexural strength. Horizontal Stack Bond, a commonly specified alternate to running bond, was one of the patterns found to have a higher flexural capacity when spanning vertically between supports.

For walls spanning horizontally between supports, stack bond construction was shown to be considerably weaker in flexure. One example of this condition is a CMU shaft at the perimeter of a building (Fig. 3). In this case, one or more walls are exposed to exterior wind pressure and do not connect to a diaphragm to allow the wall to span vertically. The exposed wall must span horizontally between the perpendicular walls at the corners of the shaft. Testing of this condition showed that the flexural strength of the stack bond masonry was approximately 30% of running bond construction.

It is important to note that the testing was performed on hollow-block panels that did not contain any reinforcement. The effect of reinforcement on the capacity of masonry walls is discussed in the following sections.

The most significant impact of the bond pattern is on the flexural strength for horizontally-spanning elements. Even though tests showed masonry exhibits some flexural strength with bond patterns other than running bond, the code sets a flexural tension stress value for unreinforced masonry to 0 psi when using these patterns in horizontally-spanning walls (refer to tables 8.2.4.2 or 9.1.9.1 (TMS 402-22)). To compensate for this reduction in flexural strength compared to running bond, masonry designed to span horizontally with patterns other than running bond must include horizontal reinforcement.

When using patterns other than running bond, TMS 402 requires a minimum horizontal reinforcement ratio of 0.00028 over the gross vertical cross-sectional area of the wall. The maximum spacing of this reinforcement is 48 inches. This requirement applies to all non-running-bond walls, regardless of whether they span horizontally or vertically, and is needed to provide continuity across the head joints. Designers may use horizontal bond beams or joint reinforcement to meet this requirement. Bond beam reinforcement should be centered in the beam or constructed with bars symmetrically placed about the wall centerline because of the potential for load reversals. The choice of using bond beams or joint reinforcement to meet this minimum requirement will depend on several factors, which are discussed in the following paragraphs.

Although the primary impact to the strength is when walls span horizontally, vertically-spanning walls are subject to a few unique requirements as well. The ability for head joints in “not laid in running bond” patterns to transfer flexural compressive stresses under bending forces in vertically-spanning walls is limited. If horizontal bond beams are provided at 48 inches, the effective width of the compression area is the same as running bond construction (Fig. 5a), however when masonry is not laid in running bond and does not have bond beams spaced at 48 inches or less, the effective width of the compressive area is limited to the lesser of either the length of the masonry unit or the spacing between vertical bars. This requirement is especially relevant for masonry that uses joint reinforcement instead of bond beams: while all “not laid in running bond” masonry must have horizontal reinforcement, if joint reinforcement is used to meet this requirement instead of bond beams, the flexural compression zone width will be limited as indicated in Figure 5b.

Concentrated loads do not distribute across head joints in walls “not laid in running bond.” The distribution of concentrated loads through masonry walls is limited by the head joint placement, however bond beams located at the point of application can be used to distribute the load over a greater width (Fig. 6). This provision also applies to bearing calculations: the increase in bearing area for a larger supporting element is limited to an area that must be truncated by the head joints when masonry is not laid in running bond.

The in-plane shear strength of unreinforced masonry is heavily influenced by how the wall is constructed, specifically which bond pattern is used and whether the masonry units are open-ended. Open-ended units allow grout to fill the void space between cells along the length of the wall, interlocking them and limiting the influence of the weak head joints that occur between units (Fig. 7). For walls not laid in running bond, those constructed using open-ended units can realize a shear strength of at least two times the strength of walls that do not have open-ended units. Running bond patterns provide an additional increase in shear capacity, nearly four times the strength of masonry that is not laid in running bond and not constructed with open-ended units.

In the 2016 edition of the code, the in-plane shear provisions did not include allowances for partially grouted walls not laid in running bond. The 2022 edition of the code provides an allowance for in-plane shear in these walls, but limits the allowable strength to 23 psi (using strength design provisions), a result of the integrity that the required prescriptive horizontal reinforcement provides. This lack of shear strength can be overcome by constructing fully-grouted walls with open-ended units, or by reinforcing the wall to invoke reinforced design provisions. The design equations for reinforced masonry do not differentiate capacities for in-plane shear based on running bond versus “not laid in running bond” patterns.

If the masonry wall is a Special Reinforced Shear wall, increased reinforcement must be provided for walls constructed with “not laid in running bond” patterns (Fig. 8). Non-participating masonry elements have similar increased requirements.

Non-structural partitions and veneer are typically subject to an even wider variety of layout patterns than structural masonry, and the unique requirements applicable to “not laid in running bond” patterns for these elements may be encountered more often.

TMS 402 requires that veneer blocks laid in other than running bond patterns have horizontal joint reinforcement of at least one W1.7 wire spaced at a maximum of 16 inches on-center. This is equivalent to the required cross-sectional area for Concrete Masonry Units, applied over a single-wythe 4-inch nominal wall thickness.

For horizontally-spanning masonry partition walls, the prescriptive requirements of TMS Chapter 15 are not applicable to walls with “not laid in running bond” patterns. This follows the test results and provisions in the TMS that pertain to the lack of allowable flexural tension stress parallel to bed joints. If designers wish to span non-running-bond partition walls horizontally between supports, they must be constructed with continuously grouted sections (using open-ended units and designed as unreinforced masonry), or reinforced and designed in accordance with relevant sections of Chapters 8 and 9 of the TMS.

Bond patterns in masonry can be used to achieve a unique aesthetic and are a simple way to articulate the visual context of a building. While the structural capacity of masonry walls is typically based on a running bond pattern, other patterns may be used with some reductions in strength and careful attention to detailing. A summary of the requirements for structural masonry constructed in other than running bond patterns is as follows:

Table 1 can be used by designers as a reference for which provisions of the TMS apply to walls “not laid in running bond.” While the content of this article was developed using the provisions from the 2022 edition of the code, the relevant sections of the 2016 edition are also provided.