Words: Jim Tann, Director of Technical Services of The Belden Brick Company
Photos: The Belden Brick Company, Kaminski Studio
Knowing the advantages of all brick masonry attributes, designers constantly look for improved systems to use brick in modern, economical ways. Larger brick sizes can assist in this and through-wall methods to avoid the cost of backup systems. The durability of brick is unmatched. It provides structure, fire resistance, an efficient and resilient building envelope, and aesthetics. Using a building material to its fullest potential is a sustainable practice, and brick masonry as a load-bearing wall system accomplishes these needs.
The term “structural brick” to define a specific unit configuration is a bit misleading. All full-depth brick units are structural. They can be built in a wall so that the solid wall thickness carries loads applied by the building weight, occupancy, and environmental factors acting upon the wall, such as wind and earthquakes. Many times, to achieve sufficient strength, the wall system needed to be several wythes thick. One example of this is the Monadnock building in Chicago, IL. The building is a solid masonry load-bearing structure of 16 stories in height, and the wall thickness at its base is six feet of structural brick and granite.
Several other examples of load-bearing (structural) brick walls exist in many historic residential areas of cities. The German Village in Columbus, OH, is one where homes, built in the 19th century, were constructed with a load-bearing brick wall of 8 to over 12 inches in thickness.
Due to sheer weight and cost, reinforcing steel began to be added to load-bearing brick walls to slenderize the wall system and increase strength. In its introduction, reinforced brick masonry walls were built as double-wythe brick walls having a space (grout space) between the wythes. Reinforcing was incorporated in this space, and it was grouted solid.
Some thinner wall sections were created by building hollow pilasters at regular spacings. The walls incorporated reinforcing and grout into the pilasters, while mason contractors built other walls with two wythes of brick. The brick units were cut at standard spacings to create grout pockets, and these pockets were reinforced and grouted.
Though these developments significantly reduced cost and construction time, innovation was needed to keep viable, affordable structural systems on load-bearing brick walls. Today, reinforced, through-wall brick units are available to help achieve such designs and minimize construction costs. These through-wall units can be 6-inch to 12-inch thick and contain hollow cells that can receive reinforcing and grout.
For sufficient reinforcing steel and grout to be occupied within the hollow cells of these brick units, the cell dimensions are typically more prominent than the traditional core holes in facing brick. As such, the net bedding area (total bedding area of brick minus area of the hollow spaces) of the units often is less than the minimum allowed for facing brick at 75%. Thus, these units qualify as hollow brick and are classified in ASTM International’s Standard Specification for Hollow Brick, ASTM Standard C652. This Standard is very similar to ASTM International Standard Specification for Facing Brick, ASTM Standard C216 in its minimum Grade and Type requirements. ASTM Standard C652 includes Grades SW (Severe Weathering) and MW (Moderate Weathering).
Also included are Types: HBX, HBS, and HBA (the prefix HB stands for Hollow Brick). These designations apply to hollow brick to be used for facing applications. ASTM Standard C652 also introduces one additional Type, HBB, for hollow building units where the brick wall will be covered or out of view.
In addition, ASTM Standard C652 allows for a greater void area. Two classifications for the void area are included in ASTM Standard C652. These include Class H40V and Class H60V, where up to 40 percent void or up to 60% void is allowed, respectively.
Face sizes of these units are commonly a nominal 4 inches in height and 16 inches in length. Other face sizes, such as 8-inch heights and 12-inch lengths, are available at some manufacturers.
The beauty of these units is that they can construct a single wythe load-bearing wall. They can provide necessary fire ratings for the structure, and they can offer a functional, resilient building envelope and provide the aesthetics and durability expected of brick masonry.
Similar to reinforced walls built with concrete masonry units (CMU). Through-wall brick can be fully grouted or partially grouted (grouted at the cells containing the reinforcing). Fully grouted walls can provide up to 4 hours of fire resistance.
Partially grouted walls likely provide 2 hours of fire resistance, depending on unit thickness and percentage of hollow area in the unit cross-section. The International Building Code (IBC) contains a Table where the fire resistance of these walls can be determined based upon the unit’s equivalent thickness.
Like all brick walls, The R-value of the units is low, typically ranging close to R-1. These walls, however, have high heat capacity, which leads to improved thermal performance. Insulation can also be introduced into un-grouted cells to increase R-value, but rigid insulation boards are often used behind the exterior brick.
Furring strips can be built into the wall, and Insulation can be placed between these strips. The wall can be closed with a proper vapor barrier and interior board. A through-wall flashing system with weeps should also be incorporated in exterior walls to manage water penetration.
In some cases, for exterior walls, the through-wall units are used as a structural backup to a brick-facing wythe. This provides maximum protection to water penetration and allows for better insulation strategies. By placing the finished face of the backup’s through-wall brick unit in the interior space, the beauty of brick can be realized in the interior and exterior. From a sustainability standpoint, this can minimize risks associated with the use of materials and wall coverings that may increase VOC exposure or increase the fire load of the structure.
The Brick Industry Association (BIA) has its Technical Notes on Brick Construction, Number 41, discussing hollow brick and proper design and detailing recommendations. BIA’s Technical Notes on Brick Construction, Number 26, can be used for single-family residential applications.
For structural design, the strength of brick is superior, and brick unit compressive strength often exceeds 15,000psi. Matched with a Type S mortar, the assumed design strength is if the brick masonry (f’m) can exceed 4,000psi. Often, however, the compressive grout strength will control the design strength.
The design procedures in the TMS 402 – BUILDING CODE REQUIREMENTS FOR MASONRY STRUCTURES are used to design Brick Masonry load-bearing walls. These provide rational and efficient designs for most applications seen in masonry wall loading.
Through-wall (Structural) Brick, Reinforced Structural Brick Masonry is not a new wall system. Reinforcing steel and grout have been used with brick masonry for over 100 years. With brick masonry’s high compressive strength, reinforcing makes for an economical, high-strength wall compared to other reinforced masonry systems.
Advantages of Thru-Wall Structural Brick
- Greater design flexibility
- Greater seismic resistance and more ductility.
- Often lower construction costs
- Reduced backup requirements
- Less restrictive deflection requirements of the backup structure
- Greater resistance to cracking
- Reduced number of tie connections, which provides a more continuous moisture barrier
- Reduced cost of the backup system
- Fire-resistant structure