Example of a properly braced wall using wood.
If you are familiar with the design or construction of concrete masonry walls, you may know that there are two very distinctive phases in the performance of masonry walls over their lifetime: the construction and service phases. When in service, and when properly designed and constructed, walls perform really well on their own. However, they need a little help during the construction phase to stay up and reach the next phase. Have you ever heard of walls blown over by wind during construction? That’s when walls are most vulnerable and in need of help: here comes the temporary bracing.
Generally speaking, masonry walls under construction behave very differently than in service. First, it takes some time for masonry to gain strength after it is built. Second, support points are different and the wall may be subjected to internal stresses that it was not designed for. As a result, the temporary bracing is installed to provide lateral support to the wall and minimize the risk collapse.
Recognizing the risk associated with masonry walls under construction, the Occupational Safety & Health Administrator (OSHA) requires that “All masonry walls over eight feet in height shall be adequately braced to prevent overturning and to prevent collapse unless the wall is adequately supported so that it will not overturn or collapse. The bracing shall remain in place until permanent supporting elements of the structure are in place” (Standards 29 CFR 1926.706). But what exactly is “adequate” for bracing masonry walls, and what are the most effective approaches for the typical job site? Fortunately for the mason and the design professional, there is an industry standard for bracing masonry walls: Standard Practice for Bracing Masonry Walls Under Construction, developed by the Council for Masonry Wall Bracing and published by the Mason Contractors Association of America (MCAA). It is important to notice that the only lateral load addressed by the Standard is wind. Other loading conditions that may occur during construction, such as impact, seismic, etc., need to be evaluated separately.
The primary objective of the Standard is to provide an acceptable level of life safety for masons and other construction workers on the job site. Designing a bracing system to prevent the collapse of a masonry wall during construction may be impractical. Wall bracing systems are generally designed for wind speeds that are lesser that the wind loads used in the structural design of the building. Masonry work is allowed to continue during low wind conditions but, because there is still a risk of collapse, evacuation of the area around the wall is required when wind speeds exceed the critical values established by the Standard.
Is this an adequate bracing? Definitely not: bracing does not have any connections or anchorage, and a pile of brick debris does not qualify as footing. Talk to your engineer for an adequate bracing system.
Care should be exercised on the construction site to implement all safety requirements listed in the Standard. The first step is to establish a restricted zone, defined as the area on each side of the wall equal to the wall height plus four feet. A competent person, who is trained and has the authority and knowledge to make prompt decisions, should monitor the wind speed during the initial and intermediate periods and evacuate workers under his jurisdiction from the restricted zone when the critical speed is exceeded. The initial period, limited to 24 hours, is the time during which masonry is being laid above its base or the highest line of bracing, during which mortar is assumed to have no tensile strength and any reinforcement is not operative. Resistance to overturning during the initial period is provided by the wall self-weight only and the Standard limits the maximum height that a wall can be built without bracing. The Intermediate Period is the time following the initial period and until the wall is connected to the elements (roof and floor framing) that provide its final lateral support. Critical wind speeds of 20 miles per hours and 35 miles per hours are defined by the Standard for the initial and intermediate periods, respectively. The evacuation wind speed of 35 mph, when all persons must exit the restricted area, is based on the assumption that the masons will not be able to work at higher wind speeds. However, the bracing is designed for 40 mph wind speed. The difference between the two values provide time for the workers to evacuate the restricted area. Once in place, the bracing should be inspected on a regular base and any damaged or weakened elements repaired or replaced. And remember, the bracing system cannot be removed until the walls have received their final lateral support, which is established by design and typically includes cross walls, pilasters, structural frame elements, floors or roofs. The Standard also provides a series of acceptable bracing solutions and their connections to the wall and ground anchorage point.
Mason contractors are generally responsible for bracing masonry walls under construction. Often, the general contractor requires a wind bracing plan to be submitted prior to beginning of work. Masons can refer to the Masonry Wall Bracing Handbook, which is published by the MCAA and contains over 700 different wall bracing configurations, to identify the type and method of bracing they will need on their job site. A general wind bracing plan can be as simple as a reference to the MCAA publications with details showing how bracing will be accomplished. Bracing components can then be designed by an engineer.