Masonry is the only major building material that can be designed to be self-supporting during construction, even for walls of significant height. Mason contractors can improve safety on the job site and reduce construction costs (this is one of those rare opportunities where both can be accomplished at the same time) with effective use of various methods for temporary bracing of masonry walls under construction.
National Standards Two primary national standards are in existence for temporary bracing. The first, and most well-known is OSHA Standard 29 CFR 1926.706. Its basic requirements include this statement: 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. Bracing shall remain in place until permanent supporting elements of the structure are in place.
This standard is too vague and unrealistic. For the permanent design of buildings, engineers design using building code-specified wind speeds and pressures that will withstand most (but not all) conditions. When nature delivers unusually high winds (tornados and hurricanes being extreme examples), the attention turns not to designing every structure for the most severe wind event that might take place but instead, to providing early warning and having occupants seek a place of safety (perhaps a basement, storm shelter or even an evacuation to another community). In contrast, OSHA establishes no clear guidance on design wind speeds or pressures. This vagueness, combined with the reality that mason con – tractors are not required to seek engineering guidance, results in field installations that may lack a known and reliable level of safety for construction workers.
In contrast, the Standard Practice for Bracing Masonry Walls Under Construction (Standard Practice), developed by the Council for Masonry Wall Bracing and published by the Mason Contractors Association of America, is based on these fundamental principles:
• Life safety is the primary goal.
• Acknowledgement that walls are vulnerable to collapse during construction.
• Basic premise:
– Design walls to resist specific wind speeds.
– Monitor wind speeds continually at the jobsite.
– Evacuate areas close to the walls, referred to as the restricted zone, prior to actual wind speeds reaching specified wind speeds.
The Standard Practice also establishes the engineering criteria that govern the structural design of the temporary bracing systems.
Temporary Masonry Bracing Systems Flowchart: Bracing Systems Design Options presents a graphical depiction of both the specified wind speeds used in the Standard Practice and bracing options, depending on the age of the masonry wall.
Obviously, newly placed (green) mortar and/or grout have no strength. The Standard Practice identifies the zero strength condition as the Initial Period, lasting no more than the first 24 hours. (Research1 has shown that masonry will achieve 50% of its design strength within 12 hours.) The only factors considered to prevent wall collapse during the Initial Period are the weight and thickness of the wall. This is the only time that a wall can truly be considered unbraced (although it is certainly possible for a wall to be inadequately braced). The Standard Practice specifies an evacuation wind speed of 20 mph for the Initial Period, and establishes maximum allowable wall heights for various masonry unit densities, thicknesses and grouting.
During the Initial Period, masonry attains 50% of the design compressive and flexural strengths (which is a wonderful quality unique to masonry) and reaches what is termed the Intermediate Period. Not until the wall has developed this strength can a bracing system be considered effective. Whatever system is utilized, it is designed for a wind speed of 40 mph as specified in the Standard Practice for the Intermediate Period. As shown in the Flowchart, the three available bracing systems are:
• Externally braced
• Internally braced – unreinforced
• Internally braced – reinforced
Let’s take a look at each of these systems in more detail. Many masonry projects will benefit by using all three of these methods, applying each system’s advantages to corresponding wall conditions.
Externally Braced Advantages: Allows for the highest overall wall heights for a given vertical steel reinforcement size and spacing. It may be the only practical bracing option for walls with wide openings at the base of the wall and for walls
laid up on long spanning steel beam lintels. Limitations: Conventional lightweight pipe bracing systems used by mason contractors installed in the typical manner will not have adequate capacity for wall heights of 32’ and greater. At these heights, the pipe brace becomes too slender; its safe working load capacity rapidly declines, while the forces to be resisted increase with increasing wall height. Additionally, deadman requirements are much higher than often realized. (Dead – man: the weight installed at the bottom of the pipe brace to resist brace loads.) For example, the required weight of a surface deadman for wall heights of 20′ is typically around 2500 lbs, and over 4000 lbs for walls of 30′ height.
Internally Braced – Unreinforced Advantages: This method is great for stair and elevator shafts. Provided the shaft walls are built concurrently (which is typically the case), walls can be designed to span horizontally (corners/intersecting walls provide support for each uninterrupted length of wall, provided there are no control joints between the supporting elements). As an example, 8” CMU walls using Type S mortar cement can span horizontally up to 24′. Shaft wall heights of 75′ or more can be achieved with no other required temporary bracing or intermediate lateral support.
Limitations: The allowable wall heights for other than shafts are rather modest. For example, 8” ungrouted CMU using Type S mortar cement has a maximum allowable wall height of 10′-0”. One possible application is for high-lift grouting (the grout pour height can be of the same height as the allowable internal unreinforced wall height). Foundation overturning needs to be checked. (See Tables 4A-C) Unreinforced walls with flashing at the base cannot be considered internally braced.
Internally Braced – Reinforced Advantages: The majority of walls are specified with enough vertical steel reinforcement that, when analyzed, is also sufficient for internal bracing. One might thus regard this temporary bracing system as free. As can be seen in Tables 3A and 3B, 8” reinforced CMU is good for internal bracing for heights of about 15′ to 25′ and 12” reinforced CMU is good for heights of approximately 18′ to 40′ depending on the size and spacing of the vertical reinforcement. Even greater heights are obtainable for double wythe (two layers) reinforced walls.
Limitations: Full lap splices are necessary, including at the dowels extending from the foundation (which achieves a fixed base, more on that later). Full lap splices as defined in the Standard Practice are a minimum 48 bar diameters in length. Foundation overturning needs to be checked. (See Tables 4A-C)
Importance of a Fixed Base Except for walls with frequent intersecting walls and/or corners (as described for shafts), masonry walls function structurally as a free-standing vertical cantilever during construction. The only way for such a wall to be stable under wind load is to develop fixity at the base. Without a fixed base, the wall will simply rotate along with its base and collapse, regardless of how strong or heavily reinforced it may be above the base. Short dowels extending out of the foundation are unable to develop the required fixity. While some degree of fixity is developed just by the mortar alone (see Internally Braced – unreinforced), the strength of this connection is of modest magnitude. Even for walls to be braced by external braces, until the brace is actually installed, the wall will require a fixed base for stability. So in essence, most masonry walls spend at least some of their lives as an internally braced wall.
For each of the bracing systems discussed in this article, detailed design tables are available at daileyengineeringinc.com. Design Tables 4A-C to assist in checking foundation overturning are also available.