Masonry Magazine February 1999 Page. 52
After almost eighteen months of development work by the Council for Masonry Bracing, a draft of the proposed "Standard Practice for Bracing Masonry Walls During Construction" is ready for public review. This new Standard is the first industry supported document giving specific procedures for bracing masonry walls during construction. Copies of the proposed Standard are available for review and comment from the Mason Contractors Association of America. Public review will end on October 31, 1998 and the final document will be published in January of 1999.
The primary goal of this Standard is to provide life safety for masons and other workers on a construction site during the time when a masonry wall is being constructed and is finally connected to the rest of a structure so that it can resist its final design loads. The Standard applies to temporary bracing of walls subjected to wind speeds of not more than 35 miles per hour during the construction period.
The Standard recognizes that masonry walls are different from precast concrete or tilt-up concrete walls. When these latter types of walls are erected, they have most of their final design strength and can resist all but the most severe design wind loads. Hence, when bracing for precast or tilt-up concrete walls are designed, they are designed for wind speeds of about 70 miles per hour. Masonry walls however are subjected to wind loads while they are still being erected. In this case, the mortar holding the wall together will not have developed its full design strength and it is not realistic to design a system to resist the same wind speeds as for precast or tilt up.
Recognizing that it is impossible to prevent the collapse of a masonry wall during construction and that life safety is the primary concern, the Standard has developed a procedure whereby the wall and area around the wall is evacuated at prescribed wind speeds. The critical wind speed resulting in evacuation is dependent on the age of the wall being constructed. Three new terms are defined to handle this approach. They are "Initial
The Council for Masonry Bracing is an ad hoc masonry industry group sponsored by the Mason Contractors Association of America, the Brick Industry Association, the National Concrete Masonry Association, the Laborers International Union and the Portland Cement Association
48 MASONRY- JANUARY/FEBRUARY, 1999
| NOMINAL WALL THICKNESS (in) | HOLLOW UNITS | SOLID UNITS | HOLLOW UNITS | SOLID UNITS | HOLLOW UNITS | SOLID UNITS |
|---|---|---|---|---|---|---|
| 4 | 8.0 | 8.0 | 8.0 | 8.0 | 8.0 | 8.0 |
| 6 | 8.0 | 8.3 | 8.0 | 9.2 | 8.1 | 10.7 |
| 8 | 10.8 | 15.3 | 11.9 | 16.9 | 14.2 | 19.8 |
| 10 | 17.0 | 24.3 | 18.8 | 26.8 | 22.0 | 32.0 |
| 12 | 23.2 | 35.8 | 25.7 | 39.5 | 30.6 | 47.1 |
Table 1. Maximum unbraced height of masonry walls above grade of highest line of lateral support measured on each side of the wall.
Initial Period
The Initial Period is the period of time, limited to a maximum of one working day during which the masonry being laid above its base or highest line of bracing. During this period, the mortar is assumed to have no strength. Wall stability is accomplished from its own dead weight only. Based on this assumption and a wind speed limit of 20 miles per hour, walls can be built to the height shown on Table 1 without bracing. If wind speeds exceed 20 miles per hour during the initial period, the workers must stop working on the wall and the Safe Zone on both sides of the wall must be evacuated. An exception to this requirement is that the Safe Zone need not be evacuated during the Initial Period for walls up to eight feet above grade for winds up to 35 miles per hour. This exception stems from a long-standing OSHA requirement.
Intermediate Period
The Intermediate Period is the period of time following the Initial Period until the wall is connected to the elements that provide its final lateral stability from adjacent structural elements, its design configuration or its design strength. During this period, the mortar is assumed to be about 24 hours old and the masonry is assumed to have about half the strength of 28 day old masonry. Once the reduced strength of masonry is established, the design of masonry between bracing points can be executed in accordance with the provisions of the Masonry Standard Joint Committee's "Building Code Requirements for Masonry structures," (Code). Development length of reinforcement during this period is increased over that required by the Code by 1.67 to account for the lower strength of grout. Provisions are included for Working Stress Design or Strength Design methods to be used. Connections to masonry can be designed using reduced masonry strengths and design formulas included in the Standard.
During the Intermediate Period, the design wind speed is 35 miles per hour. When the wind speed exceeds this amount, the workers must stop laying masonry and the Safe Zone must be evacuated.
Safe Zone
The Safe Zone is the area on each side of a wall with a distance equal to the height of the constructed wall plus four feet, measured at right angles to the wall and continuing for the length of the wall (Figure 1). In situations where the wind speeds ex-
Building Code Requirements for Masonry Structures, AC! 530-95/ASC 5-95/TMS 402-95, The Masonry Society, Boulder, CO.