1 An information series from the national authority on concrete masonry technology BRACING CONCRETE MASONRY WALLS UNDER CONSTRUCTION TEK 3-4C Construction (2014) INTRODUCTION WALLS SUBJECT TO WIND LOADS Building codes typically place responsibility for providing a reasonable level of life safety for workers during construction on the erecting contractor. Various methods are employed to protect workers while newly constructed masonry walls are curing and/or until the roof or other structural supports are in place. This TEK provides guidelines for masonry wall stability to resist the lateral loading effects of wind during construction. It is based on principles set forth in the Council for Masonry Wall Bracing's Standard Practice for Bracing Masonry Walls Under Construction (ref. Restricted zone 1), but has been updated in accordance with the design provisions of the 2011 Building Code Requirements for Masonry Structures (MSJC, ref. When other lateral loads such as impact, seismic, scaffolding, and lateral earth pressure are present, they need to be considered and evaluated separately. The Walls Subject to Backfilling section at the end of this TEK discusses bracing and support of basement walls during backfilling operations.
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There are several strategies and considerations for protecting life safety on the jobsite. These include internal bracing, external bracing and evacuation zones.
The combination of strategies appropriate for a particular job may depend on the type of masonry construction, masonry wall heights, the time elapsed since construction, and wind speeds at the site. The industry term internal bracing is relatively new. Internal bracing refers to the stability of a masonry assembly Length Height + 4 ft (1.22 m) Height + 4 ft (1.22 m) Height Restricted zone Figure 1 Restricted Zone for Masonry Walls Length + 8 ft (2.44 m) (4 ft (1.22 m) beyond each end of wall) Related TEK: 14-4B, 14-7C Keywords: backfilling, basement walls, bracing walls, construction loads, internal bracing, lateral loads, plain concrete masonry, restricted zone, unreinforced concrete masonry, wind loads NCMA TEK 3-4C (replaces TEK 3-4B) 1 2 to resist wind loads through self-weight and allowable flexural stresses within the masonry. The use of evacuation zones recognizes that it may be impractical to prevent the collapse of a masonry wall during construction when subjected to extreme loading conditions and that life safety is the primary concern.
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At prescribed wind speeds (taken as three-second gusts measured at the job site), the wall and the area around it is evacuated. The critical wind speed resulting in evacuation depends on the age of the wall being constructed and involves the three terms: restricted zone, initial period, and intermediate period. Restricted Zone The restricted zone is the area on each side of a wall subject to the effect of a masonry wall collapse. It is defined by a length equal to the height of the constructed wall plus 4 ft (1.22 m) on both sides of the wall, and a width equal to the wall length plus 4 ft (1.22 m) on both ends of the wall, as shown in Figure 1.
Effective for shorter masonry walls historically found. Changes in design and engineering of present day masonry walls have led to taller, thinner reinforced masonry walls. The standard now references The Standard Practice for Bracing Masonry Walls Under Construction chapters 5 & 6 and the Masonry Wall Bracing Design Handbook. Bracing design whether the masonry walls are braced in the traditional manner using external components or by Internal Bracing, that is, the more recent method of using the constructed masonry’s inherent strength to provide stability during wall construction.
When wind speeds exceed those allowed during the initial and intermediate periods, there is a chance that the masonry wall could fail, and the restricted zone must be evacuated in order to ensure life safety. Table 1 Maximum Unbraced Height of Ungrouted Hollow Concrete Masonry Walls During the Initial Period, ft-in. (m) Nominal wall thickness, in. (mm) Density of masonry units, g, lb/ft 3 (kg/m 3 ) Lightweight units 95.