Thursday, May 8, 2014

What is Wall Bracing?

Because our industry uses the term braced walls so much, I searched the Internet for some easy to understand definitions and answers to modular home builders and their customers's most oft asked questions.

What is wall bracing?
A system of specially constructed exterior, and often interior, wall segments attached to the roof, floor and foundation that resist lateral (sideways) loads from wind and earthquakes.


Wall Bracing Requirements
Wall bracing requirements are based on code provisions from the International Residential Code (IRC), the International Building Code (IBC) and the Uniform Building Code (UBC). The majority of the country uses the IRC to brace residential homes. All codes require wall bracing in conventional light-frame wood construction. The IRC has 8 traditional methods and two newer methods that provide narrower alternatives. In general, the 8 options require a minimum of a 4' x 8' panel of specified material nailed into stud framing at a specified location with a specified stud quality required. The most common wall bracing is a 4' x 8' sheet of wood structural panel sheathing (OSB or Plywood) nailed over studs 16" to 24" on center. With the number of windows and doors in today's home designs, it is sometimes difficult to fit in four continuous feet of wall to act as a brace. In these cases, alternate methods and prefabricated panels are often employed. To find specific information regarding conventional wall bracing, see Conventional Wall Bracing.

Prescriptive vs. Engineered
In most residential construction, prescriptive design is allowed using the IRC requirements as a 'menu' of building options. When using prescriptive design, a Design Professional of Record (DPOR) is not required. In general, areas with a Seismic Design Category (SDC) of D2 or less and a wind speed category under 110 mph allow for prescriptive design. Some large or complex custom homes may require a DPOR simply because the design is such that IRC methods cannot support the structure. A DPOR is usually required for homes built in SDC greater than D2 and wind zones above 110 mph. Additionally, some counties or municipalities require a DPOR for all homes built in those areas. It is always best to check with a local building official to determine if a plan for a particular area can be built using prescriptive design.

Wall Bracing Terms
Braced Wall Panel:
A braced wall panel is a section of a braced wall line that has the ability to resist lateral forces created by winds and seismic events.


Braced Wall Line:
A braced wall line per the International Residential Code (IRC) is an exterior or interior wall line that contains braced wall panels within 12.5 feet of both ends (or 8 feet in Seismic Design Categories D0, D1 or D2), and 25 feet on center throughout the length of the braced wall line. Braced wall lines must occur every 25-35 feet depending upon your Seismic Design Category and wind zone. Braced wall panels are permitted to be offset from the braced wall line up to 4 feet on either side of the braced wall line. An offset greater than 4 feet is treated as an additional braced wall line.


Prefabricated Panel:
A prefabricated panel is a manufactured wall panel that is equivalent to a braced wall panel for use in a narrow section of wall. These panels can be used as a substitute for the 8 traditional methods of wall bracing. When calculating the percentage of bracing within a braced wall line, each narrow prefabricated braced wall panel counts as 4 feet of bracing.


Eight Traditional Methods of Bracing:
Methods used to construct braced wall panels used in light framed structures (See Wall Bracing 401 for individual descriptions, recommendations and images).


Continuous Sheathing:
Continuous sheathing is a method of bracing a wall that requires the entire wall line to be covered with wood structural panel sheathing meeting all the provisions set forth in the IRC, which includes special nailing details at corners.


Conventional Construction:
Conventional construction is a level of design generally based on traditional construction methods and materials that have a history of good performance for specific building types and sizes. Both conventionally-specified and pre-calculated members and connections are prescriptively specified and may be combined to form a structure or structural assembly.


IRC:

IRC is an abbreviation for International Residential Code. This code details acceptable building practices used in one- and two-family dwellings and townhouses. It is commonly referred to as a prescriptive building practice.


IBC:

IBC is an abbreviation for International Building Code. This code provides guidance to designers and engineers for the design and installation of building systems through requirements emphasizing performance.


Shear Wall:
A wall designed (engineered) to resist lateral forces parallel to the plane of the wall. The difference between a shear wall and a braced wall panel is that a braced wall panel has predetermined construction details (materials, length, nailing patterns, anchorage, etc.), whereas the construction details for a shear wall are determined through calculations by a design professional.


Ductility: 
Ductility is the measure of the ability to resist loads while experiencing large movements that are outside the elastic range. This attribute increases life safety by helping the structure remain standing while being subjected to large displacements.


Damping:
Damping is the ability to absorb energy in a controlled, predictable manner to reduce movement of the structure in an earthquake. Products with good damping characteristics act like the shock absorbers in your car - absorbing energy during movement so the car shakes less.


R Factor:
R factor or Response modification factor is a seismic design load reduction factor based upon the entire structural frame system that accounts for ductility, damping and other compatibility characteristics.

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