Wall bracing provides steel-framed structures both lateral and longitudinal stability. Bracing transfers the stress of loads across the walls from brace to brace, dispersing the load at any given point. Every building, no matter the size must be able to resist any anticipated structural loads to avoid collapse.
All structures experience a variety of load conditions caused by wind shear, seismic activity, water or other phenomena. Wall bracing can take on different forms depending on its location and the design of the structure.
How Wall Bracing Works
Longitudinal forces typically occur due to gravity (downward) and wind uplift. Most building designs take those forces into account and resist them using various methods and techniques, bracing being but one.
Lateral forces, however, require more from the building design. Wind and seismic forces acting on the walls or foundation of a building cause it to move side to side, called "racking" and without additional rigidity and strength, the building can collapse.
Bracing consists of steel rods or cables inserted diagonally into the frame to form an “X” shape from the eave strut to the wall columns that define the length of the wall. In buildings with multiple bays, bracing is often used in discrete bays for stabilization. Bracing can be placed every other bay or, at a maximum, every fourth bay.
The rods and cables receive the load at one end and transfer it to the next brace or frame member to distribute the load across the wall. They resist pulling as well because they are firmly fastened at their endpoints to the building’s frame.
A braced wall is not the same as a shear wall — the shear wall includes hold-down brackets that are not used with braced walls. Also, not all sheathed walls can be considered braced walls.
Wind pushes against one wall of a building and simultaneously pulls on the opposite wall. The remaining walls, called bracing walls, must keep the structure from moving. (There are also additional wind uplift forces on the roof that are resisted with a different structural component.)
Seismic activity causes the ground beneath the building to accelerate horizontally and/or vertically, creating forces at locations of concentrated mass. Inertia keeps the structure from moving while the foundation shifts, creating a similar condition to wind blowing against the wall.
Since wind and seismic movement can come from any direction, all walls must be able to act as braces when necessary. The structure must withstand torsion, compression, shear and lift. Smaller buildings (under 60 feet wide) only need the diaphragm action of the walls and roof. Larger buildings, on the other hand, require additional strengthening in the form of bracing.
Systems and Types of Bracing
There are two bracing systems to stabilize structures against horizontal forces: vertical bracing and horizontal bracing.
Vertical Bracing - bracing placed between columns in the vertical plane to create a load path to transfer horizontal forces to the ground. Most framed buildings require horizontal bracing for both directions to resist torsion about a vertical axis.
Horizontal Bracing - bracing at each floor level creates a path for load transference of horizontal forces to the planes of vertical bracing. Each floor level requires horizontal bracing, but the floor system may also provide resistance. Roofs may require bracing also.
Types of Bracing
- Single diagonals - also called trussing or triangulation, single diagonals are formed by inserting diagonal structural members into the rectangular areas of the frame for stabilization. Single diagonals must be able to resist tension and compression.
- Cross-bracing - also called X-bracing, cross-bracing uses two diagonal members crossing at the midpoint. Cross-bracing is only required to be resistant to tension as one brace acts to resist horizontal forces at a time depending on load direction. Steel cables are often used for cross-bracing. This method limits available space within a facade or opening and tends to cause the greatest bending of floor beams.
- V-bracing - Two diagonal members extended from the top two corners of a horizontal member and meeting in the midpoint of the lower horizontal beam creates the signature V-shape. Inverted V-bracing (chevron bracing) is another form of V-bracing. The compressive brace is likely to have a lower buckling capacity than the tension yield capacity of the tension brace. Once both braces reach resistance capacity, the load must be resisted by the bending of the horizontal beam member.
- K-bracing - K-braces connect to the columns at mid-height and create the appearance of a sideways V. These braces provide more flexibility for openings and floor beams bend the least with this option. However, K-bracing is not a good option for seismically active regions because the potential for column failure is high if the compression brace buckles.
- Eccentric bracing - commonly used in seismically active regions, eccentric bracing allows for doorways and corridors in the braced bays. Similar to V-bracing, the bracing members meet instead at a center point with space between them at the top connection. The braces connect to separate places on the beam, so the "link" between bracing members absorbs energy from the seismic activity through plastic deformation. Single diagonals can also be used for eccentric bracing.
The amount of bracing required for a particular structure is specified by the local building code and depends on the Seismic Design Category (SDC) or wind speed plus the number of stories above the braced wall line and the bracing method used.
Alternatives to Cross-Bracing
Two methods exist to resist wind forces hitting a building parallel to the ridge: a cantilevered column system and a moment resisting beam.
- A cantilevered column system consists of poles embedded into the earth in pier footings. The footing depth resists the overturning force of wind or seismic forces.
- A moment resisting beam is fashioned from a strong connection fabricated and installed at each end of the beam. The moment resisting beam then connects the columns of two frames and resists the bending force created by the wind pushing against the end wall (a moment force).
Wall bracing increases resistance against wind and seismic forces to prevent structures from collapsing. The windward wall is pushed by the wind while the leeward side is pulled. Without the additional bracing, the structure may begin to tumble and rack.
Wall bracing also protects structures from damage due to the forces of momentum that occur when the ground beneath a structure moves suddenly in the horizontal frame or suffers high compression forces when the ground drops from underneath.
Several types of bracing exist. The International Building Code regulates when bracing is required, how much bracing must be used and the type of bracing method allowed for specific sets of conditions. The amount of bracing required is determined according to the Seismic Design Category or wind speed, the number of stories above the braced wall line and the method of bracing used.