Bracing Systems
Diagonal members that keep a frame from swaying under lateral loads.
Quick Answer
Bracing systems are structural elements, usually diagonal steel members, that stiffen a frame so wind and earthquake forces travel to the foundation without the frame racking sideways. Concentric braced frames meet at a common point, while eccentric frames offset the connection so a link yields first. Bracing also provides stability during erection.
The Full Picture
A frame of beams and columns with simple connections is stable under gravity but can rack sideways under wind or seismic loads. Bracing fixes that by forming triangles, which do not change shape without a member stretching or shortening. The brace carries the lateral load as axial tension or compression, and the load passes through beams, collectors and columns to the foundation. Shear walls and moment frames are alternative ways to provide the same lateral stability.
In a concentric braced frame the centerlines of the braces, beams and columns meet at a common point, which keeps forces mostly axial. Common arrangements are X-bracing, single diagonals, and chevron or V-bracing, where two diagonals meet at the beam. K-bracing places the diagonals so they meet a column at mid-height rather than at a joint. Because that can put a large unbalanced force on the column, K-bracing is generally avoided in seismic force-resisting systems, though it may appear for wind or in older buildings.
An eccentrically braced frame deliberately offsets a brace connection from the beam-column joint, creating a short beam segment called a link. In an earthquake the link is designed to yield first and absorb energy while the braces and columns stay elastic, trading some stiffness for ductility. Special concentric and eccentric frames carry detailed design requirements and are used where seismic risk is high. Which system applies is the structural engineer's decision under the governing code and ASCE 7.
Stability is also a construction issue. Federal steel erection rules state that structural stability shall be maintained at all times during the erection process, so temporary and permanent bracing, bolted joist bridging and other measures must go in as the frame rises. For a contractor, the permanent bracing appears on the structural elevations and plans as diagonals and braced-frame schedules, with member sizes, gusset plates and connection details.
Real Examples
Common Misconceptions
People assume: Bracing is just an optional stiffener.
Actually: In many frames, bracing is the lateral system. Without it, or without shear walls or moment frames, the structure would sway excessively or become unstable under wind or seismic loads.
People assume: Stability and strength mean the same thing.
Actually: A member can be strong enough for its load and still be part of an unstable arrangement. Stability is about whether the whole frame, and each member within it, resists buckling and sideways movement, during service and during erection.
People assume: Bracing members can be moved or removed to open up a wall.
Actually: Braced bays are part of the lateral load path. Relocating a diagonal for a door or window opening usually needs the structural engineer's redesign, since the load has to go somewhere else.
Frequently Asked Questions
What is the difference between concentric and eccentric bracing?
In concentric bracing, the brace centerlines meet at a common point, so the forces are mostly axial and the frame is stiff. In eccentric bracing, a connection is offset to create a short link beam that yields in a severe earthquake, giving more ductility at the cost of some stiffness.
What is K-bracing?
K-bracing uses two diagonals that meet at the middle of a column, forming a K. It can put a large lateral force on the column at mid-height, so it is generally avoided in seismic force-resisting systems. The code and the engineer of record decide whether it is acceptable on a project.
How are bracing systems quantified in a takeoff?
Brace members are counted or measured by length and weight per foot, often as part of the structural steel tonnage, with gusset plates, bolts and connection material carried separately. The estimator finds them on framing plans, braced-frame elevations and schedules.
What does stability mean in structural terms?
Stability means a structure or member holds its position and shape under load without buckling or collapsing. It covers the finished frame and the partially built frame, which is why erection bracing is a safety requirement as well as a design issue.