What Is a Knee Brace in Construction: Facts and Risks

June 12, 2024

Knee braces are inclined diagonal lumber members designed to connect the sidewall columns to the face of trusses, enhancing the lateral resistance of post frames, especially against lateral wind forces.

Function and Intent

Knee braces aim to improve the stability of post frames under lateral loads and help reduce the unsupported length of columns, minimizing buckling risks. They serve as temporary braces, particularly useful before roofing and siding are installed when the structure is more flexible.

Issues with Effectiveness

The effectiveness of knee braces depends heavily on the stiffness of their connections to both the post and the truss. If the connections are not sufficiently stiff—such as when only a few nails are used—the roof diaphragm bears most of the load, rendering the knee braces ineffective. Conversely, though more nails or bolts can stiffen connections, they risk overloading the trusses.

"my knee in a brace" by D.L. is licensed under CC BY-SA 2.0. To view a copy of this license, visit https://creativecommons.org/licenses/by-sa/2.0/.

Structural Analysis and Risks

Knee braces can induce bending moments in truss chords, affecting the structural integrity. Comprehensive load sharing among the truss, post, knee brace, connections, and roof diaphragm is essential in the structural analysis to ensure safety.

Research and Testing

Studies, such as the 1984 Johnston and Curtis study, have found that under increased loads, knee bracing becomes insignificant. Similarly, research by Gebremedian and Woeste in 1986 indicated that knee braces add minimal stiffness to structures. According to Jerry Barbera’s 1988 presentation, stress issues on trusses can arise if knee braces are placed randomly; thus, effective design requires clear communication between truss and building designers. Walker and Woeste’s 1992 book, "Post Frame Design," describes knee braces as a "no-win" solution, creating more issues than benefits.

"New robocop knee brace 2010" by Bekathwia is licensed under CC BY-SA 2.0. To view a copy of this license, visit https://creativecommons.org/licenses/by-sa/2.0/.

Practical Application and Building Codes

The Ohio Residential Code (2019) requires knee braces for any non-engineered post frame building, as per Section 328.6. To avoid using knee braces, building plans must be submitted without them and sealed by a Registered Professional Engineer in Ohio. Proper structural analysis must encompass load sharing between all components to avoid improper load distribution that can lead to structural failures.

Potential Risks and Misconceptions

The idea of using knee braces in pole buildings may stem from a lack of knowledge. Rather than providing structural benefits, knee braces can introduce harmful loads to roof trusses, risking catastrophic failures.

Practical Advice

If knee braces are mandated by local building officials due to initial plan submissions, updated plans sealed by a professional engineer may be necessary to remove the knee braces from the design.

"Tron-ified knee brace with EL wire" by Bekathwia is licensed under CC BY-SA 2.0. To view a copy of this license, visit https://creativecommons.org/licenses/by-sa/2.0/.

In essence, while knee braces can seem beneficial in theory, their practical effectiveness is often limited by connection stiffness and load distribution challenges. Careful consideration, proper design, and thorough structural analysis are crucial to avoiding the drawbacks and ensuring the structural integrity of post frame buildings.

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