A "fly brace" in construction, specifically in steel structures, is a short, diagonal brace, often made of angle sections or straps, that connects to an unrestrained flange of a beam or column. Its primary purpose is to provide lateral support and prevent the restrained member from buckling or twisting due to loads like compression or wind forces. Essentially, it enhances the stability of the structure by resisting lateral torsional buckling.

Fly braces are crucial for preventing the compressed flange of a beam or column from buckling or twisting, especially when subjected to compressive forces or lateral loads.

They are typically found connecting the bottom flange of a rafter or beam to a purlin or other supporting member.

By providing lateral support, fly braces allow for the use of smaller, more economical beams or columns while maintaining structural integrity.