Components of the secondary frame, girts and purlins act as attachment points and support for the wall and roof panels. That is all they are meant to do. While they can survive small stresses, the girts and purlins are not designed to withstand high wind loads or other events that twist or lift the panels.
The primary frame protects the structure from outright destruction, but without additional bracing, the girts and purlins will become too damaged to fulfill their function.
An Overview of Secondary Framing
Girts and purlins, along with metal eave struts, are made of flat, high-grade steel that has been formed into the shape of a Z or C, essentially a piece of steel with flanges on each side. A Z-section has flanges extending in opposite directions. A C-section has both flanges extending the same direction. When seen from the end, the letter shape appears.
- Girts and purlins act as struts to resist longitudinal loads applied to the building panels.
- Purlins brace the roof panels while girts brace the wall panels.
- They provide lateral bracing to the compression flanges of the primary frame members to increase frame capacity.
- Like other frame members, they are available with a galvanized finish and factory paint.
Stress on the walls or roof panels can twist, flex or bend the webs or flanges of the secondary frame members, pulling them out of shape and reducing their effectiveness.
Categories of Bracing
To stabilize and protect the purlins and girts, a variety of bracing techniques and components are used to stiffen secondary members. Effective bracing provides lateral bracing, to keep the flange from moving sideways, as well as restrain the members from the rotation at the flanges or within the web. Bracing also keeps the entire secondary assembly and panels from sideways movement.
Braces can be categorized by function, the method of interaction between the braced points or performance criteria. The categories merely exist to define the use of the bracing system and may overlap. A brace could perform more than one function under multiple or single loading conditions.
- Function involves stability, strength and serviceability. Bracing serves to resist buckling of a single member or the entire structure. Continuous sheathing attached to the stud flanges is an example of the functional category.1
- Method of interaction is differentiated into relative and nodal. A relative brace controls the movement of the braced point with respect to the adjacent braced points. A nodal brace controls the movement of the braced pint without interaction with the adjacent braced points.1
- Performance criteria refer to relative, discrete, lean-on and continuous bracing. A relative brace controls the movements of brace points along an affected beam or column. Braces named for performance criteria come in a wide range of shapes and sizes.
Regarding performance criteria, relative bracing controls the movements of adjacent stories or other brace points along a beam or column. Discrete bracing stabilizes each purlin or set of purlins at the concentration point of the load.
Lean-on bracing occurs when a column or beam relies on an adjacent connected structural member for support. Continuous bracing creates a column or beam with a theoretical unbraced length of zero around the braced axis. (Sputo)
Cold-formed vs. Hot Rolled Steel Bracing Designs
Hot-rolled members are created by rolling steel at high temperatures, around 1700 F, taking the steel above its recrystallization temperature where molding and shaping are easily performed.
Hot-rolled steel is typically formed into doubly-symmetrical members such as wide flanges or hollow sections with a locally stable plate. Hot-rolled steel sections do not generally exhibit distortional buckling or local buckling.
Cold-formed steel comes coiled and is fed through sets of rollers, presses and cutters to yield the desired shape. Cold-formed members tend to be singly symmetrical (C-Section) or point symmetrical (Z-section).
These members are not as stable as hot-rolled members and are prone to secondary torsion and flexure. Bracing is designed to prevent these secondary forces from developing.
Brace systems are needed to provide stability for cold-formed steel components, which have consistently lower demands placed upon them.
Sloped Roofs and Sag Rods
Three basic equations are used to calculate the bending of cold-formed sections such as girts and purlins. The values for the laterally unsupported length between fasteners on the compressed flange and between the discrete bracing supporting both flanges against torsion are also used to calculate specific torsional or flexural factors.
The calculations are used to determine the appropriate discrete bracing systems according to a published table that provides brace type depending on whether the roof is a through-fastened or standing seam roof system.
Purlins in highly sloped steel roofs are sensitive to torsion. Dead, live and snow lateral loads increase the torsion effect. Some form of discrete bracing is required to reduce the strain on the secondary members and strengthen the roof structure.
Sag rods, which are used for aligning girts and purlins during assembly, are occasionally used as discrete bracing as well, but they are not effective in all cases.
For example, if a sag rod is positioned in the center of a cold-formed Z or C-section, it will provide only light torsion resistance because the flanges are still free to move. Little protection from torsion is provided.
However, if the sag rods are installed in the proximity of the purlin’s bottom flange with sheathing on the upper flange, they make effective discrete bracing systems.
Girts and purlins are not, by themselves, are not sufficient to stabilize panels against high lateral loads. These secondary members require additional bracing to reduce torsional strain and flexure to maintain the stability of the load point and the entire structure.
Bracing systems are labeled according to their function in terms of strength, stability and serviceability. They may also be categorized according to the method of interaction or certain performance criteria.
Care must be taken to calculate the bending resistance of cold-formed steel frame members properly. The results are critical for identifying the appropriate brace system for the specific loads experienced by the metal structure’s secondary frame. If the purlins and girts are not properly braced, the entire structure may become unstable in high load conditions.