Secondary Members in Steel Buildings: Purlins, Girts, and Bracing Systems
Secondary members play a critical role in steel buildings by supporting cladding, transferring loads to primary frames, and stabilizing the overall structural system. Although they are not the main load-bearing elements, improper design of secondary members can directly affect safety, serviceability, and construction efficiency.
Lead: Why Secondary Members Matter
In steel structures, loads do not act on primary frames alone. Roof panels, wall cladding, wind pressure, and seismic effects are first resisted by secondary members before being transferred to columns and beams. Therefore, purlins, girts, and bracing systems are essential for ensuring load continuity, lateral stability, and long-term performance.
Design standards such as ASCE 7 – Minimum Design Loads and AISC 360 – Specification for Structural Steel Buildings explicitly consider the role of secondary members in load paths and stability.
What Are Secondary Members in Steel Buildings?
Secondary members are structural elements that support building envelopes and assist in load transfer, rather than carrying primary gravity loads directly. Common secondary members include:
- Purlins – roof-supporting members
- Girts – wall-supporting members
- Bracing systems – elements providing lateral stability
These components are especially common in steel structure warehouses, workshops, and industrial buildings.
Purlins: Roof Load Distribution Members
Purlins are horizontal members that support roof panels and transfer roof loads to the main frames or trusses. They primarily resist dead load, live load, wind uplift, and snow load, as defined in ASCE 7 – Roof Load Provisions.
Key Characteristics
- Typically made from cold-formed steel sections (Z-purlins or C-purlins)
- Span between portal frames, rigid frames, or trusses
- Designed for bending, shear, and sometimes uplift
According to AISC 360, purlins must satisfy strength and serviceability limits, including deflection control to prevent roof damage.
Girts: Wall Load Transfer Members
Girts function similarly to purlins but are installed along walls instead of roofs. They support wall cladding and transfer wind pressure and suction to columns or frames.
Design Considerations
- Horizontal girts are commonly used in industrial buildings
- They must resist out-of-plane bending caused by wind loads
- Deflection limits are critical to prevent wall panel deformation
Wind design for girts is governed by ASCE 7 – Wind Load Provisions, which defines pressure coefficients and load combinations.
Bracing Systems: Structural Stability Elements
Bracing systems provide lateral stability and load redistribution, particularly under wind and seismic actions. Unlike purlins and girts, bracing directly affects the global behavior of the structure.
Common Bracing Types
- Roof bracing
- Wall cross bracing
- Longitudinal bracing systems
In seismic regions, proper bracing detailing is essential. Guidelines from FEMA Earthquake Program emphasize that structural failures often result from inadequate bracing or poor connection detailing rather than insufficient material strength.
Load Path and Structural Interaction
Secondary members are integral to the structural load path, which is a core concept in steel design. Loads typically follow this sequence:
- Roof or wall panels receive environmental loads
- Purlins or girts distribute loads
- Loads transfer to primary frames or columns
- Foundations resist the final load
This load transfer mechanism aligns with the principles outlined in AISC 360 and Eurocode EN 1993 – Steel Structures.
Fabrication and Installation Considerations
Secondary members are usually prefabricated and lightweight, which improves construction efficiency. However, designers must still consider:
- Connection detailing to primary frames
- Corrosion protection for exposed environments
- Tolerances during installation
These considerations affect both cost estimation and long-term durability, especially in industrial or coastal projects.
Conclusion
Although secondary members do not carry primary gravity loads, they are essential for load transfer, stability, and envelope performance in steel buildings. Proper design of purlins, girts, and bracing ensures compliance with international standards and improves structural reliability.
Key Points
- Secondary members support cladding and transfer loads to main frames
- Purlins carry roof loads; girts resist wall wind loads
- Bracing systems ensure lateral stability
- Design must follow ASCE 7, AISC 360, and relevant Eurocode provisions
- Poor secondary member design can compromise overall structural safety