1. Primary Frame Members
The primary frame carries the main structural loads and forms the basic portal frame system.
1.1 Portal Frame Columns
Portal frame columns are the vertical primary members of the building. They support the rafters and transfer structural forces to the foundations.
Together with the rafters and their moment-resisting connections, the columns form the main transverse portal frames. Their size and configuration depend on factors such as building span, height, loading, frame spacing, and design requirements.
1.2 Rafters
Rafters are the inclined primary members that form the roof portion of the portal frame.
They carry loads from the roof system and transfer these forces to the columns through the rigid frame. Rafter geometry is influenced by factors such as building span, roof slope, loading, and architectural requirements.
For a comparison of portal frames with other large-span structural systems, see our guide to portal frame and truss systems.
1.3 Haunches
A haunch is a strengthened or deepened region of the rafter, typically located near the eaves connection.
The increased depth improves the moment resistance and stiffness of the rafter in a region where bending moments are typically high. It can also help achieve an efficient bolted moment connection between the column and rafter.
The exact size and length of a haunch depend on the structural design.
2. Secondary Structural Members
Secondary members support the building envelope and transfer relevant loads to the primary frame. Depending on the structural arrangement, they can also contribute to the lateral restraint of primary members.
2.1 Purlins
Purlins are secondary members installed along the roof between adjacent portal frames.
They support the roof cladding and transfer roof loads to the rafters. Depending on the structural arrangement and connection details, purlins can also provide lateral or torsional restraint to the rafter.
Cold-formed C and Z sections are commonly used for purlins. In some projects, rectangular hollow sections (RHS) or other suitable profiles may be used instead. The choice of section depends on the structural design, span, loading, connection details, and project requirements.
The restraint arrangement between purlins can also vary with the purlin profile. Sag rods, anti-sag rods, flat bars, or other suitable restraint systems may be used depending on the design.
2.2 Girts
Girts are horizontal secondary members installed along the building walls between the primary frames.
They support wall cladding and transfer relevant wall loads to the columns. Depending on the structural and connection arrangement, girts can also contribute to the lateral restraint of the columns.
C and Z sections are commonly used for girts. In some projects, rectangular hollow sections (RHS) may also be used where appropriate.
2.3 Eaves Struts
An eaves strut is a secondary member located at the eaves of a portal frame building.
Depending on the structural arrangement, it can serve as both the eaves purlin for the roof and the uppermost girt for the wall system, forming a structural connection between the roof and wall secondary framing.
Eaves struts are commonly designed as tubular or other suitable structural sections. Their size and connection details depend on the building configuration and structural requirements.
3. Bracing and Restraint Members
Portal frames provide primary stability within their transverse plane. Additional bracing and restraint systems are therefore required to provide longitudinal stability and control the behavior of individual structural members. The arrangement of longitudinal bracing and member restraints is an important part of portal frame design, particularly for controlling out-of-plane stability and establishing reliable load paths. The SCI guidance on the design of steel portal frames provides further technical information on these systems.
3.1 Longitudinal Tie Members
Longitudinal tie members connect adjacent portal frames along the length of the building.
They help maintain the longitudinal relationship between frames and can form part of the overall stability and load-transfer system.
In many steel building projects, longitudinal tie members are fabricated from circular hollow sections (CHS). The actual section and connection arrangement depend on the structural design.
Longitudinal tie members should not be confused with sag rods, which are installed between purlins.
3.2 Vertical X-Bracing
Vertical X-bracing is commonly arranged in selected bays along the side walls of a portal frame building.
The bracing helps transfer longitudinal forces toward the foundations and provides stability to the building in the longitudinal direction.
Circular hollow sections (CHS) are commonly used for this type of bracing. Angles, flat bars, or other suitable members may also be used depending on the structural design.
3.3 Roof X-Bracing
Roof X-bracing is arranged within the plane of the roof and connects the structural system longitudinally between portal frames.
It helps transfer longitudinal forces through the roof bracing system and provides a stable load path toward the vertical bracing bays.
Round steel bars are commonly used for roof X-bracing. Angle sections or double-angle arrangements may also be used depending on the structural design.
3.4 Anti-Sag Rods
Sag rods or anti-sag rods are installed between purlins to provide intermediate restraint and help maintain the stability and alignment of the purlin system.
Depending on the arrangement, they can provide restraint against lateral or torsional movement of the purlins, particularly during construction or under wind uplift conditions.
Round steel bars are commonly used for sag rods because they are relatively simple to install and adjust. Depending on the purlin profile and structural detailing, flat bars or other restraint arrangements may also be used.
Their spacing and connection details should be determined by the structural design.
3.5 Sag Rod Sleeves
A sag rod sleeve is a tubular component used together with a sag rod system.
The sag rod primarily acts as a tension member, while the sleeve provides compression restraint between adjacent purlins. The sleeve also helps maintain the intended geometry of the restraint system.
The exact sleeve arrangement depends on the sag rod and purlin detailing used for the project.
3.6 Flange Braces
Flange braces connect the purlin to the rafter flange and provide lateral restraint to the rafter.
They are particularly important where restraint of the rafter flange is required to control lateral or torsional movement. Flange braces are commonly fabricated from angle sections, although other suitable profiles may be used depending on the design.
The location and number of flange braces should be determined by the structural design rather than applied uniformly to every purlin.
3.7 Cladding as Part of the Restraint System
In some projects, the roof or wall cladding system may contribute to the restraint of secondary or primary members.
For example, sandwich panels may be considered as part of the structural restraint system when their connection details, panel properties, and design assumptions are specifically taken into account in the structural design.
This should not be interpreted as meaning that installed cladding automatically provides structural restraint. Its contribution depends on the panel system, fasteners, connections, supporting members, applicable design standards, and the assumptions used by the structural engineer.
4. Project-Specific Structural Members
Some portal frame buildings require additional structural systems because of their intended use. These components are not standard parts of every portal frame.
4.1 Crane Girders
Crane girders, also commonly called crane beams, support overhead travelling cranes and transfer crane-related loads to the supporting structure.
They are typically supported by columns or dedicated crane-supporting arrangements. Their design must consider the vertical, lateral, longitudinal, and dynamic effects associated with the crane system.
A crane girder should therefore be treated as an additional structural system rather than as a standard component of every portal frame building.
4.2 Mezzanine Beams and Joists
Buildings with mezzanine floors may require additional beams, girders, joists, and floor decking.
These members form the mezzanine floor system and transfer floor loads to the supporting columns or other structural members.
The size and arrangement of mezzanine members depend on the intended use of the floor, loading requirements, span, and structural layout.
