1. How the Components Work Together
A portal-frame building normally contains repeated transverse frames connected by longitudinal members. No component should be judged only by its size or appearance; its importance depends on the force it transfers and the stability function it provides.
| Component group |
Typical components |
Principal function |
| Primary transverse frame |
Columns, rafters, eaves haunches, apex region |
Resists actions in the plane of the frame |
| Secondary framing |
Purlins, girts or side rails, eaves members |
Supports the roof and wall systems and transfers loads to the primary frames |
| Longitudinal stability system |
Roof bracing, wall bracing, ties, and struts |
Transfers actions along the building and stabilizes the frame lines |
| Local member restraint |
Flange braces, stays, and related restraint components |
Helps prevent lateral movement, twisting, or buckling of individual members |
| Connections and bases |
End plates, bolts, welds, cleats, gussets, base plates, and anchors |
Transfers forces between components and into the foundations |
| Project-specific framing |
Gable posts, door framing, crane beams, mezzanines, and canopies |
Supports additional architectural or operational requirements |
The following simplified paths show how these groups interact:
| Action |
Simplified load path |
| Roof gravity or snow load |
Roof cladding → purlins → rafters → columns → bases → foundations |
| Wind on side walls |
Wall cladding → girts → columns and transverse frames → bases → foundations |
| Wind on a gable wall |
End-wall framing → roof bracing and wall bracing → foundations |
| Longitudinal roof action |
Roof system → roof bracing → eaves transfer member → wall bracing → foundations |
Actual load paths can change where a building contains large openings, internal columns, cranes, expansion joints, structural diaphragms, or proprietary systems. Portal-frame anatomy and design guidance from SteelConstruction.info provides additional technical context for these relationships.
2. Primary Transverse-Frame Members
2.1 Columns
Columns are the main vertical members of a portal frame. They support the rafters and transfer axial force, bending moment, and shear toward the bases and foundations.
Portal-frame columns commonly use rolled or welded I-sections. Tapered fabricated sections may also be used as part of an engineered building system.
Column performance depends on more than section size. Frame geometry, eaves-connection behavior, restraint positions, bracing, base assumptions, and foundation response all influence the final design. Crane brackets or other attached systems can introduce additional forces that must also be considered.
2.2 Rafters
Rafters are the inclined or pitched members forming the roof portion of the transverse frame. They receive loads from the purlins and transfer them through frame action to the columns.
A portal-frame rafter generally resists substantial bending together with axial force. This differs from a roof truss, where the triangulated members are intended to carry forces mainly through axial tension or compression.
Rafter design is influenced by span, roof slope, frame spacing, loading, deflection, stability, restraint locations, haunch geometry, connections, and transportation limits. See Portal Frame vs Truss System in Steel Structures for a system-level comparison.
2.3 Eaves Haunches
An eaves haunch is a locally deepened or strengthened region near the rafter-to-column connection. Because bending moments are often high near the eaves, the haunch can increase resistance and stiffness while helping form an efficient moment-resisting connection.
The haunch may be cut from a rolled section, fabricated from plate, or incorporated into a tapered member. It is best understood as a strengthened region of the primary frame rather than as a separate standard beam.
Its geometry, stiffeners, welds, bolts, and restraints must follow the structural and connection design. Typical proportions may help early planning, but they do not replace analysis.
2.4 Apex Region
In a double-pitched portal frame, the rafters meet near the apex or ridge. This region may contain a continuous rafter, a bolted apex connection, an apex haunch, a transport splice, or local stiffeners.
The detail must transfer the required forces while remaining practical to fabricate, transport, lift, align, and bolt. An apex haunch may principally facilitate the connection and should not automatically be assumed to perform the same role as an eaves haunch.
2.5 Internal or Valley Members in Multi-Span Frames
A multi-span building may contain internal columns, valley rafters, valley beams, or other supporting members. These components can change adjacent rafter forces, frame movement, foundation reactions, roof drainage, and erection requirements.
Removing an internal column to create a larger open area changes the structural system. It is therefore a redesign decision, not a simple material omission.
3. Secondary Roof and Wall Members
Secondary framing and bracing connect the repeated portal frames into a complete building. This article identifies their roles; their detailed selection and layout belong to dedicated design topics.
3.1 Purlins and Girts
Purlins span between portal frames and support the roof system. Girts, also called side rails in some regions, support the wall system. Both transfer loads from the cladding to the primary structure and may also participate in restraining primary members where the structural design and installed connections support that assumption.
Their size, spacing, continuity, profile, restraint function, and treatment around openings are interdependent design decisions. These subjects are covered in Purlins and Girts in Steel Buildings.
3.2 Eaves Members
An eaves member runs longitudinally near the roof-to-wall junction. Depending on the structural arrangement, it may support cladding, maintain alignment, transfer longitudinal force, or connect roof-level forces to the wall-bracing system.
Terminology note: Eaves beam, eaves strut, and eaves tie do not have identical meanings in every market. Determine the member's function from its position, section, connections, and force path rather than from its name alone.
3.3 Roof and Wall Bracing
Portal frames provide stability in their own planes, but the building also requires stability along its length. Roof bracing collects and transfers roof-level and end-wall actions, while vertical wall bracing completes the path toward the foundations. Longitudinal ties or struts may connect these parts and maintain the positions of adjacent frames.
The location and form of the bracing must be coordinated with doors, windows, equipment, cranes, expansion joints, and erection requirements. See Bracing Systems in Steel Buildings for the different systems, force paths, and layout considerations.
4. Local Restraints and Purlin Stability Components
Local restraints stabilize individual rafters, columns, purlins, or girts. They serve a different purpose from the roof and wall bracing that stabilizes the building as a whole, although the forces from local restraints must ultimately reach a stable part of the structure.
4.1 Flange Braces and Stays
A flange brace or stay connects an otherwise insufficiently restrained rafter or column flange to a suitable purlin, girt, or other supporting member. It can provide lateral or torsional restraint where the member design requires it.
This is especially important when the compression flange is not the flange directly connected to the secondary member. The critical flange can also change between gravity and wind-uplift load combinations.
A flange brace is effective only when:
- It connects to the flange requiring restraint
- Its geometry provides the intended restraint direction
- The brace and its connections have adequate strength and stiffness
- The supporting purlin or girt can receive the restraint force
- A complete force path exists from the secondary member to the braced structure
Engineering note: Attaching a purlin or girt to one flange of a rafter or column does not automatically restrain the opposite flange. A flange brace must work with a suitably stiff and continuous supporting member, not merely connect two pieces of steel.
4.2 Sag Rods and Spacer Sleeves
Sag rods, anti-sag rods, spacer sleeves, tube struts, angles, or proprietary bridging components may form an intermediate restraint system between adjacent purlins or girts.
In a common rod-and-sleeve arrangement, the sag rod acts primarily in tension, while the spacer sleeve or tube strut maintains the distance between adjacent purlins and can provide a compression path. The parts should therefore be understood as one assembly rather than as interchangeable loose accessories.
Depending on the designed system, the assembly may:
- Maintain purlin spacing and alignment
- Reduce twisting or lateral movement
- Provide discrete restraint under wind uplift
- Assist stability before the roof cladding is installed
- Transfer the downslope component of load toward an eaves or apex restraint point
The arrangement, anchorage, and force direction vary between systems. A slender sag rod should not be assumed to carry compression unless the detailing and design specifically provide that capability.
Terminology note: A spacer sleeve or tube strut within an anti-sag assembly is different from a purlin sleeve installed at a frame support to create continuity between adjacent purlin spans. The same word should not be used for both without clarification.
4.3 Cladding as Purlin or Girt Restraint
Roof or wall cladding can contribute restraint to the flange of a purlin or girt to which it is attached. Depending on the system, it may provide continuous lateral restraint, rotational restraint, or diaphragm action.
The usable restraint depends on the complete envelope system, including:
- Panel type, profile, and orientation
- In-plane and flexural stiffness
- Fastener type, position, spacing, and resistance
- Side-lap and end-lap details
- Purlin or girt geometry
- Openings and discontinuities
- Load direction and construction stage
The assumptions used in purlin load tables or design software must match the installed cladding and fastener arrangement. Before the cladding is installed, the assumed continuous restraint is absent, so temporary stability and the anti-sag system remain important. The SCI guidance on secondary steelwork and cladding restraint explains these interfaces in greater detail.
Engineering note: A sandwich panel should not automatically be treated as providing full lateral or torsional restraint merely because it is fastened to a purlin. Its contribution must be verified using an applicable design method, tested system data, or manufacturer information that reflects the panel construction, joints, fasteners, support arrangement, and governing load direction. Standing-seam and other indirectly fixed systems also require separate verification.
5. Connections, Bases, and Foundations
A structural member can perform its intended function only when its connections transfer the required forces.
5.1 Primary-Frame Connections
The eaves connection joins the rafter and column and commonly transfers bending moment, shear, and axial force. Apex connections and transport splices may carry different combinations of forces depending on their location.
Typical details can include:
- End plates
- Structural bolts
- Welded plates
- Haunch plates
- Stiffeners
- Splice plates
Connection geometry must be coordinated with the member forces, fabrication method, shipping length, and erection sequence. For a broader comparison, see Bolted vs Welded Connections in Steel Buildings.
5.2 Secondary and Bracing Connections
Purlins, girts, braces, ties, struts, and restraints may connect through cleats, brackets, gusset plates, bolts, welds, or proprietary connectors.
These small components are part of the load path. A brace cannot transfer its design force if the gusset, bolts, supporting member, or downstream connection was designed for a different action.
5.3 Column Bases and Anchors
A typical column base includes a base plate, anchor rods or anchor bolts, grout, and any required stiffeners or shear-transfer detail. It transfers axial force, shear, and, where applicable, bending moment into the foundation.
A base may be modeled as nominally pinned, partially restrained, or moment-resisting. The physical detail, anchors, foundation, and structural model must use compatible assumptions.
The foundation is not a steel-frame member, but it completes the load path. A well-designed steel frame cannot compensate for foundations or anchors designed for incompatible reactions.
6. End-Wall and Project-Specific Members
Not every portal-frame building contains the same additional components.
| Component or system |
Typical purpose |
Important interface |
| Gable posts or wind columns |
Support end-wall cladding and transfer wind actions |
End-wall girts, roof bracing, foundations |
| Door posts and headers |
Frame wall openings |
Girts, wall bracing, cladding |
| Crane runway beams |
Support overhead travelling cranes |
Brackets, columns, bracing, foundations |
| Mezzanine beams and columns |
Support an intermediate floor |
Primary frame, floor system, foundations |
| Canopies and lean-tos |
Extend the building envelope |
Main-frame forces, drainage, movement |
| Valley beams and columns |
Support multi-span roof intersections |
Drainage, frame behavior, erection |
| Service-support steelwork |
Support pipes, ducts, cable trays, or equipment |
Loads, vibration, access, fire protection |
These components must be included in the structural model and load path where they affect the main building. Adding a crane, mezzanine, solar array, suspended service, or large opening after design can change member forces, stability requirements, connections, and foundation reactions.
