What is a portal frame and where is it used?

A portal frame (or rigid frame) is the most common structural system for single-story industrial buildings, warehouses, and large-span commercial structures. It consists of steel columns and rafters connected by moment-resisting joints at the eaves (column-to-rafter) and ridge (rafter-to-rafter). The moment connections provide lateral stability in the frame plane without diagonal bracing, creating open, unobstructed interior spaces. Typical spans range from 30 to 200 feet with eaves heights of 15 to 60 feet. Key components include portal columns (W12 to W24 sections), rafters (W18 to W30), haunches at the eaves that increase rafter depth to resist peak moments, eaves moment connections, ridge connections, cold-formed purlins for roof support, girts for wall cladding, and rod or angle bracing for longitudinal stability. Portal frames are economical because they use rigid connections instead of a separate lateral bracing system.

How are haunches designed in portal frames?

A haunch (also called a knee brace or eaves haunch) increases the rafter depth at the eaves connection where bending moments are highest, reducing the rafter section required. Haunch length is typically 8-12% of the frame span, with depth 1.5-2.5 times the rafter depth. The design procedure involves six steps: (1) determine the factored moment at the column face, Meaves; (2) calculate the required section modulus at the haunch toe, Sreq = Mu / (φ × Fy); (3) size the haunch depth so stress at the toe does not exceed φFy; (4) check the haunch flange for the axial force component Ff = Meaves / (d_rafter + d_haunch − tf); (5) check the haunch web for shear transfer; (6) verify lateral-torsional buckling of the haunch segment. Haunches are typically fabricated by cutting a rafter section diagonally (most economical) or built up from plate. The haunch flange-to-rafter flange connection uses a CJP groove weld, and the web connection uses fillet welds. The taper angle is typically 15-30° for fabrication access and aesthetics.

Which load combinations govern portal frame design?

Six LRFD load combinations from ASCE 7 govern portal frame design, each controlling a different limit state. Combo 1 (1.4D) governs gravity-only columns in low-wind regions. Combo 2 (1.2D + 1.6Lr + 0.5S) governs maximum gravity with roof live load. Combo 3 (1.2D + 1.6Lr + 0.5W) governs gravity plus partial wind. Combo 4 (1.2D + 1.0W + 0.5Lr + 0.5S) typically governs column and rafter member design, with wind as the primary lateral load. Combo 5 (0.9D + 1.0W) governs uplift and overturning — this is critical for anchor bolt design, base plate design, and foundation sizing because the 0.9D term provides minimum dead load resistance to the 1.0W uplift. Combo 6 (1.2D + 1.0E + 0.2S) governs in high seismic regions. The wind uplift combination (0.9D + 1.0W) often produces net uplift on the frame, requiring careful anchor bolt and footing design to prevent overturning failure.

How is the eaves connection designed in a portal frame?

The eaves connection is the most critical joint in a portal frame, transferring moment, shear, and axial thrust from the rafter to the column. The flange force is Ff = Meaves / (d_rafter + d_haunch − tf), where Meaves is the factored moment at the column face. This flange force determines the connection type: bolted end plate connections use tension bolts in the flange zone and shear bolts in the web zone; field-welded connections use flange plates with CJP welds. The three primary connection types are: haunched end plate (most common, fabricated in shop, bolted on site), extended end plate (for lighter frames, no haunch required, 4 or 8 bolts per side), and welded flange plate (strongest, uses bolted web plate for erection). The connection must also be checked for: panel zone shear in the column web, column flange bending (prying action on bolts), stiffener requirements opposite the tension flange, web local yielding and web crippling in the column, and the interaction of axial thrust from the rafter slope with shear. AISC 358 provides prequalified end-plate moment connection designs.

What second-order effects must be considered in portal frames?

Portal frames are sensitive to second-order (P-Δ and P-δ) effects because they are typically single-story with relatively slender columns. P-Δ effects arise from gravity loads acting on the laterally displaced frame, producing additional overturning moment. P-δ effects arise from axial loads acting on the deflected member between nodes. AISC 360 requires second-order analysis when the second-order drift exceeds 1.1 times the first-order drift, or equivalently when the stability coefficient θ = P × Δ / (V × h) exceeds 0.1. For portal frames, the B1-B2 amplification method is commonly used: B1 amplifies member moments for P-δ (using Cm and Pe1), and B2 amplifies story moments for P-Δ (using ΣP and ΣPe2). The elastic critical buckling load for the frame can be estimated from an eigenvalue analysis or approximated as Pe2 = Σ(π²EI / (Kh)²) for each column. Portal frames with slender columns or large gravity loads on the roof often require second-order analysis. The direct analysis method (DAM) with notional loads and reduced stiffness is AISC's preferred approach.

Portal Frame Design — Steel Moment Frame Guide

Portal frames (also called portal frames or rigid frames) are the most common structural system for industrial buildings, warehouses, and commercial steel structures. They consist of columns and rafters connected by moment-resisting joints at the eaves and ridge. This page covers the design approach, key components, and AISC 360 requirements.

Portal Frame System Overview

Component	Function	Typical Section
Portal column	Vertical support, resists moment and axial	W12 to W24
Rafter	Horizontal/angled roof beam	W18 to W30
Haunch	Reinforces eaves moment connection	Cut from rafter or plate
Eaves connection	Moment connection (column to rafter)	Flange plate or end plate
Ridge connection	Moment or pin connection at peak	Bolted or welded
Purlins	Secondary roof framing	Z-section, C-section
Girts	Secondary wall framing	Z-section, C-section
Bracing	Longitudinal stability	Rods, angles, cables

Design Loads on Portal Frames

Load Type	Source	Direction	Combination Factor
Dead load (D)	Roof, cladding, services	Gravity	1.0
Live load (Lr)	Roof maintenance	Gravity	0.75 - 1.0
Wind uplift (W)	ASCE 7 pressure	Uplift/lateral	1.0
Wind lateral	ASCE 7 pressure on walls	Horizontal	1.0
Collateral load	MEP, sprinklers, ceiling	Gravity	1.0
Snow (S)	ASCE 7 ground snow load	Gravity	0.75 - 1.0
Seismic (E)	ASCE 7 seismic forces	Horizontal	1.0

ASCE 7 Load Combinations (LRFD)

Combo	Load Combination	Controls When
1	1.4D	Dead only (rare)
2	1.2D + 1.6Lr + 0.5S	Max gravity
3	1.2D + 1.6Lr + 0.5W	Gravity + partial wind
4	1.2D + 1.0W + 0.5Lr + 0.5S	Wind governs
5	0.9D + 1.0W	Uplift / overturning
6	1.2D + 1.0E + 0.2S	Seismic

Load combination 4 (wind) typically governs column design. Load combination 5 (uplift) governs anchor bolt design and overturning checks.

Frame Analysis

Portal frames are analyzed as 2D plane frames. Methods:

Method	When to Use	Tools
Hand calculation (portal method)	Preliminary sizing	Moment distribution
Elastic analysis (first-order)	Simple frames	Software (STAAD, RISA)
Second-order analysis (P-Δ)	Slender frames	Software required
Plastic analysis	Compact sections, limited	Specialized software

Portal Method Approximation

For preliminary sizing, the portal method gives approximate forces:

Column base shear: V = w × L / 2

Column moment (fixed base): M = w × L × h / 6 (each column)

Rafter moment (mid-span): M = w × L² / 8 - M_eaves

where w = uniform wind load on frame, L = frame spacing, h = eaves height.

Haunch Design

The haunch (also called a knee brace or eaves haunch) increases the rafter depth at the eaves connection, reducing the moment in the rafter and providing a moment-resisting connection.

Haunch Geometry

Parameter	Typical Range	Design Basis
Haunch length	8-12% of span	Moment distribution
Haunch depth	1.5-2.5 × rafter depth	Connection capacity
Haunch flange	Match rafter or wider	Flange force transfer
Haunch web	Match rafter web	Shear transfer
Taper angle	15-30°	Fabrication and aesthetics

Haunch Sizing Steps

Determine the moment at the column face (M_face)
Calculate the required section modulus at the haunch toe
Size the haunch depth so the stress at the toe is ≤ φ × Fy
Check the haunch flange for the axial force component
Check the haunch web for shear
Check lateral-torsional buckling of the haunch

Haunch Fabrication

Haunches are typically cut from the same section as the rafter (cut diagonally) or built up from plates. The haunch flange is welded to the rafter flange with a CJP groove weld. The web is fillet-welded to the rafter web.

Eaves Connection Design

The eaves connection is the most critical joint in a portal frame. It transfers:

Moment from rafter to column (through flange forces)
Shear from rafter to column (through web bolts)
Axial thrust from rafter slope

Design Forces

Flange force at eaves: Ff = M_eaves / (d_rafter + d_haunch - tf)

Shear at eaves: V = reaction shear from rafter loading

Axial thrust: H = horizontal reaction from frame action

Connection Types

Type	Pros	Cons
Welded flange + bolted web	Maximum stiffness	Field welding required
Extended end plate	Shop welded, field bolted	Heavy end plate
Haunched end plate	Best for heavy moment	Complex fabrication
Bolted flange plates	All field bolted	Lower stiffness

Column Design

Portal frame columns resist combined axial load, bending moment, and shear.

Column Sizing

Frame Height	Typical Column	Typical Rafter
15 ft	W12×45 to W12×65	W16×36 to W18×50
20 ft	W12×65 to W14×68	W18×50 to W21×62
25 ft	W14×68 to W14×90	W21×62 to W24×68
30 ft	W14×90 to W14×120	W24×68 to W27×94
35 ft	W14×120 to W14×145	W27×94 to W30×108

Column Checks

Combined axial + bending (AISC Chapter H): φPn and φMn interaction
Lateral-torsional buckling (AISC Chapter F): Braced by girts or flange bracing
Local buckling (AISC Table B4.1): Compact section requirements
Base plate design: Fixed or pinned base
Anchor bolt design: ACI 318 Appendix D

Fixed vs Pinned Base

Base Type	Moment	Column Size	Foundation	Deflection
Fixed	Moment at base	Smaller	Larger	Less
Pinned	No moment	Larger	Smaller	More
Semi-fixed	Partial moment	Moderate	Moderate	Moderate

Fixed bases reduce column size and frame deflection but require larger foundations to resist the base moment. The choice depends on foundation cost vs steel cost trade-off.

Rafter Design

Rafter Checks

Bending strength at critical sections (eaves, haunch toe, mid-span)
Combined axial + bending (frame action creates thrust)
Shear strength at eaves
Lateral-torsional buckling between purlin points
Web sidesway buckling at concentrated forces (purlin loads)
Deflection (typically L/180 for roof frames)

Roof Slope

Slope	Angle	Application
1:12	4.8°	Minimum practical slope
2:12	9.5°	Low-slope industrial
3:12	14.0°	Standard warehouse
4:12	18.4°	Higher drainage
6:12	26.6°	Steep roof, architectural

Minimum slope for drainage depends on cladding type. Standing seam metal roofs can be as low as 0.25:12 with proper sealant.

Frame Spacing

Spacing	Purlin Span	Rafter Load	Typical Use
20 ft	20 ft	Moderate	Common industrial
25 ft	25 ft	Higher	Long-span warehouses
30 ft	30 ft	High	Large clear-span
40 ft	40 ft	Very high	Special structures

Wider spacing reduces the number of portal frames but increases purlin size and rafter loading. 20-25 ft spacing is most economical for typical buildings.

Stability Bracing

Portal frames require longitudinal bracing for stability:

Bracing Type	Location	Purpose
X-bracing (wall)	End bays, alternating	Longitudinal wind resistance
Roof bracing	End bays, diaphragm	Roof diaphragm stability
Flange bracing	At haunch, mid-rafter	Rafter LTB restraint
Column bracing	At girt locations	Column weak-axis bracing
Portal brace	In plane of frame	Resistance to longitudinal load

A minimum of one braced bay per 150 ft of building length is recommended.

Frequently Asked Questions

What is a portal frame? A portal frame is a rigid frame consisting of two columns connected to a rafter (or pair of rafters) by moment-resisting connections. The frame resists lateral loads through frame action (bending of columns and rafters) rather than diagonal bracing.

When do I need a haunch? Haunches are needed when the moment at the eaves connection exceeds the capacity of the rafter-to-column connection without reinforcement. Most portal frames over 30 ft span use haunches. The haunch increases the lever arm at the connection, reducing the flange force.

Should the portal frame base be fixed or pinned? Fixed bases reduce frame deflection and column size but increase foundation cost. Pinned bases are simpler and cheaper to construct but result in larger columns and more deflection. For most industrial buildings (20-40 ft eaves height), fixed bases are preferred for deflection control.

What is the typical span range for portal frames? Portal frames are economical for clear spans from 30 ft to 150 ft. Below 30 ft, standard beam-and-column framing is usually cheaper. Above 150 ft, trusses or space frames become more economical. The most common range is 60-100 ft.

Beam Capacity — Beam design calculator
Wind Load Calculator — ASCE 7 wind loads
Moment Connection Design — AISC moment connections
Connection Types — All connection types
Base Plate Design — Base plate calculator

Disclaimer

This is a calculation tool, not a substitute for professional engineering certification. All results must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) before use in construction, fabrication, or permit documents. The user is responsible for the accuracy of all inputs and the verification of all outputs.