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 Rigid Frame for Industrial Buildings

Portal frames (rigid frames with moment-resisting eaves and ridge connections) are the dominant structural system for single-story industrial buildings, warehouses, aircraft hangars, and agricultural structures. The frame consists of tapered or prismatic columns and rafters connected by moment-resisting joints. This reference covers the engineering design of steel portal frames per AISC 360-22, including the unique aspects of haunch design, frame stability, and second-order effects.

Portal Frame Geometry

A typical portal frame has a clear span of 60 to 150 ft, an eave height of 20 to 40 ft, and a roof slope between 1:12 and 4:12 (shallow pitch for metal roofing). The frame spacing (bay spacing) is typically 20 to 30 ft along the building length. Purlins span between frames and support the metal roof deck.

The frame resists gravity loads (roof dead and live) through beam action in the rafter and column, and lateral loads (wind) through frame action -- the moment-resisting eaves joint transfers the rafter moment into the column, and the column base resists the overturning moment through a moment-resisting base detail.

Haunch Design

The eaves haunch is the defining feature of a portal frame. The rafter depth increases near the column connection, creating a deeper section at the point of maximum negative moment (the eaves). The haunch serves three purposes:

Increased flexural capacity at the eaves where the moment is highest.
Reduced rafter depth in the mid-span region where the moment is lower, saving steel weight.
Improved connection geometry -- the deeper haunch provides room for the moment-resisting bolts or welds and increases the lever arm for the tension-compression couple.

The haunch is typically fabricated by cutting a deeper section (same series) at an angle and welding it to the rafter and column. The haunch length is typically 10-15% of the frame span. The depth at the column face is typically 1.5 to 2.5 times the rafter depth at mid-span.

Haunch Design Checklist

Flange local buckling: The haunch compression flange (bottom flange at the eaves, under gravity load) is in compression. Check flange slenderness per AISC 360 Table B4.1.
Web shear buckling: The haunch web depth is significantly larger than the rafter web. Check web slenderness h/tw. If h/tw exceeds 2.24 sqrt(E/Fy), transverse stiffeners may be required.
Lateral-torsional buckling of the haunch: The haunch bottom flange is in compression. If the purlins do not brace this flange, check LTB per AISC 360 Section F2. The unbraced length is the distance from the column face to the first purlin (or fly brace).
Haunch-to-column weld: The moment at the eaves produces a tension-compression couple across the haunch depth. The weld at the haunch-to-column interface must transfer the full flange force. For a haunch depth of 36 in. and a moment of 800 kip-ft: T = C = 800 x 12 / 36 = 267 kip per flange. The weld must be sized for this force -- typically a CJP groove weld at the tension flange.

Eaves Connection Design

The eaves connection is the most critical joint in a portal frame. In a bolted end plate connection (the most common field-bolted option for portal frames), the end plate extends beyond both flanges of the haunch and is bolted to the column flange. For a typical portal frame, 6 to 8 bolts are used on the tension side.

Worked Example -- Eaves End Plate Connection

Given: Rafter W18x55 with haunch (depth at column face = 30 in.). M_eaves = 650 kip-ft. End plate A572 Gr 50. Bolts 1 in. A325, fully pretensioned. 6-bolt extended end plate configuration (6ES: 3 rows of 2 bolts on the tension side).

Tension per bolt pair: h_o = 28 in. (distance from compression flange center to tension bolt centroid). Tension per pair = M_u / h_o = 650 x 12 / 28 = 279 kip for 3 pairs = 93 kip per pair. Per bolt: 46.5 kip.

Bolt capacity: For 1 in. A325: phi Rn = 0.75 x 90 x 0.785 = 53.0 kip per bolt > 46.5 kip. OK for applied tension (before prying).

Prying check: With tp = 1.5 in., the prying amplification factor is approximately 1.12 for this geometry. Total bolt force = 46.5 x 1.12 = 52.1 kip < 53.0 kip. OK.

End plate thickness: Per DG4 yield line equation: tp_req = sqrt( (4 M_u b_p) / (phi_b Fy Y_p h_o) ) = sqrt( (4 x 650 x 12 x 10) / (0.90 x 50 x Y_p x 28) )

Y_p for 6ES approximately 180 (from DG16 Table 3.2 for this geometry). tp_req = sqrt(312,000 / (45 x 180 x 28)) = sqrt(312,000 / 226,800) = sqrt(1.376) = 1.17 in. Use tp = 1-1/4 in.

Column flange check: W14x132 column. phi Rn flange bending = 0.90 x 6.25 x 1.030^2 x 50 = 298 kip per bolt pair > 93 kip. OK.

Rafter Design

Rafters in portal frames are typically designed for combined axial load and flexure (beam-column action). The rafter carries compression from the frame thrust (the horizontal reaction at the base that resolves into the inclined rafter), plus the bending moment from gravity loads.

At the eaves (negative moment region), the bottom flange is in compression and the top flange is in tension. At mid-span (positive moment region), the top flange is in compression (with the purlins providing lateral bracing through the roof deck) and the bottom flange is in tension (unbraced, unless fly braces are provided).

Second-Order Effects

Portal frames are laterally flexible compared to braced frames. The P-Delta effect (the additional moment from the vertical load acting through the lateral displacement) must be accounted for. AISC 360 Section C1 requires a second-order analysis when the second-order drift exceeds 1.1 times the first-order drift (or equivalently, when the stability coefficient theta exceeds 0.10).

For a portal frame with an eave height of 30 ft, frame spacing of 25 ft, and a total roof dead load of 15 psf: P_story = 15 x (60 ft span / 2) x 25 ft = 11.25 kip per frame per side. First-order drift from wind: approximately H/200 = 1.8 in. for a properly proportioned frame.

theta = P_story x Delta_1 / (V_story x h) = 11.25 x 1.8 / (8.0 kip wind shear x 360 in.) = 20.25 / 2,880 = 0.007

theta = 0.007 << 0.10. P-Delta effects are negligible for this frame. Second-order analysis is not required per AISC 360 Section C1.

For taller, more heavily loaded frames (high snow load regions, larger spans), theta can approach 0.10 and second-order effects become significant. In those cases, the DM (Direct Modeling) method or the B1/B2 amplification approach (AISC 360 Appendix 8) must be used.

Column Design and Base Fixity

Portal frame columns are typically W12 to W18 sections, tapered (deeper at the base, transitioning to the rafter depth at the eaves). The column is a beam-column: axial load from the roof weight plus wind uplift, and bending from the frame action.

The column base is moment-resisting (not pinned). A pinned base in a portal frame produces unacceptably large deflections because the frame relies on fixity at both eaves and base to limit drift. The base plate and anchor rods must develop the required moment. The design is a moment-resisting base plate per AISC DG1:

Base plate: Typically 1-1/4 to 2-1/2 in. thick, with stiffeners between the column flanges.
Anchor rods: 4 or 6 rods in a rectangular pattern outside the column flanges. The tension-side rods resist the uplift couple.
Pier/footing: The overturning moment at the base is the highest in the frame (the columns resist the entire lateral overturning). The footing must be sized for settlement under the compression side and uplift resistance on the tension side.

Frame Stability -- Sway Buckling

A portal frame under gravity load alone can buckle in a sway mode if the column bases are not adequately restrained or the frame is too flexible laterally. The elastic critical buckling load of the frame can be estimated using the Merchant-Rankine approach or a finite element eigenvalue analysis.

For preliminary design, the ratio of the vertical load to the elastic buckling load (alpha_cr = P_cr / P_applied) should exceed 10 for frames analyzed by first-order methods. If alpha_cr < 10, either:

Perform a rigorous second-order analysis (with initial imperfections per AISC 360 Section C2.2b), or
Increase the frame stiffness (deeper columns, stiffer eaves connections, reduced frame spacing).

For a typical 80 ft span portal frame at 25 ft spacing, the elastic buckling load factor is typically alpha_cr = 12-18 for properly proportioned frames, satisfying the 10x requirement.

Wind Load on Portal Frames

Wind governs the design of most portal frames (except in very high snow regions). The wind pressure on the walls and roof is determined per ASCE 7-22 Chapters 27 (Directional Procedure) or 28 (Envelope Procedure for low-rise buildings).

For the main wind force-resisting system (MWFRS), the frame resists:

Windward wall pressure -- inward on the windward column
Leeward wall suction -- outward on the leeward column
Roof suction -- upward on the windward roof, upward or downward on the leeward roof (depending on roof slope and building geometry)

The combination of windward wall pressure and roof suction produces the maximum frame moment at the windward eaves and the maximum base shear.

Worked Example -- 80 ft Portal Frame Sizing

Given: 80 ft clear span, 25 ft eave height, 4:12 roof slope. Frame spacing = 25 ft. ASCE 7-22 Risk Category II, Exposure B. Wind speed V = 115 mph (3-sec gust). Roof dead load = 12 psf. Roof live load = 20 psf (ASCE 7-22 Section 4.8, non-reducible). No snow load (southern US location).

Wind load per ASCE 7-22 Ch. 27: qz = 0.00256 Kz Kzt Kd V^2 = 0.00256 x 0.85 x 1.0 x 0.85 x 115^2 = 24.5 psf at eave height. MWFRS pressures per Ch. 27 Table 27.3-1.

Windward wall (Case A, Cp = 0.8): p = 0.8 x 24.5 = 19.6 psf x 25 ft trib = 490 plf inward. Leeward wall (Cp = -0.3): p = -0.3 x 24.5 = -7.4 psf = -185 plf outward. Windward roof (Cp = -0.9 for 18.4 degree slope): p = -22.1 psf = -553 plf suction. Leeward roof (Cp = -0.5): p = -12.3 psf = -308 plf suction.

Gravity + Wind combination (ASCE 7-22 2.4.1): 1.2D + 1.0W + 0.5Lr

Preliminary sizing from AISC Manual and typical portal frame proportions:

Rafter: W18x50 at mid-span, haunched to 30 in. depth at the eaves.
Column: W14x82, tapered from W14 depth at the base to match the rafter at the eaves.
Eaves connection: Extended end plate, 6 bolts, 1-1/4 in. end plate.
Base: Moment-resisting base plate, 1-3/4 in. thick, 4 x 1-1/4 in. anchor rods.

Drift check (wind service, 10-year MRI per IBC 1604.3): Wind drift should be less than H/100 for industrial buildings with metal siding = 300/100 = 3.0 in. at the eaves. The frame analysis (first-order, neglecting P-Delta per the theta check) should be performed by a structural analysis program (RISA-3D, SAP2000, or similar) to confirm the drift is within the limit.

Related Tools and References

Disclaimer

This page is for educational and reference use only. It does not constitute professional engineering advice. Portal frame design must be performed by a licensed Professional Engineer (PE) or Structural Engineer (SE) using a verified structural analysis program. The site operator disclaims liability for any loss arising from the use of this information. Results are PRELIMINARY -- NOT FOR CONSTRUCTION.