PFC Geometry and Mono-Symmetric Properties

Parallel Flange Channels in AS/NZS 3679.1 (Grade 300) range from 75PFC to 430PFC. Unlike doubly symmetric UB sections, PFC sections are mono-symmetric about the major axis. The critical geometric consequence is that the shear centre lies outside the section profile, behind the web:

y_sc = distance from web face to shear centre (behind the web)

Section Moment Capacity — Bending About x-Axis (Clause 5.2.3)

For the major axis (x-axis), the procedure is identical to UB sections — compact sections achieve the full plastic moment:

phi_Msx = 0.90 x fy x Sx

The section is compact if the flange satisfies b/tf <= lambda_ep = 9 (Grade 300) and the web satisfies d1/tw <= lambda_ep = 82.

Most Australian PFC sections up to 380PFC56 are compact in both flange and web. The capacity factor phi = 0.90 for flexure per AS 4100 Table 3.4.

Section Moment Capacity — Bending About y-Axis (Clause 5.2.3)

For bending about the minor axis (y-axis), the section is treated as having one slender element (the web tips) and capacity is based on the elastic section modulus:

phi_Msy = 0.90 x fy x Zy

where Zy is the elastic section modulus about the minor axis. The plastic section modulus Sy is not used for minor-axis bending because the web tips would buckle before full plasticity develops.

Lateral-Torsional Buckling of PFC Sections — Clause 5.6

LTB design for PFC sections follows the same framework as UB sections but with two important differences:

1. Mono-Symmetric Section Factor — beta_x

The mono-symmetric parameter beta_x accounts for the shift in the effective shear centre when the section bends:

For a PFC loaded through the shear centre, beta_x is calculated as:

beta_x = 0.8 x d x (2 x I_cy / I_y - 1)

where I_cy is the second moment of area of the compression flange about the minor axis. For PFC sections, the top and bottom flanges are identical so 2 x I_cy / I_y is approximately 1.0, giving beta_x approximately 0.

However, when the load is applied to the top flange (typical for purlins), additional twist occurs because the load acts above the shear centre.

2. Effective Length for LTB

For PFC sections used as simply supported beams with various restraint conditions:

For purlin applications, the sheeting provides continuous lateral restraint to the top flange when positively connected at 450 mm centres or closer.

Elastic Buckling Moment Mo for PFC Sections

The elastic buckling moment for a mono-symmetric section incorporates beta_x:

Mo = sqrt[ (pi^2 x E x Iy) / Le^2 x (G x J + pi^2 x E x Iw / Le^2) ]

where Iw is the warping constant. For PFC sections, Iw is approximately:

Iw = (I_y x h_s^2) / 4

where h_s is the distance between flange centroids (approximately d - tf).

Worked Example — 200PFC30 as Floor Beam

Problem: A simply supported 200PFC30 spans 4.5 m as a floor beam supporting residential loads. Dead load = 5.0 kN/m (including self-weight 0.29 kN/m), live load = 8.0 kN/m. The compression (top) flange is laterally restrained by floor sheeting at 1.5 m centres. Grade 300 steel.

Section properties — 200PFC30: Zx = 224 x 10^3 mm^3 | Sx = 258 x 10^3 mm^3 | Iy = 1.95 x 10^6 mm^4 | J = 133 x 10^3 mm^4 | Iw = 12.8 x 10^9 mm^6 | ry = 22.8 mm

Step 1 — Section classification: Flange: b/tf = 75 / 12.5 = 6.0 < 9 (compact) Web: d1/tw = (200 - 2 x 12.5) / 6.0 = 175 / 6.0 = 29.2 < 82 (compact) Compact section — full plastic moment applies.

Step 2 — Factored design actions: w* = 1.2 x 5.0 + 1.5 x 8.0 = 6.0 + 12.0 = 18.0 kN/m M* = w* x L^2 / 8 = 18.0 x 4.5^2 / 8 = 45.6 kNm V* = w* x L / 2 = 18.0 x 4.5 / 2 = 40.5 kN

Step 3 — Section moment capacity (x-axis): phi_Msx = 0.90 x 300 x 258 x 10^3 / 10^6 = 69.7 kNm > 45.6 kNm. OK.

Step 4 — LTB check: Segment length Le = 1.5 m (top flange restrained at 1.5 m centres).

Elastic buckling moment Mo: Mo = sqrt[ (pi^2 x 200,000 x 1.95 x 10^6) / 1500^2 x (80,000 x 133 x 10^3 + pi^2 x 200,000 x 12.8 x 10^9 / 1500^2) ] Mo = sqrt[ 1,711 x 10^6 x (10.64 x 10^9 + 11.18 x 10^9) ] Mo = sqrt[ 1,711 x 10^6 x 21.82 x 10^9 ] Mo = sqrt[ 37.35 x 10^18 ] = 6.11 x 10^9 Nmm = 193 kNm

lambda_s = sqrt(Msx / Mo) = sqrt(77.4 / 193) = 0.633 alpha_s = 0.6 x [ sqrt(0.633^4 + 3) - 0.633^2 ]^0.5 = 0.6 x [ sqrt(0.161 + 3) - 0.401 ]^0.5 alpha_s = 0.6 x [ 1.778 - 0.401 ]^0.5 = 0.6 x 1.173 = 0.704

alpha_m = 1.13 (UDL on restrained segments)

phi_Mb = 0.90 x 1.13 x 0.704 x 77.4 = 55.4 kNm > 45.6 kNm. OK.

Step 5 — Shear check: Aw = 200 x 6.0 = 1,200 mm^2 phi_Vu = 0.90 x 0.60 x 300 x 1200 x 1.0 / 1000 = 194 kN > 40.5 kN. OK.

Step 6 — Deflection (serviceability): Live load only: w_L = 8.0 kN/m (4.5 m span) delta = 5 x 8.0 x 4500^4 / (384 x 200,000 x 23.4 x 10^6) = 4.6 mm L/250 = 18.0 mm. 4.6 < 18.0. OK.

Result: 200PFC30 Grade 300 is adequate. LTB governs (55.4 kNm vs 69.7 kNm section capacity). The floor sheeting restraint at 1.5 m centres is adequate.

Torsion in PFC Sections — Loading Through Shear Centre

PFC sections loaded away from their shear centre experience torsion. The torsional warping normal stresses add to the bending stresses:

sigma_total = sigma_bending + sigma_warping

For a 200PFC30 with a 4.5 m span and uniformly distributed load applied at the top flange (eccentricity e = y_sc + tw/2 = 20.9 + 3.0 = 23.9 mm from shear centre):

Torsional moment per unit length: t* = w* x e = 18.0 x 0.0239 = 0.430 kNm/m

The warping torsion analysis per AS 4100 Appendix H gives the additional longitudinal stress. For short spans (< 3 m), warping stresses can reach 20-30% of bending stress. For spans exceeding 6 m, warping stresses are typically less than 5% of bending stress and may be neglected.

Practical mitigation: Always load PFC beams through their shear centre. For top-flange-loaded purlins, provide anti-sag rods at third-points to restrain twist.

Bearing at Supports — Clause 5.13

For a PFC beam end reaction, the web bearing capacity at the support must be checked:

Rb = phi x 1.25 x bb x tw x fy

For the 200PFC30 with 75 mm bearing length: bb = 75 mm (bearing length) + (2.5 x tf) spread through flange = 75 + 2.5 x 12.5 = 106 mm Rb = 0.90 x 1.25 x 106 x 6.0 x 300 / 1000 = 214 kN > 40.5 kN. OK.

The web buckling capacity is calculated with an effective column of web height d1 = 175 mm, with the buckling curve from Clause 6.3.3 applied to determine alpha_c. For practical purposes, bearing stiffeners are only required for PFC sections deeper than 300 mm with reaction forces exceeding 150 kN.

PFC Connection Detailing

The flat back of the PFC section simplifies bolted connections. For a web side plate connection:

1. Bolt clearance: Minimum edge distance from bolt centre to flange toe = 1.5 x d_f per AS 4100 Table 9.6.1. Standard hole diameter = d_f + 2 mm for M16 and M20 bolts.
2. Gauge distance: The standard gauge g for PFC web holes is 35-45 mm from the back of the channel, depending on section depth.
3. Block shear: Check both shear rupture path along bolt line and tension rupture across the web-to-flange junction per Clause 9.1.10.

For beam-to-column moment connections using PFC sections, a bolted end plate connection is standard. The four bolts above and below the beam centreline provide moment resistance through tension/compression couple.

Frequently Asked Questions

How does mono-symmetric bending differ for PFC sections vs UB sections?

PFC sections are mono-symmetric — the shear centre lies outside the section (behind the web), and the centroid is offset from the web face. Loading through the shear centre (not centroid) prevents torsion. AS 4100 Clause 5.6.1.1 requires separate LTB checks for bending about each principal axis. For a 200PFC30 loaded through the top flange, the torsional moment from load eccentricity may govern over LTB for short, lightly loaded spans.

When should PFC sections be used instead of UB sections in Australian design?

PFC sections are ideal for: (1) purlins and girts in portal frame buildings — the flat back face simplifies bolting to cleats; (2) bridging members and bracing struts where mono-symmetric sections simplify detailing; (3) stair stringers — the channel shape naturally accommodates treads; (4) crane runway top beams where the flange lip provides lateral restraint. For primary beams, UB sections are preferred because they avoid torsion complications from the shear centre offset.

What is the effect of load eccentricity on PFC beam capacity?

When a PFC is loaded through the top flange rather than the shear centre (a distance of approximately y_sc + tw/2), a distributed torsional moment acts along the beam. For short spans under heavy loads, the warping normal stresses from restrained torsion can reduce the effective bending capacity by 15-25%. For spans exceeding 6 m, warping stresses are typically under 5% of bending stress. Providing anti-sag rods or fly braces at third-points restrains twist and eliminates the torsion penalty.

What are the AS 4100 requirements for PFC web side plate connections?

Per AS 4100 Clause 9.1.10, web side plate connections to PFC webs must consider: (1) bearing and tear-out at bolt holes in the PFC web (thin element, tw as low as 5.0 mm); (2) block shear failure paths that include the web-to-flange junction as a tension failure boundary; (3) the reduced shear area when the bolt holes are near the flange root radius. Minimum bolt gauge from web back face is typically 35 mm for PFC 100-150 and 45 mm for PFC 200-430 to clear the flange radius.

Related Pages

• AS 4100 Beam Design — UB Flexure & LTB
• Australian LTB Reference — alpha_m and alpha_s
• AS 4100 Torsion Design — Warping & St Venant
• Australian Beam Sizes — UB & PFC Sections
• AS 4100 Connection Design — Bolted & Welded
• Beam Capacity Calculator — Free Online Tool
• All Australian Steel Design References

Design Resources

Calculator tools

• Beam Capacity Calculator
• Steel Beam Deflection Check
• Bolted Connection Calculator
• Steel Column Calculator

Design guides

• AS 4100 Beam Design Guide
• AS 4100 LTB Complete Reference
• AS 4100 Shear Tab Connection Guide
• AS 4100 Base Plate Design Example

This page is for educational reference only. PFC design per AS 4100:2020 Clause 5 and AS/NZS 3679.1. Verify section properties against current ASI design capacity tables. All structural designs must be independently verified and sealed by a licensed Professional Engineer. Results are PRELIMINARY — NOT FOR CONSTRUCTION.