Course · lesson 2 of 6

Purlins and girts

Cold-formed lipped channels and zeds on the roof and walls: spans and laps, bridging, gravity against uplift, the buckling checks the STATIX purlin calculator makes to SANS 10162-2, deflection, and girts.

1. Learning outcomes

After this lesson you can:

choose between simple, continuous and lapped purlins;
place bridging or sag rods and say what they do;
check a purlin under gravity and under wind uplift;
explain the local, distortional and lateral checks the calculator makes;
check deflection, and design a girt the same way.

2. Before you start

Lesson 1 (the building and its loads) and Loads 1, lesson 5 (roof zone pressures).
The free cold-formed purlin calculator.
A pocket calculator and about 50 minutes.

3. Purlins, girts and their sections

Purlins carry the roof sheeting between the frames; girts do the same on the walls. In South Africa they are mostly cold-formed lipped channels (C) or zeds (Z), with the sheeting screwed through to one flange. That flange is held by the sheeting; the other, free flange is not.

The section. A lipped channel is described by depth, flange width, lip and thickness. Thin steel buckles before it yields: the plate elements buckle locally, the flange and lip rotate together (distortional buckling), and an unrestrained flange can buckle sideways. The Direct Strength Method takes the weakest.

4. Spans, laps and bridging

Spans. A simple-span purlin is the easiest to fix and the heaviest; continuous or lapped purlins share the moment with the supports and deflect less. Lapped Z-sections nest over the rafters, doubling the section where the moment is largest; SANS 10162-2 3.3.3.4 gives their rules (laps at least 13 % of the span for three or more spans).

Bridging and sag rods. Rows of bridging (or sag rods) tie the purlins together across the roof: they hold the free flange against twisting and carry the slope component of the load. STATIX uses the number of rows: none, one or two.

5. Gravity, uplift and buckling

Gravity against uplift. Under gravity the top flange is in compression and the sheeting holds it, so only local and distortional buckling remain. Under wind uplift the free bottom flange is in compression with nothing holding it: lateral (flexural-torsional) buckling becomes the limit, and on a light roof uplift often governs.

How the engine checks the moment. Three elastic buckling moments (global, local and distortional, the last from the Lau and Hancock formula of AS/NZS 4600 Appendix D) each give a strength curve; the smallest is the member capacity. With the flange held by sheeting, the global term is the yield moment.

Lateral buckling under uplift as STATIX checks it. SANS 10162-2 3.3.3.4 gives a reduction factor R on the section capacity for C and Z purlins with the sheeting on the tension flange, within its limits on the section and span. Without cyclone washers the clause points elsewhere: STATIX then asks for a rational M_cre rather than guessing.

6. Deflection, girts and supports

Deflection. Checked at service loads with the gross section: dead plus imposed downward, and dead plus wind upward. The default limit is span/200; sheeting makers and clients may ask for less.

Girts on the walls. The same sections span between the columns and carry wind: pressure pushes the sheeted outer flange into compression, suction puts the free inner flange in compression, like uplift on a purlin. Sag rods carry the girt’s own weight and the sheeting, which act about its weak axis.

At the support. A purlin sitting on a rafter or cleat must carry its reaction through the thin web: the calculator checks web bearing and its interaction with bending when you give the bearing length; lapped Z-sections over interior supports have their own rule (3.3.7(c)).

In STATIX: the purlin calculator takes the roof loads in kPa and the purlin spacing, runs every SANS 10160-1 combination (both wind cases only when you give a wind pressure), and reports the governing check.