Course · lesson 4 of 6

Rafters and columns as beam-columns

The members of the lesson 1 frame under portal actions: where the moments are, which buckling lengths apply in and out of the frame’s plane, what purlins, girts and fly braces restrain, sway, and serviceability, with the STATIX steel member calculator.

1. Learning outcomes

After this lesson you can:

read portal frame actions and choose the check sections;
set in-plane and out-of-plane lengths for a column and a rafter;
use purlins, girts and fly braces as restraints;
allow for sway when the calculator does not;
check eaves sway and apex deflection against agreed limits.

2. Before you start

Steel Design 1, lesson 4 (combined actions, U₁, braced and unbraced: not repeated here) and lesson 1 (the frame and its loads).
The free steel member calculator.
A pocket calculator and about 55 minutes.

3. Portal actions

Portal actions. A rafter carries a large moment and a small axial force; a column carries axial force and the eaves moment, falling to zero at a pinned base. The STATIX 3D model analyses the frame; for the free steel member calculator you bring the actions, from the model or from a frame analysis of your own.

Where to check. The column just below the haunch, the rafter at the haunch tip where the deeper haunch ends, and the rafter near the apex. Inside the haunch the section is deeper; its design is lesson 5.

4. Buckling lengths and restraint

A column has two buckling lengths. In the plane of the frame it sways with the frame: take K = 1 on the storey height with the sway effects in the moments. Out of the plane the girts hold the outer flange, and fly braces from a girt to the inner flange make a real restraint point.

A rafter’s lengths. In plane, the rafter length from eaves to apex is a safe practice value. Out of plane, the purlins hold the top flange at their spacing, but the inner (bottom) flange is held only where fly braces are fitted. Which flange is in compression decides which length counts.

Fly braces. An angle from a purlin (or girt) down to the inner flange turns the purlin’s restraint into a restraint of the compression flange. Where the inner flange is in compression and has no fly braces, the lateral-torsional buckling length is the whole region, and the capacity drops sharply.

The lengths in one table, as this course uses them. The in-plane values are safe practice choices for a frame whose sway is in the moments; a full frame stability analysis can justify less.

The haunch region, in outline. A haunch deepens the rafter at the eaves, where the moment is largest, and makes room for the bolted end plate. Its tip, where the plain rafter starts, is a check section with its own restraint; the connection itself is lesson 5.

5. Sway, uplift and serviceability

Sway. A portal is unbraced in its own plane, so its moments grow as it sways (the P-Δ effect). SANS 10162-1 amplifies the sway moments by U₂; the STATIX steel member page has no U₂ (as Steel Design 1 lesson 4 noted): choose the unbraced frame type, and enter moments that already include the sway amplification.

Uplift. Under 0.9G + 1.6W the moments reverse. Along the middle of the rafter the inner flange goes into compression, where gravity never put it: that is where fly braces earn their place.

Serviceability. Check the sideways movement of the eaves under wind and the vertical movement of the apex under the roof loads, at service values. These limits are practice, not code values: agree them with the client and the cladding supplier (crane runways and brittle walls need much stiffer frames).

In STATIX: the frame actions go into the steel member calculator one check section and one combination at a time, with the lengths that match the restraints, the frame set to unbraced, and the moments including sway. The interaction rules are those of Steel Design 1 lesson 4.