Steel Buildings in Europe

Part 4: Detailed Design of Portal Frames 4 - 27 1 Stay Figure 6.3 Example of combined lateral and torsional restraint Purlins attached to the top flange of the rafter and side rails attached to the outer flange of the column provide stability to the rafter in a number of ways:  Direct lateral restraint, when the outer flange is in compression.  Intermediate lateral restraint to the tension flange between torsional restraints, when the outer flange is in tension.  Torsional and lateral restraint to the rafter when the purlin is attached to the tension flange and used in conjunction with rafter stays to the compression flange. In all cases, the purlins and side rails should be tied back into a system of bracing in the plane of the rafters (see Section 9). Generally, the assumption that the forces are carried back to the bracing system via the roof diaphragm is accepted in many countries, even without supporting calculations. In other countries calculations are necessary, or the purlins can only be assumed to provide restraint if they are aligned directly with the bracing system. The position of the purlins and side rails will be a balance between the capacity of the purlins themselves, and the necessary spacing required to restrain the primary steel members. The maximum spacing will usually be determined from manufacturers’ load tables. Spacing may have to be reduced to provide restraint to the inside flange at strategic points along the rafter or column, so it would be common to provide purlins at reduced spacing in zones of high bending moment, such as around the eaves haunch. Normal practice is to locate one purlin at the ‘sharp’ end of the haunch, and one near the apex. The intervening length is split at regular spacing – typically about 1,6 to 1,8 m. A purlin is often located near the end plate of the rafter, and depending on the length of the haunch, one, two or more purlins in the length to the ‘sharp’ end of the haunch, usually at lesser spacing than the main length of rafter. Additional purlins may be required to carry drifted snow – these may also be used to provide restraint. Side rails are usually located at positions to suit the cladding, doors and windows. The inside of the flange at the underside of the haunch always requires restraint – it is common to position a side rail at this level. Purlins and side rails must be continuous in order to offer adequate restraint, as shown in Figure 6.3. A side rail that is not continuous (for example, interrupted by industrial doors) cannot be relied upon to provide adequate restraint.