Steel Buildings in Europe

Part 4: Detailed Design of Portal Frames 4 - 54 1 2 (a) 1 2 2 (b) 1 Apex 2 Gable under suction 1 Apex 2 Gable under suction Figure 10.2 Wind loads on gables The internal pressure or suction contributes to the net loads on the gable. When the net loads are equivalent to an external pressure, the outside flanges of the gable columns are in compression, but are restrained out-of-plane by the side rails. When the net loads are equivalent to an external suction, the inside flanges of the gable columns are in compression. This design case may be the most onerous of the two conditions. It may be possible to reduce the length of the unrestrained inside flange of the gable columns by introducing column stays from the side rails, as illustrated in Figure 6.3. 10.3 Gable rafters If the gable is of the form shown in Figure 10.1, the gable rafters are generally simply supported I section members. In addition to carrying the vertical loads, the gable rafters often act as chord members in the roof bracing system and this design case must be verified. If a portal frame is adopted as a gable frame, it is common to adopt an identical frame size, even though the vertical loads on the end frame are rather less. Generally, the reduced vertical loading will mean that the rafter can accommodate the axial force as part of the roof bracing system without needing to increase the section size.