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With equal strength, use cold-rolled steel girts or purlins allow to reduce the amount of steel in the structure of a building.
Due to its low weight, the cold profile offers both:
Purlins are structural elements on which the roof of a building is laid. Their main role is to support the cover and transmit the efforts to the frames, but also:
The girts are at the walls what the purlins are at the roof of a building. The girts are structural elements on which are fixed the cladding and joineries of a building but can also act as anti-buckling brace component of the carrier structure.
They can be placed in front of the columns or above the rafters to realize continuity joints.
They can also be placed between the structural elements to limit their overflow.
The purlins located at the ridge at the rooftop are called «ridge purlins», purlins located near the gutter at the bottom of the cover are called «eave purlins».
The distance between purlins varies according to the type of cover (from about 1.38m for fiber cement to 4m for sandwich panels).
The span of discontinuous purlins can reach 10m, that of continuous purlins (sleeved) can exceed 15m.
To know which section and how many purlins are needed to build a building, it is necessary to perform a calculation.
Here is the method:
2/ The loads and the dimensions of the constructive assembly (the purlins, sleeves ...) are transmitted to the mechanical solver which determines the forces and deformations in each point of the pieces.
3/ The software «Lisa.blue» takes into account the lateral and rotational restraint of thin sections purlins by the cover. When the cover and the purlin comply with certain conditions of geometric proportion and thickness, Eurocode 3 allows to take into account a restraint of the purlin in lateral bending and to lateral instabilities (buckling, torsional buckling of the maintained sole). This is an asset that can reduce the number of purlins or limit the section. Thus it is important to choose the right cover because it can influence the price of the purlins.
4/ The software will determine if, at each point, the resistance of the purlin («Ultimate Limit States»), and its rigidity («Service Limit States») are sufficient in relation to the criteria given in the EN 1993-1-3 («Eurocode 3 - Design of steel structures - Part 1-3: General rules - Supplementary rules for cold-formed members and sheeting»).
For the Ultimate Limit States, the software will control various points indicated in the Eurocode EN 1993-1-3 standard, particularly:
For the Service Limit States, the software will control various points indicated in the standard Eurocode EN 1993-1-3 §7 by integrating the loss of stiffness due to local buckling and the distortion of the section (efficient section), in particular :
The limit criteria depend on the type of use (roofing inaccessible / accessible) or the presence of solar panels, ceilings, etc., for which the deformation criteria of their supports can depend on additional documents.
They can be placed in continuity with sleeves. Their web provided with profiled stiffeners allows section profiling of important heights. To make frames, the profiles are usually assembled back to back by bolting, this allows to refocus the gravity center and torsion center of the section thus composed.
They can be placed in continuity by interlocking by extending their extremities. The Z section is generally less prone to twisting because the shear center is at the web level, except in the case of a strong asymmetry of the flanges.
More often used for purlins of moderate height, to constitute the frame of house or the assembly of steel trusses.
The profile Omega is mostly used as secondary frame but can also be used as a purlin or a girt.
When the purlins are inclined in the plane of the roof, they bend perpendicularly to the web according to their low inertia. It is possible to reduce this phenomenon by arranging anti-sag bars (round / rectangular tubes, cables, angles or flat irons) perpendicularly to the purlins. These anti-sag bars act as intermediary lateral supports and transmit the forces in the slope direction (traction or compression) to the braces.
The tension wires (cables, angles or flat irons) are placed in association with the anti-sag bars. They transmit the efforts of the liernes towards the frames and are strained because their slenderness does not allow compression work. The tension wires are usually set up at the top of the slope so that the anti-sag bars are mostly strained. When tension wires are placed at the bottom of the slope, the anti-sag bars are compressed and must therefore be checked in buckling (the cables can not then be used as antisag bars).
The sleeves act as a continuity of purlins that reduces the effects in spans. They are sheet thicker than the purlins because the moment acting on support is then maximum. The holes made for their bolting include an assembly space, it must be taken into account in the calculations by a decrease in the moment on support and an increase in the moment and the deflection on span compared to an ideally continuous beam. Zed purlins do not need sleeves to ensure their continuity, they are nested inside each other (one of the two flanges is smaller than the other, so as to reverse it to each span). The bolts must be checked in simple shear and more particularly in diametric pressure on the side of the purlin which is thinner.
To manufacture cold-rolled sections, high yield strength galvanized steel strip coils are required (for example: S350GD + Z275) and a roll forming line composed of: