The longitudinal structural rigidity of portal steel frame structure is relatively weak in the length direction, so it is necessary to set up bracings along the longitudinal direction to ensure its longitudinal rigidity and longitudinal stability. The main function of the bracing system is to transfer the wind, crane load and earthquake load imposed on the longitudinal structure from its point of action to the main load-bearing structure, and finally to the foundation. At the same time, the horizontal frame is connected into a geometrically invariable system to ensure structure could bear the load. This article will introduce the bracing of steel structure from Three aspects: bracing arrangement, bracing form, bracing rod design.
Upper horizontal bracing should be set on the top surface of beam, and inter-column bracings shall be set between the columns. These bracings should be set in the same room. Because the resultant force of rind bracing force to roof beam is finally balanced by the bracing system, then support plane is required to be as close as possible to the plane where the rust is located, which could avoid eccentricity of the entire roof longitudinal force transmission system. For the crossed scissor brace, if the rod is selected from the round steel, it can be achieved by drilling the web near the upper flange, or welding the connecting plate on the upper flange directly as the connection point. If angle steel is used, the connecting plate can still be welded to the upper flange.
The bracing system of light portal steel frame structure includes roof horizontal bracing and inter-column bracing. According to the working mechanism, it could be divided into flexible bracing and rigid bracing. According to the structure, the inter-column bracing is divided into oblique cross bracing and rigid-connected column bracings.Flexible bracing components are galvanized steel wire ropes, round steel, strip steel or angle steel. Due to the relatively large length of the components, they can hardly be compressed. Under the action of a longitudinal load in one direction, the component will be out of work when the other one is pulled. But a square tube or a round tube can withstand tension and pressure as rigid bracing part. The structure of cross brace contains the advantages of simple, direct force transmission, less materials, and greater rigidity. It is a standard bracing system for roofs, side walls and gables in light steel structure buildings. Due to the requirements of building function and appearance, cross bracings cannot be set in some bays, and portal bracings can be set at this time. This type of bracing can be longitudinally fixed in the space between two side pillars or between the inner pillars of a multi-span structure. Column support with column foot fixed to the foundation around the weak axis is mostly used to shielding building structure. For example, the most used is steel carport structure, which requires the wall to be completely open for cars to enter and exit. These buildings are usually very long, with low eaves and many bays.
Bracing Rod Design
In the design of the rigid frame bracing, there are two node structures that need to be tested, the first one is the connection method between the top and the frame column, the second one is the treatment of bracing foundation. The top connection usually adopts a method of extending a section of beam from the web of frame column, welding the connecting plate at the end of the beam, and connecting with the flange of bracing column by bolts. This method is simple in design and convenient to install, and has achieved good results in engineering practice.
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