Steel Structure Support System: How Much Do You Know?

[Steel Structure•Technology] 

The support system is a minor part of the steel structure system, but it is also an essential part. In the portal steel structure, the system has important functions, which are mainly reflected in:

It ensures the stability of the overall structure and every components, transmits horizontal effects to the foundation and auxiliary installation works, etc. For structures with complex layout, the support system is also conducive to the adjustment of structural stiffness, making the stress on the structure uniform and reasonable, and improving its integrity. 

1. Support system setting principles

In portal frame steel structures, the support system can be divided into roof horizontal supports, column supports and other auxiliary support systems.

The basic principles supporting system setup are:

① Transfer longitudinal load clearly, reasonably and simply, and shorten the force transmission path as much as possible;

② Ensure the out-of-plane stability of the structural system and provide lateral support points for the overall stability of the structure and components;

③ Convenient structure installation;

④ Meet the necessary strength and stiffness requirements and have reliable connections.

In portal frame light steel structure, there are still some regulations and requirements for different types of supports.

• Roof horizontal support

The roof horizontal support and the inter-column support are a whole, which jointly maintain the stability of the structure and transmit the longitudinal horizontal load to the foundation through the roof horizontal support and the inter-column support.

In order to ensure that the transmission path of the longitudinal load on the structural gable is short and fast, the horizontal horizontal supports of the roof should be set in the openings at both ends of the temperature section of the building to directly transmit the gable load.

If it cannot be set up in the first bay, it can be set up in the second bay, but it must be noted that the corresponding rods that transmit horizontal loads in the first bay should be designed as pressure rods. When the building or temperature expansion section is long, one or more horizontal supports should be added, and the spacing should not exceed 60m.

When the structure is simple, symmetrical and the heights of each span are consistent, the roof horizontal support is relatively simple, that is, it can only be provided between the end openings as long as the length of the temperature zone is met.

In a building, when the columns have different column sapce, or when the building has different spans, longitudinal horizontal supports should be set up to improve the integrity of the structure, and the distribution of the lateral stiffness of the structure should be adjusted to reduce the size of each rigid frame. The difference in lateral horizontal displacement of the column makes the structure stress uniform and reasonable.

When the building layout is irregular, such as local protrusions and concavities, pulled out columns, etc., in order to improve the overall lateral force resistance of the structure, a longitudinally and transversely closed continuous horizontal support system must be installed in the above areas.

When setting up the support system, it is necessary to set pressure bars at the roof ridge and column tops and pay attention to the structure of the nodes to ensure the transmission of horizontal forces.

• Wall support between columns

Column supports are generally set in the middle of the columns. For this reason, the horizontal ties at the top of the columns need to be designed as rigid ties to transfer the load borne by the roof horizontal supports to the inter-column supports. If the building is long, an inter-column support can be added.

The supports between the two columns are placed at 1/3 of the longitudinal direction. When the column supports cannot be set at the ideal position required by the structural design due to building usage requirements, they can also be set away from the middle of the column lines.

The supports between columns can be designed in a cross shape, a figure-eight shape, a door shape, or even a rigid frame.

It is best to use one type of wall column support in the same building, and it is not advisable to mix several types of supports. If there are functional requirements such as opening doors, windows or other factors, rigid frame support or truss support can be used.

When support systems must be mixed, their stiffnesses should be as consistent as possible. If the stiffness requirements cannot be met, the longitudinal horizontal force borne by each support should be analyzed specifically to ensure the stability and safety of the structure. At the same time, attention should be paid to the support settings. symmetry.

• Knee bracing

Knee braces are unique to solid-web portal-type rigid frame lightweight steel structure building. The knee brace is set between the lower flange of the rigid frame inclined beam and the purlin or between the inner flange of the rigid frame side column and the wall beam. It supports the stability of rigid frame inclined beams and rigid frame side columns.

The knee brace is not an independent system. The function of the rigid frame diagonal beam corner brace is to prevent the diagonal beam from lateral instability when the lower flange is compressed.

2. Structure of support system

• Roof horizontal support

The horizontal support of the roof is generally composed of bracing and rigid ties.

In a portal-type rigid frame light steel structure building, the bracing for the horizontal support of the roof can be designed as round steel, but a tensioning device should be added to facilitate the tensioning of the tie rods and avoid excessive deflection of the round steel, which will not be able to perform the load-bearing function force effect.

The cross bracing can also be designed as angle steel, but the deflection caused by the long support due to its own weight must also be considered, and necessary measures should be taken to overcome it. The angle between the cross pole and the vertical pole should be within the range of 30°~60°.

Angle steel is used to make horizontal supports for supporting tie rods. The connection points are similar to ordinary steel structures. The key is to ensure that the rods must intersect at the center of the nodes. The nodes of tie rods made of tensioned round steel can be connected with single bolts.

Rigid ties can be made of steel pipes or double angle steels.

• Support between columns

When the span and height of the building are low, round steel with tensioning devices can be used to make cross-shaped tie rods for inter-column support. Angle steel or channel steel can also be used.

In addition to angle steel, steel pipes can also be used for the cross bars supported between columns in tall buildings. Steel pipes save materials, are simple to make, and appear solid and beautiful in construction.

The connecting nodes of inter-column supports are similar to horizontal supports. It must be noted that the upper end of the support between columns and the horizontal pressure rod must be at one point with the center of the column. Similarly, the lower end of the support between columns should intersect with the center of the column on the bottom surface of the column foot as much as possible to avoid eccentric force.

If two adjacent columns are supported between columns and need to bear the horizontal reaction force together, a pressure rod should be added between the two. The pressure rod can be made of steel components. If the column base is higher than the ground, a reinforced concrete beam can be added below the ground.