The Difference Between Various Joints in Steel Structure

Sometimes for a better appearance or specific requirements, there will be concrete outside the steel structure buildings, and some even use concrete construction as bottom half of the building. This kind of buildings are often deformed under the action of external factors, leading to cracking or even destruction. Deformation joints are structural joints reserved for this situation. Deformation joints can be divided into expansion joints, settlement joints, and earthquake-proof joints.

Expansion Joints

Due to changes in factors such as temperature and humidity, building components will undergo expansion and contraction deformation. For this reason, vertical joints are usually installed in appropriate positions of the steel structure building, and the walls, floor, roof and other components of the construction are disconnected from the foundation, and the building is separated into several independent parts. The purpose of seam’s setting is to overcome the excessive temperature stress , and the foundation can not be broken.

Since the temperature change and concrete shrinkage can be effectively controlled in the underground part of the concrete structure, when the expansion joint is set, the double-column foundation of the frame and bent structure can not be disconnected. The setting of settlement joints for uneven settlement structures is not included, and the design can be controlled according to the specific situation.

Settlement Joints

Between the various parts of the upper structure, due to the large difference in the number of layers, or the large difference in the load, or the large difference in the compressibility of the foundation, in a word, when uneven settlement of the foundation may occur, it is necessary to set joints to divide the structure into several parts. So that the settlement of each part is more uniform, to avoid additional stress in the structure. This joint is called a "settlement joint" and must be completely separated from the foundation to the upper structure.

When setting settlement joints for the foundation of a high-rise building and the foundation of the podium connected to it, consideration should be given to the reliable lateral restraint and effective buried depth of the main high-rise building foundation. If there are no settlement joints provided, effective measures should be taken to reduce differential settlement and its impact.

Earthquake-proof Joints

The earthquake-proof joints are set up to divide the large building into smaller parts to form a relatively independent earthquake-proof unit to avoid damage caused by the uncoordinated overall vibration of the building due to the earthquake, and the foundation can not be disconnected. The installation of earthquake-proof joints is not always recommended for complex buildings. Due to the pros and cons of whether to install earthquake-proof joints, there have always been different opinions. The general tendency is:

1. When joints can be set or not, there is no joint. The installation of seismic joints can make the structural seismic analysis model simpler, easy to estimate its seismic effect and take seismic measures, but the torsional seismic effect needs to be considered, and the width of the joint should be determined according to the provisions of each chapter of this code.
2. When the earthquake-proof joints are not installed, the structural analysis model is complicated, the local stress concentration at the connection needs to be strengthened, and the possible adverse effects such as the earthquake torsion effect should be carefully estimated. During seismic design, the widths of settlement joints and expansion joints must meet the requirements for the width of earthquake-proof joints.

Key point: Distinguish "deformation joint" and "construction joint"

Due to the needs of the construction team or the restriction of the construction process, the joints are left between the construction units. The construction joint is not a real "seam", but a surface. It is only because the post-pouring concrete exceeds the initial setting time, and there is a joint surface with the first-poured concrete, which is called a construction joint.

The joint surface formed by successive concrete pouring is prone to various hidden dangers and quality problems. Therefore, different structural projects need to be cautious in the treatment of construction joints. The usual method of construction joints is: the construction joints of beam and slab are left at the 1/3 span of the small span, and the column construction joints are left 50mm below the floor beams. The biggest reason why the column construction joints stay here is that it is convenient for construction.

Construction Joint of Stairs

In addition to the expansion joints, settlement joints, and earthquake-proof joints mentioned above, construction joints also include structural joints, split joints to prevent continuous collapse, and so on. Different types of construction joints are to eliminate the influence of the following unfavorable factors: concrete shrinkage, expansion and contraction deformation caused by temperature changes; uneven settlement of the foundation; sudden changes in stiffness and quality and local stress concentration. In addition to permanent structural joints, temporary joints such as construction joints, post-pouring belts, and control joints should also be considered to eliminate some temporary adverse effects.

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