Local reinforcement is the reinforcement of a rod or connection node with insufficient load-bearing capacity. There are methods of increasing the section of the member, reducing the free length of the member, and the method of reinforcing the connection node. In this issue, we mainly introduce the more commonly used connection node reinforcement methods.

The connection problem in reinforcement generally has two situations: the reinforcement of the original connection due to insufficient bearing capacity (that is, the reinforcement of the connection, including the reinforcement of the node), and the connection between the reinforcement and the original member.

The reinforcement of the connection and the connection method of the reinforcement should be determined according to the reason, purpose, stress state, structure and construction conditions of the reinforcement, and considering the connection method of the original structure. The methods of riveting, welding, high-strength bolt connection, and mixed connection of welding and high-strength bolts can be used. Riveted connection has the least rigidity (except for ordinary bolt connection). The welding connection has high rigidity and good integrity, and the high-strength bolt connection is somewhere in between. The selection of the reinforcement connection method must meet the requirements of not damaging the original structure function, but also participating in the work. At present, riveting has been gradually eliminated due to complicated construction, and welding is often preferred because it does not require drilling and other processes. However, welding has the highest requirements on the material properties of the steel. In the case of incomplete original structure data and unknown material properties, the welding reinforcement must be retested to ensure the weldability.

Reinforcement principle

(1) Steel structure reinforcement connection method, that is, the choice of welding seam, rivet, ordinary bolt and high-strength bolt connection method. It should be determined according to the reason, purpose, stress state, structure and construction conditions of the structure that need to be reinforced, and considering the original connection method of the structure.

(2) In the reinforcement of the connection of the same force-bearing part, it is not advisable to use those with a large difference in rigidity. For example, a mixed connection method in which welds and rivets or ordinary bolts are subjected to common force, except when only the connections with greater rigidity (such as welds) are subjected to the full force. If there is a basis, a mixed connection in which welds and friction-type high-strength bolts are subjected to common force can be used.

(3) The material used for the reinforcement connection should match the properties of the structural steel and the original connection material, and its technical indicators and strength design values should meet the requirements of the "Code for Design of Steel Structures".

(4) Reinforcement of connections under load, especially when end welds or bolts are used to reinforce the original connection, and when expanding or adding nail holes, reasonable construction techniques and safety measures must be taken, and calculations must be made to ensure the structure (Including the connection) has sufficient bearing capacity under the reinforced load.

Reinforced connection

The plates (reinforcement) added to strengthen the structure must not only have sufficient design bearing capacity and rigidity, but also have a reliable connection with the reinforced structure to ensure that the two work together well.

The connection between the reinforcement and the reinforced structure shall be determined according to the design force requirements by calculation and considering the structure and construction conditions. For axial force-bearing members, it can be calculated according to the following formula. For bending members, the calculation should be based on the maximum possible design shear force; for compression bending members, the calculation can be based on the larger value of the above two.

When only adding intermediate support members (points) to reduce the free length of the compression member for reinforcement, the connection force between the support rods (points) and the reinforcement can be calculated by the following formula, where At is the cross-sectional area of the original member. The calculation of the welds, bolts, rivets and other connections of the reinforcement can be carried out in accordance with the provisions of the "Code for Design of Steel Structures". However, when calculating, the design value of the strength of the diagonal weld should be multiplied by 0.85, and the design value of other strength or bearing capacity should be multiplied by a reduction factor of 0.95. For example, if single angle steel is connected on one side, the design value of fillet weld strength is multiplied by a factor of 0.85x0.85=0.72.