Accidents occur in the application process of the space frame, ranging from bending of the space frame rods to excessive deformation and even collapse of the space frame.
In conclusion, there are many causes of space frame accidents, which can be summarized in the following four reasons.
Once an accident has occurred on a space frame, the serious damage needs to be removed and rebuilt, while partial damage to the structure needs to be reinforced.
The general approach to space frame reinforcement is to reinforce the rods and nodes.
The reinforcement of the space frame rods and nodes takes the rod tension and compression forces into account and uses different reinforcement measures accordingly, which are mainly the following.
When the rod is used as a tension member, because the tension member meets the general stability requirements, it is enough to consider its strength requirements for the reinforcement calculation of the chord and web rod under tension in the center of the space frame. The reinforcement calculations can use basic central tensile member calculations to carry out the reinforced rod calculations. Therefore, the reinforcement of such members needs to adopt a method of symmetrically increasing the cross-sectional area of a single rod.
For rob symmetrical reinforcement, the length of the reinforcing member is equal to the length of the bar being reinforced.
Using round steel reinforcement, the width of the weld ≥ d/3 and the length of the weld ≥ 3d, the d is the diameter of the round steel.
Using equilateral angle reinforcement, the width of the weld ≥ (d + 1) mm, the length of the weld ≥ 2b, the d is the limb thickness , and the b is the limb width.
For welded reinforced members, the welding deformation produced during welding has an impact on the member’s load-bearing capacity, but this is generally not taken into account in the calculations, with certain construction processes used to minimize this impact. The general method is to select certain fixing points to fix the reinforcement and the reinforced member as a whole with spot welding, and then to complete the final welding of the reinforced member with a continuous or interrupted weld. When the rod is a compression rod, the rod must not only meet the strength check calculation, but also meet the force stability. Therefore, in the reinforcement design of the member, not only the cross-sectional area of the member should be calculated, but also the moment of inertia and the radius of gyration of the member should be calculated, and the slenderness ratio of the member should be checked.
2.Welded ball reinforcement
In the welded ball space frame reinforcement, if the insufficient load capacity of the welded ball is caused by the absence of stiffening ribs inside the welded ball, the design solution considers placing the stiffener from inside the ball on the outside of the ball and adding cross-shaped stiffener between the stringers to reinforce the ball nodes as follows.
Stiffener reinforcement scheme: when the ball and the rod are welded, a casing must be added; the stiffener thickness is greater than or equal to the ball wall thickness, the stiffener width must be between D/4 and D/3 of the welded ball diameter, and the stiffener must be connected to the ball and rod. The parts shall be grooved with full penetration welding, and the weld quality shall be grade two.
3.Bolt ball joint reinforcement
The bolt ball is made of 45# steel. Due to the limitation of the welding process, the reinforcement of the bolt ball node is generally not by direct welding, but the steel of equal strength and cross-section is used to connect with the two adjacent bars in the same line of the node. There are three ways to strengthen the connection:
(1) Round steel connection reinforcement
This reinforcement method is suitable for the nodes where the node members are in tension, as the use of round steel reinforcement is convenient and simple, and easy to operate on-site;
Adopt round steel reinforcement.
The use of round steel reinforcement, the use of round steel and the reinforced rods have the same strength and equal section, the cross-sectional area of the pad is not less than the cross-sectional area of the round steel, and 4 pieces are used, which are arranged symmetrically. The weld width is ≥ d/3, the weld length is ≥ 3d, and d is the round steel diameter.
During the reinforcement process, first select a certain fixing points to fix the reinforcement and the reinforced member as a whole with spot welding, and then to complete the final welding of the reinforced member with a continuous or interrupted weld.
(2) Hoop – steel plate connection
This reinforcement connection is a hinged connection, mainly relying on the friction between the hoop and the rod and the shear force of the limit plate to transfer the force, to achieve the purpose of simultaneously receiving the force with the space frame rod, so there is no welding stress during the installation process.
This reinforcement method can transmit the tension and pressure of the joints. When considering the pressure transmission, not only the strength of the joints, but also the stability of the joints should be considered. The hoop is made of a round tube one size larger than that of the reinforced rod. The inner diameter of the hoop is the same as the outer diameter of the rod, and the thickness is not less than the rod wall thickness.
Due to the characteristics of the steel binder, this reinforcement joint has the following special processes:
Repair the seam → measure → repair the gap → grind the rod to the original color of the steel → paint the steel binder → install one side of the steel hoop → install the other side of the steel hoop in the correct position → install the high-strength anchor bolt → tighten the force → complete
The operation points are as follows:
①Repair the gap between the bolt ball and the rod. Use electric welding to repair the gap, and then smooth it with a grinder. ② Use a tape measure to accurately measure both sides of the bolt ball to determine the installation position of the steel pipe hoop and mark it with a stone pen on the pole. ③Rust removal and roughness treatment is carried out at the location where the steel pipe hoop is to be installed. Use an abrasive wheel grinder to polish out the metallic lustre. The greater the roughness of the sanding, the better the sanding pattern should be perpendicular to the direction of the steel force, followed by wiping clean with cotton wool dipped in acetone. ④ Paint steel binder on the steel pipe hoop after grinding, paint evenly and not too thick; (the construction process of steel binder refers to the product manual). ⑤ Install the steel pipe hoop accurately until the steel plate hoop and the space frame pole are bonded and fixed. Checking method: after pasting, use hand hammer to gently knock the steel plate along the pasting surface, such as no hollow sound, indicating that the paste has been dense, otherwise it should be peeled off the steel plate, replenish the glue and paste again. ⑥Install the other side of the steel hoop in the same way as above. ⑦ Use a torque wrench to fasten the penetrating high-strength anchor bolts. Ensure that the screw is exposed by 2 to 3 wires. Use the hammer method to check whether the bolts are loose; this joint has been tested by physical objects and meets the design requirements.
This paper combines the quality problems in the construction and use phase of the space frame, analyzes the damage to the space frame, and proposes targeted reinforcement measures for different nodes with different stress members, which effectively improves the tensile and compressive buckling bearing capacity of the members. Not only it meet the design requirements, but also, the construction is simple, fast and effective, saving resources and reducing costs to the maximum extent.