Existing large-span roof space frame structure reinforcement and renovation (part 3)

4、Partial rods of space frame stability does not meet the reinforcement method

Existing space frame structure due to changes in load, some local rods will be in the stability test stress exceeds the allowable stress, some small tension ties into compression rods and bending deformation. For the case of these local rod stability does not meet the requirements, when the use of welded hollow ball frame should be considered when wrapped steel pipe to improve its overall stability. For the bolt ball frame, wrapped steel pipe can also be used to improve the overall stability; and for small specifications of the rod, because it belongs to the structure of the secondary rod, can also be considered in the structural test under the premise of safety to replace the rod method to be solved.

For some rods with longer lengths of compression rods, for the stability does not meet requirements, the most effective method of reinforcement is to add support structure rod in the mid-span of these rods, can effectively reduce the calculated length of the rod; The compression rod casing restraint reinforcement method can also be used to achieve the purpose of reinforcing the stability of the rod.

5、Local reinforcement method for high stress in local bars of the space frame

Existing space frame structure if there is a local rod stress over the design strength, for safety reasons, in principle, should be reinforced by increasing the cross-section of the rod, reinforcement design should review the stress level of the rod in the standard value of constant load, and the stress level as a starting point to verify the stress increment of the original rod cross-section and stress design value after the reinforcement of the increased section in the main load combination conditions, so that the stress design value of the original rod to meet the specification requirements.

For hollow welded ball space frame, the reinforcement method of steel pipe jacket should be considered, because the original steel pipe has higher stress, in principle, the casing is not welded with the original steel pipe, the casing should be reliably welded with the hollow ball node.

And bolt ball node space frame, because the bolt ball material is 45# steel, its weldability is poor, the main problem for 45# steel welding is welding crack, with a certain sensitivity to cold cracking. Preheating to 200°C to 250°C before welding of this steel can effectively prevent cold cracking, and after welding, tempering and holding at 600°C to 650°C should be carried out to ensure the toughness of the welded nodes. The requirement of preheating before welding and tempering after welding is difficult to implement in the site of reinforcement of the space structure. For the tie bar to consider increasing the cross-section to the original rod with a cross-node slat reinforcement, but the slats to be welded with the original rod in large quantities, when the original rod stress level is high due to welding high temperature and produce adverse effects on the original rod force. Therefore, for the bolted ball node frame using increased rod cross-sectional reinforcement must be cautious.When the high-strength bolts and cones of the tie rod can meet the requirements of tensile bearing capacity, while the tensile bearing capacity of the steel pipe slightly does not meet, you can consider using the paste carbon fiber reinforced composite material (CFRP) for local rod reinforcement, the method for the rod along the ring winding paste a number of layers of CFRP cloth, in order to achieve the purpose of improving the tensile bearing capacity of the steel pipe. CFRP cloth has strong tensile strength, corrosion resistance, light material, and easy and fast on-site construction.

6、reinforcement of the lower support structure

In addition to reinforcing the upper roof space frame structure in the large span space frame structure, the lower support structure should also be tested, evaluated and reinforced for safety performance.

If the roof is changed from heavy roofing to light roofing, the bearing capacity and seismic testing of the lower support structure can generally be met, but for areas with seismic intensity of 7 degrees and above, the seismic level of the lower concrete frame support structure is 3 levels and above, the reinforcement rate of the frame beams and columns, hoop encryption zone and other structural requirements will have a greater increase than the original design, the reinforcement of the lower structure should be considered this impact.

If the self-weight of the roof is unchanged or slightly increased, the bearing capacity and seismic performance of the lower structure should be verified in detail, and when verified according to the current code, the load effect will be increased, and the cross-sectional bearing capacity of the structure and members will be increased as required, and the reinforcement construction requirements of the members should be increased as required. When the member cross-section and cross-section longitudinal reinforcement difference is more should consider increasing the cross-section reinforcement or external angle steel reinforcement. When the bearing capacity of the lower structure under constant load is basically satisfied, and the seismic test is not enough, the lower supporting structure can be considered to add another inter-column support (steel support or anti-buckling restraint support), local additional seismic shear wall, etc., in order to improve the seismic performance of the lower supporting structure.