Existing large-span roof space frame structure reinforcement and renovation (part 2)

2、Additional tension chord reinforcement method for three-dimensional truss

For the reinforcement of three-dimensional truss, the additional tension chord reinforcement can be adopted. The upper and lower chords of three-dimensional truss are generally continuous rods with less variation of pipe diameter and wall thickness, which can withstand larger axial force, and it is convenient to install nodal plates on the fittings with clamps, which is more suitable for adding tension chords to form a three-dimensional truss structure. Adding tension chords to the lower chord will reduce part of the interior headroom, which is an ideal reinforcement solution for the additional tension chord reinforcement of three-dimensional truss as long as the interior headroom can be satisfied.

The additional tensioned chord under the three-dimensional truss can effectively balance the self-weight of the roof structure and roof by prestressing the tensioned chord, and change the original bending of the three-dimensional truss(upper chord under compression and lower chord under tension) to the ideal stressing condition of the three-dimensional truss where the upper and lower chords are under compression and the lower string is under tension. The deflection of the structure can be effectively regulated and controlled by the prestressing effect while effectively reducing the internal force of the truss members.

After the modification, the tension of the lower chord cable will be balanced by the chords of the truss, and the structure is a self-balancing system, which will not cause additional force to the lower supporting structure.

3、Space frame structure tension chord bracket reinforcement method can also be used in the lower chord to add another prestressing tie or additional tension cable in the lower chord., these prestressing tie or lower chord tension chord anchored in the end of the space frame, through the prestressing effect, improve the stress performance of the space frame, to achieve the effect of structural reinforcement. But in fact, the space frame structure is designed according to full stress, the cross-section of chord rod is large in span and small near the support, once the prestressing tie or lower chord tensioning cable is anchored at the support end of the space frame, it will cause great pressure on the near support chord of the space frame, generally these small size support chord can’t bear this great pressure.

Therefore, when the structural load-bearing capacity of the space structure is not satisfied or the structural deflection is not satisfied, the reinforcement method of adding new prestressing ties or additional tension chords cannot be simply used, but the reinforcement method of adding new tension chords to support the structure should be used.

The basic requirement of the reinforcement method of the space frame additional tensioned chord bracket structure is to add a new independent bracket system on the space frame, this independent additional tensioned chord bracket system under the action of prestressing, only produce upward bracket force on the space frame structure in vertical direction, balance part of the roof load, reduce the force effect of the space frame structure, reduce the internal force of the space frame rod, and effectively reduce the deflection of the original space frame structure.

The additional tensioned chord bracket structure, a through-length compression bar is set in the upper chord of the space frame along the span direction of the space frame, and the pre-stressed tensioned chord cables supporting the nodes of the lower chord of the space frame are arranged in the structural height of the space frame. In order to ensure the overall stability of the through-length compression bar of the upper chord of the support structure, this compression bar is set on the small column of the space frame, and the stability of the through-length compression bar is ensured by the role of the small column of the space frame. The lower chord tension cables are arranged in the form of folding lines, and the nodes of the space frame are connected with joists to form the support for the lower chord nodes of the space frame. By tensioning the pre-stress of the chord cables, the reinforcement of the space structure is completed.

A key project in Beijing adopts the positive release four-angle tapered bolt ball node space frame, the plane size is 54.5×31.1m, the space frame is supported on three sides, the 31.1m span direction is short to open, the roof is light weight compression steel plate composite plate roofing. However, due to the design, production and installation, the deflection of the space frame exceeded the 1/250 limit requirement stipulated in the regulations after the installation of the space frame and the roofing was laid, and it could not pass the normal engineering acceptance. The general contractor commissioned the China Academy of Building Research to conduct testing, analysis and treatment. After the actual measurement of the cross-section of the space frame and the calculation and analysis of the space frame, it was found that most of the stresses of the upper and lower chords were close to 215MPa, and the individual stresses exceeded the design allowable value, and the space frame did not meet the safety requirements and the use performance requirements without reinforcement and treatment. Due to the special characteristics of the project, the space frame could not be dismantled and rebuilt, and finally adopted the reinforcement plan of tension chord bracing. A total of 5 bays of 31.1m span tensioned chord bracing system were arranged along the length of the space frame, and the upper chord of each bracket system is a through-length HW250×250×9×14 hot-rolled H-beam, which is erected on the shortened small column, and the lower chord is four 7φ5 high-strength steel strands of 15.2mm diameter, which are connected to the lower chord node of the space frame through the steel beam. The tension of the lower chord is directly balanced by the H-beam compression bar of the upper chord, which constitutes an effective tension chord bracket system. The prestressing tensioning of the chord cables effectively achieves partial unloading of the original space frame structure, ensuring the safety of the space frame structure and making its structural deflection meet the design requirements.