The roof of a sports complex is composed of 10 joists of 15.1 meters high with a span of 149.7m×141.6m, which mainly introduces the construction steps of joist and carries out finite element calculation and analysis and on-site monitoring for various working conditions in the construction process to ensure safety, reliability and fine quality of roof steel structure construction.
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Project Overview
A stadium project is a large-span space prestressed cable truss structure with a sagittal height of 25.3 meters, a transverse length of 239.8 meters and a longitudinal length of 141.6 meters, with a total construction area of 72010 square meters, which is a Class A stadium with a capacity of 9,842 seats. The total construction area is 72010m2, which is a Grade A gymnasium with a capacity of 9842 seats. The structure includes the roof of the gymnasium, the roof of the exhibition hall and the enclosure of the gymnasium.
Introduction of the string truss
The gymnasium is composed of three main parts: ring truss, tensioned chord truss and radial truss. The roof prestressing steel structure consists of 10 rounds of tensioned truss, which is made up of the top chord rigid body, bottom chord and the brace between them. The top chord is an inverted triangle truss, while the bottom chord is made up of double cables with cold cast anchors and complicated cable end connection nodes. The 10-bay trombone joist has different span and the cable strength varies greatly under the same cable equivalent pretension (1700KN), which puts forward higher requirements on construction organization.
The BEAM188 beam unit was used for joist members and beam members, two end articulated rod units were used for the spar, Link10 unit simulated for the cable and Link10 unit simulated for the tire carrier were used for the tire carrier. At the same time, the tensile points, cable force monitoring points and strain gauges of the main stressed members were arranged during the whole cable tensioning process to consider the whole structure construction scheme.
Construction Technology of Tensioned Truss
- Selection of construction machinery
A stadium is divided into 4553 lifting units, of which 1443 lifting units for the stadium. By analyzing the lifting capacity of the representative steel structure lifting units and lifting machinery in this project, a 120-ton auto crane, a 100-ton crawler crane and two QTZ-63 tower cranes were finally selected as the main construction machinery.
- Truss Fabrication
The tire carrier is made of I-beam, which is composed of column, support, positioning block and movable bolt, and the adjustable support is set in the cattle leg part of the tire carrier. When assembling the tire carrier, empty space is set at the disconnecting surface to leave welding space and the welder can weld on the temporary platform when welding under the interface.
Each chord joist is numbered before fabrication, and the solid proofing is made for each section of joist with different models and sizes according to the design and deepening drawings. Before seating, the tire carrier is set up according to the elevation and axis of chord beam segmentation points to make the elevation basically reach the design position and fine adjustment is made after seating. When positioning, pay attention to the control of the gap and elevation of the interface, and make the position of the segmented points basically in the center of the tire carrier. After adjusting and correcting, welding is carried out, and the welding interface needs to be polished smoothly. In the process of fabrication, in order to ensure that the radian of the original state of the tensioned beam is in accordance with the design, warp and weft instrument and level were used to precisely determine the position of the truss assembly to meet the design requirements.
Six sets of tire carrier were set up because the lifting members were similar and the tire carrier could be recycled. Two sets of tire carrier are set up inside the site and one 120-ton auto crane is arranged for steel member docking; four sets of tire carrier are set up outside the site and one 100-ton crawler crane is arranged for steel member docking.
- Fabrication and installation of temporary support frame
Considering the large span of single joist, in order to facilitate construction and temporary load-bearing, temporary support frame is set at each joist segment point, each support frame is made of two round steel pipes with 350mm diameter and 8mm wall thickness, and is connected and fixed with angle steel of 50 x 5 in the middle. A ladder is set on the temporary support and an operating platform is set on the top.
Use 100 tons of crawler cranes to lift the temporary support frame to the design position, the foundation is tamped, the bottom pad 200mm thick steel plate, after lifting in place with four cable wind rope fixed. The elevation was guided to the temporary support by level to facilitate the subsequent installation of the joist.
- Lifting and Assembly of Open Chord Joist
One 120-ton auto crane and one 100-ton crawler crane were used to travel in the span, and steel road box was laid at the crane travel and lifting station. At the same time, the QTZ-63 tower crane on site was used for lifting.
To ensure that the joist will not be destabilized out of plane and to ensure the safety of joist lifting, the tensioned string joist is lifted in sections at four points. After the string joist is lifted into position, it is promptly corrected by using a hand chain hoist according to the measurement and positioning axis on the top of the supporting frame, and the interface part is temporarily fixed with the support and welded to the interface.
During lifting, removable movable lugs are set on the top chord of the joist. This not only reduces the workload and material consumption during joist fabrication, but also does not require cutting after lifting, thus reducing the work process and speeding up the construction progress, as well as avoiding the disadvantage of damaging the coating layer caused by the wire rope binding method of lifting.
After the first joist was fixed, the second joist was lifted, positioned and reinforced, and the bracing system between the joists was connected in time. After the two joists, intermediate bracing system and purlins are fully connected and installed, the next joist will be lifted into position after checking that there are no hidden safety hazards in every detail.
- Installation and Tensioning of Prestressing Cable
Steel cable using extruded double protective layer of high-strength steel wire twisted twisted strand, a single wire diameter of 7mm tensile strength of not less than 1670MPa, yield strength of not less than 1410MPa, cable tensile modulus of elasticity (E) should be not less than 1.9 × 105MPa, cable installation and tension required for the operation of the platform using a steel welded to facilitate the construction of cable, while ensuring the construction of safety.
During construction, use crane or manual hoist to lift the ground numbered braces one by one and connect the nodes on the braces to the steel structure. Use multiple hoists to lift the cable and connect it to the cable clamp at the lower end of the span. According to the markings on the surface of the cable, determine the position where the cable clip is connected to the cable, and install the clip.
According to the actual situation of construction organization plan and structure form, it is difficult to tension these cables at the same time, and they need to be tensioned in batches. Considering the symmetry of the structure, the cables are divided into 5 batches, and the number of roots in the same batch is 2, and the order of tensioning in batches is symmetrical from both ends to the middle. In order to ensure the synchronization of tensioning, the same batch is divided into five tensioning procedures: 0% → 25% → 50% → 75% → 90% → 100%.
Due to the insurmountable errors and other factors in the processing and installation of the steel structure, the cable tensioning control adopts the dual control principle of controlling the cable force and deformation, in the graded single tensioning process, the first four stages mainly control the deformation and the last stage mainly controls the cable force.
- Tensioning process monitoring
The cable pre-tensioning construction process is a dynamic process of structural state change. The actual structural state differs from the analytical model due to steel erection errors, cable fabrication, installation and tensioning errors, analytical errors and environmental influences. Therefore, it is necessary to monitor the cable prestressing construction process and compare the difference between the theoretical analysis value and the actual structural response, so as to grasp the structural state of each key construction stage, ensure the whole process of cable construction in a controlled state, and ensure the structural safety of the construction process.
From the above analysis, it can be seen that the maximum vertical deformation of the structure and the equipotential force of the steel structure change continuously during the construction process, and the maximum vertical deformation of the structure is 63.9mm, which is similar to the deformation value of 64.3mm under the designed quasi-state; the maximum axial stress is 133.8MPa, which is very little different from the maximum stress value of 123MPa under the designed quasi-state, and meets the requirements.
Conclusion
The gymnasium tensor truss structure of the stadium project has a large span, high requirements for construction precision and great difficulty in construction. The whole construction process was simulated and calculated to make a scientific and feasible construction plan, and through strict process measures and technical management methods, good results were obtained and valuable construction experience was accumulated, which has an important reference value for the design and construction of related projects in the future.
