Zhuojin Pipe Fitting Co., LtdFull-Process Experimental Investigation of Multi-Chamber Polygonal Steel Tube Mega-Columns
Literature Overview
Published in Construction Technology (Chinese and English) in 2021 by Weng Bangzheng and colleagues from China Construction Second Engineering Bureau and Beihang University, this paper presents a comprehensive full-process experimental study of multi-chamber polygonal steel tube mega-columns used in the Chunzhi Eye Commercial Center project. The research, funded by the China State Construction Engineering Corporation Science and Technology Research Program, focuses on the critical interface behavior between the steel tube and high-strength self-consolidating concrete (SCC) in a complex composite column system. The study addresses two major concerns: the lateral pressure exerted on the steel tube wall during concrete placement and curing, and the shrinkage strain of the C65 high-strength concrete during the early curing period.
Structural Configuration and Material Properties
The mega-column system under investigation features a multi-chamber polygonal steel tube configuration, which is a departure from conventional circular or rectangular steel tube columns. The polygonal geometry provides enhanced structural efficiency by distributing the cross-sectional area more uniformly, while the multi-chamber design allows for staged concrete placement and improved structural redundancy.
The following table summarizes the key material and geometric parameters:
| Parameter | Specification | Remarks |
|---|---|---|
| Concrete grade | C65 self-consolidating concrete | High early-age shrinkage |
| Steel tube grade | Q345 or Q390 | Typical structural steel |
| Concrete flowability | 260-300 mm (slump flow) | SCC requirement |
| Cementitious material content | 450-550 kg/m³ | High content increases hydration heat |
| Column height | 30-80 m (project-specific) | Mega-column classification |
| Number of chambers | 3-6 | Depends on cross-section size |
| Wall thickness | 20-40 mm | Structural requirement |
Experimental Program and Key Findings
The experimental program was designed to capture the full lifecycle of the mega-column construction, from concrete placement through to the end of the curing period. The instrumentation included strain gauges on the steel tube inner wall to measure lateral pressure, temperature sensors to monitor hydration heat, and displacement transducers to track concrete shrinkage.
The key findings can be summarized as follows:
Lateral Pressure on Steel Tube Wall
During concrete placement, the lateral pressure on the steel tube wall increases with the height of the concrete column, reaching a maximum value at the base of the column. The measured lateral pressure was found to be consistent with theoretical predictions based on the fluid pressure model, with values ranging from 15 to 35 kPa depending on the concrete density and column height. During the curing period, the lateral pressure gradually decreased as the concrete hardened and the hydrostatic component diminished.
Concrete Shrinkage Strain
The C65 SCC exhibited significant early-age autogenous shrinkage, with a measured shrinkage strain of 200-350 microstrain within the first 7 days. This is substantially higher than the shrinkage of conventional concrete grades, primarily due to the high cementitious material content and the dense packing of the SCC mix. The shrinkage strain continued to develop at a decreasing rate over the subsequent 28 days, reaching a total of 400-550 microstrain at 28 days.
| Time After Placement | Shrinkage Strain (microstrain) | Cumulative Shrinkage |
|---|---|---|
| 1 day | 80-120 | 80-120 |
| 3 days | 50-80 | 180-250 |
| 7 days | 30-50 | 250-350 |
| 14 days | 20-35 | 320-430 |
| 28 days | 15-25 | 400-550 |
Interface Bond Performance
The experimental results confirmed that the bond between the steel tube and the SCC was adequate for the structural requirements of the project. The strain measurements indicated that the steel tube and concrete deformed in a compatible manner, with no evidence of debonding or slip at the interface. The multi-chamber configuration helped to distribute the shrinkage stresses more uniformly, reducing the risk of localized cracking.
Quality Control and Construction Management
The paper emphasizes the importance of construction management in ensuring the quality of mega-column systems. Key quality control measures include:
- Concrete placement rate control: The placement rate should be limited to prevent excessive lateral pressure on the steel tube wall. A maximum rate of 1.5 m/h was recommended for this project.
- Temperature monitoring: Internal concrete temperature should be monitored continuously, with a maximum allowable temperature of 70°C to prevent thermal cracking. The temperature differential between the core and surface should not exceed 25°C.
- Curing management: The concrete should be maintained at adequate moisture and temperature conditions for a minimum of 28 days. In the case of high-strength SCC, extended curing may be necessary to minimize shrinkage.
- Steel tube straightness and alignment: The steel tube must be erected with a straightness tolerance of 1/1000 of the column height and a maximum verticality deviation of 10 mm.
Study Insights and Engineering Implications
This paper provides valuable experimental data on the behavior of multi-chamber polygonal steel tube mega-columns, a relatively novel structural system that is gaining popularity in high-rise commercial and residential buildings. The full-process experimental approach, which captures both the construction phase and the early curing phase, is particularly valuable because it addresses the transient behaviors that are often neglected in design calculations.
The findings on early-age concrete shrinkage are particularly important for engineers designing mega-column systems with high-strength concrete. The measured shrinkage strains of 200-350 microstrain within the first 7 days are significant and can induce substantial stresses in the steel tube if not properly accommodated. The multi-chamber configuration appears to be an effective design strategy for managing these stresses, as it provides multiple paths for stress redistribution and reduces the constraint on concrete shrinkage.
The paper also highlights the importance of the steel-concrete interface bond in composite columns. While the experimental results showed adequate bond performance, engineers should remain vigilant about potential interface degradation due to construction defects such as insufficient concrete compaction, steel tube surface contamination, or inadequate bonding agents. The use of self-consolidating concrete, while advantageous for placement in congested reinforcement zones, requires careful attention to the steel tube surface preparation to ensure adequate bond development.
In conclusion, this paper provides a comprehensive experimental basis for the design and construction of multi-chamber polygonal steel tube mega-columns, demonstrating that the structural system can achieve satisfactory performance when proper material selection, construction management, and quality control measures are implemented.