Development of an Optimized Column System for Top-Down Construction

Introduction

Constructing underground structures in densely populated urban areas is becoming increasingly difficult due to:

  • Proximity to adjacent buildings and roads

  • Legal risks from noise, dust, and vibration complaints

  • Limitations of traditional bottom-up excavation methods

To address these issues, the top-down construction method has emerged as a viable and efficient alternative. This method enables the simultaneous construction of substructures and superstructures, offering improved safety, reduced construction time, and lower environmental impact.

Cities like Taipei, Singapore, Shanghai, Tokyo, and Seoul are already implementing this technique to great success.

Proposed Innovation: CCFT Columns with Novel Shear Connection System

This study proposes an optimized column system specifically tailored for top-down construction. The system uses:

  • Concrete-filled Circular Steel Tubes (CCFT)

  • A novel shear connector system using welded stud shear bands between the CCFT columns and flat slabs

Why CCFT Over H-Shaped Steel Columns?

Compared to conventional H-shaped columns, CCFT columns offer the following advantages:

  • Easier installation in tighter boreholes due to their circular cross-section

  • Reduced steel usage

  • Improved structural performance, even without accounting for slenderness effects

  • Capability to adjust column wall thickness or diameter based on exposed column length during excavation, further optimizing material use

Shear Connection Development and Testing

A new welded band-type shear connection system was developed and tested prior to field application. The tests demonstrated:

  • Effective shear capacity

  • Adequate deformation behavior

  • Ease of construction integration with the top-down process

Field Implementation and Results

The CCFT column system with the new shear connectors was successfully implemented in a real top-down construction project. Key findings include:

  • No reduction in load-bearing capacity of columns, even with smaller borehole diameters

  • Significant cost and time savings due to simplified construction and reduced steel material

  • Efficient coordination between column installation and slab casting during excavation

شکل 1: تصویر پروسه نصب برای سیستم اتصال برشی: (a)و (b) قرار دادن ژاکت برشی بر روی باند متحمل برش و (c) اتصالات ژاکت برشی

Figure 1:

Installation sequence of the shear connection system:
(a) and (b) Placement of shear jackets over the shear zone
(c) Final shear jacket connections

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شکل 2 : تصویر مفهوم سیستم اتصال برشی جدید : (a) به کاربرده شده در ورکشاپ و (b) بکار برده شده در فیلد

Figure 2:

Conceptual representation of the shear connection system:
(a) Workshop testing
(b) On-site field application

Conclusion

The development of a CCFT-based column system with innovative shear connectors demonstrates a highly practical and efficient solution for top-down construction in urban environments. The system offers:

  • Improved constructability

  • Reduced material costs

  • Faster construction timelines

  • Enhanced compatibility with confined sites

This advancement supports the growing demand for sustainable and adaptable construction methods in crowded metropolitan regions.