Zhuojin Pipe Fitting Co., LtdSteel Pipe Diagonal Brace Foundation Pit Reinforcement Construction Technology for Existing Structures
Literature Overview
The 2025 paper by Wang Shidong, Yang Guangde, Xu Qinglu, Zhuo Shihua, and Yang Yongfu, published in Construction Technology (Volume 54, Issue 8, pages 103-107), presents a novel construction technology for reinforcing foundation pit support systems in existing structures using steel pipe diagonal braces combined with concrete waist beams on the inner side of the exterior wall. This technology was developed under the research project "Design and Construction Technology for Industrial Heritage Renovation and Expansion in Urban Organic Renewal" by China Construction Third Engineering Bureau. The approach addresses the challenge of reinforcing foundation pit support systems while protecting the existing basement exterior wall and floor slab from damage, which is a common problem in urban redevelopment projects where existing structures must be preserved during adjacent excavation.
Technical Principle and Construction Method
The core principle of this technology is to transfer the lateral earth pressure from the foundation pit support system through the steel pipe diagonal braces to the existing basement floor slab, while protecting the exterior wall from excessive lateral loading. The construction sequence involves several key steps that must be carefully executed to ensure structural safety and construction quality.
Construction Sequence and Key Steps
The construction begins with the installation of the concrete waist beam on the inner side of the basement exterior wall. The waist beam is designed with the same width as the force-transfer strip of the foundation pit support system, ensuring that the lateral forces are distributed uniformly across the wall. The waist beam is cast using high-strength concrete and is anchored to the existing floor slab through chemical anchors or mechanical anchors, providing a stable reaction point for the diagonal braces.
The steel pipe diagonal braces are then installed between the waist beam and the foundation pit support system (typically a diaphragm wall or sheet pile wall). The diagonal braces are designed to transfer the lateral forces from the support system to the waist beam, thereby reducing the load on the exterior wall. The connection details between the diagonal braces and the waist beam, as well as between the braces and the support system, are critical for ensuring effective force transfer.
The final step involves the construction of a concrete shear groove in the existing floor slab to resist the horizontal forces transmitted through the diagonal braces. This shear groove is designed to prevent the floor slab from sliding or heaving under the horizontal load, and it must be cast with sufficient depth and reinforcement to provide the required shear resistance.
| Component | Material Specification | Key Dimension | Function |
|---|---|---|---|
| Steel pipe diagonal brace | Q345B, φ219×8 | Length 6-12 m | Force transfer |
| Concrete waist beam | C40 | Width 600 mm, Height 500 mm | Wall protection |
| Shear groove | C30 | Depth 400 mm | Horizontal resistance |
| Chemical anchors | M24 | Length 200 mm | Wall anchorage |
| Brace connection plate | Q345B, t=20 mm | 300×300 mm | Force transfer node |
Steel Pipe Diagonal Brace Design and Fabrication
The steel pipe diagonal braces are fabricated from seamless or welded steel pipes with a typical diameter of 219 mm and wall thickness of 8 mm. The pipes must meet the requirements of GB/T 8163 or GB/T 9711 for mechanical properties and chemical composition. The fabrication process includes cutting, drilling for connection plates, welding of end fittings, and surface treatment (priming and painting for corrosion protection). The welding of the end fittings must be performed according to qualified welding procedures, with visual inspection and magnetic particle testing (MT) of all welds to ensure weld quality.
The connection plates at both ends of the diagonal braces are designed to provide a rigid connection with the waist beam and the support system. The bolted or welded connections must be designed to resist the full axial force in the brace, including the effects of eccentricity and moment transfer. The design must also account for the construction sequence, where the braces may be installed in stages as the excavation progresses.
Engineering Practice Challenges and Solutions
The implementation of this technology presents several practical challenges that must be addressed. First, the installation of the waist beam on the inner side of the exterior wall requires careful planning to avoid damaging the existing waterproofing layer. The waterproofing must be protected or repaired after the waist beam is cast. Second, the installation of the diagonal braces in a confined space between the existing structure and the excavation face requires precise positioning and alignment to ensure proper force transfer. Third, the construction of the shear groove in the existing floor slab may require temporary support to prevent structural instability during the cutting and casting operations.
The quality control plan for this technology must include verification of the steel pipe material properties, dimensional inspection of the braces and connection plates, welding quality inspection (RT or UT for critical welds), and load testing of the connection details. The construction monitoring plan must include measurement of the lateral displacement of the support system, the axial force in the diagonal braces, and the deflection of the waist beam and floor slab throughout the construction process.
Study Insights and Conclusions
This construction technology represents a practical and effective solution for reinforcing foundation pit support systems in existing structures, particularly in urban redevelopment projects where the preservation of existing buildings is essential. The approach offers several advantages over traditional reinforcement methods, including faster construction speed, lower cost, and minimal disruption to subsequent basement construction activities. The use of steel pipe diagonal braces provides a reliable and efficient means of force transfer, while the concrete waist beam and shear groove ensure the protection of the existing exterior wall and floor slab. For engineers involved in similar projects, this technology provides a valuable reference for the design and implementation of foundation pit reinforcement systems in existing structures, and the lessons learned from this project can be applied to a wide range of urban renewal and industrial heritage renovation projects.