Recently, China Construction Eighth Engineering Division Corp. Ltd. Third Construction Co., Ltd. successfully applied the "prestressed fish-belly beam assembled steel support" technology, solving the problems of high cost, long construction period, dense internal support, uncontrollable deformation, and large amounts of construction waste and noise pollution caused by traditional concrete internal support. This is another beneficial attempt of new foundation pit support technology after the application of the double-spliced steel internal support developed by the Engineering Research Institute of China Construction Eighth Engineering Division.

On-site application of "prestressed fish-belly beam assembled steel support"

The prestressed fish-belly beam assembled steel support (abbreviated as IPS) is a technologically advanced and environmentally friendly large-span foundation pit internal support structure developed using the prestressing principle. This technology uses H-shaped steel struts (or corner braces) to press both ends of the fish-belly beam. By applying pre-tension to the lower chord steel strands of the fish-belly beam, the entire large-span fish-belly beam tends to displace outward from the pit, activating the passive earth pressure outside the pit, thereby achieving displacement control. Pre-compressive stress is applied to the supports at both ends of the fish-belly beam, forming a large-span, closed, and stable force system.

The system consists of upper chord beams (perimeter purlins), vertical web members, diagonal web members, lower chord steel strands, struts, corner braces, and other steel components. It is a recyclable and reusable assembled foundation pit internal support system.

Plan layout of prestressed fish-belly beam

On-site application of "prestressed fish-belly beam assembled steel support"

Project Introduction

The Senlan International Phase VI (D3-10) project is located in Pudong New Area, Shanghai, with a building area of 44,000 m² and a site area of 9,687 m². The foundation pit excavation area is about 8,136 m², with a perimeter of about 380 m and a maximum excavation depth of 10.6 m. The basement has two levels, and there are four commercial office buildings above ground. To the east of the foundation pit is Harrow International School, and to the north are Shanghai Zhuyuan Middle School and Haoqimiao Kindergarten. Among them, Harrow International School is less than 15 m from the pit edge. On the east side, at 9 m and 11 m from the pit edge, there are rain and sewage pipelines within the Harrow School campus; on the north side, at 4.8 m from the pit, there is a gas pipeline. The environmental protection level on the north and east sides of the foundation pit is Level 2, with high environmental protection requirements.

Project rendering

Problems Solved

In view of the complex surrounding environment, considering construction period, economy, safety, and other aspects, the second reinforced concrete support in the original scheme was changed to a prestressed fish-belly beam assembled steel support. Its cost is controllable, construction period is controllable, and environmental protection is controllable. After expert demonstration, the scheme was gradually implemented on site. During the process, the axial force changes of the steel support and the deformation of the foundation pit were monitored, and ultimately the deformation of the north pipeline and the east school was effectively controlled, ensuring the safety of surrounding pipelines and buildings.

On-site application of "prestressed fish-belly beam assembled steel support"

Comprehensive Benefits

The prestressed fish-belly beam assembled steel support is green and environmentally friendly, energy-saving and emission-reducing, convenient for construction, and shortens the construction period. It is a statically indeterminate structure with high redundancy, safe and reliable.

Economic benefits: After adopting the prestressed fish-belly beam steel support for the second support, the curing time of cast-in-place concrete is eliminated, shortening the construction period by 9 days and reducing costs by 520,000 yuan.

Social benefits: The prestressed fish-belly beam assembled steel support uses standardized steel components and bolted connections, making installation and removal convenient. There is no concrete curing, which shortens the project duration; the fish-belly beam has a large span, providing larger space for earthwork excavation and underground structure construction, and better construction convenience; the support can be reused after removal, saving costs; dismantling the support does not produce noise, dust, or construction waste, saving energy and protecting the environment; the fish-belly beam steel support effectively controls foundation pit deformation by applying large prestress, and the prestress can be adjusted during construction, effectively protecting the safety of underground pipelines and surrounding buildings, with significant social benefits.

On-site application of "prestressed fish-belly beam assembled steel support"

Appendix

Similar Projects

In traditional foundation pit engineering, the common support methods for retaining structures are: crisscrossing concrete or steel internal supports, resulting in small excavation space and laborious and time-consuming installation and removal...

Traditional concrete internal support

Traditional steel structure internal support

Inconvenient installation, removal, and construction of traditional internal support

At the Jiangyin Urban Construction South Gate Commercial Block project, we witnessed a new type of foundation pit retaining structure support method: the prestressed fish-belly steel support system. Its construction site looks like this:

Overall view of prestressed fish-belly steel support site

Partial view of prestressed fish-belly steel support site

Under direct comparison, the first impression of this new support method is: the support members are sparse, and the excavation construction space is very large; the support uses few materials and standard components, making installation and removal convenient and recyclable...

Comparison of construction space between two schemes in the same project

So, what is the principle and composition of this new support system?

Let's look at the overall schematic diagram. The support system structure consists of four major components: ① perimeter beam, ② fish-belly beam, ③ strut/corner brace, ④ column, as well as connectors between components and internally.

Overall schematic diagram

It is called a "prestressed" support system because the entire structure resists the force and deformation of the foundation pit retaining by applying prestress. Let's look at the force schematic diagram: the strut/corner brace applies pre-compressive force through jacks and force-keeping boxes, and the fish-belly beam applies pre-tension through steel strands, resisting the lateral pressure of the foundation pit retaining.

Force schematic diagram

Next, let's go deep into the site and analyze the specific composition of these four major components with drawings.

1. Perimeter beam

H-shaped steel perimeter beam: continuously set along the perimeter of the foundation pit crown beam with bolts, bearing pressure on both sides. Bracket: set below the perimeter beam, connecting the retaining pile body and the perimeter beam, playing a supporting role.

2. Fish-belly beam

Upper chord beam: namely the perimeter beam

Vertical web member, diagonal web member: form a truss structure to transfer force between the steel strands and the perimeter beam

Bridge frame: located at the end of the vertical web member, through which the steel strands pass

Lower chord steel strand: can provide pre-tension after tensioning

Connector: used to connect the fish-belly beam and the strut/corner brace

Anchorage: tensions, locks, and maintains the pre-tension of the steel strands

3. Strut/corner brace

H-shaped steel strut/corner brace: transmits axial support prestress

Pressurizing cross beam: perpendicular to the support direction, pressure is applied to the support between the cross beams through jacks

Force-keeping box: after the jack applies pressure, the force-keeping box is inserted into the pressurizing cross beam to maintain axial prestress

Cover plate: set at the splicing node to strengthen the weak part of the strut to form a whole

Tie rod: set between adjacent cover plates to provide lateral restraint and improve overall stability

4. Column

Square steel tube concrete column: used to bear the weight of the support system itself

Joist, bracket: transfer the weight of the strut/corner brace to the column

Hoop: fixes the joist and strut/corner brace

Diagonal brace/scissor brace: improves the overall stability of the column

On site, we also saw that in addition to the advantages mentioned at the beginning, such as large excavation space and few recyclable materials, this support system uses a fully automatic continuous monitoring system, which can dynamically apply prestress to more effectively control foundation pit deformation.