Project: Tridge
Architect and imges: Atelier ZZ
Structural Design: XinY
Location: Fujian China
Completed: 2023

• steel plates and rods only

1.General
1.1 Spatial Design
This design requires the construction of a small bridge connecting the three rooms on the second floor in a semi-outdoor space with a height of 7,700mm. The architect hopes that this connecting bridge will bring a modern artistic sense to the building and become a cohesive gathering space. A concrete or truss structure bridge would be heavy and occupy more space. The engineer's solution enabled the initial vision of a lightweight, 6mm-thick trident bridge to become a reality. The structural design of this trident bridge integrates modern computer-aided design, industrial-era craftsmanship, and contemporary artistic elements.





1.2 Structural Principle
The structure of the trident bridge consists of two structural systems: "slab" and "reinforcement":
(1) A structural system composed of steel bars welded together, with 15 steel bars distributed in the three limbs of the bridge, welded point-to-point with embedded steel bars in the floor slab. The structure is complete when wooden boards are laid directly on the steel bars. This structural system can also be replaced with other components, such as steel cables.
(2) A structural system in which embedded steel bars in the floor slab are welded to the steel plates of the three limbs, with a minimum welding length of 30mm between the steel plate and the embedded steel bars in the floor slab. This is also a complete structural system, so the trident bridge has a double-insurance structural system.
(3) If there is the ability to apply prestress on-site, it can increase the rigidity of the bridge, but applying prestress is not a mandatory option. (4) The curved surface of the bridge deck is cut from a spherical surface, and this non-developable curved shape enhances the structural strength and rigidity.
In summary, the 7m span and 2.5KN/㎡ live load-bearing capacity of the bridge are completed through the collaboration of five 25mm diameter steel bars at the bottom for tension resistance and 6mm thick steel plates at the top for compression resistance. The lightweight effect of the trident bridge is achieved by applying prestress.
The steel plate is cut near the floor slab, leaving a 5cm gap, honestly revealing the five steel bars and expressing the characteristics of the material through this detail.





1.3 Form-finding Considerations
When the force is evenly distributed along the shape of the component, the shape should be a catenary; when distributed horizontally, it should be a hyperbola; when distributed along the normal direction, it should be a spherical cap. From a force perspective, the trident bridge should be between a catenary and a hyperbola, leaning toward a hyperbola. However, the structure is designed deliberately with a spherical cap, and the curved surface of the bridge deck's steel plate is cut from a spherical surface, which enhances structural strength and rigidity with this non-developable curved shape.

1.4 Construction Method
The design strategy targets the typhoon-affected coastal villages in this area. The client invited local retired shipbuilders to weld the bridge, completing the form-finding process during construction and applying prestress before installing the steel plates by temporarily suspending weights on the steel bars.
Common hand chain hoists were used to apply the prestress, with 1.5 tons of prestress applied. After cutting the center bracket for a while, the lowest point of the center steel plate rebounded less than 20mm.





2.Planet
2.1 On "Design Efficiency, Flexibility, and Response to Local Conditions"
The structural design includes multiple structural systems, as previously mentioned: 1 is the steel bar structure system, 2 is the steel plate and embedded steel bar structure system, and 3 is the supplementary application of prestress. The overlapping of multiple structural systems provides error tolerance space for construction, ensuring safety and feasibility.

2.2 In this coastal town with a rainy and typhoon climate, the structural design considers the construction conditions of the town and makes use of local craftsmanship. The welder, Mr. Chen, and his partner, both over 65 years old, have participated in various large-scale projects. The welder's skills are once again applied in this project, with head-to-head welding, which also preserves a certain technique. After retirement, the welder obtained this challenging construction project, gaining more professional respect.

Using steel plates and steel bars as materials, the structural engineer provided simple and feasible construction steps. Due to the small amount of steel used and the light and thin bridge body, three workers completed the construction in just two working days, using the most common local tools. The overall construction cost, including materials and labor, was only 21,000 RMB.

3.People
3.1 The design aligns with the local fishing industry tradition, and the building retains the local stone masonry craftsmanship tradition. Through the Tridge, the traditional craftsmanship of the industry is preserved. This also creates opportunities for recognition and more job opportunities for construction workers.

3.2 The Tridge brings imagination to the architectural space, offering an artistic experience beyond the ordinary.

4.The structural engineer has achieved multiple lightweight structural cases. The structure of the Tridge is a combination of material properties, with steel bars for tensile resistance and steel plates for compressive resistance.

4.1 Originality and Scalability
From the appearance, the Tridge can be extended to similarly stressed structures like three-way bridges, four-way bridges, five-way bridges, etc. The span and load-bearing capacity of the bridge can be achieved through the coordination of the lower steel bars and the upper steel plates, realizing a lightweight effect.
4.2 Implementation of lightweight structure
(1) A structural system welded by steel bars, which can be replaced with steel cables or other components.
(2) A structural system with embedded steel bars in the floor slab and three-limb steel plates welded together, which is also a complete structural system.
(3) Applying prestress can increase the rigidity of the bridge, but it is not a mandatory option.
(4) The curved surface of the bridge deck steel plate is cut from a spherical surface, and this non-developable curved surface form enhances the structural strength and rigidity.

4.3 Efficiency and Economy
During the design process, through repeated communication with construction workers, adjustments to the shaping and prestress application schemes were made, providing implementation methods for the limited construction conditions.
For example, the emphasis on material characteristics, including the pre-treatment of embedded steel bars. The embedded steel bars are 20cm in length. Before welding the steel bars of the bridge, the embedded steel bars need to be bent with tools, bent downward at an arc at a position about 7-10cm outside the concrete, and then welded with the bridge's steel bars. Instead of waiting for gravity to bend the steel bars in the future. This shape after bending can prevent the steel bar bridge from bending and deforming significantly when subjected to tension in the future.

4.4 Material Characteristics and Artistry
The lightweight design of the Tridge solves the problem in this relatively high semi-outdoor space, reflecting craftsmanship and artistry. The combination of steel bars and steel plates, jointly stressed. Various details in the design aim to highlight the inherent qualities of these materials.