Curved Box Trusses — Where Geometry Meets Fabrication

A straight pipe truss is demanding enough: precise tube cutting, intersecting curve (contour) preparation, multi-pass welding, and non-destructive testing at every joint. A curved box truss adds an entirely new dimension of difficulty.

The box section — a rectangular or square hollow section (RHS/SHS) — must be bent to a specific curvature while maintaining its rectangular cross-section. Unlike round pipe, which can be bent by rotary draw machines with symmetric deformation, a rectangular box bends asymmetrically: the outer flange stretches while the inner flange compresses, and the web must accommodate both. Distortion of the cross-section during bending — flattening, buckling, twist — is the primary quality risk.

Then the curved box sections must be joined into a truss with welded connections that maintain the curved geometry. Welding distortion on a curved surface is cumulative — each weld's shrinkage pulls the member in a direction that may conflict with the design curvature. Without precise prediction and compensation of weld shrinkage, a curved box truss assembled from individually correct members will not fit its designed geometry.

This is why curved box truss fabrication is classified as one of our 10 core manufacturing technologies — it requires equipment, procedures, and expertise that cannot be improvised. Most steel fabricators can make a straight truss. Far fewer can make a curved box truss that meets geometric tolerance on digital pre-assembly.

Stage 1: Multi-Axis Bending

The box section is bent to the design curvature using our self-developed hydraulic bending machine. This machine was designed and built in-house specifically for the challenges of bending rectangular steel sections without cross-sectional distortion.

The machine applies bending force through a series of hydraulic rams that support the section on both the inner and outer flanges simultaneously, controlling the deformation of the cross-section during bending. Key parameters controlled during bending include:

• Bend radius (determined by structural design — typically 3–10× section height)
• Spring-back compensation (steel bends back after force removal — over-bend required)
• Cross-section distortion monitoring (flange flatness, web squareness)
• Incremental bend angle per stroke (controlled for smooth curve generation)

For three-dimensional curves, the member is bent in one plane, then rotated and bent in the orthogonal plane — a sequential multi-axis process that requires precise calculation of intermediate geometries.

Stage 2: Weld Distortion Compensation

Every weld shrinks as it cools — typically 1–2mm per weld in structural steel. In a curved truss with dozens of joints, this shrinkage accumulates and can deflect the truss geometry by centimeters from design. Our fabrication process compensates for this through:

• Weld shrinkage prediction: FEM analysis of each joint's shrinkage based on weld size, process, and restraint
• Pre-bend compensation: Members are bent to a modified curvature that accounts for the predicted shrinkage, so the final geometry after welding matches design
• Welding sequence optimization: Joints are welded in a calculated order that balances shrinkage forces and minimizes cumulative deflection
• Restraint fixtures: Welding jigs hold members in position during welding to resist distortion in real time

Stage 3: Digital Pre-Assembly

Before any physical assembly, the entire truss is digitally pre-assembled in 3D modeling software. Every member, every joint, every weld preparation is modeled to verify that the fabricated components fit together within geometric tolerance. Discrepancies are identified and corrected digitally before steel is cut — eliminating the costly rework that occurs when members do not fit during physical assembly.

After physical fabrication, key dimensions are surveyed using total station or laser tracker and compared to the digital model. Acceptance tolerances are typically ±3mm on member length and ±1/500 on overall truss curvature.

Curved box trusses are designed and fabricated to the following standards:

• GB 50017 — Standard for design of steel structures (member design, stability, connection design)
• GB 50018 — Technical code for cold-formed steel structures (where applicable to RHS/SHS)
• JGJ 7 — Technical specification for space frame structures (curved truss geometry)
• GB 50205 — Standard for acceptance of construction quality of steel structures (fabrication tolerance, NDT acceptance)
• GB 50661 — Code for welding of steel structures (WPS qualification, welder certification, inspection)
• GB/T 11345 — Non-destructive testing of welded joints — Ultrasonic testing (acceptance criteria)
• GB 50011 — Code for seismic design of buildings (seismic detailing for curved truss connections)
• GB/T 1591 — Low-alloy high-strength structural steels (Q355B/Q390B material specification)

Manufacturing Capability

Our curved box truss fabrication is supported by the following production capability at our 300+ mu manufacturing base in Xuzhou:

• Self-developed hydraulic bending machine — capable of bending RHS/SHS sections from 100×100 to 500×300mm, with wall thickness up to 20mm
• CNC plasma cutting — for intersecting curve preparation on box section ends
• SAW welding stations — for thick-plate welded box section fabrication
• GMAW/FCAW welding — for truss joint welding with preheat capability
• 3D digital pre-assembly — BIM software verifies every joint fit before fabrication
• Laser tracker survey — post-fabrication dimensional verification of curved geometry
• 100,000m² workshop area — sufficient space for large-scale curved truss assembly
• 200,000 tons annual capacity — volume capability for large project demand

Curved box truss fabrication is classified as one of our 10 core manufacturing technologies — capabilities that distinguish Permanent Steel from general steel fabricators. The self-developed hydraulic bending machine at the heart of this capability was designed, built, and patented in-house because no commercially available machine met our requirements for bending rectangular steel sections without cross-sectional distortion.

The Jiaozuo Tai Chi Culture Exchange Center — with its 423-meter arc and 40-meter cantilever — is our flagship curved box truss project. It demonstrated that our bending, weld distortion compensation, and digital pre-assembly processes can deliver geometric accuracy at a scale and complexity that most fabricators cannot attempt. The 423m arc was surveyed to ±5mm overall tolerance — a level of precision that would have been impossible without our integrated design-to-fabrication workflow.

What Sets Us Apart

• Self-developed bending equipment — designed for RHS/SHS sections, not adapted from round pipe machines
• FEM-based weld distortion prediction — pre-compensates for shrinkage before fabrication begins
• Digital pre-assembly — every joint verified in 3D before steel is cut
• Laser tracker verification — post-fabrication geometry confirmed to ±3mm
• 50+ R&D engineers for complex curved geometry design and analysis
• 100,000m² workshop with space for large-scale curved truss assembly
• 6 top-level qualifications covering steel structure design, manufacturing, and construction supervision
• 100+ patents including innovations in bending, welding, and digital fabrication workflows
• International project delivery with documentation packages meeting international engineering standards
• ISO 9001 QMS with full material traceability from mill certificate to final geometry verification report

For architects and engineers designing buildings with curved roof forms, sweeping canopies, or dramatic cantilevers, the question is not just whether the design can be calculated — it is whether it can be fabricated. Permanent Steel's curved box truss technology answers that question. Contact our engineering team today to discuss your architectural steel fabrication requirements and receive a comprehensive technical proposal.