Curved-Roof Grid Structures for Column-Free Airport Concourses

An airport terminal concourse is, at its core, a circulation space. Passengers move through it from check-in to security to gate, often distances of several hundred meters. Retail and food service areas line the concourse walls. Boarding bridges project from the airside facade at intervals of 60-100m. For this program to function, the concourse interior must be column-free across its full width — typically 60 to 90 meters for a wide-body gate concourse.

Conventional steel framing — beams supported by columns at 18-24m intervals — is structurally possible at these spans but architecturally and functionally unacceptable. Interior columns would obstruct passenger flow, break up retail sightlines, and interfere with the baggage handling systems that run below the concourse floor. A spatial grid structure solves this by distributing the roof load through a three-dimensional network of steel members to the perimeter columns, achieving clear spans of 60-90m with a structural depth of only 5-7 meters (a depth-to-span ratio of approximately 1:12 to 1:15).

Grid Geometry for Curved Architectural Forms

The architectural vision for a modern airport terminal is rarely a flat box. Terminal roofs incorporate barrel-vault curvature for daylighting, wave forms for visual identity, and leading-edge cantilevers at the landside facade to shelter arriving passengers. A double-layer curved grid structure follows these forms while maintaining uniform structural depth across the span.

The bolted sphere grid system is particularly suited to curved geometry because each node is independently CNC-drilled at the precise angles the geometry demands. In a flat rectangular grid, all nodes share the same bolt angle pattern. In a curved grid, no two nodes are identical — each sphere is drilled to its unique position in the three-dimensional surface. Our CNC drilling equipment reads the node angle data directly from the 3D structural model, eliminating the manual angle calculation that introduces errors in less automated systems.

For larger terminal projects where the architectural form includes both curved and straight sections, we combine pipe truss structures with grid structures. Pipe trusses provide the linear span elements (straight concourse sections, cantilevered canopies), while curved grid sections handle the geometric transitions. The two systems share common support points and are coordinated in the structural model to ensure load transfer at the interface is correctly engineered.

Vibration Control for Passenger Comfort

A long-span roof structure has a natural frequency that depends on its span, depth, mass, and boundary conditions. For a 75m span grid roof with a 5m depth and typical cladding mass, the fundamental vertical natural frequency is typically 1.5-2.5 Hz. This range is problematic because it is close to the frequency of human walking (1.6-2.0 Hz) and the excitation frequency of electric baggage carts (2-3 Hz).

If the roof's natural frequency coincides with an excitation frequency, resonance occurs and the roof oscillates with perceptible amplitude. Passengers do not feel the roof strength — they feel the roof move. Even a 5mm oscillation at 2 Hz is perceptible and creates a sense of structural unease that undermines the terminal's architectural intent of stability and permanence.

We address this through three strategies, applied selectively based on the specific dynamic analysis of each project:

• Frequency tuning: The grid depth, member stiffness, and support conditions are adjusted in the structural model to shift the natural frequency away from the excitation band. Increasing depth from 5m to 6m on a 75m span may shift the fundamental frequency from 1.8 Hz to 2.4 Hz — enough to break resonance.
• Mass distribution control: Heavy elements (HVAC units, catwalks) are positioned at node points rather than mid-span, which changes the effective mass distribution and natural frequency without adding structural material.
• Damping augmentation: For particularly sensitive configurations, viscous dampers or tuned mass dampers are integrated into the grid at locations of maximum dynamic response. These devices absorb vibrational energy and reduce oscillation amplitude by 50-70%.

Baggage Handling System Integration Below

Modern airport terminals route baggage handling conveyor systems below the concourse floor, often suspended from the roof structure above rather than supported from the slab below. This means the grid roof that passengers see from above is also the structural frame that carries the baggage system from below. The bottom chord of the grid must accommodate:

• Conveyor rail supports: Steel angles welded to bottom-chord nodes at regular intervals, each carrying 500-2000 kg of conveyor and baggage load
• Maintenance catwalks: Grated walkways suspended from the grid for baggage system access, adding dead load and requiring guardrail attachments
• Electrical and data conduits: Cable trays routing power and control signals to the baggage system motors
• Fire suppression piping: Dry-pipe sprinkler mains sized for the baggage system area, which can be 100-150mm diameter steel pipe

Each suspended element adds dead load to specific grid members. The structural analysis must include these point loads at their actual positions — not distributed as uniform area load, which is the simplification that many fabricators use. A point load of 2 tonnes on a bottom-chord member changes the force in that member, in the adjacent members, and in the node connections. If the analysis does not capture this, the member may be undersized and the connection may fail under fatigue loading (baggage conveyors vibrate continuously, creating millions of load cycles over the terminal's design life).

Integrated 3D Structural Modeling

Every airport terminal roof we engineer begins as a complete 3D structural model. The architectural surface — whether flat, curved, or freeform — is imported into our structural analysis software. Grid topology is generated on the architectural surface, with member sizes initially assigned by engineering judgment and then optimized through iterative analysis. Load cases include dead load (steel self-weight, cladding, MEP, ceiling, baggage system), live load (maintenance access, snow), wind (pressure coefficients from wind tunnel testing or CFD), seismic (response spectrum analysis with vertical component), and temperature differential (for exposed or semi-exposed structures).

Member forces are extracted for every member in every load combination. The governing force — compression, tension, or reversal — determines the member section size. Connection forces at each bolted sphere node determine the bolt diameter, sphere hole angles, and sealing plate specifications. This is not a spreadsheet calculation applied uniformly across the grid; it is a member-by-member, node-by-node analysis that produces a fabrication package where no two spheres in a curved grid are identical.

CNC Fabrication and Trial Assembly

40Cr forged steel spheres are CNC-drilled at the angles defined by the 3D model. Q355B steel tubes are cut to design length on automated cutting lines, with conical sealing plates welded to tube ends using certified welding procedures and non-destructive testing verification. The entire grid is trial-assembled in our factory in sections to verify geometric compatibility before shipment — a quality control practice that catches fabrication errors in the factory rather than on the construction site, where correction costs are 10-50 times higher.

For airport terminals, trial assembly is particularly critical because of the curved geometry. A flat grid can be assembled on a flat factory floor and measured against a 2D drawing. A curved grid must be assembled on a curved jigs that replicate the roof's final geometry, with 3D surveying verifying that each node position matches the model within tolerance. Any deviation is corrected before shipment — not after the crane has lifted the section 20 meters into the air.

Erection Sequencing and Deflection Monitoring

A long-span grid roof cannot be lifted into place as a single unit. It is assembled section by section on temporary shoring towers, with each section checked against the 3D model before the next is installed. The partially completed structure must support itself and resist construction-period wind loads before the grid achieves self-supporting geometry through closure of the final connections.

During erection, deflection is monitored at critical points using survey instruments. Predicted deflection under self-weight is compared to actual measured deflection as each section is installed. Deviation exceeding 15% of predicted triggers an engineering review. Pre-camber — a slight upward curvature built into the fabrication — compensates for dead-load deflection so the roof settles to its design position under permanent load rather than sagging below it.