GAC New Energy Production Capacity Expansion

This project is a production capacity expansion for one of China’s leading new-energy vehicle (NEV) manufacturers. When an EV brand scales from tens of thousands to hundreds of thousands of units per year, the constraint is rarely the assembly line — it is the building that contains it. The entire 85,000 m² facility — body-shop floor, battery-pack integration bays, final assembly lines, and process-support mezzanines — sits under a single continuous roof structure: a double-layer orthogonal square-pyramid flat grid spanning the full width of the production bays without intermediate columns.

Permanent Steel’s scope covered the full engineering chain: structural analysis and connection design, fabrication of all grid members and nodes at our Xuzhou manufacturing base, surface treatment, trial assembly, transport to site, and erection of the complete structural system. Client and consultants received one accountable counterparty for the steelwork — not a chain of sub-vendors each deflecting responsibility onto the next. That single-accountability model is the reason the project held its structural programme while the surrounding civils and services packages moved in parallel.

The structural brief was unusually demanding precisely because the building is ordinary. A 21st-century EV plant is a just-in-time machine: every square metre of floor must be usable, every column location is a conflict with a robot cell, an AGV route or an overhead utility spine. The client therefore specified maximum free span, maximum floor flexibility and maximum speed of enclosure. A double-layer grid fulfilled all three; this case study explains how, and why the joint strategy mattered as much as the geometry.

A double-layer flat grid is a three-dimensional truss in which all members lie on one of two parallel planes — the top chord plane and the bottom chord plane — connected diagonally in between. The result behaves like a very deep plate: bending is carried primarily by the chord planes acting in tension and compression, while the diagonals transfer shear between them. Because the two chord planes are separated by the depth of the grid (typically 1/12 to 1/18 of the span in this class of building), the structure is many times stiffer than a single-layer plane frame of the same weight.

Why it won the comparison for this site

• Against the portal frame: the portal’s moment connections are fabrication-intensive, and its practical clear span is limited by member depth and crane clearance. At 30 m+ spans the grid’s three-dimensional action uses roughly 15–25% less steel for the same serviceability envelope.
• Against the plane truss: a lattice truss carries load in one direction only. The grid carries load in two directions, so critical members are smaller, local damage or overload is redistributed, and the two-way pattern permits balanced support layouts that a truss cannot exploit.
• Against the space frame with complex geometry: an orthogonal grid keeps node coordinates on a regular mathematical lattice. That regularity is the difference between a structure that can be numerically optimised in hours and one that consumes weeks of geometry classing — and it is decisive for fabrication repeatability.

The orthogonal square-pyramid (four-pyramid) configuration was specified because it is the most uniform, most repetitive two-way grid form: the standard module can be repeated in both plan directions with identical or near-identical members, which compresses engineering time, tooling setup in the workshop, and the inventory of unique parts.

Many grid roofs are marketed as “bolted” or “welded” as if the two were ideologies. In practice, the best-performing large industrial grids are hybrids, and this project is a clear case study of where each technology earns its place.

The bolted sphere joint

A bolted sphere node is a machine-turned steel sphere with drilled and tapped holes on its faces. Members terminating at the node end in a conical connector (a shear-key fitting) that is bolted to the sphere with high-strength bolts (grade 8.8 or 10.9). The connection is assembled in minutes with a torque wrench, requires no fire-permit hot work on site, and can be fully Quality-Controlled by measurable bolt torque rather than by welder skill. On this project, bolted nodes were used for the large majority of connections — the medium-load interior members that dominate the member count of a grid.

The economics of bolting are compelling at volume: welding a sphere node on site requires positioning the member, tacking, multiple passes of qualified welder effort, NDT inspection and rework risk. A bolted node on the same member is installed in under a fifth of the labour time and with zero welding consumables. On an 85,000 m² grid with tens of thousands of joints, that difference is measured in months of erection programme — and a windproof, rainproof bolting operation is far less weather-sensitive than site welding.

The welded sphere joint

The welded sphere is the classic Chinese spatial-structure node first developed in the 1960s precisely for grids of this kind: tubular members are profile-cut and butt-welded onto a hollow sphere shell. Its advantage is connection capacity — a welded node can accept substantially higher member forces in all directions than a practical bolted node, because the bolted node’s capacity is limited by the bolt group geometry, sphere thread engagement and the fit-forging precision of the connectors.

On this project, welded nodes were specified wherever member forces sat above the efficient domain of the bolted node:

• in the high-stress zones adjacent to support columns, where chord forces peak;
• around concentrated load points — rooftop plant platforms, heavy equipment suspension points and crane-runway integration lines;
• at column-to-grid connection details where a welded node gives cleaner force transfer to the column cap;
• in edge and corner zones where asymmetric member forces would overload the most convenient bolt pattern.

Cost optimisation without losing redundancy

The hybrid strategy is a direct cost-vs-robustness optimisation, and it works because the two joint types fail differently. A bolted connection shows measurable slip before ultimate load and behaves semi-rigidly; a welded connection is rigid until fracture. Placing welded joints on the hottest load paths and bolted joints on the distribution grid means the structure’s overall rigidity is governed where it matters (the high-force skeleton), while the huge population of ordinary joints remains fast, economic and field-replaceable. If a bolted node is ever damaged, it can be unbolted and swapped without cutting. That maintainability is genuinely valuable in a production building that cannot pause for roof surgery.

The design rule we applied was simple and defensible: bolt where the fasteners can carry the envelope of forces with reserve; weld where they cannot. Where both were acceptable, the bolted solution won on programme and total cost. The result is a hybrid system whose redundancy is strictly greater than either uniform system, because a failure in the bolted population is ductile and local, while the welded skeleton keeps the overall grid coherent.

A factory roof is judged by deflection and durability more than by strength — a grid that is strong but sags measurably will crack its cladding sheeting, bind its roller doors and alarm the process engineers. The serviceability criteria for this structure were therefore set tighter than the minimum code provisions:

• Deflection: the grid was proportioned so that dead-plus-live deflection stays within the code limit for this span class, with additional pre-camber built into the bottom chord plane so that the finished soffit appears level under dead load — the aesthetic expectation of a process hall where suspended services follow a level datum.
• Wind: uplift and suction on a large flat roof plane were evaluated per GB 50009 with regional wind parameters; uplift-critical bolted connections were checked for the connection’s ability to carry tension as well as shear, and anchor/column-cap detailing was proportioned for net uplift at the perimeter.
• Seismic: the two-way grid’s redundancy translated directly into favourable seismic behaviour — many equal members and ductile connections allow energy dissipation across the whole structure rather than concentration in a few plastic hinges.
• Thermal movement: expansion provisions were placed at the perimeter supports with sliding bearings and guided details, so that a 200 m+ roof expanse breathes thermally without locking internal stresses into the grid or the cladding.
• Fire: the roof members receive fire-resistance treatment in the critical zones per the building code’s classification of the occupancy, with the coating specification selected for adhesion on blast-cleaned primed steel.

The structural completion was verified by independent surveys against the design model: node coordinates within tolerance, camber confirmed after de-propping, and the finished grid exhibiting no measurable settlement differential across the support lines three months after completion — the end-of-story of every large-span roof delivered by a team that keeps its own erection records as evidence, not as an afterthought.