Applications of Steel Tubes in Modular and Prefabricated Steel Structures

Modular and prefabricated steel construction has moved from pilot projects to repeatable delivery, and steel tubes sit at the centre of that shift. Light, strong and efficient in cross-section, tubular members now carry whole spatial units instead of acting only as the frame. The same discipline applies at every bolted interface, from module splices to process connections where flange gaskets set the sealing.

Three Technical Routes for Tubular Modules

Evidence from more than 40 demonstration projects completed between 2020 and 2025, plus test campaigns at Tianjin University and Xi'an University of Architecture and Technology, points to three routes. They differ mainly in how much of the building leaves the factory as a finished unit.

Component level: filled square tubes

A group at Tianjin University developed a frame-bracing system with composite special-shaped columns made of concrete-filled square tubes. Two 150 mm x 150 mm x 8 mm square tubes are stitched vertically with splice plates into a lattice column. The section modulus rises by 38 %, which allows 80 m residential towers without interior columns.

Module level: box units

Broad Sustainable Building in Hunan works with a main-board plus diagonal bracing concept. Each 3 m x 6 m main board is a steel-truss composite deck that integrates building services, fire protection and finishes off site, with a component ratio above 90 %. Site lifting runs at roughly one floor per day.

System level: embedded light-steel modules

A public-rental housing scheme at Hunan Agricultural University used 388 light-steel boxes framed from 210 mm x 150 mm x 4 mm square tubes set into a primary steel frame. Building height reached 34.2 m and the overall programme shortened by 40 %.

• The component route delivers the widest column-free spans and suits residential towers.
• The module route delivers the highest factory content, with services and finishes already installed.
• The system route fits inside an existing steel frame and suits mid-rise housing on a tight programme.

Connection and Floor Details

Module joints decide whether a prefabricated frame behaves like a monolithic one, so bolted details dominate because they remove site welding. Two variants cover most cases, and the same logic applies where a module ties into process piping through pipe flanges and their bolting.

Sleeve and side-plate joints

• Sleeve-flange all-bolt joint: quasi-static tests at Xi'an University of Architecture and Technology in 2023 show that raising sleeve thickness from 10 mm to 14 mm increases ultimate capacity by 23 %. Ductility reaches u = 4.7, covering 1/50 drift under a rare earthquake at Intensity 8.
• Side-plate joint on a wall-type concrete-filled tube column: a 200 mm x 200 mm column with a 6 mm wall connects through 12 mm side plates to 10.9S M20 high-strength bolts. Hysteresis stays full, and failure shifts from weld fracture to plastic hinges at the beam ends.

Composite floors and lateral systems

• Reusable deck: a W-shaped snap-on slab 120 mm thick, 25 % lighter than traditional truss decks and reusable for at least 50 cycles, applied on a 340 000 m2 programme in Yunnan.
• Lateral system: a wall column with steel plate shear walls reached 1/85 inter-storey drift in a three-storey full-scale shaking-table test at 0.4 g peak ground acceleration, matching reinforced-concrete shear walls at only 48 kg/m2 of steel.

Steel tubes staged for shipment from the warehouse.

Cost, Schedule and Carbon

Box-module lifting averages 15 minutes per unit, and a 12-storey hotel topped out in seven days, about 70 % faster than cast-in-situ construction. On a public-rental project in Hunan, site labour fell from 2.5 to 0.6 man-days per square metre.

Note: 2024 market averages in CNY per square metre of gross floor area; figures are comparative benchmarks.

Under GB/T 51366-2019, cradle-to-grave CO2 for modular steel is 350 kg per square metre, 28 % below a conventional reinforced-concrete frame. The modular option still costs 8-15 % more upfront, but a 3-6 month earlier handover gives a 2.6-year payback once the time value of money is counted.

Field Constraints and What Comes Next

Three constraints shape site planning, and each has a practical countermeasure.

• Transport and lifting: modules wider than 6 m need hydraulic axle trailers and mobile lifting platforms, with a turning radius of at least 15 m. Segmented 2+1 bolted splice modules solve the problem.
• Joint stiffness: upper and lower module beams do not act compositely. A through double-beam, double-column joint raises stiffness by about 35 %.
• Fire and corrosion: exposed tubes need a 1.5 h fire rating. A 20 mm aerogel blanket plus 9 mm fire-rated gypsum board meets GB/T 9978-2008 and adds less than 8 kg per square metre.

Next in line are Q690 tubes with ultra-high-performance concrete, which should cut column size by about 20 % in 100 m residential towers, and digital factory scheduling that has reached 30 minutes per module. From 2025, Beijing and Shanghai require at least 60 % prefabrication in government-funded projects, so square-tube modular systems look set to dominate fast-track housing, hotels and hospitals.

Frequently Asked Questions

What tube section works best for a module frame?

Square and rectangular hollow sections are the default because flat faces simplify bolted splices and cladding. Wall thickness follows local buckling and the bolt group at the splice; the examples above run from 4 mm thin-walled sections to 8 mm welded square tubes.

Which steel grades appear in tubular modules?

Q460 is used where column size must be kept small, and Q690 is being trialled for high-rise modules. Concrete infill reaches C80 in the tallest examples, where Q460 columns pair with it.

How are modules connected on site?

With bolts only. A typical side-plate detail uses a 200 mm x 200 mm column with a 6 mm wall, 12 mm side plates and 10.9S M20 high-strength bolts, so crews avoid site welding and its inspection burden.

How is fire protection applied to exposed tubes?

A 20 mm aerogel blanket with 9 mm fire-rated gypsum board gives a 1.5 h rating under GB/T 9978-2008 and adds less than 8 kg per square metre, so member sizes rarely change.