Desde fábricas hasta sitios. El viaje de una edificación modular

The journey of a modular building begins with a design process specifically oriented toward manufacturing and assembly, known as DfMA (Design for Manufacture and Assembly). Unlike conventional design, where the project is developed with progressive levels of detail (LOD 100-200 in preliminary phases, LOD 300-350 in detailed design), modular design requires reaching LOD 400 (fabrication level) before production begins: every component, connector, service penetration, surface finish, and dimensional tolerance must be defined in the BIM model. This level of detail demands 4-6 weeks of intensive design with multidisciplinary teams (architect, structural engineer, MEP engineer, manufacturing manager, and logistics coordinator) working simultaneously on a federated BIM model using platforms such as Autodesk Revit, Tekla Structures, or Trimble Connect. The cost of modular BIM design is 15-25% higher than that of an equivalent conventional project, but this investment is recovered through the elimination of on-site contingencies.

Design decisions modulate the efficiency of the entire downstream process. The modular grid defines module dimensions, constrained by three factors: road transport limitations (maximum width 4.20 m without police escort, 5.00 m with escort; maximum height 4.50 m including trailer; maximum length 18 m), crane lifting capacity (typical residential modules: 8-15 tonnes; concrete modules: up to 25-30 tonnes), and internal structural spans (bays of 3.6-4.2 m for housing, 6-9 m for offices and hotels). Geometric modularity is complemented by services modularity: each module leaves the factory with 80-90% of services integrated (electrical wiring, plumbing and drainage pipes, ventilation ducts, lighting points), and inter-module connections are resolved through standardised connectors (push-fit for plumbing, quick connectors for electrical) that reduce connection time to 15-30 minutes per module.

Factory manufacturing is the core of the modular process. Volumetric module factories operate as industrial production lines with sequential workstations: station 1 — base structure (galvanised steel frame or CLT timber frame, 2-4 hours); station 2 — floor enclosure with integrated insulation (sprayed polyurethane of 80-120 mm, conductivity 0.022 W/m·K, 1-2 hours); station 3 — vertical structure (studs, noggings, opening reinforcements, 2-3 hours); station 4 — embedded MEP services (3-5 hours); station 5 — external enclosure and roof (2-4 hours); station 6 — interior finishes (partitions, linings, joinery, sanitary fittings, 4-8 hours). The total manufacturing time for a housing module (3.6 x 12 m, 43 m²) ranges from 16 to 28 net working hours, spread over 3-5 calendar days allowing for curing and drying times.

The controlled industrial environment (temperature 18-22°C, relative humidity 45-55%, illumination 500-750 lux) enables quality levels unattainable in conventional on-site construction. Control protocols include: dimensional verification with laser total station (tolerance ≤ ±2 mm over lengths of 12 m, compared to ±10-20 mm in in-situ construction), airtightness testing of ventilation ducts at 100% of units (class D per EN 1507: leakage ≤ 0.003 l/s·m²), plumbing pressure tests at 1.5 times the nominal pressure for 30 minutes, and electrical continuity and loop impedance testing. The rejection rate in established factories stands at 0.5% of modules produced, compared to defect rates of 5-12% in conventional construction inspections (NHBC Foundation, 2016). The Volumetric Building Companies (VBC) factory in Tracy (California, 27,000 m²) produces 12 modules/day with a workforce of 250 workers, equivalent to 2,500 homes/year at full capacity.

Transporting volumetric modules from the factory to the building site is the phase most conditioned by external factors: traffic regulations, road infrastructure, distance, and terrain. In Spain, the General Traffic Regulations and DGT instructions establish that special transport loads wider than 3.00 m require administrative authorisation; modules of 3.60-4.20 m width need pilot vehicle escort and restricted-hours circulation (typically 22:00-07:00 on inter-urban roads). Transport costs range from 80 to 250 EUR/km per module depending on dimensions and escort requirements, typically representing 5-10% of total module cost. The economic distribution radius is 200-400 km; distances beyond this increase transport costs above 15%, reducing the competitive advantage over local construction. The optimal strategy combines regional factories with 300 km radius service areas.

On-site assembly is carried out with mobile telescopic cranes (capacity 100-500 tonnes) or tower cranes (tip capacity: 2-8 tonnes at 40-60 m radius). The assembly rate for lightweight volumetric modules (timber or steel frame, 8-15 t) reaches 6-10 modules/day with a 250 t mobile crane and a team of 6-8 operatives. Each module is positioned on steel mechanical connectors (bolted plates or socket connections with ±5 mm tolerance) previously anchored to the foundation or the module below. The company Laing O'Rourke assembled the 679 rooms of the citizenM Tower of London hotel (2014) at a rate of 8 modules/day, completing the 12-storey structure in 10 weeks, compared to the 24 weeks estimated for conventional construction. The post-assembly finishing and connections phase (linking services between modules, sealing joints, finishing corridors and common areas) requires 2-4 additional weeks with traditional trades, representing 10-20% of the project's total labour.

Quality control in modular construction operates at three levels: factory control (during production), transport control (protection and post-transport verification), and site control (verification of assembly and connections). In the factory, each module passes through 15 to 25 documented inspection points on a digital checklist linked to the BIM model via QR codes: each component has a code linking to its material datasheet, supplier certificates, inspection result, and georeferenced photograph. The BOPAS (Buildoffsite Property Assurance Scheme) in the UK certifies modular systems following an audit process that evaluates 12 technical criteria (structural, thermal, acoustic, fire safety, durability) with a validity of 7 years. In Spain, modular dwellings must comply with the CTE and obtain decennial insurance, which requires the involvement of a technical control body (OCT) to verify performance compliance during both manufacturing and assembly.

Protection during transport includes: wrapping of modules in 200 μm polyethylene film (protection against rain and dust), metal corner protectors against impact, lashing with 5 t breaking-load straps (minimum 4 tie-down points per module), and tilt and vibration sensors (triaxial accelerometer) that record transport conditions to document that the 1.5 g maximum allowable acceleration was not exceeded. Following assembly, building handover includes: air permeability testing (Blower Door per EN 13829, target n₅₀ ≤ 3.0 ach), infrared thermography for thermal bridge detection (maximum allowable difference: 3°C relative to the homogeneous field), comprehensive hydraulic testing of the plumbing and drainage network, electrical installation verification per REBT (Low Voltage Electrical Regulation), and in-situ acoustic measurement (airborne sound insulation DnT,w ≥ 50 dB between dwellings per CTE DB-HR). The complete journey from factories to sites is completed in 14-20 weeks for a residential building of 4-6 storeys and 20-40 dwellings, compared to 40-60 weeks for equivalent conventional construction.