Materials: specification and feasibility
Concept stage. No Stratum test data: specification targets and published literature.
Steel: grades and connections
Module frames are hollow structural sections (HSS) with plate gussets and bolted corner connectors. Grade choices follow AISC practice.
| Element | Specification | Fy / Fu (min.) | Notes |
|---|---|---|---|
| Columns, rails (rectangular HSS) | ASTM A500 Gr. C | 50 / 62 ksi | AISC’s preferred rectangular HSS grade. Round A500 Gr. C is lower (46 ksi yield); do not mix the two in calculations. AISC |
| Floor and roof beams (W shapes) | ASTM A992 | 50 / 65 ksi | Higher minimum tensile than A500 Gr. C; yield governs most member design. |
| Alternative HSS | ASTM A1085 Gr. A | 50 / 65 ksi | Tighter wall-thickness and mass tolerances than A500; AISC expects it to supersede A500 over time. Availability and cost to be checked per supplier. |
| Gussets, base and connector plates | ASTM A572 Gr. 50 (A36 where unstressed) | 50 / 65 ksi (A36: 36 / 58 ksi) | Weld-compatible with A500 and A992 under AWS D1.1 procedures. |
| Corner and inter-module bolts | ASTM F3125 Gr. A325 | 120 ksi tensile | Pretension per RCSC. Located so every bolt is reachable from inside the module. |
| Foundation anchor rods | ASTM F1554 Gr. 55 | 55 ksi yield | Cast-in; grout under base plate after leveling. |
| Corrosion protection | Hot-dip galvanizing (ASTM A123) or duplex paint system | n/a | Frame is inside the thermal envelope except at base plates and exposed connectors; those get the heavier system. |
Open on steel: the connection reviews we relied on test local joint behavior under gravity and seismic loads, but the effect of different inter-module connections on global lateral stability of a stacked building “needs more investigation” (LJMU review). Single-story and two-story residential is the lower-risk end of that gap. We have not run any physical connection tests.
Concrete: mix, reinforcement and weight
Floors, the wet-wall core and shear keys are precast. The weight of that concrete, not its strength, is the main feasibility question for a transportable module.
| Property | Specification / method | Target | Notes |
|---|---|---|---|
| Compressive strength | ASTM C39 cylinders per batch | f′c ≥ 5,000 psi at 28 days | Typical precast range. Design to ACI 318 and PCI MNL-120. |
| Reinforcement | ASTM A615 Gr. 60 (A706 where welded) | Fy 60 ksi | Epoxy or galvanized bar at exposed edges. |
| Sleeve and key grout | Non-shrink cementitious, ASTM C1107 | Per manufacturer, verified by C109 cubes | Used in grouted sleeve splices. Pour temperature limits apply on site. |
| Fire resistance | ASTM E119 assembly test | Rating by thickness and aggregate type | E119 heat-transmission end point: unexposed face rises 250 °F average or 325 °F at one point. PCI fire overview |
| Tolerances | PCI MNL-135 | Per element | Governs how floor panels fit the steel frame. |
Module weight check (our estimate, not a test)
A 16 ft × 30 ft module is 480 sq ft. Normal-weight concrete is about 150 lb/ft³, so a solid 4 in floor slab is 50 lb/ft², or 24,000 lb before the steel frame, core, glazing, panels and finishes. Lightweight aggregate concrete (about 110 lb/ft³) would bring the same slab to about 17,600 lb. Our drawing shows an 8.4 in slab; a hollow-core or topped-plank section is the likely way to hold the weight down.
| Floor option | Weight, 480 sq ft | Consequence |
|---|---|---|
| 4 in solid, normal weight | ≈ 24,000 lb | Floor alone is 12 tons. Heavy-haul permit and a large mobile crane for the set. |
| 4 in solid, lightweight | ≈ 17,600 lb | About 27% lighter; costs more per yard and has different shrinkage and fire properties. |
| Steel deck with 3 in topping | to be calculated | Likely the lightest concrete floor; composite design to AISC 360 and ACI 318 required. |
Width is the other transport limit: a federally legal load is 102 in wide, so a 16 ft module moves on an oversize permit and the rules vary by state. Sixteen feet is a common modular width and it adds cost.
Glass: standards and performance targets
Insulating glass units are glazed into thermally broken frames in the factory. We have not selected a glazing supplier, so these are targets and standards, not certified values.
| Requirement | Standard | How it is verified |
|---|---|---|
| Insulating glass units | ASTM E2190 | Manufacturer certification and unit testing |
| Heat-treated or tempered lites | ASTM C1048 | Mill and fabricator certificates |
| Wind and snow load resistance | ASTM E1300 | Calculation per lite size and design pressure; state design wind speed governs |
| Whole-window U-factor and SHGC | NFRC 100 / 200 | NFRC-rated product label; compared against the state energy code climate-zone table |
| Windborne debris zones | ASTM E1886 / E1996 | Required only in wind-borne-debris regions; laminated glass option |
Finish panels: where each one can fail
| Finish | Material standard | Main limitation | Where to be careful |
|---|---|---|---|
| Weathering steel | ASTM A606 Type 4 / A588 | Patina only forms with wet–dry cycles; run-off stains adjacent concrete and soil | Coastal and de-icing salt areas, continuously wet or shaded walls. ASTM G101 warns corrosion can be much higher when steel stays wet or is salt-contaminated. G101 |
| Board-formed GFRC | PCI MNL-128 (recommended practice) | Panel weight and anchorage design; surface crazing and efflorescence if curing is poor | Freeze–thaw states: specify air entrainment and sealer |
| Zinc standing seam | Pre-weathered rolled zinc (EN 988) | Labor-heavy install; dents; contact with copper or treated lumber run-off | Needs a ventilated cavity behind it |
| Terrazzo / cement panel | Precast per PCI | Heaviest of the six per sq ft; edge chipping | Transport and anchorage loads |
| Ceramic rainscreen | Extruded terracotta, EN 14411 / ASTM C1088 family | Brittle; clips and rails are the real structural system | Impact zones; hail |
| Brushed aluminum | ASTM B221 6063-T5, anodized (AAMA 611) | Dents easily; high embodied energy; galvanic contact with steel | Isolate from steel fasteners |
Thermal bridging matters more than the finish does. For steel framing, nominal cavity insulation overstates performance: one ASHRAE-based figure puts R-19 batts in 2×6 steel studs at 16 in. on center at an effective R-7.1 (Building Enclosure). Continuous exterior insulation is the usual fix, and clips or girts that bypass it break continuity. This is why the rail system must be thermally broken, and why we have to model it before claiming any wall R-value.
Studies
| Source | Finding | Limits for us |
|---|---|---|
| McKinsey (2019), Modular construction: from projects to products | Modular projects have cut schedules by 20–50%. Cost savings are “the exception rather than the norm”; operators optimizing for scale could reach more than 20%. Costs can rise by up to 10% if logistics and materials outweigh labor savings. | Mostly multi-unit and commercial. Savings depend on repeating a few designs, which a four-model product line is built around, but it is unproven for single homes. |
| LJMU life-cycle assessment of modular steel social housing (UK) | 91.3 tCO₂e over 50 years; embodied (A1–A3) 38.2%, operational energy and water 48.1%; 366 kgCO₂e/m² (A1–A5), comparable to high-performing CLT buildings. Low-carbon concrete, bio-based insulation and steel optimization could cut upfront emissions by roughly 8–10%. | Light-gauge steel, not HSS frame plus precast floors. Our heavier mix will probably carry more embodied carbon; no conventional-build control in the study. |
| MDPI Buildings 13(7) 1727, review of inter-module connections | Classifies connections as bolted, grouted, self-locking and tie-rod types and compares their mechanical behavior. | Reviews lab tests, mostly on steel modules. Grouted sleeves into precast concrete are less covered. |
| LJMU review of structural elements in modular steel buildings | Local connection performance is well studied; effect on global lateral stability is not. | The same gap noted under steel above. |
| FHWA weathering-steel research (bridges) | Uses the ASTM G101 composition index to compare weathering steel with plain carbon steel. | Bridges, not thin cladding sheet; patina behavior at small thickness differs. We found no stabilization data for A606 Type 4 specifically. |
Pre-shipment checks
| Item | Check | Record |
|---|---|---|
| Steel | Mill test report per heat checked against A500/A992/A572 chemistry and strength | Heat numbers traced to each member |
| Welds | AWS D1.1 qualified procedures; 100% visual; MT or UT on sampled critical joints | Weld map per module |
| Bolts | Lot certificates; pretension verified per RCSC | Torque log at set |
| Concrete | Cylinders (C39) per pour; grout cubes (C109); cover and rebar placement before pour | Batch tickets and break results |
| Glazing | NFRC label, IGU certification, air and water test on a sample per lot | Unit serials |
| Dimensions | Frame squareness and length against the ±1.5 mm shop target (a target, not a measured capability) | Survey sheet before panels go on |
| Module | Water test and blower-door test before shipping | Test report with state data plate |
What would stop this working
- Weight. The concrete floor decides transport permits and crane size. Floor system selection and a full module weight takeoff come first.
- Embodied carbon. HSS plus precast concrete is a heavier carbon profile than the light-gauge steel studied by LJMU. We need our own LCA before making any carbon claim.
- Thermal performance. No whole-wall R-value exists for the rail-and-panel system. It needs a two- or three-dimensional thermal model and, later, a guarded hot-box test.
- Connection testing. Grouted sleeves and bolted corner plates at module scale, under cyclic load, have not been tested by us.
- Cost. Steel and precast are not cheaper per pound than lumber. Any cost advantage has to come from labor, schedule and waste, and McKinsey’s range includes a 10% cost increase.
- Code path. Each state’s modular program decides the inspection and approval route for a non-wood system; see Areas served.