Structural Engineer — Volumetric Modular Steel (PPVC), China Experience

1. Purpose and how your output will be used We are developing a repeatable, factory-built low-rise modular apartment product. We need you to resolve the structural approach and present the options, so that: your recommendations can be handed to our architect to develop the architecture around a sound structural system, and they can be detailed by an overseas (China-based) module factory for fabrication. You sit upstream of the architect in this process. We are asking for a structural design basis and options memo with concept sketches — not stamped design at this stage. 2. The product Type: low-rise residential, 1–3 storeys. Includes a 2-storey residential format as a core target — in both arrangements: stacked single-level flats (one dwelling per level), and 2-storey maisonette / townhouse units where a single dwelling spans two stacked modules with an internal stair (note the resulting floor penetration at the intra-unit stair — please account for both arrangements). Construction: volumetric modular — fully-finished steel module boxes (PPVC-style), factory-built and delivered to site for stacking. This is the approach we are pursuing. Markets: two design regions — Australia and a tropical, high-seismic regional market — with differing wind and seismic regimes. Commercial driver: the advantage is offsite manufacturing plus repetition — this is a product, not a one-off. Structural decisions that maximise standardisation and minimise module variants are preferred. 3. What we need you to resolve and recommend Lead the concept with the module split logic. Please drive the concept from the module breakdown itself — transport dimensions, the steel frame grid, wet-zone clustering, service routes, module junctions and the site assembly sequence should shape the design from the outset, not be applied after an architectural concept is fixed. Everything below sits within that discipline. 3.1 Module structural system Confirm or improve a corner-supported steel module approach for 1–3 storeys. Recommend frame member strategy (corner posts, edge beams) and floor system options (steel deck, composite, or concrete) with the trade-offs for acoustic/fire/weight. Roof/top-module strategy. 3.2 Inter-module connections (priority) Vertical (stacked) connection — corner connector design and load path: gravity, uplift/tension (wind, seismic), and shear; a self-locating detail (guide pin/spigot) for fast craned erection; erection tolerance; bolt access. Horizontal (adjacent) connection — corner-to-corner tie plates and their diaphragm role (preventing independent module sway / swing-out). Confirm these are designed as load-bearing, not alignment-only. Indicative connector types and capacities, and where bolting is accessible during erection. Connection must be concealed in the finished building (mandatory). We do not want exposed external connectors or visible ISO-style corner castings (container/portable-building aesthetic). The connection should be accessed internally during erection — e.g. through small floor/ceiling access points at the corners that are subsequently closed and sealed — leaving no hardware showing externally. This is a permanent residential product and the connection must read as conventional construction. Please propose a concealed connection system that meets this while still delivering the load path and tolerance above. 3.3 Lateral stability and global behaviour Stability strategy at 1–3 storeys for wind (Australian AS 1170.2 regions, including potential cyclonic sites; severe/cyclonic wind in the regional market) and earthquake (AS 1170.4; the regional market is highly seismic and likely governs there). Bracing strategy, diaphragm action via the tie-plate system, and base fixity — including whether the lift/fire-stair core provides primary lateral stability (see Section 3.5). 3.4 Foundations Footing/slab options for stacked modules, including point loads at corner posts. Differing ground conditions and seismic demand between the two markets; levelling and the foundation-to-module tolerance interface. 3.5 Circulation, lift and fire-isolated stair — structural implications The building requires a lift and a fire-isolated escape stair. Please advise how these are best constructed and how they interact with the module stack: Lift and fire stair core: recommend whether the lift shaft and fire-isolated stair are built as a separate core (steel or concrete, conventional) or as dedicated shaft/stair modules, addressing shaft plumb/alignment, lift pit and overrun, fire-rating of the stair enclosure, and egress compliance. Core as stability element: advise whether this core can act as the building's shear/stability core, taking lateral load and simplifying bracing within the residential modules (cross-reference Section 3.3). Module-to-core connection: how the modules tie to and bear against the core, including movement/tolerance and fire separation at the interface. We are also weighing three resident-access circulation types; please advise how each is structured and recommend per use case: Stair-cluster (walk-up): stair construction (modular steel stair vs conventional), in addition to the required fire-isolated stair. External gallery access: a separate steel walkway bolted to module faces — its structure, footings/hangers, and the module-face connection (favoured for the tropical market's climate). Internal double-loaded corridor: corridor floor spanning between two module rows, landing on module ledges. 3.6 Transport, lifting and handling Module behaviour (racking/distortion) under lifting and transport; engineered lifting points; transit bracing to protect prefinish. Module size is governed by the transport envelope (sea freight + road at each end). We have not fixed preferred module size limits — please propose recommended module size limits for Australia / Pacific transport during the concept stage, with the option to refine against a defined envelope later. Develop the steel frame grid and module split around those limits. 4. Deliverables Structural design basis for the product (load cases, system, code basis per market). Inter-module connection recommendation — vertical + horizontal, with concept details and indicative capacities. Lateral stability and foundation strategy. Circulation structural options compared, with a recommendation per use case. Concept sketches / sketch details sufficient for the architect to design to and the factory to detail from. Risks, assumptions and a clear statement of what requires full design and certification later, by whom and when. 5. Boundary This is an advisory / concept engagement. Design-of-record and certification for any built project will be engaged separately with engineers registered in the relevant jurisdiction, carrying professional indemnity cover. We are not asking you to certify a built structure at this stage. 6. Process — how your work feeds the next steps Your structural recommendations → architect's concept → factory detailing → in-jurisdiction design-of-record and certification.