A frame that works on a fixed site does not automatically work on a unit that will be lifted onto a truck, driven for two days and set down on a prepared foundation. The difference is not the material; it is the loads the structure sees before anyone uses the building.

Four load cases, not one

Most structural conversations begin with gravity and wind. A transportable unit has at least four conditions to satisfy:

  • Lifting. The frame carries its own weight plus finishes at four or six pick points, which concentrates load in places the finished building never sees.
  • Transport. Vibration, braking and road-induced torsion act on a long, relatively light box. Bracing and connection stiffness matter more than in a site-built structure.
  • Installation. Level tolerances between unit and foundation, temporary supports and the sequence of connecting adjacent modules.
  • Use. The ordinary life of the building: occupants, furniture, snow, wind, and the movements caused by temperature and moisture.

What each frame type is good at

Steel frames offer predictable behaviour under concentrated lifting loads, allow precise connections and keep dimensions stable when the unit is closed up. They also make it easier to create larger openings, which is why glazed view-facing units tend to use them. The trade-off is thermal bridging at every member unless the envelope is separated from the frame.

Timber frames are lighter, which reduces transport and lifting loads, and they handle thermal performance more simply. They need careful detailing at connections and protection during transport, because a timber frame that gets wet on the road is a problem that appears months later.

Composite and hybrid solutions are common in practice: a steel primary frame with timber infill, or a steel chassis with a timber superstructure. The choice should follow the span, the opening sizes and the way the unit will be handled, not a preference for one material.

Connections decide the outcome

Transportable units fail at connections more often than at members. Bolted connections need tolerance, because a unit that is 3 mm out after a long drive cannot be hammered into alignment on site. Designers who specify zero-tolerance interfaces usually create installation problems that are solved with cutting discs.

The same applies between adjacent modules. If two units are joined, the joint has to accommodate the tolerance of both frames plus the foundation. A visible, adjustable connection is more honest than a hidden one that assumes everything arrives perfectly square.

The foundation interface

How the unit meets the ground determines what the frame has to resist. Screw piles, strip footings, concrete pads and temporary supports each impose different point loads and different levelling requirements. Deciding the foundation strategy after the unit design is a reliable way to redesign the unit.

Choose for the second life

A mobile building may be installed three times in its life. The frame that performs best is not the lightest or the cheapest per tonne; it is the one whose connections can be undone, inspected and re-tightened without destroying the finishes around them.