Single-span portal frame sheds

How does Single-span portal frame sheds work, step by step?

1. 1

   Step 1: Fix the operational brief before the geometry

   The building is a container for an operation, so the operation is described first. That means the racking type and aisle widths, the truck the operator will run, the pallet height, the number and type of dock levellers, the yard depth needed for a full articulated turn, and whether sprinkler protection will be in-rack or roof-only. Those inputs set clear height, span and bay spacing far more directly than any architectural preference. On a speculative building where no occupier is signed, the developer sets a specification aimed at the widest plausible market, and the structural engineer designs to that. Either way the brief is written down and frozen before the steel is designed, because a change to clear height after fabrication starts is expensive in a way almost nothing else on the project is.

2. 2

   Step 2: Design the frame and its foundations together

   A portal frame pushes outwards at its feet as well as downwards, and the foundation design has to deal with that horizontal thrust as well as the vertical load. The structural engineer decides between pad foundations with a tie, piled bases, or restraint taken through the ground floor slab, and that decision is made with the geotechnical designer against the ground investigation rather than assumed. Bay spacing, rafter depth, bracing bays, the wind bracing arrangement and the crane-lift weight of each assembly all come out of the same design. Member sizes, connection details and foundation dimensions are the structural engineer's, and nothing on site changes them.

3. 3

   Step 3: Build the platform, the bases and the drainage

   Groundworks strip the site, deal with any weak or made ground the investigation found, and form a working platform the erection cranes can stand on safely. Column bases are cast to tight positional tolerance with holding-down bolts set in templates, because a base out of position is a problem that follows the whole frame up. Underslab drainage, ducts and any deep services go in at the same time, since almost nothing can be added under a warehouse floor once it is poured. The platform is a designed and certified element in its own right, and it is re-inspected as tracked plant chews it up.

4. 4

   Step 4: Erect the frame in a controlled sequence

   Frames are assembled at ground level where practical, then lifted and held on temporary bracing until the permanent bracing bay is complete. Erection works outwards from a braced bay so the standing steel is always stable, and the temporary works designer signs off the propping and the sequence before anything is lifted. Wind is the governing constraint on the day - a part-erected portal frame with no bracing and no cladding is far more vulnerable than the finished building, and wind limits for lifting are set in advance and obeyed. Plumbing and levelling of each frame is checked and recorded before the crane is released.

5. 5

   Step 5: Close the envelope from the steel down

   Purlins and side rails go on immediately behind the frame, then roof sheets, rooflights, gutters and wall cladding follow bay by bay. Closing the envelope progressively gets the building watertight sooner, which is what protects the floor pour later. Roof safety is the dominant risk here: fragile rooflights, leading edges and access all sit under a specific plan, and access to the roof after handover is designed in rather than improvised. Personnel doors, dock doors, level access doors and the canopy steel are set out from the yard design, not from the elevation drawing.

6. 6

   Step 6: Fit out the interior and the fire strategy

   Sprinkler mains, the tank and pump house, lighting, power distribution, heating and ventilation, offices and welfare pods are all installed inside the shell. The fire strategy drives a lot of this: compartment walls, smoke ventilation and the sprinkler arrangement all have to be resolved with the racking layout rather than after it, because in-rack sprinklers change both the racking and the services. On most projects the office pod is a separate structure inside or attached to the shell with its own frame and floor, and it runs on its own sub-programme.

7. 7

   Step 7: Pour the floor last and protect it

   The ground floor slab goes in once the building is watertight and the main overhead work is done, because a floor poured under an open roof is a floor at the mercy of the weather. Curing conditions matter enormously and are controlled deliberately. The floor specialist and the structural engineer set the slab design, the joint arrangement and the acceptance criteria against the known racking layout and the traffic, and the floor is protected from following trades until handover. Almost every long-running warehouse dispute starts with a floor that was rushed at the end of a programme that had already run late.

8. 8

   Step 8: Commission the yard, the doors and the building services

   The yard is finished with its own falls, drainage and interceptors, marked out for the turning circles the operator actually needs, and surfaced to carry loaded artics rather than cars. Dock levellers, shelters and doors are commissioned and tested with a real vehicle. Building services are commissioned, the fire systems are witnessed, and the operations and maintenance information is handed over with the roof access arrangements. The building only works once the yard works, and a yard that is a metre short of a comfortable turn will be complained about every day of the lease.