A container freight station is described as a warehouse with a big door. It is closer to a road junction with a roof over it.
What happens inside is that vehicles arrive, are positioned, are opened, unloaded or inspected, and leave — and the building's job is to allow that to happen in the rain without anyone being hit. The steel is a long clear span over a traffic layout, and the traffic layout is what makes the span long.
1. The span comes from the turning envelope
The controlling dimension in an inspection shed is not the container. It is the circle a vehicle needs to turn in, or the aisle it needs to reverse in.
A tractor unit with a 12 m (40 ft) trailer needs a substantial swept path, and a shed that lets it drive in and out in a straight line at both ends is a different building from one where it must reverse. The straight-through arrangement needs doors and openings at both ends and a span that covers the aisle plus the working strip either side — commonly 24 to 30 m (80 to 100 ft) clear.
The reversing arrangement needs even more width and a much taller door, because reversing under a roof is done with mirrors and a banksman, and the building has to give both of them room. In practice, most sheds are designed for straight-through traffic because it is safer and faster, and the span is accepted as the price.
2. Clear height: the container on the chassis, plus the lift
Height is set by three things stacked: the chassis deck, the container, and whatever lifts it.
A container on a chassis puts its top at roughly 4 m (13 ft). A reach stacker or a forklift handling a container needs room above that for the mast, the spreader and the lift itself — and if the machine works inside the shed, the clear height to the lowest obstruction is in the order of 8 to 10 m (26 to 33 ft), not 5 m.
That is the number that decides whether the shed is a warehouse or a portal frame with a high eaves, and it is also what makes the roof drainage difficult: a large, tall, low-pitched roof has a lot of area draining to a small number of outlets.
3. Roof drainage on a big low roof
A shed of 5,000 m² (54,000 sq ft) at a shallow pitch collects an enormous volume of water in a storm, and the failure mode is not the roof falling in — it is water arriving where it should not.
Three decisions control it. The number and position of outlets, distributed so that no point of the roof is more than a short distance from one. The fall of the gutters, which on a long shed accumulates and has to be designed rather than left to the erector. And the overflow, which is the item nobody draws: an outlet that blocks, with no secondary path, sends water over the edge and down the wall, and through the door if the door is at the low end.
On a shed with a large door, an overflow scupper above the door head is a small detail that prevents a memorable failure.
4. The doors: where the wind gets in and the water follows
A container shed has large openings, often open most of the day, and those openings change the building's behaviour.
Structurally, a shed with two big openings at opposite ends is a tube, and wind entering one end loads the frame and the cladding inside as well as outside. The bracing has to be arranged so that it does not rely on the panels that the openings removed, which usually means braced bays in the side walls rather than the ends.
Practically, an open door is where rain arrives sideways. A canopy over the opening, sized to the local weather rather than to a nominal metre, and a door threshold that is raised or drained, keep the working floor usable. A shed that floods at the door every wet season is a shed that stops working at the moment it is most needed.
The door itself is also a wind problem. A large roller or sliding door presents a broad face to the wind when it is closed, and the track and guides take that load before the frame does. Doors on an exposed elevation should be detailed with that in mind, and the structure behind the track should be designed as a fixing point rather than left to the cladding rail.
5. Inside: hardstanding, markings and the people between vehicles
The floor of an inspection shed takes container handling equipment, and it also carries people who are working around vehicles.
The floor has to be designed for the wheel loads of the largest machine, with joints that do not sit where the machines turn. The markings are a structural decision in disguise: the lanes, the pedestrian route and the no-go zones have to be laid out so that a person never has to cross a reversing path, which often means a raised walkway along one side rather than a painted line.
That walkway then needs its own structure — a plinth or a mezzanine with stairs at intervals — and it is much cheaper to include in the original design than to add to a finished shed.
6. What I want before drawings
- The vehicle types, and whether they drive through or reverse.
- The handling equipment used inside, and its height and wheel loads.
- Whether the shed is open-sided or closed, and which openings stay open all day.
- The rainfall intensity used for drainage, and where water may discharge.
- Whether a pedestrian route separate from the traffic is required.
- The wind speed to be used, taken from the applicable code at the project site rather than a round number, plus the large-opening detail — an open-ended shed is loaded internally as well as externally, and the fixings at the open ends fail before the frame does.
A freight station is a traffic building. Fix the swept paths and the clearance, and the shed is simply the enclosure that keeps the operation dry.