A cross-dock is a door building with a very short middle
A cross-dock moves goods from an inbound trailer to an outbound trailer with as little storage as possible. Trailers dock on both long sides, and the goods cross the floor between them.
That single idea produces a building with very specific proportions: wide enough for the staging lane, long enough for the doors, and with a floor that is flat from one end to the other.
Door count sets the length
The number of doors on each side comes from the trailer arrival pattern and the time each trailer spends at the dock. That is a traffic calculation, and its output is the building length.
Door spacing then follows the trailer width plus the clearance needed to open a door and walk past it. Getting that dimension wrong produces a dock where drivers cannot open their doors fully, which is discovered on the first day of operation.
The floor has to be level and it has to be flat
A dock leveller works within a limited range. If the apron settles or the floor falls away from the door, the leveller runs out of travel and the trailer cannot be loaded.
That makes the slab and the apron a single design problem. Both need to be designed for the same loading, both need to be jointed so they move together, and the joint at the door line needs to be detailed so it does not open up.
The apron is the part that fails first
Trucks arrive, reverse, wait and leave on the apron. The pavement takes static loads from a stationary trailer and turning loads from a manoeuvring one, and the turning loads are much more severe.
The apron therefore needs a heavier pavement than the access road, and it needs to drain away from the doors rather than towards them. Water running to a dock door in a tropical climate is a daily problem during the wet season.
Canopies over the doors are a queue calculation
A canopy keeps rain off the trailer and off the goods during transfer. Its length is set by how many trailers are being worked at once and how long the transfer takes; its width is set by the trailer and the handling equipment.
A canopy that is too short is the same as no canopy for the trailer at the end of the row.
A cross-dock has almost no storage, which changes the fire case
Because goods cross the floor rather than sit in it, the fire load in a cross-dock is lower than in a warehouse of the same area. That is worth telling the insurer, and it is worth reflecting in the structure rather than building a warehouse and using it as a cross-dock.
What I want before drawings
- The door count on each side and the door spacing.
- The dock height and the trailer fleet heights.
- The apron layout, the turning envelope and the drainage falls.
- The canopy length and the number of trailers worked at once.
- Whether any storage is intended, and how much.
- The wind speed for the site in km/h from the applicable local code, not a round number.
A cross-dock is one of the few buildings where the door schedule is genuinely the design. Settle the doors and the floor, and the rest follows.
Written by Johnny Joe, who advises owners and project managers in Africa and South America on steel buildings. If this decision is live on a project, send the layout and the constraint that worries you and I will tell you what it changes.
Related: the dock height decision · traffic layout before the frame · canopy sizing from the queue · a floor added inside a shed
