A loadout canopy gets specified as "cover over the loading bay", and then someone measures the widest truck and adds a metre. That produces a canopy that is too short on a busy day and unnecessarily wide every day.
The two dimensions answer two different questions, and neither of them is the truck's width. Length is a queueing question: how many vehicles stand under cover at once, and for how long. Width is a cycle-time question: how many operations happen side by side under that cover, and how much room a vehicle needs to get in and out without touching the structure.
1. Canopy length is a queue calculation
A loadout canopy exists to keep product dry between the silo and the truck, so its length has to cover the vehicles that are actually being loaded, plus the ones waiting their turn under cover.
The queue length comes from the cycle. If a vehicle spends twenty minutes under the spout and arrives every twelve minutes, there will be a queue, and the queue will stand in the rain unless the canopy covers it. That arithmetic is more useful than any rule about canopy length, and it usually produces a longer canopy than the budget assumed — or a second loading point, which is the cheaper answer.
The other factor is whether the canopy also covers the driver's paperwork and sampling position. On most sites it should; a sampling point in the open is a sampling point that stops in bad weather, and the canopy is the cheapest way to keep dispatch running through the wet season.
2. Canopy width is a cycle-time calculation
Width is decided by the number of lanes and the manoeuvring space between them.
A single lane needs the vehicle width, plus clearance to the columns on both sides, plus enough room for the driver to open a door and for a person to walk past. That is rarely less than about 8 m (26 ft) between column faces, and on sites where vehicles are reversed in, more.
Two lanes side by side is not simply double. It needs a separation that prevents one vehicle clipping another while manoeuvring, and it needs the columns positioned so that neither lane loses its turning envelope. In practice two lanes plus a central column line is often worse than two lanes under a wider span, because the central column sits exactly where the vehicles want to swing.
The honest rule: decide the lanes and the manoeuvre first, then let the span follow. A canopy that is 18 m (60 ft) wide with no internal columns is a different structure from one that is 18 m wide with a row down the middle, and only one of them lets a long vehicle turn.
3. Height: the fifth wheel and the raised body
Canopy height is set by the tallest vehicle with its body raised, not by the vehicle in transit.
A tipper body raised to discharge is the obvious case, and it is usually accounted for. What gets missed is the equipment on the vehicle: a raised tarpaulin frame, a loading arm, an operator platform, or a conveyor that folds out. Each of those adds to the clearance that has to exist under the canopy at the point where the vehicle actually stops, which is not the same as the clearance at the entrance.
The practical figure is the raised height plus a working margin, measured at the loading position rather than averaged across the canopy. A canopy that clears the raised body at its centre but not at the edge is a canopy that gets damaged by the first vehicle that parks slightly off line.
4. Dust and spillage under the canopy
A loadout point generates dust and drops material, and the canopy is where both land.
Dust under a canopy is worse than in the open because there is no wind to disperse it, so it settles on the structure. In a steel canopy the settlement surfaces are the purlins and the tie members, and the accumulation becomes a load and a fire risk at the same time. The design answer is a structure with as few horizontal surfaces as possible above the loading position, and a roof profile that sheds rather than traps.
Spillage is a floor question. Material dropped at the spout has to be cleared, and clearing it means a machine entering the canopy — which means the floor has to take that machine's wheel loads, and the clearance under the canopy has to accommodate it. A canopy designed only around trucks often cannot fit the loader that keeps it clean.
5. The structure between silo and canopy
The canopy usually attaches to something: a silo, a headhouse, or a separate frame. Each option has a different consequence.
Attaching to a silo means the canopy's loads go into a structure that was designed for a different purpose and is often already built. Silos settle, canopies are loaded by wind, and the junction between them is where differential movement shows up as a cracked flashing or a torn sheet. Attaching to a separate frame avoids that, at the cost of a second set of columns and foundations in exactly the space where vehicles manoeuvre.
The third option — a canopy cantilevered from a single line of columns on the far side of the lanes — is often the best compromise, because it leaves the vehicle side completely clear. It also concentrates load in those columns, which means they need to be designed for it rather than inherited from the silo structure.
6. What I want before drawings
- The loading cycle time and the arrival interval, so the length comes from arithmetic.
- The number of lanes, and whether vehicles drive through or reverse in.
- The tallest raised configuration of the largest vehicle, at the loading position.
- What clears the spillage, and how heavy it is.
- Whether the canopy attaches to an existing structure or stands free.
- The wind speed to be used, taken from the applicable code at the project site rather than a round number, plus the open-side edge detail — a canopy with three open sides is loaded by uplift on the roof and by internal pressure at the open edges, and that is a fixing question before it is a frame question.
A canopy is a small building with a large job: it decides whether dispatch stops in the rain. Size it from the queue and the cycle, not from the width of a truck.