A crushing and screening plant is described by its throughput, so the building gets sized by its throughput. It is the first mistake on almost every project of this type.
Tonnage tells you how big the equipment is. It does not tell you how big the hall has to be. What decides the hall is the maintenance lift: the height needed to lift a mantle, a screen box or a drive assembly out of the machine and swing it clear, and the clear floor beside the machine where the part can be set down. On most plants I look at, that single requirement adds two to three metres of height above anything the process itself demands, and it is the number cut when the budget tightens.
Cut it, and the plant runs for two years and then discovers it cannot change a screen deck without dismantling the roof.
1. The maintenance lift sets the height, not the throughput
Start with the heaviest removable component and the lift that removes it. A screen box on a large unit can be several tonnes, and the crane or hoist that lifts it needs a hook height above the machine's top, plus the height of the sling, plus clearance to swing the load sideways out of the frame.
In practice a primary station with a jaw or gyratory unit wants clear height in the order of 12 to 15 m (40 to 50 ft) to the hook under the roof structure, not to the eaves. That is a different number from the one on the process drawing, and it is the one the steel should be drawn to.
The same logic applies to the access: a maintenance platform at the right level is worth more than an extra metre of height, because a fitter working off a ladder inside a running plant is a stoppage waiting to happen.
2. Feed opening and bin drop: the two elevations that fix everything
Two elevations dominate a crushing hall, and they are set by different people.
The feed opening is set by the haul truck or the loader that charges the plant — the discharge height of the largest unit that will ever tip into it, plus a margin so the operator is not guessing. The bin drop is set by the discharge conveyor or the stockpile below. Between those two elevations sits everything else: the machine, the platforms, the chute angles, the dust collection.
When those two numbers are fixed early, the hall designs itself. When they are left to the end, the hall gets a mezzanine that nobody planned for and a conveyor that runs at an angle the material will not accept.
Chute angles are worth a note because they cause more retrofits than the structure does. Wet, sticky feed needs a steeper angle than dry, angular rock, and the difference is measured in degrees that translate into several hundred millimetres of height at the discharge end. Ask what the feed will be on its worst day, not its design day.
3. Vibration is a foundation problem before it is a structure problem
Screens and crushers put dynamic load into whatever they stand on, and the building is standing next to them.
The mistake is to treat the machine foundation and the building foundation as one job because they are poured at the same time. They behave differently. A screen frame that is tied rigidly to the building structure transmits its motion into the frame and into every bolted connection above it, and the result shows up as loose bolts and cracked welds within a season.
Separating them costs a joint detail and a bit of coordination. Sharing them costs years of tightening.
The same applies to the structure directly above a screen: walkways, chute supports and handrails should be supported from the machine frame or isolated from it deliberately, not connected to whichever member is nearest.
4. Dust, spillage and the floor that has to take both
A crushing hall is a dusty building with a floor that receives material it did not ask for.
Dust settles on every horizontal surface, and in a steel building those surfaces are the purlins and the top flange of every rafter. That is a cleaning problem and a fire load at the same time, and it is solved by detail, not equipment: members that shed dust, no inaccessible pockets above the plant, and a roof profile that does not trap a layer of fines.
The floor has a different job. Spillage from chutes, washdown water and rain off the feed material all arrive at the same place, and the floor has to take an abrasive, heavy load while draining. Slopes in the order of 1 in 100 toward a collection point, and a slab designed for the actual wheel loads of a loaded loader rather than a nominal warehouse figure, are the two decisions that matter. A slab that cracks under a loader does not fail visibly — it fails by spalling at the joints, and the repair is a shutdown.
5. Where the plant will grow, and why that decides the bay layout
Almost every crushing plant is built in phases, and the second phase usually arrives within three years.
That has one design consequence that is cheap now and expensive later: keep one full bay clear of anything that cannot be moved. Conveyor foundations, transformer plinths, water tanks and buried cable routes all end up exactly where the next stage wants to go, and moving any of them means stopping the plant.
A bay slightly longer than today's layout needs is the cheapest option available. It also gives the maintenance lift somewhere to land, which is the same argument from the other direction.
6. What I want before drawings
- The heaviest component that will ever be lifted, and how it leaves the machine.
- Feed elevation and discharge elevation, with the worst-case material rather than the design material.
- Whether the machine foundations are separate from the building, and who is responsible for the joint between them.
- The wheel loads of the largest loader or truck that will work inside the hall.
- The phase 2 layout, even if it is only a sketch.
- The wind speed to be used, taken from the applicable code at the project site rather than a round number, together with the roof edge detail — an open-sided or partially clad hall behaves very differently from a closed one, and the difference shows at the eaves first.
None of these are steel questions. All of them change the steel drawing, and the maintenance lift is the one that gets cut first and regretted longest.