A feed mill's pelleting hall is set by loads, not by bags
Ask for the size of a feed mill and you will get a throughput: so many tonnes an hour, so many bags a day. Ask what the building has to carry and the answer usually goes quiet.
That gap is where the frame gets decided by default, and the default is the cheapest section that will pass a uniform load check. A pelleting hall is not a uniform load. It is a small number of very heavy things standing on a floor that also has to be walked on, swept, and opened up for maintenance.
The brief arrives in tonnes; the frame needs kilograms
A pelleting line is a conditioner, a pellet mill, a cooler, a crumbler and a sieve, stacked so that material falls from one to the next. The mill and its conditioner sit as one assembly, and that assembly stands on a small footprint with a high centre of gravity.
The beam under it is therefore not sized by a floor live load from a table. It is sized by a concentrated load at a known position, with a dynamic factor, plus the vibration that comes with a rotor turning under load. That number exists in the equipment supplier's datasheet and should be in the brief before anyone prices a frame.
When it is not in the brief, the design still gets done — against a sensible-looking floor load. The building is then correct on paper and marginal in service, which is worse than being wrong, because nobody re-checks it.
Pelleting is a vertical process, so the building is a stack of floors
Feed moves up before it moves down. Intake, grinding, batching and mixing happen at the bottom; the material is then lifted to the top of the tower and falls through conditioning, pelleting, cooling, crumbling and screening on the way back down.
The consequence for a steel building is that the floor framing carries more of the design than the roof does. A shed of the same footprint with the same span would be a different structure: lighter purlins, fewer beams, no floor openings. What you are pricing is a multi-level frame with a roof on top.
This is also why eave height is the wrong first question here. In a mill the controlling height is the top of the elevator leg and the headroom above the highest machine, and the eave follows those. A mill designed from an eave height backwards usually ends up with a tall building and a cramped top floor.
Dust relief decides the envelope before the span does
Grain dust is combustible, and a mill is designed with that in mind: the envelope is given areas that are intended to relieve pressure outward rather than let the building fail as a whole.
For the steel frame this has three direct consequences. The relief area has to exist somewhere in the walls or roof, so the envelope is not simply a sealed box. The panels that form it must be fixed so that they release in the intended direction, which constrains panel orientation and fixing pattern. And nothing can be hung from a panel that is designed to blow out — no cable tray, no duct bracket, no signage.
I have seen the third point missed more often than the first two. A tray bolted to a relief panel converts a designed opening into a projectile.
Every duct is a hole in the roof
Aspiration ducting, cyclones and bag filters all want to be outside. Each one is an opening, and each opening is a structural and a waterproofing problem at the same time.
A roof-mounted dust collector is heavy and it stands on a small footprint, which means it needs its own framing and a curb, brought down to a rafter or a column line. It cannot be carried by purlins, and it cannot be bolted to the sheeting. Where collectors are grouped, the openings should be grouped too: three holes in one bay with one upstand is a detail; the same three holes spread across a roof is three future leaks and three trimmers.
The elevator leg is a separate tower
The bucket elevator runs from the pit to the top of the mill, and it is its own braced tower standing next to the frame.
The detail that matters is the connection. The leg moves — thermally, and under wind — and so does the building, but not identically. If the leg is welded rigidly to the frame at every floor, the differential movement goes into the frame and into the leg connections. Tying it at floors with slotted or sliding connections lets each structure move and keeps the tie doing only the job it was meant to do.
The same principle governs the junction between a long mill and a long building's expansion joint: once two structures move differently, the detail has to say so.
Silos settle; the mill does not
A silo battery beside the mill carries a very different load pattern on a very different foundation, and the two settle differently over the first years.
Where a bridge or a conveyor connects the silos to the mill, that connection has to allow the differential movement. Made rigid, it becomes the member that transfers settlement into the mill frame, and the symptom appears as cracked finishes and loose bolts at the opposite end of the building rather than at the silo.
A mill is not one corrosion environment
Most of a feed mill is dry. The conditioning stage adds steam, the liquid addition area handles molasses and oils, and the intake area handles wet grain in season. One global corrosion class across the whole building is either expensive in the dry zones or inadequate in the wet ones.
Set the class per zone, and let the coating and the detailing follow — the way it is done for a pump house, where the wet corner and the dry corner are not the same specification.
What I want before drawings
- The equipment list, with weights, footprints and support points, not only the throughput.
- The floor-by-floor layout with every opening, including openings reserved for a future line.
- Where relief venting is expected, and what stands in front of it.
- Collector and cyclone positions, with their weights and their support points.
- Elevator leg height and how the leg is intended to be tied to the frame.
- Silo layout, and whether silos and mill share a foundation.
- The wind speed for the project site in km/h, taken from the local code that applies there rather than from a round number — a tall, narrow tower beside a low shed is exactly the shape where the two get loaded differently and a single assumed value hides it.
- The erection sequence you expect, because a mill is built around its own equipment.
None of these are steel questions. All of them change the steel drawing.
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: eave height · expansion joints in a long building · corrosion class as a selection input · erection sequence
