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Contents
The question most people ask is what holds a full silo down.
The answer is that a full silo holds itself down. Ten thousand tonnes of wheat are not going anywhere. The anchors are not there for that.
The empty silo is the dangerous one
An empty cell is a tall, light, closed cylinder with an enormous side area. Wind passing it does not just push sideways — it creates suction on the far side and on the roof. The result is a force that lifts.
That is why anchorage is sized for the empty silo at the maximum wind for the site, not for the full one. It is the worst case, and the manufacturer calculates it for each type of cell and each wind zone.
It also explains why a silo that stood full through three winters without trouble can suffer in the first summer it stands empty and a storm comes through.
What actually holds the silo down
The wall of the cell does not bear uniformly on the concrete. The load comes down through the vertical stiffeners — the profiles that wrap the corrugated sheet on the outside. Each stiffener ends in a base plate, and the base plate is bolted to the raft.
So you are not anchoring a cylinder. You are anchoring a few dozen points, each with its own load, on a circle whose diameter is given in the drawing.
There is also the base ring, sealed to the concrete so water cannot get in and grain cannot get out. That anchors nothing — it seals. Worth knowing the difference, because on site the two get confused often.
Two ways of setting the bolts
Cast into the concrete. The bolts are positioned before the pour, on a steel template, and stay in the mass of the concrete. This is what almost every manufacturer asks for, because the bolt works over its whole length and the capacity is the one in the calculation.
Installed after curing. The hole is cored or hammer-drilled in hardened concrete and a mechanical or chemical anchor goes in. This is the answer for a retrofit, for a cell added beside existing ones, or for a correction.
The second option forgives position errors. In exchange, it depends on things you can no longer verify afterwards: the real strength of the concrete, the distance to the edge of the raft, how well the hole was cleaned of dust before injection. A chemical anchor set in an uncleaned hole has a fraction of the capacity in its data sheet — and it does not show.
The template decides everything
The holes in the stiffener base plates come from the factory. They are not reamed, not moved, not cut — any of that voids the warranty and weakens the piece exactly where it works hardest.
Which means the bolts have to be where the drawing says, not near it. Position tolerances for an anchor bolt are measured in millimetres, not centimetres.
So a steel template is made from the manufacturer's drawing, braced rigidly, and checked three times:
- before the pour — position, circle diameter, verticality, how far the bolt projects above the finished level
- during the pour — falling concrete and the vibrator will move a badly braced template, and that happens more often than anyone would guess
- after stripping — measured again, before the first truck of equipment arrives
The threads are protected before the pour. A thread coated in cement laitance is hard to clean and easy to damage, and a bolt with a damaged thread can no longer be tightened to the specified torque.
The mistakes that cost
The foundation poured before the documentation exists. The most expensive of them all. The silo arrives, the anchor points do not line up, and concrete does not move. It gets broken out and poured again, two or three weeks gone and an extra invoice. The right order is: contract with the manufacturer, documentation, then foundation.
The bolt too close to the edge of the raft. Pulled upward, a bolt tears a cone of concrete out around itself. If the edge is close, the cone has no room to form and the concrete breaks well below the design load. The edge distance is in the drawing for a reason.
Tightening before the concrete reaches strength. Fresh concrete feels hard after a week. It is not. A bolt tightened too early loses its bond in the mass.
No grout under the base plate. If the plate sits on concrete with voids underneath, the load is not transferred across the surface — it concentrates in the bolt. Levelling grout is there to stop that.
Not re-checking the torque after the first fill. The structure settles under load. Torques are checked again after the first fill-and-empty cycle — an hour of work that is almost always forgotten.
Tightening is not "as hard as the wrench goes"
Joints are tightened to the torque in the drawing, with a torque wrench, in the sequence the manufacturer gives — usually crosswise or in passes around the circumference, not bolt after bolt around the circle.
A bolt tightened too hard stretches and loses its elasticity; one too loose lets the stiffener move, and the movement wears the hole. Both show up only years later.
Water at the base
The anchorage zone is exactly where rainwater running down the wall of the cell collects. A galvanised bolt, grout done properly and a fall that takes water away from the raft are the difference between an anchorage that lasts thirty years and one that rusts in five.
Worth checking at every maintenance visit: look at the base of the stiffeners, not only inside the cell.
What we do
We build the foundations with the anchor bolts positioned within the tolerances in the manufacturer's documentation, and we do the mechanical assembly to the same documentation — so the two stages can no longer blame each other.
We also work where the foundation was poured by someone else: we check the levels and the position of the anchors before assembly starts, while something can still be corrected.
If you have a project in preparation, or a foundation already poured that you are not sure about, write to us with the location, the capacity and the equipment manufacturer.
See also foundations and concrete infrastructure and grain silo mechanical assembly.
