Steel Baseplates And Holding Down Bolts

A steel column base has to take the whole load of the column and spread it into concrete that is a fraction as strong.

It also has to be installed to a tolerance the concrete trade cannot hold, which is why the base is the most adjusted and most compromised connection in a steel frame.

What the base has to do

Spread the load. Steel can carry stresses an order of magnitude above what concrete can bear, so the plate has to distribute the column load over enough area that the concrete and the grout beneath it are not overstressed.

Transfer moment, if there is any. A pinned base transfers vertical and horizontal load only. A fixed base also transfers moment, which means tension on one side of the plate and a much heavier bolt group.

Transfer horizontal load, by friction, by the bolts in shear, or by a shear key cast into the concrete.

Hold the column down, which matters under wind uplift on a light structure. On a carport, an awning or a shed, uplift rather than compression is frequently the governing case, as it is in [bracing and tie down in timber framed houses](/blog/bracing-and-tie-down-in-timber-framed-houses).

Pinned or fixed, and why it changes everything

The distinction is the single largest decision at the base.

A pinned base is a small plate with bolts close to the column, designed to rotate. The frame above has to provide its own lateral stability, which usually means bracing or a core.

A fixed base is a larger plate with bolts set well out from the column, designed to resist rotation. It gives the column a shorter effective length, which can halve the required section, and it puts a moment into the footing, which makes the footing larger.

Which means the choice is a trade between the column and the footing, and the column's own capacity depends on it, as set out in [columns, slenderness and buckling](/blog/columns-slenderness-and-buckling).

The error is to draw a fixed base and build something that rotates. A plate too thin, bolts too close in, or grout that was never properly packed all turn a nominally fixed base into a partly pinned one, and the column above was designed on the assumption that would not happen.

Plate thickness is usually the limit

The plate spans between the bolts and the column, and it bends.

A plate too thin yields and dishes, which lets the bolts stretch and the base rotate. It is a quiet failure that shows as a column slowly going out of plumb rather than as anything dramatic.

Thickness is governed by the bolt layout rather than by the load alone: bolts set further from the column need a thicker plate to span to them. So moving the bolts out to gain moment capacity costs plate thickness, and the two are designed together.

Stiffeners are the alternative to thickness. They work and they add fabrication cost and they create more welds to inspect.

Holding down bolts

The item most often wrong on site.

Cast in, not drilled, wherever possible. A cast-in bolt with an engineered head or plate develops its capacity through the concrete. A post-installed anchor into a footing that was poured without provision is a different and generally weaker proposition, and it is the subject of [anchors into concrete and why they fail](/blog/anchors-into-concrete-and-why-they-fail).

Edge distance and embedment are the capacity, not the bolt diameter. A large bolt close to the edge of a footing fails the concrete in a cone, not the bolt in tension.

Position tolerance is tight and the concrete trade's tolerance is not. Which is why the bolts are set in a template rather than by measurement, and why the plate holes are oversized with thick washers to absorb what remains.

Bolts bent into position is the failure to watch for. A bolt bent to line up with a hole has lost a proportion of its capacity and introduced a prying action nobody calculated. Where the position is wrong, the remedy is a new plate, not a persuaded bolt.

The grout, which is structural

The gap between the plate and the concrete is not a tolerance allowance, it is a load path.

The plate is set on shims or levelling nuts, plumbed, then the gap is filled with a non-shrink cementitious grout that transfers the compression into the concrete. A partially filled gap means the whole column load is going through the shims, which is a point load on an area a fraction of the plate.

Which makes grouting a hold point rather than a finishing task. Voids in the grout are invisible once the base is covered and they are a common cause of bases that rotate.

Corrosion, because the base is always the first to go

A column base sits at the floor or ground level, where water collects and stays.

Which makes it the single most common place structural steel loses section in Sydney, as set out in [corrosion protection for structural steel](/blog/corrosion-protection-for-structural-steel). The detailing answers are to raise the base above the finished surface, drain the area, encase the base in concrete where appropriate, and never leave an exposed plate sitting in a puddle.

Assessing an existing frame starts at the bases for that reason.

ACSES provides structural engineering and connection design across Sydney. Talk to us about a project.