Every steel column ends at a base plate, and every base plate is held by anchor bolts cast into the concrete foundation. This small connection is where the steel structure meets the civil works - and where many site problems begin, because it depends on coordination between the steel supplier and the local foundation contractor. A few millimeters of anchor bolt error can stop an entire erection.
What the Connection Does
The base plate spreads the column's concentrated load onto the concrete foundation, and the anchor bolts hold the column in position against uplift and horizontal forces. In moment frames, the base connection also transfers bending into the foundation, requiring a stiffer base plate and more bolts. The design is normally done by the structural engineer following the steel design standard (e.g., GB 50017) and the foundation code (e.g., GB 50007).
Anchor Bolt Layout and Tolerances
Anchor bolts are positioned to the base plate hole pattern, typically with a tolerance of ±3 to ±5 mm in plan. To make this workable on site:
The steel supplier issues an anchor bolt plan showing bolt group positions, diameters, projection above concrete, and thread length.
The foundation contractor sets the bolts using a template or the bolt group drawing, not freehand.
The base plate holes are usually oversized by a few millimeters, and slotted holes are provided in one direction where adjustment is needed.
Any deviation beyond the tolerance is corrected with approved methods (sleeves, re-drilling only with the engineer's approval) - never by bending the bolts.
Base Plate Types
Simple (pin) base: a flat plate with bolts, carrying axial load and shear only - typical for braced frames and single-storey columns.
Moment base: a thicker plate with more bolts, sized to carry bending from the column into the foundation - typical for portal frame columns in seismic regions.
Column base with cap: a cap plate welded to the column over a concrete pedestal, used where the column sits above grade for corrosion protection.
Grouting
After the columns are erected, plumbed, and leveled, the gap under the base plate is filled with non-shrink grout. This transfers load uniformly to the concrete. Grout must be mixed and placed correctly, allowed to cure before the frame is fully loaded, and protected from frost and rain during curing.
Buyer's Checklist
Confirm the supplier issues an anchor bolt plan before foundation works start.
Send the bolt plan to your foundation contractor in writing - this is the single most common site miscommunication.
Verify the grouting procedure and that grout is non-shrink.
For export projects, check that anchor bolts and the fixing set ship with the first container or by courier - missing bolts stop erection completely.
The column-to-foundation connection is simple in principle and critical in practice. A supplier that coordinates the bolt plan with your civil works is protecting your schedule; one that sends steel without drawings leaves the most common erection problem to chance.