Primary Framing
Base Plates and Anchor Bolts for Steel Buildings
Definition: A base plate is a steel plate welded to the bottom of a column that spreads its load onto the foundation. Anchor bolts cast into the concrete hold the plate in place and resist uplift, shear and, where designed, moment from the steel building.
Key facts
- Pinned bases use a small number of bolts near the column centerline.
- Fixed bases use larger plates and bolt groups to resist moment.
- Anchor bolts are supplied by the building supplier or the foundation contractor as agreed.
- Accurate bolt setting with templates is critical for fit-up.
- Grout is placed under the plate after frames are plumbed.
Role in the structure
All loads in the frame end at the base. Downward loads bear through the base plate and grout onto the concrete. Uplift from wind is resisted by the anchor bolts in tension. Horizontal thrust and shear are resisted by bolts in shear, by shear lugs, or by tie rods or slab reinforcement, depending on the foundation design.
How do anchor bolts resist horizontal thrust?
Gable frames push outward at the base under gravity load. This thrust can be resisted by anchor bolts in shear, by shear lugs welded under the plate, by hairpin bars into the slab, or by tie rods across the building.
The foundation engineer selects the method based on soil, slab design and loads. The steel supplier provides the base reactions for each load case to support this design.
Who designs the foundation and anchor bolts?
Typically the steel building supplier designs the base plate and specifies anchor bolt diameter and layout, while a local foundation engineer designs the concrete, embedment and anchorage to the local code.
Clear responsibility should be agreed in the contract. Anchor bolt layout drawings and reactions are among the earliest deliverables so foundations can start before steel arrives.
Why is accurate anchor bolt setting important?
Misplaced bolts are one of the most common causes of erection delay. Plywood or steel templates hold bolts at the correct spacing and projection while concrete is placed.
Before erection, a survey confirms bolt positions, projection and pier elevations. Corrections must be approved by the engineer rather than improvised on site.
Materials & coatings
- Structural steel plate for base plates and shear lugs.
- Anchor rods to recognized anchor bolt product standards, plain or galvanized.
- Nuts, plate washers and leveling nuts matched to the rod grade.
- Non-shrink cementitious grout.
Design considerations
- Column reactions for all load combinations, including uplift.
- Base fixity: pinned or fixed.
- Method of resisting horizontal thrust and shear.
- Anchor embedment, edge distance and concrete strength.
- Corrosion protection at grade or in wet areas.
- Tolerances for bolt position and projection.
Fabrication
- Cut and drill base plates, usually with oversized holes for tolerance.
- Weld base plates square to columns.
- Supply templates and anchor bolt layout drawings.
Erection
- Survey anchor bolts and pier elevations before steel delivery.
- Set columns on leveling nuts or shims.
- Install plate washers over oversized holes.
- Grout after the structure is plumbed and bolted.
Inspection & quality
- Check bolt diameter, projection and thread condition.
- Verify position against layout drawings.
- Inspect grout placement and curing.
- Confirm nuts are tightened as specified.
What to include in your RFQ
- Who supplies anchor bolts and who designs foundations.
- Soil and foundation information if available.
- Preferred thrust resistance method, if any.
- Finished floor level and pier heights.
- Corrosion environment at grade.
3D model & IFC of this component
Rotate the model, isolate the component and download its IFC4 file for your BIM coordination model.
IFC entity: IfcPlate + IfcMechanicalFastener (ANCHORBOLT). Illustrative geometry for coordination and learning. Final member sizes come from the project design.
Frequently Asked Questions
Why are base plate holes oversized?
Oversized holes accommodate normal concrete construction tolerances. Thick plate washers are then used to transfer load properly.
Can I weld a column to embedded plates instead?
Embedded plates are an alternative in some projects, but the detail must be designed by the engineer and coordinated with the steel supplier.
When should grout be placed?
After the columns are aligned, plumbed and the bay is braced, and before the structure is subjected to full loads.
Engineering content is general guidance. Every World Steel building is designed by qualified engineers to the code and loads of the project.
