Foundations and the steel-to-concrete interface

What the interface is and who is responsible

In a pre-engineered building the steel supplier designs the superstructure and the foundations are normally designed and built by others – the owner's civil engineer and contractor. The interface between them is the base plate and anchor bolt connection, and it is defined by three documents from the PEB designer:

• Anchor bolt plan: grid lines, bolt positions, projections, diameters and base elevations for every column.
• Reaction table: unfactored column reactions for each load case (dead, collateral, live, snow, wind, seismic, crane) so the foundation engineer can form the local load combinations.
• Embedment and detail notes: anchor type, plate washers, shear lugs, grout and any slab-tie requirements.

A clear interface is one of the most important coordination items in a PEB project, because foundations are poured long before the steel arrives.

Foundations in the load path

The footing is the last element of the load path. It receives vertical compression and uplift, horizontal thrust from the portal frames, longitudinal forces from braced bays and, where bases are fixed, moment.

Frame thrust deserves special attention: gable portal frames push their bases outward under gravity load and may pull inward under uplift. It can be resisted by the footing in passive soil pressure and friction, by tie beams between opposite footings, by hairpin bars anchored into the slab, or by piles designed for lateral load.

Typical foundation types

Perimeter upstand or block walls (dado walls) are common under the wall sheeting; their top level and setback must be coordinated with the base trim and girt layout of the wall sheeting.

Design considerations

• Use the reactions with the local concrete code (ACI 318, CSA A23.3, AS 3600, EN 1992-1-1) and load combinations for the site.
• Check uplift: light steel buildings in high-wind areas often need heavy footings to resist net uplift.
• Anchorage design for breakout and pull-out: ACI 318 Chapter 17, CSA A23.3 Annex D, AS 5216 or EN 1992-4.
• Pedestal top level allows for grout (typically 25–50 mm) and finished floor level.
• Future expansion – provide footings and anchors for expandable endwalls where planned.
• Crane buildings – fixed bases and crane loads significantly increase footing size; see crane buildings.

Pinned versus fixed bases and the slab-on-grade

The slab-on-grade is normally poured after the steel is erected and the roof is on, so that it is protected from weather. Its thickness and reinforcement are set by forklift wheel loads, rack post loads and the subgrade – industrial slabs of roughly 150–250 mm are common. At the columns:

• The base plate is usually set below finished floor level on a pedestal, then encased or boxed out; isolation joints around columns (circular or diamond-shaped blockouts) prevent restraint cracking.
• Where the slab acts as a tie for frame thrust, hairpin bars around the anchor bolts are lapped into the slab reinforcement, and saw-cut joints must not cut through the tie path.
• At door openings the slab edge is rebated or thickened to suit door tracks, and the base trim and girt level are coordinated with the slab edge and any dado wall.
• Under-slab services, valley-gutter downpipes and crane rail foundations must be set out before the pour.

Setting anchors, tolerances and grouting

Anchors should be set with rigid templates, ideally the steel templates supplied with the anchor bolt package, and surveyed after the pour. Typical limits in the AISC Code of Standard Practice include 1/8 in between rods within a group and 1/4 in between adjacent groups; AS/NZS 5131 and EN 1090-2 give comparable limits for anchor and base positions. See erection tolerances.

1. Fix templates to formwork and check grid and level before pouring.
2. Protect threads with sleeves or tape.
3. Survey anchors and pedestal levels after curing and report deviations before the steel ships.
4. After erection and alignment, fill under base plates with non-shrink grout.

Common mistakes and quality checks

• Foundations designed before the final anchor bolt plan is issued – always design from the "approved for construction" drawing revision.
• Thrust ignored or tie beams omitted.
• Anchors displaced during concreting by vibrators.
• Slab joints conflicting with anchor groups or hairpins.
• Endwall and wind-column anchors missed on the civil drawings.

World Steel issues the anchor bolt plan and reaction tables early, and its engineering team coordinates with the foundation designer when the client requests it.