15.1 Anchor Bolt Layouts, Base Plate Details & Embed Schedules

15.1 Anchor Bolt Layouts, Base Plate Details & Embed Schedules

Start with the foundation plan, the column mark and the grid intersection. Follow the detail callout to the anchor assembly and the base plate section. The plan locates the assembly. The schedule identifies the required rod, plate or embed type. The section shows what the plan view cannot, including the elevation of the concrete, the bearing surface of the plate and the depth below the concrete surface. On a 47P exhibit, check which dimension applies to each of those surfaces before selecting an answer.

1. Verify the rod and the plate as a matched assembly

ASTM F1554 identifies anchor rod grades 36, 55 and 105 by their minimum yield strength in ksi. If the approved schedule requires weldable Grade 55, check for the specified S1 supplementary requirement in the material documentation. If a detail calls for Grade 105, identify it by the approved material documentation and refer any proposed welding or heating for design review. The number by itself describes a strength grade and does not authorize a field modification.

Check rod diameter, length and thread length separately. Match the base plate hole pattern, plate washer and nut to the approved detail. The AISC Steel Construction Manual gives recommended maximum anchor rod hole sizes and associated washer details for accommodating rod placement variation. The approved assembly may call for smaller holes or particular washers. A rod can satisfy its setting tolerance while still conflicting with a hole or washer in the specified assembly. Document the two checks separately. If the plate cannot be seated, refer the mismatch through the discrepancy process. Do not enlarge the holes, bend the rod or substitute a washer on the inspector’s authority.

2. Check setting tolerances and projection separately

Read the project’s referenced standard and any tighter limits on the approved embedment detail. Under ACI 117-10 Sections 2.3.4.1 and 2.3.4.2, the top elevation of each anchor bolt may vary by ±1/2 inch from the specified elevation. Individual horizontal centerline deviation is limited as follows.

For steel erection where AISC 303-22 applies, Section 7.5.1 uses the same numerical limits and checks horizontal deviation anywhere along the exposed projection. Section 7.5.2 requires the axis to be perpendicular to the theoretical bearing surface unless the contract documents specify otherwise. If a rod leans, checking the centerline at its top and near the concrete surface helps identify a conflict that a single measurement can miss.

Set up the survey from the project grids and benchmark. Compare each rod with its specified position, then verify the plate fit separately. The top elevation is a coordinate on the project datum. Projection equals the rod top elevation minus the actual concrete top elevation. If the concrete top is 1/4 inch high while the rod top is on its specified elevation, the projection is 1/4 inch shorter than it would have been at the specified concrete elevation.

3. Work through the base plate stack

The example detail shows a 1-1/2-inch distance from top of concrete to the underside of the base plate, a 1-3/4-inch base plate, a 3/8-inch top washer and a 1-inch nut. Any leveling hardware within the 1-1/2-inch gap is already included in that distance. The rod has eight threads per inch. The drawing shows 1/2 inch of nominal protrusion beyond the nut and requires at least two exposed full threads. Assume the threaded portion reaches through the nut and the rod end has no incomplete threads in the dimensions used below.

Each thread pitch is 1/8 inch, so two pitches occupy 1/4 inch. The shortest projection that provides the specified protrusion in this example is 4.625 + 0.250 = 4.875 inches. Compare the actual projection to this minimum and the actual top elevation to the specified top elevation. For this survey, the concrete top is at its specified datum, so elevation changes correspond to projection changes.

A and B meet both numerical checks. C is at the elevation tolerance boundary but misses the specified protrusion. A nut that reaches the rod end may still engage its threads fully, depending on the usable threads and actual hardware. For C, the measured deficiency is the specified exposed thread length. Document the deficiency and obtain a disposition. When reading another detail, use its own nut, washer, bearing gap and thread requirements. Do not assume the sample two-thread requirement applies to every anchor assembly.

4. Distinguish embedment, projection and rod length

For a cast-in headed rod, ACI 318-19 Chapter 2 and Fig. R2.1 measure effective embedment depth to the head’s bearing contact surface. In the sketch, that surface faces the exposed end of the rod. For a hooked rod or post-installed anchor, use the geometry and embedment specified for that system. Chapter 17 design provisions distinguish the different mechanisms for load transfer and applicable failure modes.

Top of rod                    ───┐
                                 │ projection
Actual top of concrete        ───┼───┐
                                 │   │ effective embedment
Bearing face of cast-in head  ──┬┴┬──┘
                                └─┘

If the head is 1/2 inch thick and the head’s far face lies 13 inches below the surface in this example, the bearing face is 12-1/2 inches below the surface. With a 5-1/8-inch projection, the rod’s overall length is 18-1/8 inches. Record the two depths with their respective datums. One number cannot substitute for the other.

5. Read miscellaneous embed schedules and confirm installation

For an embed plate, match the piece mark to the detail. Check the plate orientation and elevation relative to the concrete face, the pattern and size of welded anchors, and the required distance from edges and adjacent reinforcing steel. For a cast-in anchor with attachments or anchor reinforcement, ACI 318-19 Section 26.7.2(a) requires secure positioning in the forms and orientation as detailed, with consolidation around the anchor and anchor reinforcement during placement. A braced template with dimensions taken from the approved layout is one way to control anchor movement. Confirm that the template can maintain the specified position during discharge and consolidation and that concrete can reach under the plate and around the anchors.

Before concealment, measure and record the location, top elevation, projection and applicable embedment dimensions. Verify inaccessible attachments and clearances at this stage. If the work shifts, identify the affected anchor mark and document the measured location, discrepancy, notification and disposition. In a steel erection application, OSHA 29 CFR 1926.755(b) requires approval from the project structural engineer of record for repair, replacement or field modification of column anchor rods, and written notification from the controlling contractor to the steel erector before column erection.