How the base plate calculator works
This free base plate calculator designs a steel column base plate with the method of AISC Design Guide 1 and AISC 360-22. Choose a W, HP, HSS or pipe column from the AISC Shapes Database, set the plate size and thickness, the concrete pedestal and the anchor rods, then enter the axial load, moment and shear for each load case. The calculator checks concrete bearing, the required plate thickness, anchor rod tension and shear, in LRFD or ASD.
What it checks
- Concrete bearing by AISC 360-22 §J8: Pp = 0.85f'cA1√(A2/A1) ≤ 1.7f'cA1, with the pedestal confinement found from the pedestal size
- Plate thickness for concentric load from the cantilevers m, n and λn′ of Design Guide 1
- Plate thickness for axial load and moment with a uniform bearing stress: small eccentricity with full bearing, or large eccentricity where the anchor rods carry tension and the bearing length Y comes from the Design Guide 1 quadratic
- Net uplift with the rods on both sides in tension
- Anchor rod tension, shear and combined tension and shear for ASTM F1554 Grade 36, 55 and 105 rods by AISC 360-22 §J3
- Shear transfer by friction between the plate and grout, with the rods taking the shear when friction is not enough
Method
The ASCE 7-22 load combinations are built from your service loads, with wind in both directions, and every combination is checked. For each one the calculator decides whether the base is concentric, has a small moment, a large moment or net uplift, then finds the bearing stress, anchor rod force and required thickness. The plate thickness comes from the plastic moment of a unit strip of plate at the critical section, tp = √(4Mu/φFy) in LRFD.
Assumptions and limitations
- The moment acts about the column strong axis, and the anchor rods are in two rows at ±f from the plate center.
- Concrete anchorage of the rods (breakout, pullout and side-face blowout per ACI 318-19 Chapter 17), shear lugs, grout and the column-to-plate weld are not checked.
- Plate bending at the tension side uses the full plate width B, as in Design Guide 1.
Frequently asked questions
How thick should a column base plate be?
It depends on the bearing pressure and how far the plate projects past the column. The calculator finds the required thickness from the largest of the cantilevers m, n and λn′ for axial load, or from the bearing and anchor rod forces when there is moment, and compares it with the plate you choose.
What is A2/A1 in base plate design?
A1 is the plate area and A2 is the largest area of the supporting concrete that is geometrically similar to and concentric with the plate. Confinement raises the bearing strength by √(A2/A1), up to a factor of 2.
When do anchor rods carry tension?
When the eccentricity e = M/P exceeds the critical value ecrit = N/2 − P/(2qmax), the bearing block alone cannot balance the moment, so the rods on the tension side carry the difference. Net uplift also puts the rods in tension.
Does it check anchor rod embedment?
No. It checks the steel strength of the rods. The concrete breakout, pullout and side-face blowout strength of the anchorage, ACI 318-19 Chapter 17, must be checked separately.
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For educational and preliminary design purposes only. Always verify results with a licensed PE.