Steel Base Plate (LRFD, ACI 318-19 / AISC 360-16)

How it calculates

The Steel Base Plate (LRFD, ACI 318-19 / AISC 360-16) calculator applies AISC Steel Design Guide 1 (2nd Edition) for plate geometry and bending, with anchor limit states to ACI 318-19 Chapter 17. Load combinations come from the building code set in your project defaults.

Load combinations

Unfactored dead, live, roof live, snow, rain, wind, and seismic loads are combined into LRFD strength combinations. Bearing uses the maximum factored axial load. Anchor tension is calculated for every combination, and the largest tension governs. Combinations that produce net uplift on the plate are not supported.

Concrete bearing capacity (ACI 318-19 Cl 22.8, AISC 360-16 Cl J8)

The factored concrete bearing strength, with phi_c = 0.65:

phi × P_p = phi_c × 0.85 × f'c × A_1 × min(sqrt(A_2/A_1), 2)

Where grout is considered, the lesser of the grout strength (no confinement increase) and the concrete strength governs.

Utilization = P_u / phi × P_p ≤ 1.0

Plate bending (AISC DG1 Cl 3.3 - 3.4)

The plate is checked at the bearing interface and tension interface in both X and Y axes. The critical plate moment demand M_u,pl (kip-in/in of plate width) is the maximum of the cantilever bending from bearing pressure over projections m and n.

Required plate thickness from bending:

t_min = sqrt(4 × M_u,pl / (phi_p × F_y))

Utilization = M_u,pl / phi × M_n ≤ 1.0

Anchor rod tensile limit states (ACI 318-19 Cl 17.6)

Steel tensile capacity is always checked. The concrete checks run when you include them:

• Steel tensile capacity (Cl 17.6.1): phi_t × N_sa = phi_t × A_se × f_uta per rod
• Concrete breakout in tension (Cl 17.6.2): CCD method with projected area A_Nc, modification factors for edge distance, eccentricity, and cracking, including group effects
• Pullout (Cl 17.6.3): phi × 8 × A_brg × f'c for headed anchors, with a separate equation for hooked anchors
• Side-face blowout (Cl 17.6.4): single anchor and group, applies when h_ef ≥ 2.5 × c_a1

Utilization = N_u / phi × N_n,g ≤ 1.0

Anchor rod shear limit states (ACI 318-19 Cl 17.7)

Three shear limit states are checked in each principal axis:

• Steel shear capacity (Cl 17.7.1): phi_v × V_sa = phi_v × 0.6 × A_se × f_uta, reduced by 0.8 where grout is considered
• Concrete pryout (Cl 17.7.3): based on the tensile breakout strength of the group
• Concrete shear breakout (Cl 17.7.2): projected area method with edge distance factors

Utilization = V_u / phi × V_n,g ≤ 1.0

Pryout and shear breakout run only when the concrete checks are included.

Tension and shear interaction (ACI 318-19 Cl 17.8)

When the anchors carry both tension and shear, the tension and shear utilizations are added:

Interaction = N_u / phi × N_n,g + V_u / phi × V_n,g ≤ 1.2

Frictional shear capacity (ACI 318-19 Cl 22.9)

An optional, separate check compares the shear demand to friction at the plate interface. It does not add to anchor shear capacity:

phi × V_f = 0.75 × mu × P_u, limited to the lesser of 0.2 × f'c × A_1 and 800 psi × A_1

The friction coefficient mu is 0.55 with grout and 0.70 without. Grout strength replaces f'c in the limit where grout is considered.

Seismic provisions

Earthquake loads can be entered and are included in the load combinations. The ACI 318-19 Section 17.10 seismic anchor provisions are not applied. The calculator shows a warning when earthquake loads are present.