Steel Base Plate (LRFD)

How it calculates

The Steel Base Plate (LRFD) calculator uses AISC Design Guide 1, 3rd Edition (2024) for plate geometry and bending design, ACI 318-19 Chapter 17 for all anchor limit states, and ASCE 7-22 for LRFD load combinations. This is the current-code version of the base plate calculator.

Load combinations

Unfactored gravity, wind, and seismic load components are combined using ASCE 7-22 Chapter 2 LRFD combinations. The calculator evaluates each combination and reports the governing demand for every limit state - compressive axial typically governs bearing while the combination with minimum compression and maximum moment governs anchor tension.

Concrete bearing capacity (AISC DG1 3rd ed., Cl. 3.1)

The factored bearing strength:

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

where A_2/A_1 is the geometric confinement factor, capped at 4.0 when the concrete frustum is fully confined.

Utilization = P_u / phi × P_p ≤ 1.0

Plate bending at bearing interface (AISC DG1 3rd ed., Cl. 3.3 - 3.4)

When the plate is in the small-moment regime (full bearing), the critical bending demand at the plate cantilever projections m and n is:

M_u,pl = q × max(m, n)^2 / 2

where q is the factored bearing pressure per unit length. The required plate thickness is back-calculated from:

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

Utilization = M_u,pl / phi × M_n ≤ 1.0

Plate bending at tension interface (AISC DG1 3rd ed., Cl. 3.3 - 3.4)

For large-moment conditions, anchor tension T_u pulls upward on the plate and creates plate bending at the tension interface. The moment demand and required plate thickness at the tension face are checked independently in both axes.

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

Five tensile failure modes are evaluated for the anchor group:

• Steel tensile strength (Cl. 17.6.1): phi × N_sa = phi_t × A_se,N × f_uta
• Concrete breakout in tension (Cl. 17.6.2): CCD projected area method with modification factors psi_ec,N (eccentricity), psi_ed,N (edge distance), psi_c,N (cracking), and psi_cp,N (post-installed)
• Pullout (Cl. 17.6.3): phi × N_pn = phi × 8 × A_brg × f'c per headed anchor
• Side-face blowout (Cl. 17.6.4): when h_ef / c_a1 ≥ 2.5
• Interaction of shear and tension: combined action ratio per Cl. 17.8

Utilization = N_u / phi × N_n,g ≤ 1.0

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

Shear failure modes evaluated in both principal axes:

• Steel shear capacity (Cl. 17.7.1): phi × V_sa = phi_v × 0.6 × A_se,V × f_uta
• Concrete pryout (Cl. 17.7.3): phi × V_cp = phi × k_cp × N_cbg
• Concrete shear breakout (Cl. 17.7.2): projected area A_Vc with edge distance and eccentricity modification factors

Utilization = V_u / phi × V_n,g ≤ 1.0

Frictional shear capacity (ACI 318-19 Cl. 22.9)

Friction beneath the base plate contributes to shear resistance: V_friction = mu × P_u. The friction coefficient is 0.55 for steel on grout or 0.70 for steel directly on concrete.

Minimum required plate thickness

The governing t_min is the maximum value across the bearing interface checks and tension interface checks in both axes - this single value is reported as the design output for plate sizing.