Steel Base Plate (LRFD)

How it calculates

The Steel Base Plate (LRFD) calculator follows AISC Design Guide 1, 3rd Edition (2024) for plate sizing and bending. Anchor rods in concrete follow ACI 318-19 Chapter 17. AISC 360-22 covers bearing, welds and combined rod tension and shear. Load combinations come from the building code set in your project defaults.

Load combinations

Unfactored dead, live, roof live, snow, rain, wind and earthquake loads are combined into LRFD strength combinations. Bearing and friction use the largest factored compression. Anchor tension is calculated for every combination in each axis, and the largest tension governs. Combinations with net uplift are designed for, with the anchors taking the tension.

Concrete bearing capacity (ACI 318-19 Cl 22.8, AISC 360-22 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)

A_2 is derived from the plate footprint and the concrete base depth, capped at the concrete base footprint. 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)

The plate is checked at the bearing interface and the tension interface, about each axis. Each axis uses the small-moment (full bearing) or large-moment (anchor tension) approach, and the pure-moment procedure when there is no axial load. With full bearing beyond the cantilever projections m and n, the demand per unit width is:

M_u,pl = f_p × max(m, n)^2 / 2

where f_p is the bearing pressure. At the tension interface, the anchor tension acts on the plate over its lever arm to the bending line.

phi × M_n = 0.9 × F_y × t_p^2 / 4

Utilization = M_u,pl / phi × M_n ≤ 1.0

When moment acts about both axes, a biaxial interaction ratio adds the squared bending utilizations for each axis and is checked against 1.0.

Minimum required plate thickness

The required thickness from bending is t = sqrt(4 × M_u,pl / (phi_b × F_y)), with phi_b = 0.9. The governing t_min is the largest of these requirements, with a 1/8 in floor:

• Pure axial bearing
• Bearing interface and tension interface, about each axis
• One-way bending about the web or flange under net uplift, with the rods inside the anchor group extents
• Outset anchor rod bending, for rods outside the column footprint

The plate thickness you enter is checked against t_min. Left at its default, the thickness equals t_min and the check reads 100%.

Anchor rods outside the column footprint

For wide-flange and HSS columns, rods outside the column footprint load the plate locally at the nearest section corner. Three checks cover this case:

• Plate bending under net uplift: a cantilever over the diagonal from rod to corner, with an effective width of up to 5 in
• Weld strain: the rod tension the column-to-plate weld can take before it is overstrained, based on weld throat and plate thickness
• Weld strength: the rod tension the weld can transfer, using only the weld length local to the rod

Column-to-plate weld (AISC 360-22 Cl J2)

The column is assumed fillet welded to the plate with E70XX electrodes. Axial tension, resultant shear and moment are combined into one weld demand and compared with the weld group's available strength. The weld size is checked against the minimum for the parts joined and the maximum for the column element thickness. On HSS columns, the weld length discounts the rounded corners.

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 × N_sa = 0.75 × A_se × f_uta per rod. Group capacity uses the rod tensile stress reduced for combined shear, per AISC 360-22 Cl J3.8
• Concrete breakout in tension (Cl 17.6.2): projected area method with eccentricity, edge distance and cracking factors, 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

When moment acts about both axes, an anchor rod biaxial interaction ratio is also checked against 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_sa = 0.65 × 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): edge distance, cracking and member thickness factors, for shear toward and parallel to the edge. The lower capacity governs

Which anchor row resists shear breakout depends on rod welding to the plate and on whether you allow shear to redistribute between rows.

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 utilization and the larger shear utilization are added:

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

Frictional shear capacity

An optional, separate check compares the larger 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 taken as 0.4. 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.