Base plates
A column base is not one calculation. It is a stack: a plate that bends, a grout bed that carries the pressure down, concrete that can crush or cone out, anchor rods that can yield or pull out, and a shear path that has to be chosen. The app treats each of those as a separate element with its own limit states, so the panel tells you which part of the stack governs, not just that the base passed.
The plate and the concrete under it
Under compression the plate is always checked for two things: the concrete crushing under the contact area, and the plate yielding in bending over its cantilevers. Both come out of the same pressure distribution, which is why the eccentricity of the load decides how they are computed.
Concrete bearing
concrete_bearingThe contact pressure against the strength of the concrete. Under AISC the capacity is 0.65 · 0.85 · fck · A1 · √(A2/A1), capped at 0.65 · 1.70 · fck. Under NBR 8800 it is fck/(γc·γn) · √(A2/A1), capped at plain fck, with γc = 1.40 and γn = 1.40. The ceiling matters: without it, a large pedestal would appear to raise the capacity without limit.
AISC 360 J8 · NBR 8800 6.6.5
Plate bending
plate_bendingThe plate spans as a cantilever from the profile out to its edge. The lever is the largest of the Murray, Thornton and Design Guide 1 cantilevers, and the required thickness comes from it. This is the check that decides whether 16 mm is enough or 22 mm is needed.
AISC Design Guide 1 §3
Confinement, A2 over A1
a2_confinement_factorWhen the plate sits on a pedestal larger than itself, the surrounding concrete confines the contact area and the bearing capacity rises by √(A2/A1), truncated at 2, which is the 1:2 pyramid spreading down from the plate. This element never fails: it reports the factor and feeds it back into the bearing check. Leave it off and the base is simply computed without the bonus.
AISC 360 J8 (A2/A1 ≤ 4) · NBR 8800 6.6.5 · NBR 6118 21.2.1
Levelling grout
grout, grout_strengthThe grout bed levels the plate and carries the compression down uniformly. There is one rule that has nothing to do with load: the grout has to be at least as strong as the foundation, otherwise it becomes the weak link in a path that assumed it was not. A plate whose smaller side is over 600 mm also needs an injection and vent hole, or the bed will trap air.
AISC Design Guide 1 · NBR 8800 6.6.5
When the load is not centred
A base with moment has an eccentricity e = M/N. While e stays within L/6 the whole plate is in compression and a uniform pressure is a fair model. Past L/6 part of the plate lifts, the pressure becomes triangular, and the anchor rods on the far side go into tension. That is a different problem, not a worse version of the same one.
Within L/6
Full contact. Bearing and bending are computed from the average pressure, and the anchors carry no tension from the moment.
Beyond L/6
The pressure block of Design Guide 1 solves the base: it returns the contact length Y, the anchor tension T and the peak pressure, so bearing, bending and anchor tension all have real numbers to size against.
Older projects behave differently on purpose. A project stamped by the legacy engine computes everything from the average pressure, which ignores the moment, and past L/6 it emits a regime warning instead of a solution: a failure with no way out, because there is nothing to size against. The corrected engine, which every new project uses, runs the pressure block instead. If a base you know is fine reads as failing on eccentricity alone, that is the engine version talking.
Anchor rods: four separate ways to fail
Steel and concrete fail differently, and the app never mixes them into one number. The rod can yield, the head can punch through the concrete around it, the concrete can cone out, and near an edge it can break sideways. Each of those is a row of its own.
Rod steel
anchor_tension, anchor_shear, anchor_tension_shear_interactionTension from uplift or moment, shear, and the interaction between the two on the same rod. The demand is the design force split across the rods that are working.
AISC 360 J3.6 and J3.7 · NBR 8800 6.3.3 · EN 1993-1-8 Tab. 3.4
Head or hook
anchor_pulloutThe mechanical anchorage, meaning the part that grips the concrete against direct extraction. A headed rod with a nut and washer plate is checked as 8·Abrg·f′c; a J or L bolt is checked on its bend as 0.9·f′c·eh·da, with the lever kept inside the range the code allows. Cracked concrete is assumed, which is the conservative side.
ACI 318-19 17.6.3
Concrete in tension
anchorage_lengthThe breakout cone, the pullout of the head and, close to an edge, side-face blowout. The cone is the pyramid projected at 1.5 times the embedment, so the embedment depth and the distance to the edge are what move this number, not the diameter of the rod.
ACI 318-19 17.6.2, 17.6.3 and 17.6.4
Concrete in shear
anchor_shearEdge breakout, pry-out behind the rod, and the tension and shear interaction on the concrete side. Edge distance dominates here: the same base moved 200 mm closer to the edge of the pedestal is a different calculation.
ACI 318-19 17.7.2, 17.7.3 and 17.8
Supplementary reinforcement, meaning hairpins or confinement bars at the top of the pedestal, is an element of its own. Fully anchored and sized for the whole uplift, it takes over from the concrete cone and lets the concrete-side strength reduction factor rise from 0.70 to 0.75, because failure is then controlled by steel that yields.
Choosing how the shear gets out
Horizontal force at the base leaves through exactly one of three paths, and the three are mutually exclusive: turning one on turns the others off. This is a design decision, not something the app can read from the model.
Friction at the interface
friction_capacityVf = μ·P, with μ = 0.40 against grout. It is real, and it is free, but it only exists while there is net compression. Under uplift the capacity goes to zero and the ratio blows up, which reads as a catastrophic failure when the truth is that the path does not apply. That is why it is not the default.
AISC Design Guide 1 §3.5 · ACI 318-19 17.7
Through the anchor rods
anchor_shear + plate bearingThe default. Shear goes straight into the rods, checked on the rod steel and on the plate crushing against the shank. In a grouted joint the force arrives with eccentricity, which bends the shank, so a reduction of about 0.80 is applied and an oversized hole is discouraged: the bigger the hole, the less of the rod is working.
AISC 360 J3.10 · AISC Design Guide 1
Shear lug
lug_concrete_bearing, lug_flexureA plate welded under the base plate and embedded in the grout, transferring shear by contact. Three limit states: bearing on the concrete in front of the embedded face, bending of the lug plate itself as a cantilever, and the weld between the lug and the plate on both sides. It is what you reach for when the shear is too large for the rods.
AISC Design Guide 1 §3.5 · ACI 318-19 17.7 · NBR 6118 21.2.4
The hardware that gets forgotten
Two details are usually drawn without being checked, and both have a limit state.
Plate washer over an oversized hole
washer_min_sizeHoles in a base plate are deliberately loose, to absorb the tolerance of casting the rods. A square plate washer puts the bearing area back over the nut so it cannot pull through. Its thickness has to reach db/3, and the punching around the oversized hole is checked under rod tension.
AISC Design Guide 1 Tab. 2.3 · AISC Manual Tab. 14-2
Levelling nuts and the standoff
anchor_shear (bending on the shank)Levelling the plate on a nut underneath leaves a gap between the concrete and the plate, and across that gap the shank works in bending: Mu = V · standoff against φb·fy·Z, with Z = d³/6 for the solid round. With no standoff the nut bears directly on the plate and there is nothing to check. This is the one that surprises people, because the gap is often 40 mm of nothing.
AISC Design Guide 1 §3.5, double-nut standoff
Stiffeners
stiffener_fit + weldTriangular gussets between the shaft and the plate stiffen it, cut the local bending of the plate and help anchor the moment couple. Checked on the free-edge slenderness b/t and on the weld to both the plate and the shaft.
AISC Design Guide 1 · EN 1993-1-5 · NBR 8800 6.2
Every row on this page opens into its calculation memo: the formula, the values that went in, and the clause it comes from. The base plate family is the one reproduced against the widest set of published examples, and the memo names which one each check follows.