Column base plates

Where the steel meets the concrete. This lesson designs pinned and moment column bases to EN 1993-1-8: the bearing strength f_jd, the equivalent T-stub and the width c, the moment base by Table 6.7, the anchors with EN 1992-4, and shear by friction and anchors, by hand and on the EN route of the STATIX base plate calculator.

1. Learning outcomes

After this lesson you can:

work out the bearing strength f_jd with β_j and k_j, and the width c;
find the compression resistance of a pinned base from the effective area;
find the moment resistance of a base by Table 6.7 from its tension and compression sides;
list the anchor checks of EN 1992-4 and check the anchors’ steel and cone;
check the shear by friction and anchors, and a base in the STATIX base plate calculator.

2. Before you start

Lesson 5 (bolts) and lesson 6 (welds).
The free STATIX base plate calculator, set to EN 1993-1-8 + EN 1992-4.
A pocket calculator and about 60 minutes.
The same lesson on the SA code: Steel Design 1, lesson 7 (SANS 10162-1).

3. What a base plate does

A column base. The column is welded to a steel plate, which sits on a layer of grout on the concrete and is held down by anchors cast into it. The base spreads the column’s compression over the much weaker concrete, and the anchors take any tension and help with the shear.

4. Bearing on the concrete

The bearing strength of the joint f_jd (6.2.5(7)). Concrete under a small loaded area is confined by the concrete around it, so it can take more than f_cd; the concentration factor k_j grows with the concrete beyond the plate, to a limit of 3. Grout of at least 0.2 f_ck and not thicker than 0.2 of the plate width gives β_j = 2/3.

The equivalent T-stub in compression (6.2.5). The plate bends around the column’s flanges and web; only a strip of width c around the outline is stiff enough to press on the concrete at the full bearing strength. For an H column the effective area is the outline of the flanges and the web, widened by c, within the plate.

The width c (6.2.5(4), eq. 6.5). The plate cantilevers out from the column face and bends under the bearing pressure; c is the width it can carry at f_jd before it yields. A thicker or stronger plate, or weaker concrete, gives a wider strip.

f_cd = α_cc f_ck / γ_c; f_jd = β_j k_j f_cd, β_j = 2/3 (6.2.5(7)) k_j = min(a₁/L, b₁/B, 3); a₁ = min(L + 2 × overhang, 3L, L + h) (the page’s rule) c = t √[f_y / (3 f_jd γ_M0)] (6.5)

5. Pinned bases

A pinned base. The column carries axial force and shear only; the effective area times f_jd is the compression resistance. The calculator works it out from the column outline and c, clipped to the plate.

6. Moment bases

A moment base (6.2.8.3). The moment is carried by a couple: tension in the anchors on one side, compression under the opposite flange. The lever arm z runs from the anchor row to the centre of the compressed flange. Table 6.7 gives M_j,Rd from the two sides’ resistances and the eccentricity e = M/N.

The tension side. The plate, held by the anchors and pulled by the column flange, is an equivalent T-stub in tension (6.2.6.11): it can yield in bending, or the anchors can break, or both. Anchors through grout and a thick base are long and stretch enough that the plate edge does not lift onto the concrete, so there are no prying forces.

z = z_T + z_C (anchor row to column centre + half the distance between flange centres) large moment, N in compression (e = M/N, the tension side governs one bound): M_j,Rd = min[ F_T,Rd z / (z_C/e + 1); −F_C,Rd z / (z_T/e − 1) ] (Table 6.7, e taken negative here as on the page)

7. Anchors and shear

Anchors in concrete (EN 1992-4). EN 1993-1-8 checks the anchor as a bolt; EN 1992-4 adds the ways the concrete around it can fail. The calculator runs these for cast-in headed anchors (and post-installed anchors with the product’s values), with the edges you declare.

Shear. With compression on the base, friction between the plate and the grout carries shear: 0.2 times the compression for sand-cement mortar. The anchors add their own shear resistance, which 6.2.2(7) reduces for bending in a grouted joint; a shear lug can carry larger forces.

Practical inputs. The grout rules of 6.2.5(7) make β_j = 2/3 valid; the plate’s f_y drops with thickness; α_cc is a national choice; and k_j and the anchors’ edge checks need the real concrete around the base.

Which engine the page runs (checked): the base plate page sends its inputs to the server’s base plate engine, which converts them for the STATIX app’s connection engine. Its EN route: f_ck as the cylinder strength, γ_c 1.5, α_cc 1.0 or 0.85 by choice, β_j 2/3, k_j from “Pedestal / footing” (1.0 when none is given), plate f_y by thickness (345 MPa for 25 mm S355), γ_M0 1.0, γ_M2 1.25; anchors to EN 1992-4 with the partial factors it recommends; friction 0.2 N_c,Ed. It also lists a few finite-element screening rows that are not part of the verdict.

8. In STATIX

The base plate calculator’s EN route: code “EN 1993-1-8 + EN 1992-4”; the column, plate and anchors; the concrete f_ck (cylinder), grout and edges; then N, M and V, and the check rows.