Course · lesson 7 of 7

Column base plates

Where the steel meets the concrete. This lesson designs a column base: the concrete in bearing, the plate thickness, the holding-down bolts and the anchors in the concrete, shear transfer and moment bases, with SANS 10162-1 clause 25 and the STATIX base plate calculator.

1. Learning outcomes

After this lesson you can:

find the bearing strength of grouted concrete under a base plate, and how a pedestal raises it;
find the effective bearing area of a plate of given thickness and check a pinned base;
check a moment base: the bolt tension, the concrete compression and the plate between them;
check holding-down bolts to SANS 10162-1 clause 25 and name the anchor failure modes in concrete;
check 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 and your copy of SANS 10162-1.
A pocket calculator and about 60 minutes.

3. The column base

A column base: the column is welded to a steel plate, which sits on a grout layer on the concrete and is held down by cast-in bolts. A pinned base carries N and V; a moment base also carries M, which pulls on the bolts on one side and presses the concrete on the other.

4. Concrete in bearing

Concrete in bearing. Concrete loaded on a small area of a bigger block is stronger, because the block around it confines it: k_j grows with the concrete area A_c1 that can spread the load (limited by the overhang and the depth). With no size given, STATIX takes k_j = 1. SANS 10162-1 has no rule of its own here; the page uses EN 1993-1-8 6.2.5 and says so.

f_ck = 0.8 f_cu (the SANS page asks for the cube strength) f_cd = α_cc f_ck / 1.5 (α_cc = 1.0, or 0.85 by choice on the page) f_jd = β_j k_j f_cd, β_j = 2/3, k_j = 1 unless the pedestal is given

5. The plate: bearing width and thickness

The effective bearing area. Only the plate close to the column is stiff enough to press on the grout: a strip of width c around each flange and the web. Their area times f_jd is the base's resistance to compression.

Where c comes from. The plate beyond the column face is a cantilever loaded by the bearing pressure. c is the length it can carry without yielding (EN 1993-1-8 6.2.5(4)), so the plate thickness sets the bearing area.

Thick plate is weaker. Steel is rolled to its grade's yield strength only up to 16 mm; a thicker plate gets less. STATIX takes the plate's f_y from its own thickness, and the column's from its flange.

c = t √(f_y / (3 f_jd γ_M0)) (f_y for the plate's own thickness) pinned base: N_j,Rd = f_jd × (two flange strips + the web strip between them)

6. Moment bases

A moment base. The moment and the axial force are carried by a pull T in the holding-down bolts on one side and a push C on the concrete under the other flange, a lever arm z apart. The base fails at the weaker side: the bolts and plate in tension, or the concrete in compression.

The tension side as a T-stub (EN 1993-1-8 6.2.4; SANS has no rule here). A thin plate bends and yields; a thicker one bends while the bolts pull (with prying where the bolts are short); a thick one stays flat and the bolts break. STATIX checks all three and the column web.

7. Holding-down bolts and anchors

The holding-down bolt. SANS 10162-1 clause 25 gives its tension resistance on the net area at the thread root, and an interaction with shear. The head, a nut or a plate washer, must be big enough to bear on the concrete without pulling through.

How an anchor fails in the concrete (EN 1992-4; SANS 25.2.2.2 refers to SANS 10100-1, so STATIX uses EN 1992-4 and labels it). The steel breaks, a cone of concrete pulls out, the head pulls through, or the concrete splits near an edge. Shear has its own modes: steel, pryout and the edge.

8. Shear

Shear transfer. Friction under the plate carries some shear while the column is in compression (STATIX takes 0.20 N: SANS allows friction but gives no coefficient). The rest goes into the holding-down bolts, or into a shear lug welded under the plate, which the page can add.

9. In STATIX

The STATIX base plate calculator: column and plate, the bolts, the concrete (cube strength f_cu on SANS; a pedestal or a free edge if there is one), then N, M and V. Each row names its clause: SANS where SANS has a rule, EN where it does not.

The page offers two routes: SANS 10162-1 clause 25 (the default) and EN 1993-1-8 with EN 1992-4. SANS covers the bolts, their shear and tension, bearing at the bolts and the welds; for the concrete bearing, the plate bending and the anchors in concrete SANS sends you elsewhere, so the page uses the EN rules and labels each row.