Steel Column Base: Base Plate, Anchors and Plate Thickness to DBN V.2.6-198

Column base parts and force transfer

A column base is a plate welded to the bottom of the column plus anchor bolts in the foundation. Bracket plates and cantilever ribs, where present, pass the force from the column to the projecting parts of the plate.

DBN V.2.6-198:2014 (cl. 11.1) names two ways of passing the design force to the plate:

• through the milled end of the column, which bears on the planed surface of the plate (bases with a milled end);
• through welds (bases with bracket plates and cantilever ribs).

With a milled end the design compression force is taken as passing entirely through the ends (cl. 17.13.1). A base with a milled end is used only for centrally compressed columns: there must be no moment in the post. The bearing resistance of a fitted end is Rp from Table G.4.

Plan size of the plate

The plan size of the plate follows from the strength of the foundation concrete (cl. 11.1): the pressure under the plate must not exceed the design resistance of the concrete to local compression under the standards for concrete and reinforced concrete structures. The concrete class and foundation size come from the foundation design; the forces the column passes at plate level are in Foundation for a column, mast or pole.

When a moment acts, the pressure is not uniform, and each panel is calculated with the average pressure under it (Annex M). The overhangs must hold the welds, anchors and nuts with washers, and a longer overhang calls for a thicker plate.

Plate thickness: formula (11.1)

The thickness of the plate is found from the bending strength condition (cl. 11.2, formula 11.1):

t ≥ √( 6 · Mmax · γn / ( Ry · γc ) )

Mmax is the largest of the moments on the plate’s panels, so the plate is first divided into panels and each moment is found. The other quantities are in the table.

Bending moments on the plate panels

The column, bracket plates and ribs divide the plate into panels. For a cantilever panel (the overhang beyond a flange or rib) formula M.1 applies, where c is the cantilever overhang:

M = 0.5 · q · c²

For a panel supported on two mutually perpendicular sides, or on three or four sides, formula M.2 applies:

M = αp · q · bp²

αp is taken from Table M.2 by the support scheme shown there and the ratio ap / bp; ap and bp are clear dimensions (note 1). For a panel on three sides with ap / bp below 0.35 the moment is found as for a cantilever of length ap, and between 0.35 and 0.5 by linear interpolation (note 2).

Adjacent panels that relieve the plate may be taken into account. Since c and bp enter the moments squared, a long overhang without a rib quickly raises the thickness needed.

Example: a plate for an I-beam No. 30 and N = 600 kN

The column of I-beam No. 30 is checked in the article on steel column design. The data are the same: I-beam No. 30 (height 300 mm, flange width 135 mm), N = 600 kN, γn = 1.05. The plate is taken as 400 × 400 mm of C345 steel, static load, uniform pressure; the concrete is not checked.

What the example shows

Ry depends on thickness, so 300 N/mm² (over 20 up to 40 mm) is checked once the plate is chosen: for 25 mm it holds. A 20 mm plate fails: with Ry = 315 N/mm² it would need 23.4 mm.

Only the cantilever panel is checked. The panel between the flanges, on three sides, goes by M.2 with αp from Table M.2; the largest moment governs, so 24.0 mm is a lower bound.

The plate size affects the thickness: at 350 × 350 mm (q = 4.90 N/mm², c = 107.5 mm) t ≥ 22.3 mm, at 500 × 500 mm (q = 2.40 N/mm², c = 182.5 mm) t ≥ 26.4 mm. A larger plate lowers the pressure on the concrete but needs a thicker plate or ribs.

Anchors, holes in the plate and grouting

Anchor bolts are checked for the tension from the moment and axial force; their design tension resistance is in cl. 7.7 and Table D.6 of DBN V.2.6-198. Diameter, steel, length and anchorage come from the foundation design: DBN V.2.1-10 says nothing about anchor assemblies. See also Anchor bolts for poles and masts and Metric thread of anchor and erection bolts.

The holes in the plate are made with clearance that covers the deviations of the anchors, and large-diameter washers are sometimes placed between the nut and the plate (DSTU EN 1090-2:2019, cl. 11.2.3.3). Tolerances on anchor position are in the article on erection tolerances.

The plate is levelled on levelling nuts; they may be left in place unless the specification says otherwise (cl. 9.5.4). The grout material and thickness under a column plate are set by the project.

What to put in the KMD and in the enquiry

The fabricator sees the base only as it is drawn. The KMD and the enquiry should carry:

Common mistakes

• calculating only the cantilever and not checking the panels on three and four sides: Mmax is the largest of all the moments;
• using Ry for a thin plate and then fitting a thicker one: by Table G.2 the design strength falls as thickness grows;
• using a milled-end base for a column with a moment: such bases are for centrally compressed columns only;
• leaving a column on anchor bolts alone, without guys: the standard forbids relying on them only against overturning unless this has been checked (DSTU EN 1090-2:2019, cl. 9.6.5.1).

Standards and validity

• DBN V.2.6-198:2014 «Steel structures. Design standards» with Amendment No. 1: Section 11, cl. 7.7, cl. 17.13.1, Tables 5.1, G.2, G.4 and D.6, Annex M;
• DBN V.2.1-10:2018 «Foundations of buildings and structures. Basic provisions»: foundation size and concrete class;
• DSTU EN 1090-2:2019 «Execution of steel structures and aluminium structures. Part 2. Technical requirements for steel structures» (cl. 9.5.4, 9.6.5.1, 11.2.3.3);
• DBN V.1.2-14:2018 «General principles of reliability and structural safety of buildings and structures» (factor γn).

The clauses, formulas and values from DBN V.2.6-198 are read from the 2014 edition of the text; before citing them in a project, check them against the current version with Amendment No. 1. The numbers in the example are illustrative.