Design Code Options

Simple column bases can be designed to the following EC3 code versions:

• EC3
• EC3 Finland NA
• EC3 Ireland NA
• EC3 Malaysia NA
• EC3 Norway NA
• EC3 Singapore NA
• EC3 Sweden NA
• EC3 UK NA

Base plate steel grades

Base plates will use S235, S275, S355 and S460 family groups. Strengths greater than 460 are beyond scope. User defined grades ≤ 460 are allowed.

Design method and valid sections and forces for design

Base plate design uses the equivalent T-stub method for axial forces, where an ‘Effective area’ is calculated for T-stubs in compression, and base plate bending is assessed for T-stubs in tension. Horizontal major and/or minor shear is allowed, but no moments i.e. pinned base design.

• Only I/H and hollow section steel columns are valid for design.

• Minor shear is a valid design force for base plates on all of these column sections except for Rectangular Hollow Sections.

• Axial tension is a valid design force only on I/H section steel columns with the following bolt layout conditions:

  • Doubly symmetric bolt layouts

  • Total of 2 rows of bolts in the base plate

  • Bolt rows outside the column flanges have 2 or more bolts in each row

  • Bolt rows inside the column flanges have exactly 2 bolts in each row

  Note: For bolt rows both outside and inside the column flanges, a Warning will be given if the first and last bolt in the row lies vertically on plan outside the flange tips (which follows the guidance given at the bottom of page 70 of SCI P398 (Ref. 13)).

Column position on the base plate

The column can only be concentric on the base plate.

Base plate position on the concrete foundation

The base plate can be eccentric on the concrete foundation along both the minor and major axes. Note, such eccentricities are achieved in the concrete foundation properties.

Concrete foundation design

Concrete foundation design is separate to, but cognizant of, the base plate design. Only isolated foundations are valid concrete foundations for base plate design checks in first release.

Tekla Connection Designer

Under EC regional code there is no longer an option to export a base plate to Tekla Connection Designer from Tekla Structural Designer. For EC design checks, of column splices or beam to column connections for example, Tekla Connection Designer can still be used as a standalone product.

‘Equivalent T-stub in Compression - Effective Area’ Design Method

The ‘Effective area’ method is used for design under axial compression. The principle steps in this method are as follows:

• Calculate the design bearing strength, f_jd

• Calculate the required plate area, A_reqd and the actual area provided, A_plate

• Compare A_plate and A_reqd (Note, A_plate must be greater than A_reqd to proceed)

• Calculate the stiff cantilever projection dimension, ‘c’

• Calculate the effective plate area, A_eff

• Compare A_eff and A_reqd (Note, A_eff must be greater than A_reqd to ‘pass’)

Clarification of the design bearing strength calculation

The design bearing strength, f_jd, between the underside of the base plate and the bedding material in the grout space is given by:

f_jd = β_j * α * fcd

where

β_j = foundation joint material coefficient = (2/3)

α = a coefficient which accounts for diffusion of the concentrated force within the foundation

f_cd = design value concrete compressive strength = α_cc * f_ck / ɣ_c

α_cc = coefficient for long term effects

f_ck = concrete characteristic cylinder strength

ɣ_c = partial safety factor for concrete

‘Equivalent T-stub in Tension - Plate Bending’ Design Method

The ‘Equivalent T-stub in tension’ method is used for base plate bending design under axial tension.

Per Figure 6.2 of BS EN 1993-1-8 (Ref. 12). the distances between bolt centers and flange/web welds (m_x and m respectively) are measured 20% into the leg lengths of the welds. Where a butt weld is modelled on a flange the weld leg length is taken as zero.

Yield line patterns for bolt rows outside column flanges are based on Table 5.3 of SCI P398 (Ref. 13), and for bolt rows inside the flanges the yield line patterns are based on Tables 2.2 to 2.4 of SCI P398 (Ref. 13). For rows inside the flanges, the alpha factor is derived by an iterative process discussed in Appendix G of SCI P398 (Ref. 13).

Per Clause 6.2.6.11(2) of BS EN 1993-1-8 (Ref. 12) no prying force is assumed in determining base plate thickness. The T-stub resistance calculation in first release also assumes full tension resistance of the bolts i.e. there is no allowance for prying in bolt tension design.

Horizontal Shear

Horizontal shear transfer and resistance can be achieved by one of three options: friction, shear on bolts, or combined friction plus shear on bolts. The default option is friction, with a coefficient of friction defaulted at 0.2 but editable by the user. The vector sum of major and minor shear forces is used as the design shear force, to allow for uniaxial and biaxial shear situations.

In cases where uplift is present then a combined shear and tension check is carried out.

Welds

Weld resistance, which is calculated using the simplified method given in Clause 4.5.3.3 of BS EN 1993-1-8 (Ref. 12), varies by section type and whether direct contact in bearing is flagged on or off in the base plate properties - when on, axial force on the weld is taken as zero.

• For I/H sections, flange welds resist the vector sum of axial force and minor shear force, while web welds resist major shear force. Forces and resistances are given in Force units.

• For RHS sections, flange welds (faces A & C) resist axial force, while web welds (faces B & D) resist major shear force. Forces and resistances are given in Force units.

• For CHS and SHS sections, an all around full profile weld resists the vector sum of axial force, major shear force and minor shear force. Forces and resistances are given in Force per length units.

Connection forces are established from a global analysis of the building as a whole. Column base plates in Tekla Structural Designer have a limited set of design forces for which they can be designed. Non-design forces are identified and, where their value is greater than a given limit, they are displayed to you in the results along with a Warning status. The given limits are defined on the Design Forces page of the Design Settings dialog available from the Design tab on the ribbon.

The forces from the global analysis are treated in the following manner:

• Simple column bases are designed for the positive axial (compression) force at the base of the column, the negative axial (tension) force at the base of the column, the major shear (foundation reaction) in the plane of the column web (column section minor axis), and minor shear (foundation reaction) in the plane of the column flanges (column section major axis). Bases are orientated to the column’s major and minor axes and hence there is no requirement to resolve the force when the column is rotated. Columns can only be sloped in the plane of the web and the bottom stack axial force and shear are resolved into vertical and horizontal forces in the base.

Where the global analysis includes second-order (P-Delta) effects the Ultimate Limit State design forces will include these effects also. However, for column bases the design forces for soil bearing pressure calculations are taken from an elastic global analysis of the unfactored loadcases without second-order effects. Nevertheless, EQU and GEO load combinations are not considered in the base plate design i.e. these combinations do not appear in the results. All seismic (SEIS) combinations appear in the results. However, those deriving from ELF are considered for design while those from RSA result in Beyond Scope status.