Bearing splice in a column
The design of column splices is covered in BS EN 1993-1-8 where it is lumped together with
the moment resistance of beam-to-column joints. Richard Henderson of the SCI illustrates the
design of a column bearing splice considering the strut moment with a numerical example.
The design of column splices is a subject that the SCI is asked
about from time to time, including whether a design example
is available. The Green Book1, Simple joints to Eurocode 3, P358
deals with column splices in Chapter 6. The detailing rules set
out in the Green Book do not mention the source of the design
moments in the column which are used to check if the column
is not in bearing anywhere over the cross section. Traditionally,
column splices were introduced close to floor slab level so
although the moments due to nominal eccentricity of the floor
beams (if unbalanced) were near their maximum, the internal
moments in the column were assumed to be small enough
to ignore. Requirements to provide fall protection has led to
the position of column splices being extended upwards to a
height of 1.2 m above floor steelwork level to allow the fixing of
temporary handrails. This was discussed in Advisory Desk note
AD 3142. The internal moments are larger than for a lower splice
and should be considered in the splice design.
Column Design – internal bending moment
The design of a column according to BS EN 1993-1-1 essentially
follows the Perry-Robertson approach where at failure, the
combined axial and bending stress in the extreme fibre is equal
to the yield strength of the material. The bending moment (strut
moment), is due to the assumed bow imperfection, which is
amplified by the axial load. According to the UK National Annex
to BS EN 1993-1-1 the bow imperfection must be back-calculated
from the design resistance of the column.
The theoretical treatment of elastic buckling of a strut which
leads to the elastic critical (Euler) buckling load assumes a
deflected shape of a half-sine wave. This can be used to
determine the deflection and therefore the bending moment at
any position up the column, between points of restraint.
Designers who remember the treatment of strut action in
BS 5950:2000 Annex C will recognise this as the approach
adopted there. A parabolic shape for the curvature could be
assumed but this results in larger intermediate displacements
and would therefore be on the safe side.
Other design requirements
BS EN 1993-1-8 para. 18.104.22.168(14) states that “Where members are
prepared for full contact in bearing, splice material should be
provided to transmit at least 25% of the maximum compressive
force in the column”.
Robustness requirements in Class 2B buildings demand that
vertical ties are provided over the height of the building.
According to BS EN 1991-1-7 para A.6(2) the column should be
capable of resisting an accidental tie force equal to the largest
design vertical permanent and variable load reaction applied to
the column from any one storey. Column splices must therefore
carry the vertical tie force which is an accidental load and
reduced partial factors apply as a result. Advisory Desk note
AD4153 confirms this and provides additional information.
The stiffness of the column at the splice position must also be
such that the column behaves as a continuous element.
Tolerances at the splice position
The National Structural Steelwork Specification (NSSS)4 includes
several clauses relating to permitted deviations at column splices
as indicated in Table 1, which may also be found in BS EN 1090-25.
The design of the splice must be sufficient to accommodate the
maximum deviations allowed in the specification.
Clause Parameter Requirement
7.2.3 Squareness of
ends prepared for
Ends prepared with respect to
longitudinal axis of member. Plan
or elevation of end Δ = D/1000
9.6.10 Column splice
Local angular misalignment (Δθ)
occurring at same time as gap (Δ).
Δθ = 1/500. Δ = 0.5 mm over at
least 2/3 of the area with a
maximum of 1.0 mm locally.
9.6.11 Eccentricity at
Non-intended eccentricity (e = ex
or ey) about either axis. e = 5 mm
9.6.12 Straightness of a
Location (Δ) of the column in plan
relative to a straight line between
position points at adjacent storey
levels. Δ = s/750* with s ≤ h/2
*This value is s/1000 in
BS EN 1090-2
D = width or depth of member;
s = height of splice above lower storey; h = storey height
The following example illustrates the design method. Consider a
column splice supporting five floors above. The column length
below the splice extends over three storeys. Storey heights are
4.0 m. Each floor applies a load of 2800 kN. A permanent action
of 3.6 kPa and a variable action of 5 kPa are assumed.
To calculate the design axial compression at the splice level,
the variable action reduction factor αn given in NA.2.6 of the
UK NA to BS EN 1991-1-16 has been calculated.
For 5 storeys,
10 = 1.1 – = 0.6
n = 1.1 –
According to NA.2.6 the same reduction factor is used to
calculate the design axial compression at the base of the lower
column, which supports eight storeys.
The design compression at the splice is therefore
5 × 2100 × 10-3 = 10.5 MN. The maximum design compression in
the lower column section is 16.8 MN.
Table 1: Manufacturing and installation tolerances