| Buildings
Image courtesy of A C Bacon Engineering
Column bases – The importance of getting it right
This article introduces the topic of column bases as used
in portal frame sheds and other steel framed buildings.
ften hidden from
view and
forgotten, column
bases and their
foundations are as
important as any
other building element. Not only
do they provide a stable base on
which to erect the steel frame, but
they also provide an essential link
in the load path. All loads applied
to the building must eventually
pass through the column bases
into the ground.
O
Column bases provide the
important link between the
building superstructure and the
supporting ground
A typical column base consists
of a steel plate, known as the base
plate, welded to the base of the
column and connected to the
concrete foundation by four
holding down bolts. Although two
holding down bolts are likely to be
adequate to resist the design
loads, four should always be used
for the sake of safety during
erection. For large columns
(Universal Beam sections 356 mm
depth and above), it is common
practice to locate the bolts
between the flanges (i.e. within the
depth of the web), but for smaller
columns the bolts should be
placed outside the flanges. The
plate should be raised 50 mm
above the concrete foundation to
create a gap for the grout.
Loading on column bases
Column bases need to be
designed to carry upward and
downward vertical loads in
additional to horizontal base shear.
Most portal frame bases are
classed as ‘pinned’ even though
they have four bolts. While these
bolts possess some stiffness
(sufficient to provide stability
during erection), they do not
provide any significant resistance
to bending moments. The in-plane
stability of a portal frame is
provided by the haunched
connections at the eaves and not
by the column bases. Moment
resisting bases are sometimes
used in specialist applications, but
they are expensive due to the
fabrication required (e.g. welding
of stiffeners) and require a large
moment resisting foundation.
Downward vertical loads are
taken directly into the ground in
compression, with the base plate
spreading the load to limit the
pressure on the concrete
foundation. Vertical uplift loading is
taken in tension through the
columns into the holding down
bolts and is ultimately resisted by
friction between the foundation
42 | Farming Monthly | May 2018
and the surrounding soil.
Horizontal loads are taken into the
foundations by the bolts acting in
shear. It is worth noting that portal
frame bases are subjected to
significant horizontal forces, not
only from the wind loading, but
also due to gravity loading on the
roof. For a symmetric portal frame,
the horizontal thrust at the base is
equal to the bending moment at
the haunch divided by the column
height. The base shear on
asymmetric frames can be even
greater.
Design
The design of column base
plates is covered by EN 1993
(Eurocode 3). Base plates should
be sized according to the vertical
loading in the column and the
strength of the concrete in the
foundation. The base plate should
be sized for the largest factored
column load, taking account of all
realistic load combinations. Where
frames are subjected to combined
wind and snow loading, the
resulting column loads will be
asymmetric due to the overturning
action on the frame. The base
plate should be designed to
accommodate the maximum
downward force such that the
strength of the concrete is not
exceeded. This is achieved by
assuming an ‘effective area’ of
plate around the column profile.
Once the area of the base plate is
known, its thickness may be
calculated from the ‘cantilever’
length extending around the
column (i.e. the distance from the
edge of the column to the edge of
the assumed effective area) and
the design strength of the steel.
Free calculation tools are available
for designing the base plate
(search online for ‘steel base plate
calculator’).
The holding down bolts should
be specified to resist the uplift on
the column base (due to wind
action) in tension and also to
transfer the horizontal forces in
shear. The size of bolt will depend
on the magnitude of these forces,
but M20 is typical for small and
medium size sheds. The length of
bolt will depend on the strength of
the concrete in the foundation and
the tensile force that needs to be
transmitted into the concrete. This
should be checked by a structural
engineer.
Resisting horizontal forces
As mentioned above, the
column bases are subjected to a
significant horizontal base shear,
which needs to be resisted in
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