| Buildings
some way. A common way of
achieving this is by tying the base
of the column into the floor slab
using steel reinforcement. If this
approach is used, care must be
taken when detailing the
reinforcement to avoid cracking of
the slab. Where fibre reinforced
concrete is used for the slab,
reinforcing bars or mesh should
be placed at column locations to
prevent cracking.
An alternative method of
resisting horizontal forces is the
use of passive earth pressure
acting on the sides of the concrete
foundations. This is the simplest
approach, but its effectiveness
depends on the surrounding
ground being properly compacted
and able to resist the pressure. A
drainage or service trench dug
alongside the foundations would
be significantly detrimental in this
respect.
The third option is to run steel
ties between the columns at sub-
base level and then bury them
under the slab. This is probably
the most effective approach, but is
also the most expensive and can
be inconvenient for access prior to
concreting the slab (when the ties
are above ground level).
Foundations
For a nominally pinned base, as
used for most portal frame sheds,
the foundations need only be
designed for axial load. The size of
the foundation will depend on the
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magnitude of this load and the
bearing resistance of the
underlying soil. In simple terms the
ground bearing pressure,
calculated by dividing the
downward l oad by the plan area of
the foundation, should not exceed
the allowable ground bearing
pressure of the soil. It is good
practice to dig trial holes prior to
designing the foundations in order
to measure the bearing resistance
of the soil specific to that location.
This will ensure that the foundation
is large enough without being
over-conservative. The foundations
also need to resist wind uplift. This
is achieved through friction
between the soil and concrete.
Where ground conditions are
known to be poor, a structural
engineer’s advice should be
sought, as alternative foundation
types (e.g. piles) may need to be
considered.
Installation
It is common practice to erect
the columns on a single central
pack located in between the
holding down bolts. This enables
the erectors to plumb the columns
easily and accurately. Once the
columns have been erected and
the holding down bolts secured in
place, it is essential that the voids
surrounding the bolts and under
the base plate are filled with a
specially formulated grout (i.e. a
product designed for this specific
purpose rather than general-
Image courtesy of A C Bacon Engineering: Shows a typical
column base prior to grouting
purpose mortar or concrete). The
grout should have sufficient
strength not to crush or crack
under the concentrated force from
the column and must also be
sufficiently workable to fill the
voids around the holding down
bolts. The grouting process should
be undertaken carefully to ensure
that the whole volume under the
base plate is filled right up to the
underside of the plate. It is good
practice to drill a hole in the base
plate to allow trapped air to
escape and provide visual
confirmation that the cavity has
been filled.
Conclusions
Column bases provide the
important link between the
building superstructure and the
supporting ground and their
correct design and installation is
essential. All elements of the base,
including the plate, bolts and
foundation, should be designed to
resist downward loading, uplift and
shear, but not normally bending
moment. The base plates and
bolts should be designed following
the latest Eurocode
recommendations and great care
should be taken to ensure that
they are installed correctly,
especially during the grouting
operation.
May 2018 | Farming Monthly | 43