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Note 7 Level 2
Technical
Technical Guidance Note
TheStructuralEngineer
August 2013
41
Designing a concrete
pad foundation
Introduction
The purpose of a pad foundation is to spread a concentrated force into soil.
They are one of the most simple and cost effective types of footings for
structures. Provided the founding soil is of sufficient strength and is not too
deep to reach, pad foundations are the preferred solution for foundations due
to the straight forward nature of their design and construction.
This Technical Guidance Note covers the design of concrete pad
foundations, both mass and reinforced concrete forms. It will not, however,
discuss how the bearing capacity of the soil is determined, as that is
explained in Technical Guidance Note 19 (Level 1) Soil bearing capacity. It
is suggested that you read that text in conjunction with this, in order to gain a
more comprehensive understanding of the topic.
•
•
Figure 1
Unreinforced
concrete pad
footing depth
ICON
LEGEND
• Design principles
• Worked example
• Applied practice
• Further reading
• Web resources
•
Figure 2
Reinforced
concrete pad
footing depth
Figure 3
Resultant
force location in
pad foundation
Concentrated
force spread into
distributed area
load beneath
footing
Design principles
Pad foundations are designed to spread a
concentrated force that is to be applied to a
bearing stratum. They therefore need to be
sufficiently stiff to spread the force into the
soil such that the founding pressure does
not exceed its permissible bearing stress.
This can be done by either making the pad
sufficiently deep so that the force spreads out
by a predefined angle or with reinforcement.
The angle is determined based on the
strength of the concrete and the bearing
capacity of the soil. For unreinforced concrete
This version 1.1 published October 2016.
Table 1: Depth/projection ratios for unreinforced footings
Unfactored ground
pressure s (kN/m2)
a
hf
C20/25
C25/30
C30/37
C35/45
≤ 200
1.2
1.1
1.1
1.0
300
1.5
1.4
1.3
1.2
400
1.7
1.6
1.5
1.4
a is the projection from the face of the column
hf is the depth of the footing
›
Note 7 Level 2
42
TheStructuralEngineer
August 2013
Technical
Technical Guidance Note
•
pads, Table 1 (taken from the Manual for the
design of concrete structures to Eurocode
2, where further guidance on this topic can
be obtained) should be referred to, when
determining the spread of force within a pad
footing.
Figure 5
Extent of
bending stress
in shallow pad
foundation
The level at which the pad foundation is to be
placed is also important, as to have it too deep
can result in difficult and even dangerous
conditions during construction. As a rule-ofthumb it is advisable to limit the overall depth
of a pad foundation, to no more than 1m from
ground level, for typical low-rise buildings.
The primary driver behind the size of a pad
footing is the need to prevent tension within
the concrete, which could cause the pad
foundation to crack and then fail (Figure 1).
For reinforced concrete pads, one of the
governing criteria that determines the depth
of the footing is its ability to resist ‘punching
shear’ (Figure 2). This is a shear force that
is developed around the perimeter of the
vertical element the pad is supporting, be
it a column, wall or masonry pier. The other
equally important element is its resistance
to bending, which occurs as the footing
spreads the force onto the soil by virtue of its
stiffness.
Resistance to bending moments
When bending moments are applied to a
pad footing it is good practice to size the
foundation so that the resultant force lies
within the middle third of the base (Figure 3).
Assuming the resultant force is within the
middle-third, the distribution of compression
stress across the pad foundation is defined as:
P
Pe
P
6e
p = BL !
= BL a 1 ! L k
BL2
6
Where:
p is the compressive stress in the soil under
the pad foundation
P is the applied axial load, including the
self-weight of the footing
B is the width of the pad foundation
L is the length of the pad foundation
e is the eccentricity of the applied axial load
taken at the centre of the bearing
When the resultant force lies outside the
middle third, the effective contact area
between the pad footing and the soil is
reduced. This is because the length of
the pad is reduced as one side of the pad
develops tension and so is lifted off the soil
as the resultant force is applied to it due to
its eccentricity. This creates an ‘effective
length’ of the pad along the axis of the
applied bending moment. This is calculated
by positioning the resultant force and placing
it on the point of a middle third within the
effective length (Figure 4).
such cases the designer is required to check
to see if proper provision has been made
for the connection between the structure
and the foundation, to allow for the resulting
tension forces.
In such conditions there is a possibility of
uplift occurring as the pad rotates onto the
soil. The value of the bearing stress being
applied to the soil is calculated thus:
Appropriate partial factors
2P
p = 3By
y is the distance from the line of action P to
the nearest edge of the pad footing and is
defined as
L
2 -e
It is possible for biaxial bending moments
to be applied to pad foundations. In such
instances the following expression should
be used to calculate the bearing stress in
the soil:
My
M
P
p = BL ! Z x ! Z
x
y
Where:
Mx is the applied bending moment in the
major axis
Zx is the elastic modulus of the pad
foundation in its major axis in plan
My is the applied bending moment in the
minor axis
Zy is the elastic modulus of the pad
foundation in its minor axis in plan
In some instances it is possible that an uplift
force is generated on the structure, which is
typically due to negative wind pressures. In
When designing foundations of any kind,
the partial factors to applied forces differ
depending on what is being checked within
the footing. In essence, when checking the
applied stress against the overall strength of
the soil, the partial factors are significantly
reduced, whereas when the foundation itself
is being designed, full partial factors apply.
•
Figure 4
Effective length of bearing
www.thestructuralengineer.org
43
Worked example
A 400 x 400mm concrete column has an axial characteristic load of 700 kN (980 kN
design) with an accompanying characteristic bending moment of 35 kNm (50 kNm
design) along one axis only. Check to see if a 1.85m x 1.85m x 0.6m deep pad foundation
can support the applied loads. The founding soil stratum has a bearing capacity of
250 kN/m2 and the axial load includes the self-weight of the foundation and soil
surcharge. Any reinforcement specified will be applied in both directions. Concrete grade
to be C30/70.
Worked example
A 400 x 400mm concrete column has an axial characteristic load of 700 kN (980 kN
design) with an accompanying characteristic bending moment of 35 kNm (50 kNm
design) along one axis only. Check to see if a 1.85m x 1.85m x 0.6m deep pad foundation
can support the applied loads. The founding soil stratum has a bearing capacity of
250 kN/m2 and the axial load includes the self-weight of the foundation and soil
surcharge. Any reinforcement specified will be applied in both directions.
BS EN 1990 defines the partial factors
for sub-structures and splits them into
geotechnical (GEO) and structural (STR).
When checking the bearing stress that is
being applied to the soil, the GEO partial
factors apply:
www.thestructuralengineer.org
E d = 1.0G k,j + 1.3Q k,1 + } 0 1.3Q k,2
43
Where:
Ed is the effect of the action
Gk,j is the permanent action e.g. self-weight
of the structure
Eurocode
0.
Qk,1 is theApplied
leading frequent
variable action
practice
e.g. occupancy and furniture
Qk,2 is the accompanying quasi-permanent
action
e.g. wind0: Basis of Design
BSvariable
EN 1990-1
Eurocode
ψ0 is the factor for the accompanying quasivalue
of a variable
action*
BSpermanent
EN 1992-1-1
Eurocode
2: Design
of
*These values
are drawn
from
A1.1Rules
of
Concrete
Structures
– Part
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General
EN 1990
forBS
Buildings
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BS
ENdesigning
1992-1-1 UK
National Annex
to
foundations
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the STR
Eurocode
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Design
of Concrete
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–category
Part 1-1: General
EdEN=1997-1
1.35G
QGeotechnical
BS
Eurocode
k, j + 1.57:
k, 1 + } 0 1.5Q k, 2
Design – Part 1 General Rules
Reinforcement
design Annex to
BS
EN 1997-1 UK National
The bending
stress in the pad
footing
Eurocode
7: Geotechnical
Design
– Part 1
needs toRules
be considered from the face of
General
the column it is supporting (Figure 5). The
distance ‘a’ is the length over which the
bending stress need only be considered. All
Glossary
reinforcement
in a padand
foundation must be
further
reading
designed to resist the
bending stress.
The design of reinforcement
in pad the
– in this context;
foundations
very similar
tostress
that ofisfloor
distance
overiswhich
bearing
applied
slabs.
The tension stress resulting from
to
the footing.
applied shear and bending moment(s)
are resisted
byrule
the reinforcement
that is
Middle
third
– A design practice
designedthe
in accordance
with
BSaccording
EN 1992whereby
foundation is
sized
1-1.aThis
is explained
to
centralisation
of in
theTechnical
resultantGuidance
force.
Note 3 (Level 2) Designing a concrete slab.
Pad foundation – An element of substructure that
Eurocode
0. spreads a concentrated
load fromApplied
the super-structure
into
practice
bearing soil.
Punching
shear
– stress
a flDesign
at
BS
EN 1990-1
Eurocode
0: within
Basis of
concrete element, such as a slab or a pad
foundation
caused
by a concentrated
BS
EN 1992-1-1
Eurocode
2: Design of
point load.Structures – Part 1-1: General Rules
Concrete
for Buildings
Uplift – Where applied forces are greater
thanEN
(and
actingUK
in the
opposite
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BS
1992-1-1
National
Annex
to
to) the self-weight
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Eurocode
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Concrete
Structures
to lift. Rules for Buildings
–causing
Part 1-1:it General
Further
Reading
BS
EN 1997-1
Eurocode 7: Geotechnical
Tomlinson
M. 1J.General
(2001) Foundation
Design
Design
– Part
Rules
and Construction (7th ed.) Zug, Switzerland:
Prentice Hall
›
Note 7 Level 2
43i
TheStructuralEngineer
August 2013
concrete element, such as a slab or a pad
foundation caused by a concentrated
point load.
Technical
Uplift – Where applied forces are greater
Technical Guidance Note
than (and acting in the opposite direction
to) the self-weight of the pad foundation,
causing it to lift.
Further Reading
Tomlinson M. J. (2001) Foundation Design
and Construction (7th ed.) Zug, Switzerland:
BS EN 1997-1 UK National Annex to
Prentice Hall
Eurocode 7: Geotechnical Design – Part 1
General Rules
Eurocode 0.
Web resources
Glossary and
further reading
The Concrete Society:
www.concrete.org.uk/
Effective length – in this context; the
distance over which bearing stress is applied
The
Institution
to the
footing. of Structural Engineers library:
www.istructe.org/resources-centre/library
Middle third rule – A design practice
whereby the foundation is sized according
to a centralisation of the resultant force.
TSE20_41-43.indd 43
Pad foundation – An element of substructure that spreads a concentrated
load from the super-structure into
bearing soil.
24/07/2013 16:36
Punching shear – stress within a flat
concrete element, such as a slab or a pad
foundation caused by a concentrated
point load.
Uplift – Where applied forces are greater
than (and acting in the opposite direction
to) the self-weight of the pad foundation,
causing it to lift.
Further Reading
Tomlinson M. J. (2001) Foundation Design
and Construction (7th ed.) Zug, Switzerland:
Prentice Hall
Eurocode 0.
Web resources
The Concrete Society:
www.concrete.org.uk/
The Institution of Structural Engineers library:
www.istructe.org/resources-centre/library