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Practical Design to Eurocode 2 23/11/16

Lecture 10: Foundations 1

Practical Design to Eurocode 2

The webinar will start at 12.30

Lecture Date Speaker Title

1 21 Sep Charles Goodchild Introduction, Background and Codes

2 28 Sep Charles Goodchild EC2 Background, Materials, Cover and effective spans

3 5 Oct Paul Gregory Bending and Shear in Beams

4 12 Oct Charles Goodchild Analysis

5 19 Oct Paul Gregory Slabs and Flat Slabs

6 26 Oct Charles Goodchild Deflection and Crack Control

7 2 Nov Paul Gregory Detailing

8 9 Nov Jenny Burridge Columns

9 16 Nov Jenny Burridge Fire

10 23 Nov Jenny Burridge Foundations

Course Outline



Lecture 10

25th November 2015

Foundations

Lecture 9 Exercise:

Model Answers

Fire resistance of Column



Fire resistance of column

Using Equation 5.7, work out the fire resistance of a 250

x 750 column with an axial capacity of 3750kN and an

axial load in cold conditions of 3500kN. The column is on

the ground floor of a three storey building and the length

is 4.5m. The cover is 30mm, main bars are 20mm and

the links are 10mm diameter.

Design Exercise Answers

= 120( + + + +

120).

μfi = 0.7 x 3500/3750 = 0.65 Rηfi = 83(1 - 0.65) = 28.8

a = 30 + 10 + 10 = 50mm Ra = 1.6(50 - 30) = 32

l0,fi = 0.5 x 4.5 = 2.25m Rl = 9.6(5 - 2.25) = 26.4

b’ = 1.2 x 250 = 300mm Rb = 0.09 x 300 = 27

n > 4 Rn = 12

R = 120((Rηfi + Ra + Rl + Rb + Rn)/120)1.8 = 131 minutes



Foundations

Outline – Week 10, Foundations

We will look at the following topics:

• Eurocode 7: Geotechnical design – Partial factors, spread

foundations.

• Pad foundation – Worked example & workshop

• Retaining walls

• Piles



Eurocode 7

Eurocode 7 has two parts:

Part 1: General Rules

Part 2: Ground Investigation and testing

6 (p43 et seq)

Plus NA

Plus NA

Eurocode 7

How to…… 6. Foundations

The essential features of EC7, Pt 1 relating

to foundation design are discussed.

Note:

This publication covers only the design of

simple foundations, which are a small part

of EC7.

It should not be relied on for general

guidance on EC7.



Limit States

The following ultimate limit states apply to foundation

design:

EQU: Loss of equilibrium of the structure

STR: Internal failure or excessive deformation of the

structure or structural member

GEO: Failure due to excessive deformation of the ground

UPL: Loss of equilibrium due to uplift by water pressure

HYD: Failure caused by hydraulic gradients

Categories of Structures

Category Description Risk of geo-technical failure

Examples from EC7

1 Small and relatively

simple structures

Negligible None given

2 Conventional types of

structure – no difficult

ground

No exceptional

risk

Spread

foundations

3 All other structures Abnormal risks Large or unusual

structures



EC7 – ULS Design

EC7 provides for three Design Approaches

UK National Annex - Use Design Approach 1 – DA1

For DA1 (except piles and anchorage design) there are two

sets of combinations to use for the STR and GEO limit

states.

Combination 1 – generally governs structural resistance

Combination 2 – generally governs sizing of foundations

STR/GEO ULS – Actions partial factors

Permanent Actions Leading

variable

action

Accompanying variable

actions

Unfavourable Favourable Main Others

Combination 1

Exp 6.10 1.35Gk 1.0Gk 1.5Qk 1.5ψ0,iQk

Exp 6.10a 1.35Gk 1.0Gk 1.5ψ0,1Qk 1.5ψ0,iQk

Exp 6.10b 1.25Gk 1.0Gk 1.5Qk 1.5ψ0,iQk

Combination 2

Exp 6.10 1.0Gk 1.0Gk 1.3Qk 1.3ψ0,iQk

Notes:

If the variation in permanent action is significant, use Gk,j,sup and Gk,j,inf

If the action is favourable, γQ,i = 0 and the variable actions should be ignored



Factors for EQU, UPL and HYD

Limit state

Permanent Actions Variable Actions

Unfavourable Favourable Unfavourable Favourable

EQU 1.1 0.9 1.5 0

UPL 1.1 0.9 1.5 0

HYD 1.35 0.9 1.5 0

Partial factors – material properties

Parameter SymbolCombination

1Combination

2EQU

Angle of shearing

resistanceγφ 1.0 1.25 1.1

Effective cohesion γc’ 1.0 1.25 1.1

Undrained shear

strengthγcu 1.0 1.4 1.2

Unconfined strength γqu 1.0 1.4 1.2

Bulk density γγ 1.0 1.0 1.0



Geotechnical Report

The Geotechnical Report should:

• be produced for each project (if even just a single sheet)

• contain details of:

• the site,

• interpretation of ground investigation report,

• geotechnical recommendations,

• advice

Foundation design recommendations should state:

• bearing resistances,

• characteristic values of soil parameters and

• whether values are SLS or ULS , Combination 1 or

Combination 2 values

Spread Foundations

EC7 Section 6

Three methods for design:

• Direct method – check all limit states:

• Load and partial factor combinations (as before)

• qult=c’Ncscdcicgcbc + q’Nqsqdqiqgqbq +γ’BNγsγdγiγgγbγ/2

where– c = cohesion

– q = overburden

– γ = body-weight

– Ni = bearing capacity factors

– si = shape factors

– di = depth factors

– ii = inclination factors

– gi = ground inclination factors

– bi = base inclination factors

“We just bung it in a

spreadsheet”

Settlement often critical

See Decoding Eurocode 7 by A

Bond & A Harris, Taylor & Francis



Spread Foundations

EC7 Section 6

Three methods for design:

• Direct method – check all limit states

• Indirect method – experience and testing

used to determine SLS parameters that

also satisfy ULS

• Prescriptive methods – use presumed

bearing resistance (BS8004 quoted in NA). Used in sub-

sequent slides).

Spread Foundations

Design procedures in:



Fig 6/1 (p46)

Procedure for depth of

spread foundations

Pressure distributions

SLS pressure

distributions

ULS pressure

distribution



Load cases

EQU : 0.9 Gk + 1.5 Qk (assuming variable action is

destabilising e.g. wind, and

permanent action is stabilizing)

STR : 1.35 Gk + 1.5 Qk (Using (6.10). Worse case of Exp

(6.10a) or (6.10b) could be used)

a a

bF

hF

≥ √(3σgd/fctd,pl)h

aF0,85 ⋅

where:

σgd is the design value of the ground pressure

• as a simplification hf/a ≥ 2 may be used

Plain Concrete Strip Footings & Pad Foundations:

Cl. 12.9.3, Exp (12.13)



a a

bF

hF

C16/20 C20/25 C25/30 C30/37

allowable

pressure σgd hF/a hF/a hF/a hF/a

50 70 0.65 0.60 0.55 0.52

100 140 0.92 0.85 0.78 0.74

150 210 1.12 1.04 0.95 0.90

200 280 1.29 1.21 1.10 1.04

250 350 1.45 1.35 1.23 1.17

Plain Concrete Strip Footings & Pad Foundations

e.g. cavity wall 300 wide carrying 80 kN/m onto 100 kN/m2

ground:

bf = 800 mm

a = 250 mm

hf = say assuming C20/25 concrete

0.85 x 250 = 213 say 225 mm

Reinforced Concrete Bases

• Check critical bending moments at column faces

• Check beam shear and punching shear

For punching shear the

ground reaction within

the perimeter may be

deducted from the

column load



Pad foundation

Worked example

Worked Example

Design a square pad footing for a 350 × 350 mm column

carrying Gk = 600 kN and Qk = 505 kN. The presumed

allowable bearing pressure of the non-aggressive soil is

200 kN/m2.

Answer:

Category 2. So using prescriptive methods:

Base area: (600 + 505)/200 = 5.525m2

=> 2.4 x 2.4 base x 0.5m (say) deep.



Worked Example

Loading = 1.35 x 600 + 1.5 x 505

= 1567.5 kN

ULS bearing pressure = 1567.5/2.42

= 272 kN/m2

Critical section at face of column

MEd = 272 x 2.4 x 1.0252 / 2

= 343 kNm

d = 500 – 50 – 16

= 434 mm

K = 343 x106/(2400 x 4342 x30)

= 0.025

Use C30/37 concrete

Worked Example

⇒ z = 0.95d = 0.95 x 434 = 412mm

⇒As = MEd/fydz = 343 x 106 / (435 x 412) = 1914mm2

⇒ Provide 10H16 @ 250 c/c b.w (2010 mm2) (804 mm2/m)

Beam shear:

Check critical section d away from column face

VEd = 272 x (1.025 – 0.434) = 161kN/m

vEd = 161 / 434 = 0.37MPa

ρ = 2010/ (434 x 2400) = 0.0019 = 0.19%

vRd,c (from table) = 0.42MPa

=> beam shear ok.

6/Table 6 (p47)

Concise Table 15.6



Worked Example

Punching shear:

Basic control perimeter at 2d from face of column

vEd = βVEd / uid < vRd,c

β = 1, ui = (350 x 4 + 434 x 2 x 2 x π) = 6854mm

VEd = load minus net upward force within the area of the

control perimeter)

= 1567.5 – 272 x (0.352 + π x .8682 + .868 x .35 x 4)

= 560kN

vEd = 0.188 MPa; vRd,c = 0.42 (as before) => ok

Retaining Walls

Chapter 9



Ultimate Limit States

for the design of

retaining walls

Calculation Model A

Model applies if bh≥ ha tan (45 - ϕ’d/2)

Rankine theory



Calculation Model B

Inclined ‘ virtual’ plane theory

Model applies to walls of all shapes and sizes

General Model A Model B

2

BL

2

bbL

bbBb

BtW

HbW

b

sts

tsh

c,kbb

c,kss

=

+=

−−=

γ=

γ=

BL

2

bbbL

2

tanbHbW

tanbHth

vp

hstf

f,kh

hf

hb

=

β=Ω

++≈

γ

β+=

β++=

General expressions



Overall design

procedure

9 (Figure 4)

Initial sizing

bs ≈ tb ≈ h/10 to h/15

B ≈ 0.5h to 0.7h

bt ≈ B/4 to B/3



Overall design

procedure

9 (Figure 4)

Figure 6 for overall

design procedure

9 (Figure 6)



Soil Densities

Ex Concrete Basements

Design value of effective angle of shearing resistance, φ’d

tan φ’d = tan (φ’k/γφ)

where

φ’k = φ’max for granular soils and

= φ’ for clay soils,

φ’max and φ’ are as defined as follows

γφ = 1.0 or 1.25 dependent on the

Combination being considered.




Angle of shearing resistance

Estimated peak effective angle of shearing resistance,

φ’max = 30 + A + B + C

Estimated critical state angle of shearing resistance,

φ’crit = 30 + A + B, which is the upper limiting value.


Granular Soils

Long term ≡ Granular Soils

Clay soils




Calcs –Material properties & earth pressures

9 Panel 2 (p71)

9 (Panel 2)

2

’

Overall design

procedure

9 (Figure 4)



Design against

sliding

(Figure 7)

9 (Figure 7)

9 (Panel 3)

Sliding Resistance



Overall design

procedure

9 (Figure 4)

Design against Toppling

9 (Figure 9)



Overall design

procedure

9 (Figure 4)

Design against bearing failure9 (Figure 10)



Expressions for bearing resistance

9 (Panel 4,Figure 11)

Overall design

procedure

9 (Figure 4)



Structural design9 (Figure 13)

Remember: Load and Partial Factor Combinations

Parameter Symbol Comb. 1 Comb. 2

Actions

Permanent action : unfavourable γG,unfav 1.35 1.0

Permanent action: favourable γG,fav 1.00 1.00

Variable action γQ 1.50 1.30

Soil Properties

Angle of shearing resistance γφ 1.0 1.25

Effective cohesion γc’ 1.0 1.25

Undrained shear strength γcu 1.0 1.4

Unconfined strength γqu 1.0 1.4

Bulk density γγ 1.0 1.0



Piles

Flexural and axial resistance of piles

‘Uncertainties related to the cross-section of cast in place piles and

concreting procedures shall be allowed for in design’

‘In the absence of other provisions’, the design diameter of cast in place

piles without permanent casing is less than the nominal diameter Dnom:

• Dd = Dnom – 20 mm for Dnom < 400 mm

• Dd = 0.95 Dnom for 400 ≤ Dnom ≤ 1000 mm

• Dd = Dnom – 50 mm for Dnom > 1000 mm

ICE Specification for piling and embedded retaining walls (ICE SPERW)

B1.10.2 states ‘The dimensions of a constructed pile or wall element shall

not be less than the specified dimensions’. A tolerance of 5% on auger

diameter, casing diameter, and grab length and width is permissible.



Flexural and axial resistance of piles

• The partial factor for concrete, γc, should be multiplied by a

factor, kf, for calculation of design resistance of cast in place

piles without permanent casing.

• The UK value of kf = 1.1, therefore γc,pile = 1.65

• “If the width of the compression zone decreases in the direction

of the extreme compression fibre, the value η fcd should be

reduced by 10%”

Bored piles

Reinforcement should be detailed for free flow of

concrete.

Minimum diameter of long. reinforcement = 16mm

Minimum number of longitudinal bars = 6

[BUT – BS EN 1536 Execution of special geotechnical work Bored Piles

says 12 mm and 4 bars!]

Minimum areas:

Pile cross section: Ac

Min area of long. rebar, As,bpmin

Pile diameters

Ac ≤ 0.5 m2 ≥ 0.5% Ac < 800 mm

0.5 m2< Ac≤ 1.0 m2 ≥ 2500 mm2

Ac > 1.0 m2 ≥ 0.25% Ac >1130 mm



Minimum reinforcement

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0 200 400 600 800 1000 1200 1400 1600

Min

imu

m

are

a o

f re

info

rce

me

nt,

A

s,b

pm

in(m

m2)

Pile diameter, mm

Workshop

Design a pad foundation for a 300mm square column

taking

Gk = 600kN, Qk = 350kN.

Permissible bearing stress = 225kPa.

Concrete for base C30/37.

Work out

• size of base,

• tension reinforcement and

• any shear reinforcement.



Workshop Problem

Category 2, using prescriptive methods

Base size: (Gk + Qk)/bearing stress = _____ __ _____ __

= __ __ __ _ _m2

⇒ ______ x ______ base x ______mm deep (choose size of pad)

Use C 30/37 (concrete)

Loading = γg x Gk + γq x Qk = ________________= _____kN

ULS bearing pressure = ______/________2 = _____kN/m2

Critical section at face of column

MEd = ______ x ______ x _______2 / 2 = _____kNm

d = _______ – cover – assumed ø = _______________= ____mm

K = M/bd2fck= ______

Workshop Problem

⇒ z = ____d = ______ x ______= ____mm

⇒ As = MEd/fydz = _____mm2

⇒ Provide H____ @ ______ c/c (_______mm2)

Check minimum steel

100As,prov/bd = _______

For C30/37 concrete As,min = ____ ∴ OK/not OK

Beam shear

Check critical section d away from column face

VEd = ______ x ________= ________kN/m

vEd = VEd / d = ________MPa

ρ = _____/ (______ x ______) = _____ = _____ %

vRd,c (from table) = ____MPa ∴ beam shear OK/not OK.

Table 15.5

12.3.1



Workshop Problem

Punching shear

Basic control perimeter at 2d from face of column

vEd = βVEd / uid < vRd,c

β = 1, ui = = _____mm

VEd = load minus net upward force within the area of the control

perimeter)

= _____ – ____ x ( )

= _____kN

vEd = _____MPa; vRd,c = ______ (as before) => ok/not ok

End of Lecture 10(and the course!)

Emails:

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