practical design to eurocode 2 - · pdf filepart 1: general rules part 2: ground investigation...
TRANSCRIPT
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Practical Design to Eurocode 2
The webinar will start at 12.30
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Lecture Date Speaker Title
1 21 Sep Jenny Burridge 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 Columns
8 9 Nov Jenny Burridge Fire
9 16 Nov Paul Gregory Detailing
10 23 Nov Jenny Burridge Foundations
Course Outline
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Lecture 10
23rd November 2017
Foundations
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Lecture 9 Exercise
Lap length for column longitudinal bars
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Column lap length exercise
H25’s
H32’s
Lap
Design information
• C40/50 concrete
• 400 mm square column
• 45mm nominal cover to main bars
• Longitudinal bars are in compression
• Maximum ultimate stress in the bars
is 390 MPa
Exercise:
Calculate the minimum lap length
using EC2 equation 8.10:
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Column lap length exercise
Procedure
• Determine the ultimate bond stress, fbd EC2 Equ. 8.2
• Determine the basic anchorage length, lb,req EC2 Equ. 8.3
• Determine the design anchorage length, lbd EC2 Equ. 8.4
• Determine the lap length, l0 = anchorage length x α6
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Model Answers
Lap length for column longitudinal bars
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Column lap length exercise
H25’s
H32’s
Lap
Design information
• C40/50 concrete
• 400 mm square column
• 45mm nominal cover to main bars
• Longitudinal bars are in compression
• Maximum ultimate stress in the bars
is 390 MPa
Exercise:
Calculate the minimum lap length
using EC2 equation 8.10:
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Column lap length exercise
Procedure
• Determine the ultimate bond stress, fbd EC2 Equ. 8.2
• Determine the basic anchorage length, lb,req EC2 Equ. 8.3
• Determine the design anchorage length, lbd EC2 Equ. 8.4
• Determine the lap length, l0 = anchorage length x α6
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Solution - Column lap length
Determine the ultimate bond stress, fbd
fbd = 2.25 η1 η2 fctd EC2 Equ. 8.2
η1 = 1.0 ‘Good’ bond conditions
η2 = 1.0 bar size ≤ 32
fctd = αct fctk,0,05/γc EC2 cl 3.1.6(2), Equ 3.16
αct = 1.0 γc = 1.5
fctk,0,05 = 0.7 x 0.3 fck2/3 EC2 Table 3.1
= 0.21 x 402/3
= 2.456 MPa
fctd = αct fctk,0,05/γc = 2.456/1.5 = 1.637
fbd = 2.25 x 1.637 = 3.684 MPa
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Solution - Column lap length
Determine the basic anchorage length, lb,req
lb,req = (Ø/4) ( σsd/fbd) EC2 Equ 8.3
Max ultimate stress in the bar, σsd = 390 MPa.
lb,req = (Ø/4) ( 390/3.684)
= 26.47 Ø
For concrete class C40/50
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Solution - Column lap length
Determine the design anchorage length, lbd
lbd = α1 α2 α3 α4 α5 lb,req ≥ lb,min Equ. 8.4
lbd = α1 α2 α3 α4 α5 (26.47Ø) For concrete class C40/50
For bars in compression α1 = α2 = α3 = α4 = α5 = 1.0
Hence lbd = 26.47Ø
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Solution - Column lap length
Determine the lap length, l0 = anchorage length x α6
All the bars are being lapped at the same section, α6 = 1.5
A lap length is based on the smallest bar in the lap, 25mm
Hence,
l0 = lbd x α6
l0 = 26.47 Ø x 1.5
l0 = 39.71 Ø = 39.71 x 25
l0 = 993 mm
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Foundations
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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
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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
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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.
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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
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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
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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
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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
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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
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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
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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
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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
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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).
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Spread Foundations
Design procedures in:
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Fig 6/1 (p46)
Procedure for depth of spread foundations
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Pressure distributions
SLS pressure
distributions
ULS pressure
distribution
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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)
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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)
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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
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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
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Pad foundation
Worked example
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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.
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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
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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
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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
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Retaining Walls
Chapter 9
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Ultimate Limit States
for the design of
retaining walls
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Calculation Model A
Model applies if bh≥ ha tan (45 - ϕ’d/2)
Rankine theory
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Calculation Model B
Inclined ‘ virtual’ plane theory
Model applies to walls of all shapes and sizes
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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
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Overall design
procedure
9 (Figure 4)
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Initial sizing
bs ≈ tb ≈ h/10 to h/15
B ≈ 0.5h to 0.7h
bt ≈ B/4 to B/3
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Overall design
procedure
9 (Figure 4)
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Figure 6 for overall
design procedure
9 (Figure 6)
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Soil Densities
Ex Concrete Basements
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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.
Ex Concrete Basements
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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.
Ex Concrete Basements
Granular Soils
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Long term ≡ Granular Soils
Clay soils
Ex Concrete Basements
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Calcs –Material properties & earth pressures
9 Panel 2 (p71)
9 (Panel 2)
2
’
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Overall design
procedure
9 (Figure 4)
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Design against
sliding
(Figure 7)
9 (Figure 7)
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9 (Panel 3)
Sliding Resistance
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Overall design
procedure
9 (Figure 4)
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Design against Toppling
9 (Figure 9)
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Overall design
procedure
9 (Figure 4)
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Design against bearing failure9 (Figure 10)
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Expressions for bearing resistance
9 (Panel 4,Figure 11)
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Overall design
procedure
9 (Figure 4)
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Structural design9 (Figure 13)
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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
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Piles
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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.
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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%”
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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
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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 a
rea
of
rein
forc
em
en
t,
As,b
pm
in(m
m2)
Pile diameter, mm
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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.
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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= ______
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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
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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
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Model answer: Workshop Problem
Category 2, using prescriptive methods
Base size: (600 + 350)/225 = 4.22m2
2.1 x 2.1 base x 450mm (say) deep
Use C30/37
Loading = 1.35 x 600 + 1.5 x 350 = 1335kN
ULS bearing pressure = 1335/2.12 = 303kN/m2
Critical section at face of column
MEd = 303 x 2.1 x (1.05-0.15)2 / 2 = 258kNm
d = 450 – 50 – 16 = 384mm
K = 258 x 106 / (2100 x 3842 x 30) = 0.028
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z = 0.95d = 0.95 x 384 = 365mm
As = MEd/fydz = 258 x 106 / (435 x 365) = 1626mm2
Provide H16 @ 250 c/c (1688mm2) (804 mm2/m)
Check minimum steel
100As,prov/bd = 100 x 1688 / 2100 / 384 = 0.209
For C30/37 concrete As,min = 0.151 ∴ OK
Beam shear
Check critical section d away from column face
VEd = 303 x (0.9 – 0.384) = 156kN/m
vEd = 156 / 384 = 0.41MPa
vRd,c (from table) = 0.44MPa => beam shear ok.
Model answer: Workshop Problem
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Punching shear
Basic control perimeter at 2d from face of column
vEd = βVEd / uid < vRd,c
β = 1, ui = (300 x 4 + 384 x 2 x 2 x π) = 6025mm
VEd = load minus net upward force within the area of the control
perimeter)
= 1335 – 303 x (0.32 + π x .7682 + .768 x .3 x 4)
= 467kN
vEd = 467 x 103/(6025 x 384) = 0.202MPa;
vRd,c = 0.44 (as before) => ok
Model answer: Workshop Problem