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Development of the ConcreteCode in Hong Kong

Ir Prof Paul PangBuildings Department

October 2011

1. Background of the structural use of concrete

2. Special features of the Code 2004

3. Ductility

4. 3rd edition or Code 2012

5. Acceptable details

6. Workmanship

Content

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1. Background of theStructural Use of concrete

Structural Use of Concrete

Reinforced Concrete Regulations of the London County Council 1915

London County Council By-laws 1938

London County Council, London Building (Constructional) By-laws 1952

Hong Kong Building (Construction) Regulations 1956

Hong Kong Building (Construction) Regulations 1964

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Structural Use of Concrete

Structural Use of Concrete

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Structural Use of Concrete

Structural Use of Concrete 1987 – working stress method

Structural Use of Concrete 1987 – limit state method (following BS8110:1985 with partial load factors 1.5/1.7)

PNAP 187 issued in 1995 stipulated that RSE could follow BS 8110:1985 with partial load factors 1.4/1.6 but subject to additional coring for strength tests

Structural Use of Concrete

Circular Letter to AP/RSE dated 13 December 2004 for the publication of the Code of Practice for the Structural Use of Concrete 2004 (the Code)

2nd edition of the Code published in December 2008

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Code of Practice for the Structural Use of Concrete

1st edition : December 2004

Technical Committee : January 2008

2nd edition : August 2008

3rd edition or Code 2012 : Drafting stage

Background of the Code 2004

2. Special features of the Code

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Features

a. New stress-strain curve and design formulae

b. Use of high strength concrete

c. Beam-column joint design

d. Serviceability in response to wind loads

e. Ductility detailing

a. New stress-strain curve and simplified stress block

k=0.9 for fcu ≤45 N/mm2

k=0.8 for 45< fcu ≤70 N/mm2, or

k=0.72 for 70< fcu ≤100 N/mm2

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Design formulae are provided for K<=K’ and K>K’ respectively, where Kand K’ are defined as follows:

K = M/ b d 2 fcu

For fcu <= 45 N/mm2

K’ = 0.156 for moment redistribution <= 10%; orK’ = 0.402 (βb - 0.4) - 0.18 (βb - 0.4)2 for moment redistribution > 10%

For 45 < fcu <= 70 N/mm2

K’ = 0.120 for moment redistribution <= 10%; orK’ = 0.375 (βb - 0.5) - 0.143 (βb - 0.5)2 for moment redistribution > 10%

For 70 < fcu <= 100 N/mm2 (for which moment redistribution is not allowed)K’ = 0.094

Corresponding new design formulae

b. Use of high strength concrete

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c. Beam-column joint design

BS8110 and Code of Practice 1987 made an over-simplified assumption that beam-column joints would not fail

Researches and load tests on structural frames indicated that shear stresses can be built up within beam-column joints and lead to failure

It is only since the 1970s that the attention of structural engineers has been drawn to the critical role of beam-column joints in reinforced concrete frames. Park & Paulay, 1993 refers

Clause 6.8.1.2 requires that forces resulting from gravity loads and wind forces acting on a beam-column joint shall be properly designed for

d. Serviceability in Response to Wind Loads

Excessive response to wind loads (7.3.2)

Static or dynamic analysis

Static analysis : H/500

Dynamic analysis : Limits of peak acceleration0.15 m/s2 for residential buildings0.25 m/s2 for office or hotel buildings

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e. Ductility detailing

No such provision under BS8110 and the Concrete Code 1987.

Ductility will complement the strength of a structure and enhance the overall performance

Enhances the probability of survival of a structure under extreme loads

3. Ductility

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Design objectives

3 Main objectives:

Strength

Serviceability

Ductility

Why ductility?

Improve performance of the structures

Structures have a much better chance of survival when subject to loads exceeding their strength capacities, e.g. overloading, accidental impact, earthquake, terrorist attack, etc

Hong Kong is densely populated with people live and work in multi-storey buildings. Collapse or disproportionate failure of structures will have dire consequences, must be prevented

Cost vs benefit

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Ductility - definition

Paulay and Priestley:

The term ductility defines the ability of a structure and selected structural components to deform beyond elastic limit without excessive strength or stiffness degradation.

Ductile behaviour

Deflection

Load

Brittle behaviour

Ductile behaviour – large deformation without excessive strength degradation

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Ductility design objectives

Structure shall not fail in a brittle fashion without warning

Capable of sustaining large deformations at near-maximum load carrying capacity

Gives ample warning of failure

Prevents total collapse and prevents casualties

Material properties

Reinforcing steel Concrete

strainstrain

stre

ss

stre

ss

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Analogy

The strength of a chain is the strength of its weakest link

(Diagram from Paulay and Priestley)

Ductile linkBrittle links Brittle links

n Brittle links

(a)Ductile link

(b)Ductile chain

(c)

1. Select a suitable configuration for plastic mechanism

2. Select suitable locations for plastic hinges

3. Design with suitable strength differentials

Capacity design

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Step 1 : Select suitable configuration for plastic mechanism

AvoidPreferred

appropriately detailed locations (plastic hinges)

plastic deformation at undesirable locations

Plastic hinges :

locations for plastic deformations

energy dissipation – energy is dissipated during the process when plastic hinges are formed, reversed, and so on

should be formed in the beams, or in the connections of the beams to columns, but not in the columns

Step 2 : Select suitable locations for plastic hinges

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Design plastic hinges and other regions with suitable strength differentials

Other regions remain elastic under all feasible actions corresponding to over-strength in the plastic hinges

It is immaterial whether the other regions are ductile or brittle

Step 3 : Design with suitable strength differentials

Code requirements

Weak beams

Strong columns and beam-column joints

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Stress behaviour

Uniaxial stress behaviour

Biaxial stress behaviour

Triaxial stress behaviour

f’cc = f’c + 4.1flwhere

f’cc = axial compressive strength of confined specimen

f’c = axial compressive strength of unconfined specimen

f l = lateral confining pressure

Concrete confinement

By transverse reinforcement

which provides passive confinementmaintains concrete core integrity and prevents longitudinal bar buckling

significantly increases strength and ductility

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Confinement reinforcement

(Diagram from Paulay and Priestley)

(a) Circular hoops or spiral

(b) Rectangular hoops with cross ties

(c) Overlapping rectangular hoops

(e) Confinement by longitudinal bars

(d) Confinement by transverse bars

Unconfined concrete

Stress-strain curves for confined andunconfined concrete

(Diagram from Paulay and Priestley)

Compressive strain, εc

Com

pres

sive

stre

ss, f

c Confined concrete

Unconfined concrete

f’cc

f’c

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Stringent requirements for links

e.g. 135 deg anchorage

e.g. restraint of every column main bars within critical zone

Code requirements

4. 3rd edition or Code 2012

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Add new clauses on structural layout for ductility design

Centre of mass vs centre of rigidity

Simple, regular plans preferred. Avoid articulated plans such as T and L shapes.

Symmetry in plan preferred. Avoid significant torsional response.

Integrated foundation system. Avoid partly on rock and partly on soil

Regularity (in geometry and storey stiffness) in elevation

Preferred and undesirable vertical configuration

Clause 2. Basis of design

Review by Poly U and HKU in collaboration with a number of local concrete suppliers

Results agree with current recommendations in the Code

Clause 3.1.5 Elastic deformation

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Revamp as per local researchOld: εcs = csKLKcKeKj

New:

where (εsh)u is the ultimate shrinkage

kh, εs(fcm), βPV and βRH account for effects of effective thickness, concrete grade, paste volume (or aggregate content) and RH respectively

Cs allows for the significantly larger shrinkage of local concrete

Reference : AKH KWAN et al (2010) Shrinkage of Hong Kong granite aggregate concrete. Magazine of Concrete Research

Clause 3.1.8 Drying shrinkage

7.51.15 – 1.35500C

5.01.08500B

Agt %Rm/ReGrade

CS2 : Current : Grade 460New (not yet published) :Grade 500B & 500C

2 options for update of Concrete Code :Option 1 : Grade 500B + 500C Option 2 : Grade 500C only

Clause 3.2 Reinforcing steel

Reinforcing steel

stre

ss

ReRm

strain

Agt

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Revamp

4.3.1 Normal strength concrete

4.3.2 High strength concrete

Means to mitigate spalling under fire, e.g. guidance given in

EC2 clause 6.2

Clause 4.3 Requirements for fire resistance

Clause 6.2 of BS EN 1992-1-2:2004

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Guidance re ductility design consideration

Possible control :

Change in stiffness

Storey drift

Design with suitable strength differential

Clause 5.5 Transfer structure

AKH KWAN Concrete Code HandbookWONG SHF and KUANG JS (2009) on the design of rc beam-column joints to Hong Kong Concrete Code 2004. The HKIE Transactions

Add guidance on design shear force

Add guidance on detailing

Clause 6.8 Beam-column joints

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Guidance on design shear forces

Column

Beam

Vb’

T’

Mc’

Mc

Vb

Vc’

Vc

Nc’

Nc

Beam

TC’

CVjh

Vjv

Potentialfailure plan

C = T = As1 fyC’ = T’ = As2 fyVjh = T+C’-Vc

Vjv = (hb/hc)Vjh

Instead of :

C = T = 1.25 As1 fyC’ = T’ = 1.25 As2

fy

hc

hb

Vertical joint shear reinforcement

may be provided by straight bars or inverted U-bars with adequate anchorages

surplus capacity of column longitudinal reinforcement may be used to resist all or part of vertical shear force in the joint

Horizontal joint shear reinforcement may be provided by links formed from U-bars with adequate tension lap lengths

Guidance on detailing

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Add 9.7.4 Large diameter bored piles

Clause 9.7 Foundations

Clause 9.7.4 Large diameter bored piles

Longitudinal reinforcement

min 6 bars, diameter ≥ 16 mm

shall be of high yield steel bars and not less than:

(a) 0.5% Ac for Ac ≤ 0.5 m2;

(b) 2500 mm2 for 0.5 m2 < Ac ≤ 1 m2;

(c) 0.25% Ac for Ac > 1.0 m2;

where Ac is the gross cross-sectional area of pile.

Transverse reinforcementshall comply with the requirements for columns as stipulated in clauses 9.9.2.2 and 9.9.2.3.

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Review existing requirements

Add ductility detailing for walls

Options for reinforcement fixing

Add ductility couplers

Clause 9.9 Detailing for ductility

Centre of splice (lap or coupler) may be located within middle ½ (instead of ¼) of storey height

If ΣMc ≥ 1.2ΣMb, clause 9.9.2.1 (d) may be dispensed with, where

ΣMc = sum of moment capacities under appropriate axial load of column sections above and below joint;ΣMb = sum of either clockwise or anti-clockwise moment capacities of beams on both sides of joint, whichever is the greater

Clause 9.9.2.1(d) Column longitudinal reinforcement

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1st paragraphDelete reference to “limited ductile high strength reinforced concrete”

9.9.2.2(b)Indicate that an absolute minimum link spacing of 100 mm is also acceptable

Clause 9.9.2.2 Column – transverse reinforcement within critical region

Ductility couplers

Advantage :

Can be placed in any location provided that :

the couplers in columns are staggered in 2 layers at min 300 mm apart, with the lower layer at min 300 mm above structural floor level

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Ductility couplers

Requirements :

The couplers are tested to AC 133 to establish compliance with Type 2 mechanical splices specified in ACI 318

The splice assembly shall fail in bar-break mode

The splice assembly shall have permanent elongation not exceeding 0.1 mm after loading to 0.6fy in accordance with clause 3.2.8.2 of the Code

Ductility couplers

Test to AC 133 “Acceptance Criteria for Mechanical Connector Systems for Steel Reinforcing Bars” :

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Ductility couplers

Result of test to AC 133 :

Add requirements for ductility couplers

Clause 10 General specification, construction and workmanship

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Review existing requirement that:All structural concrete should be obtained from suppliers certified under QSPSC (Quality Scheme for Production and Supply of Concrete) or equivalent unless works are in remote areas or volume of concrete <50m3

Consider if the requirement could be relaxed for normal strength concrete or concrete lower than a specified strength

Clause 11 Quality assurance and quality control

Review 12.1 Basis of design

Ductility requirements

Clause 12 Prestressed concrete

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Load tests to satisfy strength requirement only

Ductility requirement still need to be complied with

Clause 13 Load tests of structures or parts of structures

Update to align with EC, ACI and GB codes and

standards

Annex A Acceptable standards

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5. Acceptable details

Theory vs practicality

(From Moehle)

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Bar lapping for column

Couplers for column

Ordinary couplers Ductility couplers

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Transverse reinforcement for rectangular column

Transverse reinforcement for circular column

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Horizontal joint shear reinforcement

Horizontal joint shear reinforcement

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can be provided by vertical bars or inverted U-bars with adequate anchorages into column above or below the joint.

Vertical joint shear reinforcement

Shear links in beam

Using close stirrups Using open stirrups with a top locking link

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Torsional links in beam

Torsional links in beam

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6. Workmanship

Strong beam-column joints

Stringent requirements for links

2 main objectives

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Construction teamProper fixing of reinforcementIn particular : All links must be securely fixed to main bars

RSEStringent site control and supervision

BDIncreases audit checkingTakes action if defectively bent or loosely fixed reinforcement is found

Efforts required

End