introduction to concrete design

BASIC STRUCTURAL DESIGN

ARUAN EFENDY BIN MOHD GHAZALIFAKULTI KEJURUTERAAN AWAM

ARUAN EFENDY BIN MOHD GHAZALI FAKULTI KEJURUTERAAN AWAM

Learning out-comesAt the end of this lecture, the student should be

able to:

a) List out the relevant codes of practice used in concrete design (CO1)

b) Explain the concept of SLS and ULS(CO1)c) Explain a basic concept of concrete

design(CO1).

1.0 Design in Reinforced Concrete 1.0 IntroductionImportance of RC, it being one of the principal materials

used in structural design.Composite material consisting of steel reinforcing bars

embedded in concrete. Complimentary properties and cost consequences.Overall economy with the advantages of corrosion and

fire resistance.Type of application: Structural Frames Retaining Walls

Water Retaining structures. Highways Bridges

ARUAN EFENDY BIN MOHD GHAZALIFAKULTI KEJURUTERAAN AWAM

These structures are normally designed in accordance

with a variety of CODES:

BS 5400:

Code of Practice for the Design of Steel, Concrete and Composite Bridges.

BS 8007:

Code of Practice for the Design of Concrete structures for Retaining Aqueous Liquids.

BS 8110: (Which is to be the basis of the work for this semester)

Code of Practice for the Structural use of Concrete ARUAN EFENDY BIN MOHD GHAZALI

FAKULTI KEJURUTERAAN AWAM

BS8110 is divided into 3 parts;

Part 1: Code of Practice for Design and Construction.Covers material commonly required for everyday design.

Part 2: Code of Practice for Special Circumstances.Torsion, Deflection and Elastic Deformation

Part 3: Design Charts for Singly Reinforced Beams, Doubly Reinforced and Rectangular Columns.Charts for the design of singly reinforced beams, doubly reinforced beams and rectangular columns

ARUAN EFENDY BIN MOHD GHAZALIFAKULTI KEJURUTERAAN AWAM

Irrespective of the element being designed a designer

will need an understanding of :

The symbols used The Basis of Design Material Properties Loading Stress Strain Relationships Durability and Fire Resistance Symbols

ARUAN EFENDY BIN MOHD GHAZALIFAKULTI KEJURUTERAAN AWAM

Basis of Design

Limit State method - with two principal states normally

covered.1. The Ultimate Limit State - ULS.

Behaviour at failure In a variety of modes2. Serviceability Limit State - SLS.

Behaviour at working loads and principally the limit states of Deflection and cracking.

ARUAN EFENDY BIN MOHD GHAZALIFAKULTI KEJURUTERAAN AWAM

Material Properties:Characteristic Strength of Concrete fcu

Concrete Grade Characteristic Strength (N/mm2) fcu

C25 25

C30 30

C35 35

C40 40

C45 45

C50 50

ARUAN EFENDY BIN MOHD GHAZALIFAKULTI KEJURUTERAAN AWAM

Characteristic Strength of reinforcement fy

Reinforcement Type Characteristic Strength (N/mm2) fy

Hot Rolled Mild steel 250

High Yield Steel 460

ARUAN EFENDY BIN MOHD GHAZALIFAKULTI KEJURUTERAAN AWAM

Design Strength

In order to take account of the difference between actual and laboratory values, local weaknesses and inaccuracies in the assessment of the resistance of sections, the Characteristic Strengths, fk are divided by an appropriate partial safety factor for strength,γm .

Design Strength = fk / γm

ARUAN EFENDY BIN MOHD GHAZALIFAKULTI KEJURUTERAAN AWAM

Partial safety factors for Strength of Material 

Material and Stress type

Partial Safety Factor, γm

Reinforcement 1.15

Concrete - Flexure or Axial Load

1.5

Concrete - Shear, unreinforced

1.25

Concrete - bond 1.4

Concrete - other e.g. bearing

>1.5

ARUAN EFENDY BIN MOHD GHAZALIFAKULTI KEJURUTERAAN AWAM

Loading:Characteristic LoadsGk - Dead load Qk - Imposed Load Wk - Wind Load

BS 648:Schedule of Weights for Building Materials

BS 6399: Design Loadings for Buildings, Part 1: Code of Practice for Dead and Imposed loads

are useful documents in which to find values to be used

in calculations.ARUAN EFENDY BIN MOHD GHAZALI

FAKULTI KEJURUTERAAN AWAM

Design LoadsIn order to account for Variation in Loads due to:Errors in the analysis and Design Constructional inaccuracies Possible load increases

The Characteristic Loads Fk are multiplied by the

appropriate partial safety factor for loads, γf to give the

Design Loads acting on the structure

Design Load = Fk .γf ARUAN EFENDY BIN MOHD GHAZALI

FAKULTI KEJURUTERAAN AWAM

Load Combination Load Type

Dead Load, Gk Imposed Load, Qk

Wind Load, Wk

Adverse Beneficial Adverse Beneficial

Dead + Imposed 1.4 1 1.6 0 -

Dead + Wind 1.4 1 - - 1.4

Dead + Wind + Imposed 1.2 1.2 1.2 1.2 1.2

Generally the adverse factors are used to determine the design loads acting on a structure. Elastic analyses then allow the determination of maximum BM’s and Shears for which sections must be capable of sustaining

ARUAN EFENDY BIN MOHD GHAZALIFAKULTI KEJURUTERAAN AWAM

Stress-Strain CurvesStress-Strain Curve for ConcreteA typical curve for concrete is shown below:

ARUAN EFENDY BIN MOHD GHAZALIFAKULTI KEJURUTERAAN AWAM

BS 8110 makes use of a modified stress-strain curve as

shown

ARUAN EFENDY BIN MOHD GHAZALIFAKULTI KEJURUTERAAN AWAM

DURABILITY & FIRE RESISTANCE

As well as the need to design structures to withstand the

applied loads due consideration must be given to both

durability and fire resistance. In fact it can be seen that the

design of an element can not begin without considering

these factors in some way

ARUAN EFENDY BIN MOHD GHAZALIFAKULTI KEJURUTERAAN AWAM

DurabilitySigns of concrete deterioration are nowadays

far too common. Ex’s..... Repair can be very costly and

difficult. Improved durability is therefore paramount.

How can this be achieved:cover to reinforcement minimum cement content maximum water/cement ratio maximum crack widths

ARUAN EFENDY BIN MOHD GHAZALIFAKULTI KEJURUTERAAN AWAM

The table gives nominal (min+5) depths of cover to be used for a variety of exposure conditions. Note linkage with Max. water/cement ratio, Min. cement content and concrete grade

ARUAN EFENDY BIN MOHD GHAZALIFAKULTI KEJURUTERAAN AWAM

Fire Protection

Fire protection of reinforced concrete members is

largely achieved by specifying limits for:

Cover to reinforcement minimum dimensions for section

ARUAN EFENDY BIN MOHD GHAZALIFAKULTI KEJURUTERAAN AWAM

This table shows the nominal cover to ALL reinforcement to meet the specified period of

fire resistance

ARUAN EFENDY BIN MOHD GHAZALIFAKULTI KEJURUTERAAN AWAM

In addition to cover we must also consider minimum section dimensions which vary depending upon the element considered and it’s location as indicated(BS8110):

ARUAN EFENDY BIN MOHD GHAZALIFAKULTI KEJURUTERAAN AWAM

Minimum dimensions of reinforced concrete members for fire resistance

ARUAN EFENDY BIN MOHD GHAZALIFAKULTI KEJURUTERAAN AWAM