TOPIC 1_Intro RC Design 1

8/3/2019 TOPIC 1_Intro RC Design 1

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Introduction

What is structural design?

PROCESS.

Selection of appropriate materials.

Determination of suitable element size.

What is Structural design purposes?

To provide safe structure and suitable to use that can

be build and maintain with a minimum cost.


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Reinforced Concrete Structure

Being one of the principal materials used in structuraldesign.

Composite material consisting of steel reinforcingbars and concrete.

Why reinforced concrete? Concrete :

Highly in compressive strength but weak in tensilestrength.

Reinforcement (steel) :

Highly in tensile strength but weak in compressivestrength.

Overall economy with the advantages of corrosion and

fire resistance.


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Reinforced concrete is a composite material of steelbars embedded in a hardened concrete matrix;concrete, assisted by the steel, carries thecompressive forces, while steel resists tensile forces.



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Structural Design Process

Architectural Drawing

Structural Arrangement Plan Loading Analysis / StructuralAnalysis

Structural DesignDetail Drawing

Submission


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The first function in design is the planning carried outby the architect to determine the arrangement andlayout of the building to meet the clients

requirements.

The structural engineer then determines the beststructural system or forms to bring the architects

concept into being.

Construction in different materials and with different

arrangements and systems may require investigationto determine the most economical answer.

Architect and engineer should work together at thisconceptual design stage.


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Once the building form and structural arrangementhave been finalized the design problem consists ofthe following: idealization of the structure into loadbearing frames and

elements for analysis and design estimation of loads

analysis to determine the maximum moments and shears fordesign

design of sections and reinforcement arrangements forslabs, beams, columns and walls using the results fromabove

production of arrangement and detail drawings and barschedules



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Type of Application Building Structural Frames


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Type of Application

Retaining Walls


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Type of Application

Water Retaining structures


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Type of Application

Dam


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RC Structural Element

Slab

Beam

Column

Staircase

Foundation


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The Code of Practice used

The RC structures are normally designed inaccordance with a CODE:

BS 8110: 1997 & 1985 : Structural use of Concrete.

BS8110 is divided into 3 parts:

Part 1: Code of Practice for Design and Construction.

Part 2: Code of Practice for Special Circumstances.

Part 3: Design Charts for Singly Reinforced Beams,Doubly Reinforced and Rectangular Columns.


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RC Design Concept

Irrespective of the element being designed adesigner will need an understanding of :

The Symbols used

The Basis of Design

Material Properties

Loading

Stress Strain Relationships


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The Symbols Used

For purpose of design, the common symbols havebeen used are:


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The Basis of Design

Limit State method This method stated that thestructures should have enough strength, safe andsuitable to use.

Not achieved limit state along service period.

There are two principal states which are; Ultimate Limit State (ULS)

Condition in which the structure is failed and unsafe forits intended purposes. i.e : collapse

Serviceability Limit State (SLS)

Condition in which the structure is damaged andunsuitable for its intend purposes causing discomfort tothe occupants. i.e : excessive deflection and cracking.


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The aim of design is the achievement of anacceptable probability that the structure will performsatisfactorily during its life.

For reinforced concrete structures the normalpractice is to design for the ultimate limit state, checkfor serviceability and take all necessary precautionsto ensure durability.

The Basis of Design


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The structure must be designed to carry the mostsevere combination of loads to which it is subjected.

The sections of the elements must be capable ofresisting the axial loads, shears and moments

derived from the analysis. The design is made for ultimate loads and design

strengths of materials with partial safety factorsapplied to loads and material strengths.

Ultimate Limit State (ULS)


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The main serviceability limit states and codeprovisions are as follows.

Deflection The deformation of the structure should not

adversely affect its efficiency or appearance.

Deflections may be calculated, but in normal

cases span-to-effective depth ratios (L/d) can beused to check compliance with requirements.



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Cracking

Cracking should be kept within reasonable limits

by correct detailing. Crack widths can be calculated, but in normal

cases cracking can be controlled by adhering todetailing rules with regard to bar spacing in zones

where the concrete is in tension.



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Material Properties

Concrete


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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


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Material Properties

Steel Reinforcement


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Material Properties

Characteristic Strength of reinforcement fy.

Reinforcement Type Characteristic Strength (N/mm2) fy

Hot Rolled Mild steel (R) 250

High Yield Steel (T) 460

Fabric Wire Mesh (BRC) 485


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Material Properties

Design strength

In order to take account of the difference betweenactual and laboratory values, local weaknesses andinaccuracies in the assessment of the resistance of

sections, the Characteristic Strengths fk (fcu & fy) aredivided by an appropriate partial safety factor forstrength .


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Material Properties

Partial safety factors for Strength of Material

Table 2.2 : BS 8110: Part 1: 1997

Partial Safety FactorMaterial and Stress type

Reinforcement 1.15

Concrete - Flexure or Axial Load 1.50

Concrete - Shear, unreinforced 1.25

Concrete - bond 1.4

Concrete - other e.g. bearing >1.5


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Loading

All structures are subjected to loading from variessources.

The main categories of loading are :

Characteristic Dead Load, Gk

BS 648: Schedule of Weights for Building Materials

Characteristic Imposed Load, Qk

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

Characteristic Wind Load, Wk

CP3: Chapter V: Wind Loads which will eventually besuperseded by Part 2 of BS 6399


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Loading

Characteristic Dead Load, Gk

Loads which are due to the effects of gravity, i.e. theself weight of all permanent construction such asbeams, column, floors, walls, roofs and finishes.

Characteristic Imposed Load, Qk Loads which are due to variable effects such as the

movement of people, furniture, equipment and traffic.

Characteristic Wind Load, Wk

Loads which depend on wind speed at certain area is

clearly variable and its source is out with humancontrol.


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Loading

Design Load - In order to account for variation inLoads due to:

Errors in the analysis and Design

Constructional inaccuracies

Possible load increases

The characteristic loads Fk (Gk,Qk,Wk) are multipliedby the appropriate partial safety factor for loadsto give the Design Loadsacting on the structure.


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Loading

Partial safety factor for loads,


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Stress-strain curve for concrete

Actual Curve


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Design curve (BS 8110)


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Stress-strain curve for reinforcement

Actual Curve


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Design curve (BS 8110)


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DURABILITY &

FIRE RESISTANCE As well as the need to design structures to withstand

the applied loads due consideration must be given toboth durability and fire resistance.

Durability How can this be achieved ?

Cover to reinforcement

Minimum cement content

Maximum water/cement ratio Maximum crack widths


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The table below gives nominal (min+5) depths of cover tobe used for a variety of exposure conditions. Note: linkagewith Max. water/cement ratio, Min. cement content andconcrete grade.

To avoid corrosion of reinforcement BS 8110

recommends that a limit be placed on the maximum crackwidth of 0.3mm.

This requirement can generally be satisfied if thesimplified rules on detailing reinforcement are observedwrt.:

minimum area maximum spacing.....see later for beams &slabs

design.


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Conditions of exposure Nominalcover (mm)

Mild - protected from weather 25 20 20 20 20

Moderate - sheltered from weather - 35 30 25 20

Severe - exposed to severe rain - - 40 30 25

Very Severe - de-icing salts, fumes etc. - - 50 40 30

Extreme - abrasives e.g. sea water - - - 60 50

Max. Free water/cement ratio 0.65 0.60 0.55 0.50 0.45

Min. Cement Content kg/m3 275 300 325 350 400

Lowest grade C30 C35 C40 C45 C50


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Fire Resistance

Fire protection of reinforced concrete members islargely achieved by specifying limits for:


Minimum dimensions for section

The table below shows the nominal cover to ALLreinforcement to meet the specified period of fireresistance.


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Nominal cover due to fire resistance

Fireresistance

Nominal Cover (mm)

Beams Floors Columns

hrs. S.S Cont. S.S Cont.

0.5 20 20 20 20 20

1.0 20 20 20 20 20

1.5 20 20 25 20 20

2.0 40 30 35 25 25

3.0 60 40 45 35 25

4.0 70 50 55 45 25

Table 3.4 : BS 8110 :1:1997


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Minimum section dimensions

In addition to cover we must also consider minimumsection dimensions which vary depending upon theelement considered and its location as indicated:


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Minimum section dimensions

Fireresistance

Minimum Dimension

Beam Floor Fully exposed

hrs. Width Thickness column width

(b mm) (h mm) (b mm)

0.5 200 75 150

1.0 200 95 200

1.5 200 110 250

2 200 125 300

3 240 150 400

4 280 170 450


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RC Beam Design(Ultimate Limit State)

Here are some examples of Reinforced Concrete

beams that you may find in practice.


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RC Beam Design

Any of the above arrangements can be employed inconditions where the beam is simply supported orwhere it is continuous over the supports.

NOTE: When beams are used in a continuoussituation care must be taken to correctly locate thereinforcement in the tension face of the beam.


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