Post on 07-Apr-2018
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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.
Reinforced Concrete Structure
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Reinforced Concrete Structure
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Structural Design Process
Architectural Drawing
Structural Arrangement Plan Loading Analysis / StructuralAnalysis
Structural DesignDetail Drawing
Submission
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Structural Design Process
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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Structural Design Process
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
Structural Design Process
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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.
Serviceability Limit State (SLS)
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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.
Serviceability Limit State (SLS)
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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 Relationships
Stress-strain curve for concrete
Actual Curve
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Stress Strain Relationships
Design curve (BS 8110)
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Stress Strain Relationships
Stress-strain curve for reinforcement
Actual Curve
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Stress Strain Relationships
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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Cover to reinforcement
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:
Cover to reinforcement
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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To be continue