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    REINFORCEMENT CONCRETE chapter 1 REINFORCEMENT CONCRETE chapter 1 Presentation Transcript

    • WHAT IS REINFORCED CONCRETE? • Principal materials used in many civil engineering applications (buildings, retaining walls, foundations, water retaining structures, highways, bridges etc • A composite material: reinforcing bars embedded in concrete • Concrete: high compressive strength, low tensile strength
    • WHAT IS REINFORCED CONCRETE? • Steel: high tensile strength, low compressive strength • Concrete + Steel: • Economical structural material, strong in compression & tension • Concrete provides corrosion protection and fire resistance
    • STRUCTURAL DESIGN? • Process of determining , selection of and determination of for the structure to be built • Aim: ensure that the structure will perform satisfactorily during its design life
    • STRUCTURAL DESIGN PURPOSES? • • • • Fitness for purpose Safety and reliability Economy Maintability
    • Fitness for purpose • Arrangement of spaces, spans, ceiling height, access and traffic flow must complement the intended use. • The structure should fit its environment and be aesthetically pleasing
    • Safety and reliability • Structure must be strong to safely support all anticipated loadings • Structure must not deflect, overturn, tilt, vibrate or crack in a manner that impairs its usefulness
    • Economy • Overall cost of structure should not exceed the client’s budget • Designer should take into account: cost of materials, buildability, construction time, cost of temporary structures and maintenance costs
    • Maintainability • Structure should be designed to require a minimum maintenance, can be maintained in a simple fashion
    • Structural Elements • Beams: horizontal members carrying lateral loads • Slabs: horizontal plate elements carrying lateral loads • Columns: vertical members carrying primarily axial loads but generally subjected to axial load and moment • Walls: vertical plate elements resisting vertical, lateral or in-plane loads • Foundations: pads or strips supported directly on the ground that spread loads from columns or walls to the ground • Stairs: plate elements consists of a flight of steps, usually with one or more landings provided between the floor levels
    • Structural Elements
    • Structural Elements
    • Code of Practice • document that gives recommendations for the design and construction of structures • Contains detailed requirements regarding loads, stresses, strengths, design formulas and methods of achieving the required performance of complete structure
    • Code of Practice functions • Ensure adequate structural safety • Simplify the task of designer • Codes ensure a measure of consistency among different designers • Have legal validity, in that they protect the structural designers from any liability due to structural failures that are caused by inadequate supervision, faulty material and construction
    • Eurocode • EN 1990: Eurocode – Basis of structural design • EN 1991: Eurocode 1 – actions on structures • EN1992: Eurocode 2 – Design of concrete structures
    • Creep • General definition: the or of a solid material to • occurs as a result of long-term exposure to high levels of stress that are below the yield strength of the material • more severe in materials that are subjected to heat for long periods, and near their melting point. Creep always increases with temperature • The rate of this deformation is a function of the , , and the
    • Example: creep in cardboard • a largely empty box was placed on a smaller box • fuller boxes were placed on top of it • Due to the weight, the portions of the empty box not sitting on the lower box gradually crept downward
    • Concrete Creep • Definition: Continuous deformation of a member under sustained load • Characteristics: • The final deformation of member can be 3 – 4 times the short term elastic deformation • Deformation is roughly proportional to the intensity of loading and to the inverse of concrete strength • If load is removed, only instantaneous elastic deformation will recover – the plastic deformation will not • There is a redistribution of load between concrete and steel present
    • Concrete Durability • Influenced by: i. ii. iii. iv. v. Exposure conditions The cement type The concrete quality The cover to the reinforcements The width of any cracks
    • Specifications of materials (Concrete) • The selection is governed by the strength required – depends on the intensity of loading and the form and size of structural member • Concrete strength: measured by the crushing strength of cubes or cylinders of concrete made from the mix • Identified by its class. Ie: C25/30 – characteristic cylinder crushing strength (fck) of 25N/mm2 and cube strength of 30N/mm2
    • Specifications of materials (Reinforcing Steel) • Grade 250 bars: hot-rolled mild-steel bars – have smooth surface, bond with conrete is by adhesion only • High-yield Bars: Deformed bars, ribbed highyield steel bars may be classified as: • Class A: used in mesh and fabric. Lowest ductility cathegory • Class B: commonly used for reinforcing bars • Class C: high ductility, used in earthquake design
    • Example
    • Limit State Design • Design method in EC2 is based on limit state principles • Limit State: State of a structure which represents the acceptable limit of an aspect of structural behaviour • Criterion for safe design is the structure should not become unfit for use – not reach a limit state during its intended life
    • Principle types of limit state 1. Ultimate Limit State • • Deals with the strength and stability of the structure under the maximum design load it is expected to carry No part or whole of the structure should collapse, overturn or buckle under any combination of design load
    • Principle types of limit state 1. Ultimate Limit State • Divided into the following categories • • • • EQU – Loss of equilibrium of the structure STR – Internal failure or excessive deformation of the structure or structural members GEO – failure due to excessive deformation of the ground FAT – fatigue failure of the structure or structural members
    • Principle types of limit state 2. Serviceability Limit State • Deals with the conditions beyond which specified service requirements are no longer met such as excessive deflection and cracking 3. Other Limit States • May be reached including considerations of durability, vibration, and fire resistance of structures
    • Actions • • • EC2 terminology for loads and imposed deformations EC2 defines and action (F) as a force or load applied to structure Characteristic actions used in design and defined in EC2 are as follows: • • Characteristic permanent action Gk – Selfweight of structure, finishing weight etc Characteristic variable action Qk – people, furniture, equipment etc
    • Design Actions • • • • Design value of an action is obtained by multiplying the characteristic actions Fk by partial safety factor for actions γf Fd = Fk x γ f γf accounts for possible increases in load, inaccurate assessment of the effect of loads, inaccurate modelling of the load Values of γf are given in EN 1990: Annex A1
    • Combination of Action • Permanent and Variable actions will occur in different combinations. All must be considered to determine the most critical design situation
    • MODE OF FAILURE IN SECTION DURING LOADING • Reinforcing steel can sustain very high tensile strains, due to the ductile behavior of steel • Concrete can accommodate compressive strains which is much lower in comparison • The final collapse of a normal beam at ULS is usually cause by crushing of concrete in compression • Depending on the amount of RS provided, flexural failure may occur in 3 ways:
    • 1. Balanced • Concrete crushes & steel yields simultaneously at ULS • Compressive strain of concrete reaches ultimate strain and the tensile strain of steel reaches yield strain simultaneously • Depth of neutral axis is equal to 0.617d
    • 2. under-reinforced • Steel reinforcement yields before concrete crushes • Area of tension steel provided is less than the area provided in balance section • Depth of neutral axis is less than 0.617d • Give ample prior warning of the impending collapse
    • 3. over-reinforced • Concrete fails in compression before steel yields • Area of tension steel provided is more than the area provided in balance section • Depth of neutral axis is greater than 0.617d • Failure is sudden (without any sign of warning) and brittle