CC605 REINFORCED CONCRETE DESIGN

UNDERSTANDING THE DESIGN CONCEPT

INTRODUCTION TO DESIGN

SO WHAT IS A REINFORCED CONCRETE?

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 reliable structural system, selection of suitable materials and determination of optimum member sizes 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 tendency of a solid material to move slowly or deform permanently u n d e r t h e i nfl u e n c e o f s t re s s e s

• 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 material properties, exposure time, exposure temperature and the applied structural load

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. Exposure conditionsii. The cement typeiii. The concrete qualityiv. The cover to the reinforcementsv. 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 high-yield 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

PRINCIPLES OF LIMIT STATE DESIGN

INTRODUCTION TO 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 γf

• 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• This mode of fa i lure is preferred in

design practi ce

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• This mode of failure is not permitted

REFERENCE

• REINFORCED CONCRETE DESIGN TO EUROCODE 2, IR MOHAMAD SALLEH YASSIN

• REINFORCED CONCRETE DESIGN TO EUROCODE 2, BILL MOSLEY, JOHN BUNGEY, RAY HULSE