Chapter 1 بسم الله الرحمن الرحيم Design of Concrete Structure I University of Palestine Instructor: Eng. Mazen Alshorafa

Chapter 1

الرحمن الله بسمالرحيم

Design of Concrete Structure I

University of Palestine

Instructor:

Eng. Mazen Alshorafa

Page 1



Introduction

Concrete and Reinforced Concrete:

Instructor:


Concrete is a mixture of paste and aggregates (sand & rock). The paste, composed of cement and water,

coats the surface of the fine (sand) and coarse aggregates (rocks) and binds them together into a

rock-like mass known as concrete.

Some times one or more admixture are added to change certain characteristic of the concrete such as

its workability, durability, and time of hardening.

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Introduction

Concrete has a high compressive strength and a very low tensile strength.

Reinforced concrete is a combination of concrete and

steel wherein the steel reinforcement provides the tensile strength lacking in the concrete. Reinforced

concrete is used as a prime construction material universally.

The construction of reinforced concrete structures requires the use of a form to take the shape of the

built member. The reinforcement is held in place in the form during the casting operation. Once the

concrete has hardened to the required strength, only then the forms are removed.


Instructor:


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Introduction

Steel is used as a reinforcement element due to some factors, which are,

Steel improves the resistance of concrete in the tension regions.

Steel and concrete have similar thermal expansion coefficients; 0.000010 to 0.000013 for concrete and 0.000012 for steel per degree Celsius, thus causing

negligible internal stresses resulting from temperature changes, which in turn, means a good

bond between the two materials. Steel adds ductility which is required in the design

process.


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Introduction

What distinguishes reinforced concrete from other

structural materials is its durability, ability to be

formed in different shapes, rigidity, fire resistance,

low maintenance, and its economy compared to other

types of structural materials.

Advantages of Reinforced Concrete:

Instructor:


Low tensile strength (cracking occurs: need good

reinforcing detailing) Need forms and shoring

Low strength to weight ratio Time dependent properties * Shrinkage (Volume

change due to drying) * Creep (Deflection under

constant load)

Disadvantages of Reinforced Concrete:

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Introduction

Concrete design can be classified into three main categories;

Types of Concrete Design

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1- Plain Concrete Design

It is mainly used for nonstructural members. This is due to the low strength of concrete in tension.

Compressive stresses

Tensile stresses

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Introduction


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2- Reinforced Concrete Design

The compressive strength of concrete is high while its tensile strength is low. To alleviate the situation,

high tensile strength reinforcement in the form of steel bars is added in the tension regions to enhance

the capacity of concrete members

Compressive

stresses

Tensile stresse

s

Steel bars

embedded

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Introduction


Instructor:


3- Prestressed Concrete Design

Since the strength of reinforced concrete can be enhanced by the elimination of cracking,

prestressing is used to produce compressivestresses in tension regions. Prestress is applied to a concrete member by high-strength steel tendons in

the forms of bars, wires, or cables that are first tensioned and then anchored to the member.

When compared to classical reinforced concrete design, prestressed concrete design produces

lighter sections, thus allowing the economic use of much longer spans.

force force

Loads books

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Introduction

Design involves the determination of the type of structural system to be used, the cross sectional

dimensions, and the required reinforcement. The designed structure should be able to resist all forces expected to act during the life span of the structure

safely and without excessive deformation or cracking.

Analysis involves the determination of the capacity of a section of known dimensions, material properties

and steel reinforcement, if any to external forces and moments.

Design Versus Analysis

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Introduction

When a structural element becomes unfit for its intended use, it is said to have reached a limit state.

The limit states are classified into three groups:

Limit States of Reinforced Concrete Design

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1- Ultimate Limit States

These involve structural collapse of some structural elements or the structure altogether. These limit states should be prevented as they tend to cause

loss of life and property. Elastic instability, rupture, progressive collapse, and

fatigue are forms of these limit states.

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Introduction


Instructor:


2- Service Limit States

These involve the disruption of the functional use of the structure, not its collapse. A higher probability

of occurrence can be tolerated than in case of an ultimate limit state since there is less danger of loss

of life. Excessive deflections, immoderate crack widths, and

annoying vibrations are forms of these limit states.3- Special Limit States

These involve damage or failure due to abnormal conditions such as collapse in severe earthquakes,

damage due to explosions, fires, or deterioration of the structure and its main structural elements.

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Introduction


Instructor:


Generally, for buildings, a limit state design is carried out first in order to proportion the elements, and second a serviceability limit state is conducted to

check whether these elements satisfy those serviceability limit states.

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Introduction

Objectives of Structural Design:

Instructor:


The design of a structure must satisfy three basic requirements:

1) Stability to prevent overturning, sliding or buckling of the structure, or part of it under the

action of loads.2) Strength to resist safely the stresses induced by

the loads in the various structural members.3) Serviceability to ensure satisfactory performance

under service load conditions- which implies providing adequate stiffness to contain deflections,

crack widths and vibrations within acceptable limits, and also providing impermeability, durability.

There are two other considerations that a sensible designer in mind, viz. economy and aesthetics.

Page 13



Introduction

Design Codes

Instructor:


A code is a set of technical specifications that control the design and construction of a certain type of

structures.There are two types of codes; Structural code and

Building code.Structural code is a code that involves the design of

a certain type of structures (reinforced concrete, structural steel, etc.) The structural code that will be

used extensively throughout this course is The American Concrete Institute (ACI 318-05), which

is one of the most solid codes.Building code is a code that reflects local conditions

such as earthquakes, winds, snow, and tornadoes in the specifications.

IBC (UBC,BOCA and SBC).

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Introduction

Design Methods

Instructor:


Two methods of design have long prevalent.Working Stress Method focuses on conditions

at service loads.Strength Design Method focusing on

conditions at loads greater than the service loads when failure may be imminent.

The Strength Design Method is deemed conceptually more realistic to establish structural safety.

The Working-Stress Design Method

This method is based on the condition that the stresses caused by service loads without load factors

are not to exceed the allowable stresses which are taken as a fraction of the ultimate stresses of the

materials, fc’ for concrete and fy for steel.

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Introduction

Design Methods

Instructor:


The Ultimate – Strength Design Method

strength required to strength provided

carry factored loads

At the present time, the ultimate-strength design method is the method adopted by most prestigious

design codes.In this method, elements are designed so that the internal forces produced by factored loads do not exceed the corresponding strength capacities and

allow for some capacity reduction.

The factored loads are obtained by multiplying the working loads (service loads) by factors usually

greater than unity.

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Introduction

Loads on Structures

Instructor:


All structural elements must be designed for all loads anticipated to act during the life span of such

elements. These loads should not cause the structural elements to fail or deflect excessively under working

conditions.Dead load (D.L)

• Weight of all permanent construction• Constant magnitude and fixed location

Examples: * Weight of the Structure (Walls, Floors, Roofs, Ceilings,

Stairways, Partitions) * Fixed Service Equipment

(HVAC, Piping Weights, Cable Tray, Etc.)

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Introduction

Loads on Structures

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Live load (L.L)

The live load is a moving or movable type of load such as occupants, furniture, etc. Live loads used in

designing buildings are usually specified by local building codes. Live loads depend on the intended

use of the structure and the number of occupants at a particular time.

Some Typical Uniformly Distributed Live Load

Apartment Buildings: Residential areas and corridors 200 Kg/m2

Public rooms and corridors 500 Kg/m2

Office Buildings: Lobbies and first-floor corridors 500Kg/m2

Offices 250Kg/m2

Corridors above first floor 400Kg/m2

File and computer rooms 400Kg/m2

Storage Warehouses Light 600Kg/m2

Heavy 1200Kg/m2

Stairs and Exit Ways 500 Kg/m2

Schools Classrooms 200Kg/m2

Corridors above first floor 400Kg/m2

First-floor corridors 500Kg/m2

Garages (cars) 250Kg/m2

Retail Stores Ground floor 500Kg/m2

Upper floors 750Kg/m2

Wholesale, all Floors 600Kg/m2

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Introduction

Loads on Structures

Instructor:


Wind load (W.L)

The wind load is a lateral load produced by wind pressure and gusts. It is a type of dynamic load that

is considered static to simplify analysis. The magnitude of this force depends on the shape of the building, its height, the velocity of the wind and the

type of terrain in which the building exists.Earthquake load (E.L) or seismic load

The earthquake load is a lateral load caused by ground motions resulting from earthquakes. The

magnitude of such a load depends on the mass of the structure and the acceleration caused by the

earthquake.

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Introduction

Safety Provisions

Instructor:


Safety is required to insure that the structure can sustain all expected loads during its construction

stage and its life span with an appropriate factor of safety.

There are three main reasons why some sort of safety factor are necessary in structural design

• Variability in resistance. *Variability of fc’ and fy, *assumptions are made during design and *differences between the as-built dimensions and those found in structural drawings.

• Variability in loading. Real Loads may differ from assumed design loads, or distributed differently.

• Consequences of failure. *Potential loss of life, *cost of clearing the debris and replacement of the structure and its

contents and *Cost to society.

Page 20



Introduction

Safety Provisions

Instructor:


The strength design method, involves a two-way safety measure. The first of which involves using load

factors, usually greater than unity to increase the service loads. The second safety measure specified by

the ACI Code involves a strength reduction factor multiplied by the nominal strength to obtain design

strength. The magnitude of such a reduction factor is usually smaller than unity

Factored loads ≤ design strength

Page 21



Introduction

Safety Provisions

Instructor:


Load Factors

Dead only

U = 1.4D

Dead and Live Loads

U = 1.2D+1.6L

Dead, Live, and Wind Loads

U=1.2D+1.0L+1.6W

Dead and Wind Loads

U=1.2D+0.8W or U=0.9D+1.3W

Dead, Live and Earthquake Loads

U=1.2D+1.0L+1.0E

Dead and Earthquake Loads

U=0.9D+1.0E

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Introduction

Safety Provisions

Instructor:


Strength Reduction Factors

According to ACI strength reduction factors Φ are given as follows:

a- For tension-controlled sections Φ = 0.90

b- For compression-controlled sections, Members with spiral reinforcement

Φ = 0.70 Other reinforced members

Φ = 0.65

c- For shear and torsion Φ = 0.75

Tension-controlled section compression-controlled section

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Chapter 1 بسم الله الرحمن الرحيم Design of Concrete Structure I University of Palestine Instructor: Eng. Mazen Alshorafa