Eng. Mazen Alshorafa

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Chapter
1

ميحرلا نمحرلا الله مسب

Design of Concrete
Structure I

University of
Palestine

Instructor:

Eng. Mazen Alshorafa












Page
1

Design of Concrete
Structure I

University of
Palestine

Introduction

Concrete and Reinforced Concrete
:

Instructor:

Eng. Mazen Alshorafa

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.

Page
2

Design of Concrete
Structure I

University of
Palestine

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.

Concrete and Reinforced Concrete:

Instructor:

Eng. Mazen Alshorafa

Page
3

Design of Concrete
Structure I

University of
Palestine

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.


Concrete and Reinforced Concrete:

Instructor:

Eng. Mazen Alshorafa




Page
4

Design of Concrete
Structure I

University of
Palestine

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:

Eng. Mazen Alshorafa



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:

Page
5

Design of Concrete
Structure I

University of
Palestine

Introduction

Concrete design can be classified into three main categories;

Types of Concrete Design

Instructor:

Eng. Mazen Alshorafa

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

Page
6

Design of Concrete
Structure I

University of
Palestine

Introduction

Types of Concrete Design

Instructor:

Eng. Mazen Alshorafa

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

stresses

Steel bars
embedded

Page
7

Design of Concrete
Structure I

University of
Palestine

Introduction

Types of Concrete Design

Instructor:

Eng. Mazen Alshorafa

3
-

Prestressed Concrete Design

Since the strength of reinforced concrete can be enhanced by the
elimination of cracking, prestressing is used to produce compressive

stresses 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

Page
8

Design of Concrete
Structure I

University of
Palestine

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

Instructor:

Eng. Mazen Alshorafa




Page
9

Design of Concrete
Structure I

University of
Palestine

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

Instructor:

Eng. Mazen Alshorafa




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.



Page
10

Design of Concrete
Structure I

University of
Palestine

Introduction

Limit States of Reinforced Concrete Design

Instructor:

Eng. Mazen Alshorafa

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.

Page
11

Design of Concrete
Structure I

University of
Palestine

Introduction

Limit States of Reinforced Concrete Design

Instructor:

Eng. Mazen Alshorafa

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.

Page
12

Design of Concrete
Structure I

University of
Palestine

Introduction

Objectives of Structural Design:

Instructor:

Eng. Mazen Alshorafa

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

Design of Concrete
Structure I

University of
Palestine

Introduction

Design Codes

Instructor:

Eng. Mazen Alshorafa

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).

Page
14

Design of Concrete
Structure I

University of
Palestine

Introduction

Design Methods

Instructor:

Eng. Mazen Alshorafa

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,
f
c’

for concrete and
f
y

for steel.

Page
15

Design of Concrete
Structure I

University of
Palestine

Introduction

Design Methods

Instructor:

Eng. Mazen Alshorafa

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.

Page
16

Design of Concrete
Structure I

University of
Palestine

Introduction

Loads on Structures

Instructor:

Eng. Mazen Alshorafa

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.)

Page
17

Design of Concrete
Structure I

University of
Palestine

Introduction

Loads on Structures

Instructor:

Eng. Mazen Alshorafa

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/m
2


Public rooms and corridors
500
Kg/m
2

Office Buildings:


Lobbies and first
-
floor corridors
500
Kg/m
2


Offices

250
Kg/m
2


Corridors above first floor
400
Kg/m
2


File and computer rooms
400
Kg/m
2

Storage Warehouses


Light
600
Kg/m
2


Heavy
1200
Kg/m
2

Stairs and Exit Ways
500
Kg/m
2

Schools


Classrooms
200
Kg/m
2


Corridors above first floor
400
Kg/m
2


First
-
floor corridors
500
Kg/m
2

Garages (cars)

250
Kg/m
2

Retail Stores



Ground floor
500
Kg/m
2


Upper floors
750
Kg/m
2

Wholesale, all Floors

600
Kg/m
2

Page
18

Design of Concrete
Structure I

University of
Palestine

Introduction

Loads on Structures

Instructor:

Eng. Mazen Alshorafa

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.

Page
19

Design of Concrete
Structure I

University of
Palestine

Introduction

Safety Provisions

Instructor:

Eng. Mazen Alshorafa

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 f
c
’ and f
y
,
*
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

Design of Concrete
Structure I

University of
Palestine

Introduction

Safety Provisions

Instructor:

Eng. Mazen Alshorafa

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

Design of Concrete
Structure I

University of
Palestine

Introduction

Safety Provisions

Instructor:

Eng. Mazen Alshorafa

Load Factors

Dead only

U =
1.4
D

Dead and Live Loads

U =
1.2
D+
1.6
L

Dead, Live, and Wind Loads

U=
1.2
D+
1.0
L+
1.6
W

Dead and Wind Loads

U=
1.2
D+
0.8
W

or
U=
0.9
D+
1.3
W

Dead, Live and Earthquake Loads

U=
1.2
D+
1.0
L+
1.0
E

Dead and Earthquake Loads

U=
0.9
D+
1.0
E

Page
22

Design of Concrete
Structure I

University of
Palestine

Introduction

Safety Provisions

Instructor:

Eng. Mazen Alshorafa

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