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
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