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

3. Design of Concrete Structure I University of Palestine
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University of Palestine
Introduction
Concrete and Reinforced Concrete:
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.
Instructor:
Eng. Mazen Alshorafa

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University of Palestine
Introduction
Concrete and Reinforced Concrete:
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:
Eng. Mazen Alshorafa

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University of Palestine
Introduction
Concrete and Reinforced Concrete:
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; to for concrete and 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.
Instructor:
Eng. Mazen Alshorafa

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University of Palestine
Introduction
Advantages of Reinforced Concrete:
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.
Disadvantages of Reinforced Concrete:
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)
Instructor:
Eng. Mazen Alshorafa

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University of Palestine
Introduction
Types of Concrete Design
Concrete design can be classified into three main categories;
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
Instructor:
Eng. Mazen Alshorafa

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University of Palestine
Introduction
Types of Concrete Design
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
Steel bars embedded
Instructor:
Eng. Mazen Alshorafa

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University of Palestine
Introduction
Types of Concrete Design
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.
Loads
books
force
force
Instructor:
Eng. Mazen Alshorafa

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University of Palestine
Introduction
Design Versus Analysis
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.
Instructor:
Eng. Mazen Alshorafa

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University of Palestine
Introduction
Limit States of Reinforced Concrete Design
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:
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.
Instructor:
Eng. Mazen Alshorafa

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University of Palestine
Introduction
Limit States of Reinforced Concrete Design
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.
Instructor:
Eng. Mazen Alshorafa

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University of Palestine
Introduction
Limit States of Reinforced Concrete Design
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.
Instructor:
Eng. Mazen Alshorafa

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University of Palestine
Introduction
Objectives of Structural Design:
The design of a structure must satisfy three basic requirements:
Stability to prevent overturning, sliding or buckling of the structure, or part of it under the action of loads.
Strength to resist safely the stresses induced by the loads in the various structural members.
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.
Instructor:
Eng. Mazen Alshorafa

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University of Palestine
Introduction
Design Codes
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 ), 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).
Instructor:
Eng. Mazen Alshorafa

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University of Palestine
Introduction
Design Methods
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.
Instructor:
Eng. Mazen Alshorafa

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University of Palestine
Introduction
Design Methods
The Ultimate – Strength Design Method
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.
Instructor:
Eng. Mazen Alshorafa

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University of Palestine
Introduction
Loads on Structures
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.)
Instructor:
Eng. Mazen Alshorafa

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University of Palestine
Introduction
Loads on Structures
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 Kg/m2
Office Buildings:
Lobbies and first-floor corridors 500Kg/m2
Offices Kg/m2
Corridors above first floor Kg/m2
File and computer rooms Kg/m2
Storage Warehouses
Light Kg/m2
Heavy Kg/m2
Stairs and Exit Ways Kg/m2
Schools
Classrooms Kg/m2
Corridors above first floor Kg/m2
First-floor corridors Kg/m2
Garages (cars) Kg/m2
Retail Stores
Ground floor Kg/m2
Upper floors Kg/m2
Wholesale, all Floors Kg/m2
Instructor:
Eng. Mazen Alshorafa

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University of Palestine
Introduction
Loads on Structures
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.
Instructor:
Eng. Mazen Alshorafa

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University of Palestine
Introduction
Safety Provisions
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.
Instructor:
Eng. Mazen Alshorafa

22. Factored loads ≤ design strength
Design of Concrete Structure I
Page 20
University of Palestine
Introduction
Safety Provisions
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
Instructor:
Eng. Mazen Alshorafa

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University of Palestine
Introduction
Safety Provisions
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
Instructor:
Eng. Mazen Alshorafa

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University of Palestine
Introduction
Safety Provisions
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
Instructor:
Eng. Mazen Alshorafa