Chapter 1 بسم الله الرحمن الرحيم 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; 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. 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 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 ), 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 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 500Kg/m 2 Offices 250Kg/m 2 Corridors above first floor 400Kg/m 2 File and computer rooms 400Kg/m 2 Storage Warehouses Light 600Kg/m 2 Heavy 1200Kg/m 2 Stairs and Exit Ways 500 Kg/m 2 Schools Classrooms 200Kg/m 2 Corridors above first floor 400Kg/m 2 First-floor corridors 500Kg/m 2 Garages (cars) 250Kg/m 2 Retail Stores Ground floor 500Kg/m 2 Upper floors 750Kg/m 2 Wholesale, all Floors 600Kg/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 = 1.4D 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

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

Page 20 Design of Concrete Structure I University of Palestine Introduction Example# 1 Instructor: Eng. Mazen Alshorafa The beam shown in Figure [1]carries a uniformly distributed service dead load of 5 t/m, and a service live load of 3 t/m. Determine the maximum positive bending moment in the beam. = Solution W u1 = 1.4 = 1.4(5)=7.0 t/m W u2 = 1.2 D+1.6 L = 1.2(5)+1.6(3)=10.8 t/m M u,max = w u,max L 2 = 10.8 (6) 2 = 48.6 t.m m Dead = 5 t/m, Live = 3 t/m 6.0 m w u = 10.8 t/m 48.6 t.m

Page 20 Design of Concrete Structure I University of Palestine Introduction Example# 2 Instructor: Eng. Mazen Alshorafa The beam shown in Figure [2]carries a uniformly distributed service dead load of 5 t/m, and a service live load of 3 t/m. Determine the maximum positive and negative bending moments for which beam ABC should be designed for. Solution Maximum negative moment: This case is evaluated by fully loading the two spans by dead and live loads W u = 1.2 D+1.6 L = 1.2(3)+1.6(1.5)=6.0 t/m The maximum negative moment is evaluated using any available structural analysis software. M –ve (max) =18.75 t.m, as shown in figure 1.a 5.0 m A B C

Page 20 Design of Concrete Structure I University of Palestine Introduction Example# 2 Instructor: Eng. Mazen Alshorafa Maximum positive moment: This case is evaluated by fully loading one of the two spans by dead and live loads while loading the other span by dead load only. For the span loaded with dead and live loads, w u = 1.2 D+1.6 L = 1.2(3)+1.6(1.5)= 6.0 t/m For the other span, wu = 1.2 D = 1.2(3) = 3.6 t/m The maximum positive moment is evaluated using any available structural analysis software. M +ve (max) =12.0 t.m, as shown in figure 1.b.

Page 20 Design of Concrete Structure I University of Palestine Introduction Example# 2 Instructor: Eng. Mazen Alshorafa w u = 6.0 t/m t.m t.m w u = 6.0 t/m 15.0 t.m 12.0 t.m 5.0 t.m w u = 3.6 t/m Max. Negative Moment Max. Positive Moment

Page 20 Design of Concrete Structure I University of Palestine Introduction Example# 3 Instructor: Eng. Mazen Alshorafa For frame ABCD shown in Figure [3], determine the axial forces for which member AB should be designed for when the following service loads are applied: − a dead load of 1 t/m on member BC; − a live load 2.5 t/m on member BC; − a horizontal wind load of 5 tons at joint C, which acts to the right. A B D C 10.0 m 5.0 m

Page 20 Design of Concrete Structure I University of Palestine Introduction Example# 3 Instructor: Eng. Mazen Alshorafa Combination (1): D + L, U = 1.2D + 1.6L w u = 1.2 D+1.6 L = 1.2(1)+1.6(2.5)= 5.2 t/m F AB = 5.2(10/2)=26 ton (compression) as shown in figure 3.a Combination (2): D +L+ W, U = 1.2D L + 0.8W w u (vertical) = 1.2 D+1.0 L = 1.2(1)+1.0(2.5)= 3.7 t/m p u (horizontal) = 1.6 W = 1.6(5) = 8 tons F AB = 14.5 tons (compression) as shown in figure 3.b Combination (3): D + W, U = 1.2D + 0.8W w u (vertical) = 1.2 D = 1.2(1) = 1.2 t/m p u ( horizontal ) = 0.8 W = 0.8(5) = 4 tons F AB = 4 tons (compression) as shown in figure 3.c Combination (4): D + W, U = 0.9D + 1.3W w u (vertical) = 0.9 D = 0.9(1) = 0.9 t/m p u (horizontal) = 0.8 W = 1.3(5) = 6.5 tons F AB = 1.25 tons (compression) as shown in figure 3.d

Page 20 Design of Concrete Structure I University of Palestine Introduction Instructor: Eng. Mazen Alshorafa A B D C 5.2 t/m A B D C 3.7 t/m 8.0 ton A B D C 1.2 t/m A B D C 0.9 t/m 6.5 ton 4.0 ton F AB =26 ton F AB =14.5 ton F AB =4.0 ton F AB =1.25 ton Fig. 3.a Fig. 3.c Fig. 3.d Fig. 3.b Member AB should be designed for an axial compression load of 26.0 tons