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Reinforced Concrete Design. Lecture 3. Dr. Nader Okasha. Introduction to Design Concepts. Objectives of Structural Design:. The design of a structure must satisfy three basic requirements: Strength to resist safely the stresses induced by the loads in the various structural members.
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Reinforced Concrete Design Lecture 3 Dr. Nader Okasha
Objectives of Structural Design: • The design of a structure must satisfy three basic requirements: • Strength to resist safely the stresses induced by the loads in the various structural members. • Stability to prevent overturning, sliding or buckling of the structure, or part of it under the action of loads. • 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.
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 318-05), which is one of the most solid codes. Building codeis a code that reflects local conditions such as earthquakes, winds, snow, and tornadoes in the specifications.
Design Methods (Philosophies) • 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 fyfor steel.
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. The factored loads are obtained by multiplying the working loads (service loads) by factors usually greater than unity.
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 = 0.75 Other reinforced members Φ = 0.65 c- For shear and torsion Φ = 0.75 Tension-controlled sectioncompression-controlled section
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
Loads 1- Dead loads (Gravity + Lateral) 2- Live loads
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
Lateral Loads 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.
