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I.S 456-2000. AQUIB ANSARI (Assistant Professor) Department of Civil Engineering A.C.E.T Nagpur. Overall Design Process. Conception Modeling Analysis Design Detailing Drafting Costing. The Building Structural System - Physical. Building Structure. Floor Diaphragm.
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I.S 456-2000 AQUIB ANSARI (Assistant Professor) Department of Civil Engineering A.C.E.T Nagpur
Overall Design Process • Conception • Modeling • Analysis • Design • Detailing • Drafting • Costing
The Building Structural System - Physical Building Structure Floor Diaphragm Frame and Shear Walls Lateral Load Resisting System Floor Slab System Gravity Load Resisting System Sub-structure and Member Design Beams, Columns, Two-way Slabs, Flat Slabs, Pile caps Shear Walls, Deep Beams, Isolated Footings, Combined Footings
The Building Structural System - Conceptual • The Gravity Load Resisting System • The structural system (beams, slab, girders, columns, etc) that act primarily to support the gravity or vertical loads • The Lateral Load Resisting System • The structural system (columns, shear walls, bracing, etc) that primarily acts to resist the lateral loads • The Floor Diaphragm • The structural system that transfers lateral loads to the lateral load resisting system and provides in-plane floor stiffness
Building Response Objective: To determine the load path gravity and lateral loads • For Gravity Loads - How Gravity Loads are Distributed • Analysis of Gravity Load Resisting System for: • Dead Load, Live Live Load, Pattern Loads, temperature, shrinkage • Important Elements: Floor slabs, beams, openings, Joists, etc. • For Lateral Loads – How Lateral Loads are Distributed • Analysis of Lateral Load Resisting System for: • Wind Loads, Seismic Loads, Structural Un-symmetry • Important elements: Columns, shear walls, bracing , beams
The Simplified Structural System STRUCTURE RESPONSES DisplacementsStrains Stress Stress Resultants EXCITATION Loads Vibrations Settlements Thermal Changes pv
Global Modeling of Structural Geometry (f) Grid-Plate
Structure Types • Cable Structures • Cable Nets • Cable Stayed • Bar Structures • 2D/3D Trusses • 2D/3D Frames, Grids • Surface Structures • Plate, Shell • In-Plane, Plane Stress • Solid Structures
Concrete Properties Stress Strain Typical stress-strain curve for a concrete cylinder in compression
Concrete Properties • Concrete stress-strain curve shows no definite yield point; • Concrete does not have the large plastic deformation capacity of structural steel in the stress-strain curve, and so does not display the same ductile behaviour; • Concrete has a brittle failure.
Pre-cracking Behaviour of Concrete Stress At relatively low strains, the stress-strain relationship is approximately linear e<ecr Strain M < Mcr M < Mcr e<ecr f < fcf fcf is the actual flexural tensile strength where f’cf is the characteristic flexural tensile strength.
Ultimate Behaviour of Concrete Stress f’c Strain ec= 0.003 kud es M = Muo M = Muo e>ecr • As the strains increase, the relationship between stress and strain is no longer linear; • The strain distribution in the section is still assumed to be linear; • The stress distribution will be non-linear; • The beam cross-section is assumed to be at its ultimate load when the concrete extreme compression fibre reaches a strain of 0.003.
Types of Concrete Section • The calculation procedure varies depending on when the steel yields; • In under-reinforced sectionsthe steel has already yielded when the concrete reaches its ultimate state with strains of 0.003 at the extreme compressive fibre; • In balanced sectionsthe steel yields just as the concrete reaches its ultimate state with strains of 0.003 at the extreme compressive fibre; • In over-reinforced sectionsthe steel has not yielded when the concrete reaches its ultimate state with strains of 0.003 at the extreme compressive fibre;
Reinforcement • Mild steel and medium tensile steel bars conforming to IS 432 (Part 1). • High strength deformed steel bars conforming to IS 1786. • Hard-drawn steel wire fabric conforming to IS 1566. • Structural steel conforming to Grade A of IS 2062 Fe 250 Fe 415 Fe 500 Fe 550
Concrete • For concrete of grade greater than M55, design parameters given in the standards may not be applicable and the values may be obtained from specialized literatures and experimental results. • Comments: Whether RCC concrete is to be taken as ordinary concrete or Standard concrete?. If it is to be considered as standard Concrete, then minimum grade of concrete will be M25. • For water retaining structures, it must be grade M30 minimum.
Modulus of Elasticity • Ec = 5000 fck in N/mm2 • Actual measured values may differ by 20% from the values obtained from the expression. • Comments: E value does not affect the static analysis for vertical and horizontal forces, except the secondary forces. However, change in E value significantly affects the detailed dynamic analysis. The fundamental time period for SDOF system may vary from 10-12%.
Shrinkage The total shrinkage of concrete depends upon the constituents of concrete, size of the member and environmental conditions. For a given humidity and temperature, the total shrinkage of concrete is most influenced by the total amount of water present in the concrete at the time of mixing and, to a lesser extent, by the cement content . In the absence of test data, the approximate value of the total shrinkage strain for design may be taken as 0.000 3
Thermal Expansion The coefficient of thermal expansion depends on nature of cement, the aggregate, the cement content, the relative humidity and the size of sections-The value of coefficient of thermal expansion for concrete with different aggregates may be taken as below:
DURABILITY OF CONCRETE • the environment; • the cover to embedded steel; • the type and quality of constituent materials; • the cement content and water/cement ratio of • the concrete; • workmanship, to obtain full compaction and • efficient curing; and • the shape and size of the member.
INSPECTION AND TESTING OF STRUCTURES Core Test Concrete in the member represented by a core test shall be considered acceptable if the average equivalent cube strength of the cores is equal to at least 85 percent of the cube strength of the grade of concrete specified for the corresponding age and no individual core has a strength less than 75 percent.
INSPECTION AND TESTING OF STRUCTURES Load Tests for Flexural Members The structure should be subjected to a load equal to full dead load of the structure plus 1.25 times the imposed load for a period of 24 h and then the imposed load shall be removed. The deflection due to imposed load only shall be recorded. If within 24 h of removal of the imposed load, the structure does not recover at least 75 percent of the deflection under superimposed load, the test may be repeated after a lapse of 72 h. If the recovery is less than 80 percent, the structure shall be deemed to be unacceptable.
INSPECTION AND TESTING OF STRUCTURES Load Tests for Flexural Members The structure should be subjected to a load equal to full dead load of the structure plus 1.25 times the imposed load for a period of 24 h and then the imposed load shall be removed. The deflection due to imposed load only shall be recorded. If within 24 h of removal of the imposed load, the structure does not recover at least 75 percent of the deflection under superimposed load, the test may be repeated after a lapse of 72 h. If the recovery is less than 80 percent, the structure shall be deemed to be unacceptable.
INSPECTION AND TESTING OF STRUCTURES non-destructive Tests Non-destructive tests are used to obtain estimation of the properties of concrete in the structure. The methods adopted include ultrasonic pulse velocity and rebound hammer . Non destructive tests provide alternatives to core tests for estimating the strength of concrete in a structure, or can supplement the data obtained from a limited number of cores.
Methods of Design Structure and structural elements shall normally be designed by Limit State Method. Where the Limit State Method can not be conveniently adopted, Working Stress Method. Designs based on experimental investigations on models or full size structure or element may be accepted
Loads • Dead Loads • Imposed Loads • Wind Loads • Snow Loads • Earthquake Forces • Shrinkage, Creep and Temperature Effects • Other Forces and Effects • Foundation movement, • Elastic axial shortening, • Soil and fluid pressures, • Vibration, Fatigue, Impact, • Erection loads • Stress concentration effect due to point load and the like.
Load Combinations • 1.5 DL + 1. 5 LL • 1.2 DL + 1.2 LL 1.2 EQ / WL • 1.5 DL 1.5 EQ / WL • 0.9 DL 1.5 EQ / WL
Load Combinations as per IS:875-1987 (P-V) • DL • DL + IL • DL + WL/EQ • DL + IL + WL/EQ • DL + WL/EQ + TL • DL + IL + WL/EQ + TL Note: Load with only Wind loads are considered.
Load Combinations as per IS:456-2000 (Limit State of Collapse) • 1.5 DL + 1.5 IL • 1.5 DL + 1.5 EQX ( Earthquake towards left) • 1.5 DL – 1.5 EQX (Earthquake towards right) • 1.2 DL + 1.2 IL + 1.2 EQX • 1.2 DL + 1.2 IL - 1.2 EQX • 0.9 DL + 1.5 EQX • 0.9 DL + 1.5 EQX Note: Load with only eqx load is considered.
STABILITY OF THE STRUCTURE – Overturning • The stability of a structure as a whole against overturning shall be ensured so that the restoring moment shall be not less than the sum of 1.2 times the maximum overturning moment due to the characteristic dead load and 1.4 times the maximum overturning moment due to the characteristic imposed loads. • In cases where dead load provides the restoring moment, only 0.9 times the characteristic dead load shall be considered. Restoring moment due to imposed loads shall be ignored.
STABILITY OF THE STRUCTURE - Sliding • The structure shall have a factor against sliding of not less than 1.4 under the most adverse combination of the applied characteristic forces. In this case only 0.9 times the characteristic dead load shall be taken into account.
Analysis • All the structures may be analyzed by the linear elastic theory to calculate internal actions produced by design. In lieu of rigorous elastic analysis, a simplified analysis as given in 22.4 for frames and as given in 22.5 for continuous beams may be adopted. • Where side sway consideration becomes critical due to unsymmetry in geometry or loading, rigorous analysis may be required.
Effective Span • Continuous Beam or Slab - In the case of continuous beam or slab, if the width of the support is less than l/12 of the clear span, the effective span shall be as in SS Beam. If the supports are wider than I/12 of the clear span or 600 mm whichever is less, the effective span shall be taken as under: • 1) For end span with one end fixed and the other continuous or for intermediate spans, the effective span shall Abe the clear span between supports; • 2) For end span with one end free and the other continuous, the effective span shall be equal to the clear span plus half the effective depth of the beam or slab or the clear span plus half the width of the discontinuous support, whichever is less;3) In the case of spans with roller or rocker bearings, the effective span shall always be the distance between the centres of bearings.
Effective Span • Frames-Inthe analysis of a continuous frame,centre to centre distance shall be used.
Critical Sections for Moment • For monolithic construction, the moments computed at the face of the supports shall be used in the design of the members at those sections.
Stiffness • The relative stiffness of the members may be based on the moment of inertia of the section determined on the basis of any one of the following definitions:. • Gross section - The cross-section of The member ignoring reinforcement; • Transformed section - The concrete cross section plus the area of reinforcement transformed on the basis of modular ratio ; or • Cracked section - The area of concrete in compression plus the area of reinforcement transformed on the basis of modular ratio. The assumptions made shall be consistent for all the members of the structure throughout any analysis..
