Lecture 1 - Introduction PDF

Role of Structural Engineers To help in the creation of the safe built environment Nothing can function, if structural engineers do not do their job well Every other professional “Depends” on the role of structural engineers Great Pyramid of Giza Hanging Gardens of...

Role of Structural Engineers To help in the creation of the safe built environment Nothing can function, if structural engineers do not do their job well Every other professional “Depends” on the role of structural engineers Great Pyramid of Giza Hanging Gardens of Babylon Lighthouse of Temple of Artemis Alexandria  Engineer Activity Conception – Analysis – Design – Detailing, etc. Structure Buildings – Bridges – Trusses – Shells – Towers, etc. Structural Code American – British – European – Japanese, etc. Engineering Material Concrete – PSC – Steel – Timber, etc. Spectrum Model 2D Frame/Truss – 3D Frame/Truss – Full 3D FEM, etc. Analysis Linear Static – NL Static – Linear Dynamic – NL Dynamic, etc. Solution Equation Solution – Finite Elements – Programming, etc.  Conception Modelling Analysis Overall Design Design Process Detailing Drafting Costing “Integrated Design Process” Structural Design Process Conceptual Design Architectural or Functional Plans Drawings, Cost Estimate Final Design Output Structural System Trial Member/Sections Detailing Design & Detailing Modelling & Analysis Yes Revise Member/ No Acceptable? Modelling Sections Analysis Response Member Design  “Structural Design is the process of proportioning the structure to safely resist the applied forces in the most cost effective and friendly manner” Design Philosophy and Process Material Specifications Load Effects Cross Section Requirements Design Details Constraints Member sizes and Configurations Structural Analysis vs Structural Design Structural Analysis Structural Design Design Stick Models, Finite Element Structural Material (RC, PSC, Timber..) Philosophy Method (FEM) Design Code (NSCP, ACI, BS Codes, EuroCode, JIS …) Output: Element/Member Design Approach (Working stress, ultimate strength, andActions, ProcessDisplacements.. limit state..) Structural Members (beams, columns, slabs, footings) Local Construction Techniques and Practices Output: Element/ Member Cross Section, Reinforcement… Design Levels Code Based Design Full 3D, 2D/3D Linear Partial Equations, Nonlinear, The Story of Differential Closed Form with Static Inelastic Charts, Tables, Equations Approximations Dynamic FEA FEA/Matrix Rules, Limits Structural Engineering Rigorous Semi Rigorous Simplified Specified Analytical Analytical Numerical Numerical Procedures Building Industry relies on Codes and Standards Codes specify requirements Give acceptable solutions Prescribe (detailed) procedures, rules, limits Codes are based on research and experience Spirit of the code is to help ensure Compliance to the code Public Safety and provide formal/ legal is intended to meet the basis for design decision spirit Prescriptive Codes – A Shelter Public: Is my structure safe? Design Will it be damaged, how much, how long to repair? Philosophy Structural Engineer: and Process Not sure, but I did follow the “Code” As long as engineers follow the code, they can be sheltered by its provisions Design Methodologies Traditional Design Methods Prime Concern: “Balance External Actions with Internal Stress Resultants with adequate margin of safety” The Story of 𝑆 ≥ 𝐹𝑂𝑆 ∗ 𝐹 stress Structural 𝑑 𝑎 Engineering Check for: Deflections, Deformations, Durability strain Vibrations, Crack Width, Fire Protections, Permeability, Chemical Attacks Ductility and other special considerations Various Methods of Structural Design Working Stress Design Allowable Stress Design (ASD) The Story of Working Stress Design (WSD) Structural Ultimate Strength Design Ultimate Strength Design (USD) Engineering Strength Design (SD) Load and Resistance Factor Design (LRFD) Performance-based Design Pushover Analysis Capacity-based Design Various Methods of Structural Design ALLOWABLE STRENGTH ULTIMATE STRENGTH DESIGN DESIGN (USD) (ASD) DESIGN STRENGTH SAFETY FACTOR (Ω) RESISTANCE FACTOR (Φ) FACTOR [greater than 1.0] [less than 1.0] DESIGN LOADS UNFACTORED FACTORED DEMAND VS 𝑹𝒏 ≥ 𝑹𝒅 𝝓𝑹𝒏 ≥ 𝑹𝒅 CAPACITY 𝛀 Limit State Design Concept Types of Limit State Description Ultimate limit states Loss of equilibrium Rupture Progressive collapse The Story of Formation of plastic mechanism Instability Structural Fatigue Engineering Serviceability limit states Excessive deflections Excessive crack width Undesirable vibration Special limit states Due to abnormal conditions and abnormal loading such as: Damage or collapse in extreme earthquakes Structural effects of fire, explosion Corrosion or deterioration Limit State Design Concept Limit state design involves: Identification of all potential modes of failure Determination of acceptable levels of safety against occurrence of each limit state Consideration by the designer of significant limit states Safety factors The main goal is to satisfy the criteria: Material Safety factor Member factor φ𝑅𝑛 ≥ ෍ 𝛾𝑖 𝑄𝑖 Load factor Structural Analysis Factor Resistances ≥ Loads Structure Factor Limit State Design Concept 𝛾𝑚 𝛾𝑏 Characteristic value of Material safety Member safety Design member Design Strength material basic strength capacity Structure Verification Factor 𝛾𝑖 𝛾𝑎 𝛾𝑓 Structural Characteristic value of Load Factor Analysis Factor Member design Design Load Load demand Structural Components The Story of Structures Structural Members Engineering Cross- Sections Materials Structure as Noun and Verb As a Noun, structure is defined as “the arrangement of and the relations between parts or elements of something complex” As a verb structure is defined as “construct or arrange according to a plan; give a pattern or organization”. When applied to the physical and built environment, the term Structure means an assemblage of physical components and elements, each of which could further be a structure itself, signifying the complexity of the system The discipline of “Structural Engineering” refers to the verb part of the definition Source: James G. Macgregor, Reinforced Concrete: Mechanics and Design, 3rd Edition. Material Properties of Reinforced Concrete Advantages of Reinforced Concrete High compressive strength per unit cost Has great resistance for water and fire The material is very rigid Low maintenance material Has long service life Most economical material for slab, footings, basement walls, and other similar application. Can be casted into different variety of shapes The raw materials are inexpensive and mostly available anywhere Lower skill for labor is needed for erection Concrete Concrete is a mixture of sand, gravel, crushed rock, or other aggregates held together in a rocklike mass with a paste of cement and water. Sometimes one or more admixtures are added to change certain characteristics of the concrete such as its workability, durability, and time of hardening. Advantages of Concrete Concrete Composition Cement Aggregate Coarse Aggregates (Gravel or crushed rock) Fine Aggregates (Sand) Water Source: Design and Control of Concrete Mixtures, Admixture Portland Cement Association, 2011 Curing of Concrete Curing is performed by submerging the specimen underwater. This is done in order to prevent moisture loss. Rapid moisture loss leads to cracking and loss of strength of the concrete specimen Testing of Concrete ASTM C39 – Standard Method for Compressive Strength of Cylindrical Concrete Specimens Increasing compressive load is applied to a concrete cylinder (150mm dia. x 300mm height) until the concrete crushes 𝑃 𝑓 ′𝑐 = 𝐴 Stress-Strain Relationship of Concrete 𝑆𝑡𝑟𝑒𝑠𝑠, 𝑓𝑐 From NSCP 2015 C f’c For normal weight concrete (wc = 2300 kg/m3) B 𝑬𝒄 = 𝟒𝟕𝟎𝟎𝝀 𝒇′ 𝒄 𝒊𝒏 𝑴𝑷𝒂 A For other weights Ec 𝑬𝒄 = 𝒘𝒄 𝟎. 𝟎𝟒𝟑 𝒇′ 𝒄 𝒊𝒏 𝑴𝑷𝒂 D where 𝝀 – factor for the type of concrete 𝝀 = 1.0 if normal weight concrete 𝑆𝑡𝑟𝑎𝑖𝑛, 𝜀𝑐 𝝀 = 0.75 if light weight concrete A – Proportionality Limit B – Elastic Limit Modulus of rupture C – Ultimate Point 𝒇𝒓 = 𝟎. 𝟔𝟐𝝀 𝒇′ 𝒄 𝒊𝒏 𝑴𝑷𝒂 D –Rupture Point Modulus of Rupture ASTM C78 – Standard Method for Flexural Strength of Concrete A concrete specimen (150x150x750 mm) is loaded with increasing load at third points The length should be at least 3 times the depth of the specimen 𝑃 𝑃 2 2 𝑀𝑚𝑎𝑥 𝑐 𝑃 𝐿 𝑃𝐿 𝑓𝑟 = 𝑀𝑚𝑎𝑥 = = 𝐼 2 3 6 𝑃𝐿 𝑑 6 2 𝑃𝐿 𝑓𝑟 = = 𝐿/3 𝐿/3 𝐿/3 𝑏𝑑 3 𝑏𝑑 12 Why do we provide steel in concrete? Concrete is very strong in compression but relatively weak in tension The use of reinforcement in concrete construction To resist tensile forces in structural members To resist a portion of compression loading To resist diagonal tension in shear To limit crack widths and control spacing of cracks due to stresses induced by temperature changes and shrinkage. Shrinkage Creep Longitudinal bars Nominal Sizes (diameter in mm)) 10 12 Nominal Length Nominal Length 16 (in m) (in m) 20 6 6 25 7.5 7.5 28 9 9 32 10.5 10.5 36 12 12 Longitudinal bars Stress-Strain Relationship of Steel 𝑬𝒔 = 𝟐𝟎𝟎, 𝟎𝟎𝟎 𝑴𝑷𝒂 𝑆𝑡𝑟𝑒𝑠𝑠, 𝑓𝑐 C B D fy A Strain hardening Necking 𝑆𝑡𝑟𝑎𝑖𝑛, 𝜀𝑐 A – Proportionality Limit B – Elastic Limit C – Ultimate Point D –Rupture Point Review of Fundamentals A monolithic floor system consists of 100 mm thick slabs and simply supported beams with a 7.3 m span, 3 m on centers. The floor carries a superimposed dead load of 1.15 kPa and live load of 1.9 kPa. Walls weighing 2.8 kPa and 2.4 m high are directly supported by the beams. Beam dimensions as determined are bw = 275 mm and h = 500 mm. Concrete weighs 24 kN/m^3. Use f’c = 28 Mpa, and fy = 420. Design the beam based on the limit state principles. Show the D/C ratio for each limit.