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  1. Structural Engineering Sergio F. Breña STEM Education Institute Saturday Workshop September 30, 2006 University of Massachusetts Amherst

  2. Outline • Introduction to Structural Engineering • Forces in Structures • Structural Systems • Civil Engineering Materials • Some Definitions of Important Structural Properties University of Massachusetts Amherst

  3. Structural Engineering • What does a Structural Engineer do? • A Structural Engineer designs the structural systems and structural elements in buildings, bridges, stadiums, tunnels, and other civil engineering works (bones) • Design: process of determining location, material, and size of structural elements to resist forces acting in a structure University of Massachusetts Amherst

  4. Engineering Design Process • Identify the problem (challenge) • Explore alternative solutions • Research past experience • Brainstorm • Preliminary design of most promising solutions • Analyze and design one or more viable solutions • Testing and evaluation of solution • Experimental testing (prototype) or field tests • Peer evaluation • Build solution using available resources (materials, equipment, labor) University of Massachusetts Amherst

  5. Design Process in Structural Engineering • Select material for construction • Determine appropriate structural system for a particular case • Determine forces acting on a structure • Calculate size of members and connections to avoid failure (collapse) or excessive deformation University of Massachusetts Amherst

  6. Examples of Typical Structures University of Massachusetts Amherst

  7. Forces in Structures University of Massachusetts Amherst

  8. Forces Acting in Structures • Forces induced by gravity • Dead Loads (permanent): self-weight of structure and attachments • Live Loads (transient): moving loads (e.g. occupants, vehicles) • Forces induced by wind • Forces induced by earthquakes • Forces induced by rain/snow • Fluid pressures • Others University of Massachusetts Amherst

  9. Forces Acting in Structures Vertical: Gravity Lateral: Wind, Earthquake University of Massachusetts Amherst

  10. Global Stability Sliding Overturning University of Massachusetts Amherst

  11. 100 lb Compression Forces in Structural Elements 100 lb Tension University of Massachusetts Amherst

  12. 100 lb Bending Forces in Structural Elements (cont.) Torsion University of Massachusetts Amherst

  13. Arch Typical Structural Systems (1) University of Massachusetts Amherst

  14. C T C C T Forces in Truss Members Typical Structural Systems (2) Truss University of Massachusetts Amherst

  15. Frame Typical Structural Systems (3) University of Massachusetts Amherst

  16. Typical Structural Systems (4) Flat Plate University of Massachusetts Amherst

  17. Typical Structural Systems (5) Folded Plate University of Massachusetts Amherst

  18. Typical Structural Systems (6) Shells University of Massachusetts Amherst

  19. Properties of Civil Engineering Materials University of Massachusetts Amherst

  20. T Example (English Units): T = 1,000 lb (1 kip) A = 10 in2. Stress = 1,000/10 = 100 lb/in2 Stress = Force/Area Section X Example (SI Units): 1 lb = 4.448 N (Newton) 1 in = 25.4 mm T = 1,000 lb x 4.448 N/lb = 4448 N A = 10 in2 x (25.4 mm)2 = 6450 mm2 (1 in)2 Stress = 4448/6450 = 0.69 N/mm2 (MPa) Section X T T Definition of Stress University of Massachusetts Amherst

  21. T DL Lo T Definition of Strain Strain = DL / Lo Example: Lo = 10 in. DL = 0.12 in. Strain = 0.12 / 10 = 0.012 in./in. Strain is dimensionless!! (same in English or SI units) University of Massachusetts Amherst

  22. Stress – Strain Behavior of Elastic Mats. Stress E E = Modulus of Elasticity = Stress / Strain Strain University of Massachusetts Amherst

  23. Stress Stress E Strain Strain Stress (a) Linear Elastic Stress (b) Non-linear Elastic Strain Strain Plastic strain Plastic strain (c) Elastic-plastic (d) Non-linear Plastic Types of Stress-Strain Behavior University of Massachusetts Amherst

  24. Materials Used in Civil Engineering • Stone and Masonry • Metals • Cast Iron • Steel • Aluminum • Concrete • Wood • Fiber-Reinforced Plastics University of Massachusetts Amherst

  25. Engineering Properties of Materials • Steel • Maximum stress: 40,000 – 120,000 lb/in2 • Maximum strain: 0.2 – 0.4 • Modulus of elasticity: 29,000,000 lb/in2 • Concrete • Maximum stress: 4,000 – 12,000 lb/in2 • Maximum strain: 0.004 • Modulus of elasticity: 3,600,000 – 6,200,000 lb/in2 • Wood Values depend on wood grade. Below are some samples • Tension stress: 1300 lb/in2 • Compression stress: 1500 lb/in2 • Modulus of elasticity: 1,600,000 lb/in2 University of Massachusetts Amherst

  26. Concrete Components • Sand (Fine Aggregate) • Gravel (Coarse Aggregate) • Cement (Binder) • Water • Air University of Massachusetts Amherst

  27. Fiber-Reinforced Composites Composite Laminate Polyester Polymer Matrix Epoxy Vinylester Glass • Functions of matrix: • Force transfer to fibers • Compressive strength • Chemical protection Fiber Materials Aramid (Kevlar) Carbon • Function of fibers: • Provide stiffness • Tensile strength University of Massachusetts Amherst

  28. Important Structural Properties University of Massachusetts Amherst

  29. Compressive Failure Tensile Failure Engineering Properties of Structural Elements • Strength • Ability to withstand a given stress without failure • Depends on type of material and type of force (tension or compression) University of Massachusetts Amherst

  30. Engineering Properties of Structural Elements • Stiffness (Rigidity) • Property related to deformation • Stiffer structural elements deform less under the same applied load • Stiffness depends on type of material (E), structural shape, and structural configuration • Two main types • Axial stiffness • Bending stiffness University of Massachusetts Amherst

  31. T DL Lo T Axial Stiffness Stiffness = T / DL Example: T = 100 lb DL = 0.12 in. Stiffness = 100 lb / 0.12 in. = 833 lb/in. University of Massachusetts Amherst

  32. Bending Stiffness Displacement Force Stiffness = Force / Displacement Example: Force = 1,000 lb Displacement = 0.5 in. Stiffness = 1,000 lb / 0.5 in. = 2,000 lb/in. University of Massachusetts Amherst

  33. Stiffest Stiffness of Different Structural Shapes Stiff Stiffer University of Massachusetts Amherst

  34. Types of Structural Elements – Bars and Cables Bars can carry either tension or compression Cables can only carry tension University of Massachusetts Amherst

  35. Loads Compression Tension Types of Structural Elements – Beams University of Massachusetts Amherst

  36. Racking Failure of Pinned Frame Infilled Frame Rigid Joints Braced Frame Providing Stability for Lateral Loads University of Massachusetts Amherst

  37. Concepts in Equilibrium University of Massachusetts Amherst

  38. Equilibrium of Forces (Statics) • Forces are a type of quantity called vectors • Defined by magnitude and direction • Statement of equilibrium • Net force at a point in a structure = zero (summation of forces = zero) • Net force at a point is determined using a force polygon to account for magnitude and direction University of Massachusetts Amherst

  39. Moment of Force = Force x Distance To neutralize rotation about point A, moments from the two forces has to be equal and opposite: 100 lb x 3 ft = 50 lb x 6 ft A 3 ft 6 ft Moment (Rotational) Equilibrium University of Massachusetts Amherst

  40. 8 ft 10 ft Side AC Side BC = = = = 1.667 1.333 100 lb 6 ft 6 ft Side AB Side AB A 10 ft 6 ft Force  BC Force  AC 1.333 1.667 = = Force  AB Force  AB C B Force  AC = 1.667 x 100 lb = 166.7 lb Force  BC = 1.333 x 100 lb = 133.3 lb 8 ft Force Calculation in Simple Structure 36.9 University of Massachusetts Amherst

  41. 166.7 lb 100 lb 36.9 133.3 lb 1 Square = 10 lb Graphic Statics University of Massachusetts Amherst

  42. Force Transfer from Beams to Supports Force, P 1/3 L 2/3 L 2/3 P 1/3 P Span, L University of Massachusetts Amherst

  43. Force Transfer Example - Bridge 8,000 lb 32,000 lb 15 ft 45 ft 30 ft 30 ft L = 60 ft 22,000 lb* 18,000 lb** *Front axle: 8,000 lb x 45/60 = 6,000 lb Rear axle: 32,000 lb x 30/60 = 16,000 lb **Front axle: 8,000 lb x 15/60 = 2,000 lb Rear axle: 32,000 lb x 30/60 = 16,000 lb University of Massachusetts Amherst

  44. www.teachersdomain.org University of Massachusetts Amherst