Bldg Construction Chapter 03


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Bldg Construction Chapter 03

  1. 1. Building ConstructionRelated to the Fire ServiceChapter 3 — The Way Buildings are Built: Structural Design Features
  2. 2. Learning Objective 1 Explain the various loads exerted on a building resulting from environmental sources. Building Construction 3–2
  3. 3. Load and Gravity• Load – Any effect or force that a structure must resist• Gravity – Force on building through weight of building components and contents – Snow, ice, or water on building adds weight – Total weight of building exerts force on soil beneath it Building Construction 3–3
  4. 4. Wind• Basic effects – Direct pressure – Aerodynamic drag – Negative pressure• Secondary effects – Rocking effect – Vibration – Clean-off effect Building Construction 3–4
  5. 5. Calculating DirectPressure of Wind p = Cv(2) v = velocity p = static pressure C = numerical constant (0.00256) that accounts for air mass and assumptions of building behavior Building Construction 3–5
  6. 6. Earthquakes• Slippage between tectonic plates of earth’s surface• Subjects buildings to seismic forces• Occur most frequently in fault zones• Seismic loads far more complex than those for wind• Vibrational motion can be 3- dimensional (Continued) Building Construction 3–6
  7. 7. Earthquakes• Torsional, resonant forces• Force magnitude depends on factors• Most significant force is horizontal motion• Buildings with geometric irregularities more susceptible to earthquake damage• Building codes require stronger seismic bracing for buildings with larger (Continued) occupancies Building Construction 3–7
  8. 8. Earthquakes (Continued) Building Construction 3–8
  9. 9. Earthquakes• Design methods to protect against forces – Increase stiffness using symmetrical shear walls and cross bracing – Continuous structures with high degree of redundancy in structural frames – Damping mechanisms – Base isolation Building Construction 3–9
  10. 10. Soil Pressure• Exerts horizontal pressure against foundation• Can only be estimated• Magnitude depends on several factors• Active or passive Building Construction 3–10
  11. 11. Determining Soil Pressure p = Cwh p = pressure h = depth of soil w = density of soil C = numerical constant that depends on the physical properties of the soil Building Construction 3–11
  12. 12. Other Forces• Temperature• Vibration• Shrinkage Building Construction 3–12
  13. 13. Learning Objective 2 Distinguish between the classifications of loads based on origin and movement. Building Construction 3–13
  14. 14. Live Loads and Dead Loads• Classification of types of forces resulting from gravity• Dead – Weight of any permanent part of a building• Live – Any load that is not fixed or permanent (Continued) Building Construction 3–14
  15. 15. Live Loads and Dead Loads• Distribution of loads – Uniformly distributed loads – Concentrated load• Snow loads• Water loads Building Construction 3–15
  16. 16. Static and Dynamic Loads• Static – Steady or applied gradually – Dead loads – Many live loads• Dynamic – Involves motion – Wind, earthquakes, vibration, falling objects – Capable of delivering energy in addition to weight Building Construction 3–16
  17. 17. Structural Equilibriumand Reactions• Equilibrium – When support provided by structural system is equal to the applied loads• Reactions – Forces that resist the applied loads Building Construction 3–17
  18. 18. Learning Objective 3 Recognize and discuss the internal forces resulting from the loads and forces applied to a structural member. Building Construction 3–18
  19. 19. Interior Forces Createdby Exterior Loads• Tension – Pull material apart• Compression – Squeeze material• Shear – Slide one plane past an adjacent plane• Strength of materials varies with direction of interior forces (Continued) Building Construction 3–19
  20. 20. Interior Forces Createdby Exterior Loads (Continued) Building Construction 3–20
  21. 21. Interior Forces Createdby Exterior Loads• Stress – Quantity used to evaluate magnitude of interior forces• Factors of safety• Stresses typically occur in combination within a member• Keeping stresses within allowable values determines shape and size of structural members Building Construction 3–21
  22. 22. Exterior Loads Classifiedby Manner Applied• Axial – Applied to center of cross section of structural member and perpendicular to that cross section• Eccentric – Perpendicular to the cross section of the structural member but does not pass through the center of the cross section (Continued) Building Construction 3–22
  23. 23. Exterior Loads Classifiedby Manner Applied• Torsional – Offset from center of cross section of the structural member and at an angle to or in the same plane as the cross section• Loads change under fire conditions due to thermal energy released (Continued) Building Construction 3–23
  24. 24. Exterior Loads Classifiedby Manner Applied Building Construction 3–24
  25. 25. Learning Objective 4 Describe the basic structural components. Building Construction 3–25
  26. 26. Beams• Simply supported• Cantilever• Continuous• Restrained Courtesy of Ed Prendergast• Primary design consideration – Ability to resist bending from applied loads (Continued) Building Construction 3–26
  27. 27. Beams Building Construction 3–27
  28. 28. Columns• Designed to support an axial compressive load• Thin columns fail by buckling• Short, squatty columns fail by crushing Building Construction 3–28
  29. 29. Arches• Interior stresses primarily compressive• Produce inclined forces at end supports• Carry loads across a distance Courtesy of Donny Howard Building Construction 3–29
  30. 30. Cables• Flexible members used to support roofs and brace tents and restrain pneumatic structures• When used to support loads over distance, will assume shape of parabola• Stresses are tension Building Construction 3–30
  31. 31. Trusses• Framed units made up of group of triangles in one plane• If loads applied at only point of intersection of truss members, only compressive or tensile stresses• If has curved top chord, subjected to (Continued) bending forces Building Construction 3–31
  32. 32. Trusses• Provide inherently rigid frame• Potential for early failure under adverse conditions; failure of any portion of top or bottom chords results in failure of truss (Continued) Building Construction 3–32
  33. 33. Trusses Building Construction 3–33
  34. 34. Space Frames• Truss structures developed in three dimension• Suited to support uniformly distributed loads Courtesy of Ed Prendergast Building Construction 3–34
  35. 35. Truss Connectors• Wood truss – Pins, bolts, gusset plates, adhesives, brackets, metal straps• Steel trusses – Steel gusset plates, rivets, welds• If connector fails, truss will fail Building Construction 3–35
  36. 36. Learning Objective 5 Describe the basic structural systems. Building Construction 3–36
  37. 37. Structural Bearing Walls• Span elements such as beams, trusses, and precast concrete slabs• Usually exterior walls with interior support system of columns and beams• Provide lateral support along direction of the wall• Subjected to compressive loads Building Construction 3–37
  38. 38. Frame Structural Systems• Walls enclose frame but provide no structural support• Steel stud wall framing• Post and beam construction Courtesy of Ed Prendergast• Rigid frames• Truss frames• Slab and column frames Building Construction 3–38
  39. 39. Shell and Membrane Systems• Consist primarily of enclosing surface with stresses resulting from applied loads occur within the surface Courtesy of Ed Prendergast Building Construction 3–39
  40. 40. Membrane Structure• Thin stretched flexible material• Addressed in building codes if life of 180 days or more• Addressed in fire codes if life of 180 days or less• Cannot resist compressive forces• Advantages Building Construction 3–40
  41. 41. Shell Structure• Rigid three-dimensional structures having a thickness that is small compared to other structural materials• Regular geometric shapes• Most commonly constructed of concrete Building Construction 3–41
  42. 42. Summary• Forces exerted on buildings arise from many sources and determine how a building’s structural system is designed.• A variety of structural components are available to support the applied loads.• Under fire conditions, loads and stresses are subject to change in magnitude and direction. Building Construction 3–42
  43. 43. Review Questions 1. What is a load? 2. What factors determine the magnitude of the forces developed within a building during an earthquake? 3. What is a dead load? (Continued) Building Construction 3–43
  44. 44. Review Questions 4. How do dynamic loads differ from static loads? 5. What is a membrane structure? Building Construction 3–44