Ch 06 Wood Frame Construction


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  • Courtesy of APA- The Engineered Wood Association
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  • Photo courtesy of Blaze-tech Fire Protection
  • Courtesy of the estate of Francis L. Brannigan
  • Ch 06 Wood Frame Construction

    1. 1. 6 Wood Frame Construction
    2. 2. Objectives (1 of 3) <ul><li>Identify and describe six types of wood frame structures classified as Type V construction </li></ul><ul><li>Identify and describe the specific fire protection differences between balloon frame and platform frame </li></ul>6
    3. 3. Objectives (2 of 3) <ul><li>Describe the construction of a platform-frame building </li></ul><ul><li>Understand how a truss is constructed and how it performs from a fire protection perspective </li></ul><ul><li>Explain the difference between firestopping and draftstopping </li></ul>6
    4. 4. Objectives (3 of 3) <ul><li>Describe the behavior of engineered and manufactured wood products under fire attack </li></ul><ul><li>Describe the different types of wood siding and roofing materials </li></ul>6
    5. 5. Introduction <ul><li>Most fires are fought on, in, or under wood. </li></ul><ul><li>Basic facts about wood </li></ul><ul><ul><li>It is combustible </li></ul></ul><ul><ul><li>It creates combustible void spaces in which fire can hide and burst out </li></ul></ul><ul><ul><li>It can, in thin sections, have a very rapid flame spread </li></ul></ul>6
    6. 6. Definitions <ul><li>Building terms are local in origin; different trades use different terms </li></ul><ul><ul><li>Wood-framed </li></ul></ul><ul><ul><li>Wall-bearing </li></ul></ul><ul><ul><li>Curtain walls </li></ul></ul><ul><ul><li>Wooden-walled building </li></ul></ul>6
    7. 7. Wood Frame Terminology (1 of 2) <ul><li>Chamfer </li></ul><ul><li>End matched </li></ul><ul><li>Engineered wood </li></ul><ul><li>Glued laminated timbers </li></ul><ul><li>Heavy timber </li></ul><ul><li>Joists </li></ul>6
    8. 8. Wood Frame Terminology (2 of 2) <ul><li>Lumber </li></ul><ul><li>Matched lumber </li></ul><ul><li>Oriented strand board (OSB) </li></ul><ul><li>Plywood </li></ul><ul><li>Rough lumber </li></ul><ul><li>Splines and studs </li></ul><ul><li>Wood and wood lath </li></ul>6
    9. 9. Uses of Wood in Buildings <ul><li>Structural </li></ul><ul><li>Non-load-bearing </li></ul><ul><li>Roofing </li></ul><ul><li>Interior trim </li></ul>6
    10. 10. Types of Wood-Frame Buildings <ul><li>Log cabin </li></ul><ul><li>Post and frame </li></ul><ul><li>Balloon frame </li></ul><ul><li>Platform frame </li></ul><ul><li>Plank and beam </li></ul><ul><li>Truss frame </li></ul>6
    11. 11. Log Cabin <ul><li>Constructed of entire tree trunks </li></ul><ul><li>Many older buildings, even multistory buildings, are concealed log cabins </li></ul><ul><li>Many carry unexpectedly heavy loads </li></ul><ul><li>Interior surfaces of log cabins might be boards or plywood </li></ul>6
    12. 12. Post and Frame <ul><li>Has an identifiable frame or skeleton of timber fitted together </li></ul><ul><li>Mortise and tenon </li></ul><ul><li>Trunnels </li></ul><ul><li>The walls are not structural or load-bearing. </li></ul>6
    13. 13. Balloon Frame <ul><li>Wall of ordinary studs nailed together </li></ul><ul><li>Almost universal construction method for multi-story wooden buildings until the middle of the 20 th century </li></ul>6
    14. 14. Fires in Balloon-Frame Buildings (1 of 2) <ul><li>Fire can spread through all the interconnected spaces </li></ul><ul><li>Interconnected voids are one big balloon </li></ul><ul><li>Installing firestopping in an old building is costly </li></ul>6
    15. 15. Fires in Balloon-Frame Buildings (2 of 2) <ul><li>Basement is usually the worst place for a fire to start </li></ul><ul><li>Fire fighters need to anticipate the spread. </li></ul><ul><li>Extension sector </li></ul><ul><li>Vermiculite </li></ul>6
    16. 16. Platform Frame (1 of 2) <ul><li>The first floor is built as a platform </li></ul><ul><li>Subflooring is laid on the joists </li></ul><ul><li>Frame for the first-floor walls is erected on the first floor </li></ul><ul><li>No continuity from top to bottom </li></ul>6
    17. 17. Platform Frame (2 of 2) <ul><li>Inherent barriers to limit the spread of fire through the walls </li></ul><ul><li>Open stairwell </li></ul><ul><li>Soffits </li></ul><ul><li>Kitchen fire can extend through soffits </li></ul>6
    18. 18. Constructing a Platform-Frame Building (1 of 2) <ul><li>Rough carpentry </li></ul><ul><li>Sill  </li></ul><ul><li>Subflooring  </li></ul><ul><li>Header or bond course  </li></ul><ul><li>Trimmer </li></ul>6
    19. 19. Constructing a Platform-Frame Building (2 of 2) <ul><li>Bottom plate  </li></ul><ul><li>Top plate  </li></ul><ul><li>Rafters  </li></ul><ul><li>Ridge boards </li></ul><ul><li>Hip  </li></ul><ul><li>Valley  </li></ul>6
    20. 20. Plank and Beam (1 of 2) <ul><li>Built with heavier beams </li></ul><ul><li>Thick, finished tongue and groove planks are used for flooring </li></ul><ul><li>Reduces the volume of concealed space </li></ul><ul><li>Interior finishes have high flame spread and smoke-developing characteristics </li></ul>6
    21. 21. Plank and Beam (2 of 2) <ul><li>Fire in high open spaces can spread rapidly and develop into a huge volume </li></ul><ul><li>Intermediate structures should be sprinklered or be of noncombustible construction </li></ul>6
    22. 22. Lightweight Trusses and Other Wooden Members (1 of 2) <ul><li>Fire service has gradually developed knowledge of trusses </li></ul><ul><li>Contribute to lighter-weight structures </li></ul><ul><li>Allow offsite prefabrication </li></ul><ul><li>Satisfy many building requirements </li></ul>6
    23. 23. Lightweight Trusses and Other Wooden Members (2 of 2) <ul><li>Lighter in weight than solid construction </li></ul><ul><li>Provides long clear spans </li></ul><ul><li>Can be delivered prefabricated </li></ul>6
    24. 24. What Is a Truss? <ul><li>A framed structure consisting of a triangle or group of triangles arranged in a single plane </li></ul><ul><li>Loads applied at intersections of the members will cause direct stresses in the members </li></ul><ul><li>Loads applied between these points cause flexural stresses </li></ul>6
    25. 25. Rigidity (1 of 2) <ul><li>Geometric principle </li></ul><ul><li>Triangle is inherently stable </li></ul><ul><li>Economy </li></ul><ul><li>Chords </li></ul>6
    26. 26. Rigidity (2 of 2) <ul><li>Struts </li></ul><ul><li>Ties </li></ul><ul><li>Panel points </li></ul><ul><li>Web </li></ul>6
    27. 27. Principle Types (1 of 2) <ul><li>Parallel-Chord Truss </li></ul><ul><ul><li>The bottom and top chords are parallel. </li></ul></ul><ul><ul><li>Steel bar joist </li></ul></ul><ul><ul><li>Long-span parallel-chord roof trusses have a slight upward pitch to the center. </li></ul></ul><ul><ul><li>Are being used for floors and roofs </li></ul></ul>6
    28. 28. Principle Types (2 of 2) <ul><li>Triangular Trusses </li></ul><ul><ul><li>Triangular in shape in order to provide a peaked roof </li></ul></ul><ul><ul><li>Must be closely spaced </li></ul></ul>6
    29. 29. Connectors <ul><li>A problem in truss design </li></ul><ul><li>Connectors now capable of transmitting heavy loads </li></ul>6
    30. 30. Compression vs. Tension (1 of 2) <ul><li>Normal truss: </li></ul><ul><ul><li>Top chord in compression and bottom chord in tension </li></ul></ul>6
    31. 31. Compression vs. Tension (2 of 2) <ul><li>Cantilever is reversed: </li></ul><ul><ul><li>Top chord in tension and bottom chord in compression </li></ul></ul><ul><li>In sketches, compression members are often shown as thick lines, while tension members are shown as thin lines </li></ul>6
    32. 32. Truss Principles, Case #1 <ul><li>A building span of 20 feet </li></ul><ul><li>Two 10-foot ­I-beams extend from the walls. </li></ul><ul><li>Assume each beam can carry 1000 pounds. </li></ul><ul><li>If column is removed, beam would have to be 20 feet long but carry only 500 pounds </li></ul>6
    33. 33. Truss Principles, Case #2 (1 of 2) <ul><li>The column is cut off and a stub remains at the junction of the beams </li></ul><ul><li>A triangle restores the load­carrying capacity of the beam </li></ul>6
    34. 34. Truss Principles, Case #2 (2 of 2) <ul><li>A truss with single compression member extending downwards is called an inverted king post truss </li></ul><ul><li>A truss with two compression members is called a queen post truss </li></ul>6
    35. 35. Problems with Trusses <ul><li>Failure </li></ul><ul><li>Multiple truss failures </li></ul><ul><li>Rising roofs </li></ul><ul><li>Tables of allowable design stresses </li></ul><ul><li>Defective design </li></ul><ul><li>Competition in the supply business </li></ul>6
    36. 36. Truss Failure in Fires <ul><li>Economy of the truss </li></ul><ul><li>Bottom chord: under compression </li></ul><ul><li>Top chord: in tension </li></ul><ul><li>Compressive load </li></ul><ul><li>Ties </li></ul><ul><li>Large triangular trusses </li></ul>6
    37. 37. Wood Truss Floors <ul><li>Are a hazard to fire fighters </li></ul><ul><li>Heating of the gusset plate will decompose tensioned wood fibers </li></ul><ul><li>No outward indication of their presence </li></ul><ul><li>May be disclosed by smoke or fire pushing through the wall at the floor line </li></ul>6
    38. 38. Truss Void (1 of 2) <ul><li>Found within a truss roof system, this void space is between the top floor ceiling and the roof </li></ul><ul><li>Represents a large area in which explosive carbon monoxide can accumulate </li></ul><ul><li>Voids are interconnected horizontally and vertically </li></ul>6
    39. 39. Truss Void (2 of 2) <ul><li>The National Fire Protection Association (NFPA) Fire Protection Handbook </li></ul><ul><li>Concentration (in parts per million [ppm]) x duration (minutes) = 35,000 is likely to be dangerous </li></ul><ul><li>The flammability range of carbon monoxide is from 12.5 to 74 percent </li></ul>6
    40. 40. Catastrophe Potential (1 of 2) <ul><li>Lateral extension of truss </li></ul><ul><ul><li>May support a balcony that is the only exit for occupants </li></ul></ul>6
    41. 41. Catastrophe Potential (2 of 2) <ul><li>Truss passes through outside wall </li></ul><ul><ul><li>Only firestopping is typically gypsum “buttered” in place with cement </li></ul></ul><ul><ul><li>Firestopping is penetrated by lighting circuits </li></ul></ul><ul><li>Stairway exits with truss voids </li></ul><ul><ul><li>If involved in fire, can collapse before the occupants have escaped </li></ul></ul>6
    42. 42. Automatic Sprinklers and Truss Voids <ul><li>NFPA 13R </li></ul><ul><ul><li>Purpose to prevent flashover and save lives </li></ul></ul><ul><ul><li>Code describes partial systems that should not be expected to provide full fire suppression </li></ul></ul>6
    43. 43. Lightweight Wooden I-Beams <ul><li>Wooden beam </li></ul><ul><li>Sawn beam </li></ul><ul><li>Steel I-beam </li></ul><ul><li>Sawn wooden beam </li></ul><ul><li>Wooden I-beams </li></ul><ul><li>Web of the I-beam </li></ul>6
    44. 44. Truss Frame (1 of 2) <ul><li>The roof and floor trusses and studs are tied into a unitized frame </li></ul><ul><li>The small dimension lumber will burn faster than larger solid lumber </li></ul><ul><li>Unsupported spans in trussed structures are subject to total collapse </li></ul>6
    45. 45. Truss Frame (2 of 2) <ul><li>Loss of a stud could precipitate the collapse of the integral roof or floor truss </li></ul><ul><li>Design assumes that every truss member will remain in its assigned position under load </li></ul>6
    46. 46. Firestopping <ul><li>Fire spreads through hidden voids </li></ul><ul><li>No tested standards for wood firestopping or draftstopping </li></ul>6
    47. 47. Differences: Firestopping and Draftstopping <ul><li>Both limit the spread of fire </li></ul><ul><li>Firestops typically limit vertical movement </li></ul><ul><li>Draftstops limit horizontal movement </li></ul>6
    48. 48. Types of Firestopping <ul><li>Inherent firestopping results from normal building construction </li></ul><ul><li>Legal firestopping meets the requirements of a code </li></ul>6
    49. 49. Effectiveness of Firestopping (1 of 2) <ul><li>The lack of firestopping in one stud channel is sufficient to transmit fire </li></ul><ul><li>In older houses, sides of the chimney-like stud channels are combustible </li></ul><ul><li>Lack of firestopping is particularly critical in balloon-frame buildings </li></ul>6
    50. 50. Effectiveness of Firestopping (2 of 2) <ul><li>Not all building inspectors are familiar with the basic gas law: </li></ul><ul><ul><li>If the temperature rises and the volume remains the same, the pressure rises </li></ul></ul>6
    51. 51. Cutoff Ends of Joists <ul><li>Used as firestopping </li></ul><ul><li>No seal because the wood “cut out” creates a space </li></ul>6
    52. 52. West and East Coasts <ul><li>Uniformly unsatisfactory practices </li></ul><ul><li>Sheet of gypsum board “buttered” into place </li></ul><ul><li>Thin plywood or flake board poorly fitted </li></ul>6
    53. 53. Removal of Firestops <ul><li>Removed for the installation of such items as heat ducts, electrical cables, sprinkler systems, and central vacuum cleaner systems </li></ul><ul><li>Unlikely to be replaced </li></ul>6
    54. 54. Draftstopping and Truss Floors <ul><li>Truss voids in each floor </li></ul><ul><li>Truss proponents argue that firestopping will mitigate the problem </li></ul><ul><li>Severe backdraft explosion can provide a significant collapse </li></ul>6
    55. 55. Protecting Wood from Ignition <ul><li>Sobering News </li></ul><ul><ul><li>Most fires are fought by fire fighters standing on, in, or under a combustible structure </li></ul></ul><ul><ul><li>Encasing wood in concrete promotes decay </li></ul></ul>6
    56. 56. Impregnation <ul><li>Wood can be impregnated in a variety of way: pressure- or chemically-treated. </li></ul><ul><li>Wood cannot be made fireproof or noncombustible </li></ul><ul><li>It can be made fire retardant </li></ul><ul><li>Impregnated wood is not noncombustible wood </li></ul>6
    57. 57. Pressure Treatment <ul><li>Can reduce wood’s flame spread </li></ul><ul><li>Pressure treatment can reduce the hazard of wood construction </li></ul><ul><li>Treated wood will burn, although at a slower rate </li></ul>6
    58. 58. Underwriters Laboratories, Inc. (UL) <ul><li>Provides various classifications of treated lumber </li></ul><ul><li>Consult “Lumber, Treated (BPW)” </li></ul>6
    59. 59. Chemicals <ul><li>Previously used ones impregnated the wood but leached out and corroded metal connectors </li></ul><ul><li>New products cite noncorrosive materials </li></ul>6
    60. 60. Bad Practices <ul><li>Fire retardant plywood (FRT), one sheet wide, on both sides of a firewall </li></ul><ul><li>Plywood delaminates with fire exposure </li></ul><ul><li>Plywood treated with certain chemicals decays from heat </li></ul>6
    61. 61. Surface Coatings <ul><li>Intumescent coatings swell up when heated </li></ul><ul><li>People spread it thinner than recommended </li></ul><ul><li>Applying to existing installations leaves the unexposed surface untreated </li></ul>6
    62. 62. Dangerous Treated Wood <ul><li>Pressure treated plywood and lumber treated has been used for basement walls </li></ul><ul><li>Widely used for exterior structures </li></ul><ul><li>Still combustible and might cause a collapse </li></ul><ul><li>Fumes are toxic </li></ul>6
    63. 63. Plywood <ul><li>Plywood was seen as answer to wood’s lack of shear strength </li></ul><ul><li>Plywood exposed to fire delaminates </li></ul><ul><li>Plywood can be impregnated to render it fire retardant </li></ul>6
    64. 64. Spliced Timbers <ul><li>Are joined by metal connectors to transfer loads </li></ul><ul><li>Acts as a single member </li></ul><ul><li>Heated metal connections can destroy the wood and the timber may fail </li></ul>6
    65. 65. Laminated Timbers <ul><li>Plank-like sections of nominal two-inch boards are glued under pressure </li></ul><ul><li>Burn like solid heavy timbers and do not delaminate like plywood </li></ul><ul><li>Laminated wooden beams are not the same thing as wooden I-beams </li></ul>6
    66. 66. Paper Wrapping <ul><li>Laminated timbers are shipped in a protective paper wrapper </li></ul><ul><li>Paper is hemp-reinforced and coated with a bituminous moisture repellant </li></ul><ul><li>Paper ignites readily and has a high flame spread </li></ul>6
    67. 67. Planks <ul><li>Fabricated by gluing three boards together with the center board protruding on one side and indented on the other </li></ul><ul><li>A sample ignited burned like a solid piece of wood </li></ul>6
    68. 68. Finger Joints <ul><li>Made by cutting a series of long points into the end of each piece </li></ul><ul><li>The joints are glued together </li></ul>6
    69. 69. Chipboard <ul><li>Wood chips are often glued together to make flat sheets </li></ul><ul><li>Sometimes used for the floors of mobile homes </li></ul><ul><li>Some is water soluble </li></ul>6
    70. 70. Flitch Plate Girders <ul><li>A composite of a steel plate or plywood sandwiched between two beams </li></ul><ul><li>Failure of the connection between the wood and steel could cause failure </li></ul>6
    71. 71. Wood and Plastic Roof Panels <ul><li>Large plywood roof panels with a gypsum board interior surface sandwiched a thick plastic foam core to create panels </li></ul><ul><li>Panel used as a roof support </li></ul><ul><li>Dangerous to vent such a roof </li></ul>6
    72. 72. Sheathing <ul><li>Covering applied to the studs or framing of a structure </li></ul><ul><li>Exterior surface covers the sheathing. </li></ul><ul><li>Many old houses were built without sheathing </li></ul>6
    73. 73. Low-Density Black Fiberboard <ul><li>Moisture and vermin proof </li></ul><ul><li>Carries the warning, “Combustible. May burn or smolder if ignited.” </li></ul><ul><li>Plumber’s torch </li></ul>6
    74. 74. Plywood and Gypsum <ul><li>Both are used for sheathing </li></ul><ul><li>Combined with brick veneer surfacing and gypsum interior surface, gypsum is used to provide rated fire-resistive exterior walls </li></ul>6
    75. 75. Foamed Plastic <ul><li>Also used for sheathing </li></ul><ul><li>May or may not be flame-inhibited </li></ul><ul><li>In a fire, may degrade and give off noxious fumes </li></ul>6
    76. 76. Siding (1 of 2) <ul><li>Novelty siding  </li></ul><ul><li>Batten  </li></ul><ul><li>Plywood siding  </li></ul><ul><li>Shingle and shake  </li></ul><ul><li>Asbestos cement shingle  </li></ul>6
    77. 77. Siding (2 of 2) <ul><li>Asphalt felt siding </li></ul><ul><li>Vinyl siding  </li></ul><ul><li>Metal siding  </li></ul><ul><li>Corrugated metal siding  </li></ul><ul><li>Stucco  </li></ul>6
    78. 78. Brick Veneer <ul><li>Is laid up from the foundation in one wythe </li></ul><ul><li>Such a wall is unstable because it is thin </li></ul><ul><li>Galvanized steel anchors are nailed to the studs </li></ul><ul><li>Pyrolytic decomposition can be an issue </li></ul>6
    79. 79. Stone Veneer <ul><li>Natural or artificial stone and cast concrete </li></ul><ul><li>Permastone is one trade name </li></ul>6
    80. 80. Wood Shingle Roofing (1 of 2) <ul><li>Fire hazard </li></ul><ul><ul><li>Some of the greatest fire disasters in history have been due to the spread of fire by wood shingle roofs </li></ul></ul><ul><ul><li>The conflagration hazard presented by wood shingles is a serious consideration </li></ul></ul>6
    81. 81. Wood Shingle Roofing (2 of 2) <ul><li>A Strong Comeback </li></ul><ul><ul><li>Many areas have wood-shingled roofs. </li></ul></ul><ul><ul><li>Permitted wherever frame buildings are permitted </li></ul></ul>6
    82. 82. Testing Laboratories <ul><li>Rate wood shingles in accordance to NFPA 256 </li></ul><ul><li>Elements considered include flame exposure, spread of flame, and resistance to burning </li></ul><ul><li>Roofing materials are classified as A, B, or C </li></ul>6
    83. 83. Tile Roofs <ul><li>Heavy tile roof </li></ul><ul><ul><li>Collapse of thousands of pounds of tile </li></ul></ul><ul><ul><li>Truss-supported tile failure </li></ul></ul>6
    84. 84. Imitation Timber <ul><li>Watch for the following: </li></ul><ul><ul><li>Unprotected steel beams or columns boxed in wood </li></ul></ul><ul><ul><li>Unprotected steel encased in plaster </li></ul></ul><ul><ul><li>False wood beams </li></ul></ul><ul><ul><li>Polyurethane imitation wood beams </li></ul></ul>6
    85. 85. Making Wood Construction Safe <ul><li>Wood is a uniquely renewable resource </li></ul><ul><li>Unfortunately it is combustible </li></ul><ul><li>Combustibility can be dealt with only by complete automatic sprinkler protection </li></ul>6
    86. 86. Summary (1 of 2) <ul><li>Type V construction has six types of wood frame structures </li></ul><ul><li>In a balloon-frame building, fire can spread through all the interconnected spaces from cellar to attic and across the ceiling </li></ul><ul><li>Firestopping is often required by code to be installed to prevent the spread of fire </li></ul>6
    87. 87. Summary (2 of 2) <ul><li>Trusses and specially treated or constructed wood materials create unique challenges for fire protection </li></ul><ul><li>Wood cannot be made fireproof or noncombustible </li></ul><ul><li>Combustibility can be dealt with only by complete automatic sprinkler protection </li></ul>6
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