Ch 06 Wood Frame Construction
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Transcript of Ch 06 Wood Frame Construction
![Page 1: Ch 06 Wood Frame Construction](https://reader035.fdocuments.us/reader035/viewer/2022081420/555b19dbd8b42aa67a8b49d2/html5/thumbnails/1.jpg)
6 Wood Frame
Construction
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Objectives (1 of 3)
• Identify and describe six types of wood frame structures classified as Type V construction
• Identify and describe the specific fire protection differences between balloon frame and platform frame
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Objectives (2 of 3)
• Describe the construction of a platform-frame building
• Understand how a truss is constructed and how it performs from a fire protection perspective
• Explain the difference between firestopping and draftstopping
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Objectives (3 of 3)
• Describe the behavior of engineered and manufactured wood products under fire attack
• Describe the different types of wood siding and roofing materials
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Introduction• Most fires are fought on, in, or under
wood.
• Basic facts about wood
• It is combustible
• It creates combustible void spaces in which fire can hide and burst out
• It can, in thin sections, have a very rapid flame spread
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Definitions
• Building terms are local in origin; different trades use different terms
• Wood-framed
• Wall-bearing
• Curtain walls
• Wooden-walled building
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Wood Frame Terminology (1 of 2)
• Chamfer
• End matched
• Engineered wood
• Glued laminated timbers
• Heavy timber
• Joists
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Wood Frame Terminology (2 of 2)
• Lumber
• Matched lumber
• Oriented strand board (OSB)
• Plywood
• Rough lumber
• Splines and studs
• Wood and wood lath
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Uses of Wood in Buildings
• Structural
• Non-load-bearing
• Roofing
• Interior trim
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Types of Wood-Frame Buildings
• Log cabin
• Post and frame
• Balloon frame
• Platform frame
• Plank and beam
• Truss frame
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Log Cabin
• Constructed of entire tree trunks
• Many older buildings, even multistory buildings, are concealed log cabins
• Many carry unexpectedly heavy loads
• Interior surfaces of log cabins might be boards or plywood
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Post and Frame
• Has an identifiable frame or skeleton of timber fitted together
• Mortise and tenon
• Trunnels
• The walls are not structural or load-bearing.
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Balloon Frame
• Wall of ordinary studs nailed together
• Almost universal construction method for multi-story wooden buildings until the middle of the 20th century
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Fires in Balloon-Frame Buildings (1 of 2)
• Fire can spread through all the interconnected spaces
• Interconnected voids are one big balloon
• Installing firestopping in an old building is costly
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Fires in Balloon-Frame Buildings (2 of 2)
• Basement is usually the worst place for a fire to start
• Fire fighters need to anticipate the spread.
• Extension sector
• Vermiculite
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Platform Frame (1 of 2)
• The first floor is built as a platform
• Subflooring is laid on the joists
• Frame for the first-floor walls is erected on the first floor
• No continuity from top to bottom
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Platform Frame (2 of 2)
• Inherent barriers to limit the spread of fire through the walls
• Open stairwell
• Soffits
• Kitchen fire can extend through soffits
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Constructing a Platform-Frame Building (1 of 2)
• Rough carpentry
• Sill
• Subflooring
• Header or bond course
• Trimmer
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Constructing a Platform-Frame Building (2 of 2)
• Bottom plate
• Top plate
• Rafters
• Ridge boards
• Hip
• Valley
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Plank and Beam (1 of 2)
• Built with heavier beams
• Thick, finished tongue and groove planks are used for flooring
• Reduces the volume of concealed space
• Interior finishes have high flame spread and smoke-developing characteristics
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Plank and Beam (2 of 2)
• Fire in high open spaces can spread rapidly and develop into a huge volume
• Intermediate structures should be sprinklered or be of noncombustible construction
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Lightweight Trusses and Other Wooden Members (1 of 2)
• Fire service has gradually developed knowledge of trusses
• Contribute to lighter-weight structures
• Allow offsite prefabrication
• Satisfy many building requirements
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Lightweight Trusses and Other Wooden Members (2 of 2)
• Lighter in weight than solid construction
• Provides long clear spans
• Can be delivered prefabricated
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What Is a Truss?• A framed structure consisting of a triangle
or group of triangles arranged in a single plane
• Loads applied at intersections of the members will cause direct stresses in the members
• Loads applied between these points cause flexural stresses
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Rigidity (1 of 2)
• Geometric principle
• Triangle is inherently stable
• Economy
• Chords
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Rigidity (2 of 2)
• Struts
• Ties
• Panel points
• Web
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Principle Types (1 of 2)
• Parallel-Chord Truss
• The bottom and top chords are parallel.
• Steel bar joist
• Long-span parallel-chord roof trusses have a slight upward pitch to the center.
• Are being used for floors and roofs
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Principle Types (2 of 2)
• Triangular Trusses
• Triangular in shape in order to provide a peaked roof
• Must be closely spaced
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Connectors
• A problem in truss design
• Connectors now capable of transmitting heavy loads
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Compression vs. Tension (1 of 2)
• Normal truss:
• Top chord in compression and bottom chord in tension
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Compression vs. Tension (2 of 2)
• Cantilever is reversed:
• Top chord in tension and bottom chord in compression
• In sketches, compression members are often shown as thick lines, while tension members are shown as thin lines
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Truss Principles, Case #1
• A building span of 20 feet
• Two 10-foot I-beams extend from the walls.
• Assume each beam can carry 1000 pounds.
• If column is removed, beam would have to be 20 feet long but carry only 500 pounds
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Truss Principles, Case #2 (1 of 2)
• The column is cut off and a stub remains at the junction of the beams
• A triangle restores the load carrying capacity of the beam
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Truss Principles, Case #2 (2 of 2)
• A truss with single compression member extending downwards is called an inverted king post truss
• A truss with two compression members is called a queen post truss
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Problems with Trusses
• Failure
• Multiple truss failures
• Rising roofs
• Tables of allowable design stresses
• Defective design
• Competition in the supply business
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Truss Failure in Fires
• Economy of the truss
• Bottom chord: under compression
• Top chord: in tension
• Compressive load
• Ties
• Large triangular trusses
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Wood Truss Floors
• Are a hazard to fire fighters
• Heating of the gusset plate will decompose tensioned wood fibers
• No outward indication of their presence
• May be disclosed by smoke or fire pushing through the wall at the floor line
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Truss Void (1 of 2)
• Found within a truss roof system, this void space is between the top floor ceiling and the roof
• Represents a large area in which explosive carbon monoxide can accumulate
• Voids are interconnected horizontally and vertically
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Truss Void (2 of 2)
• The National Fire Protection Association (NFPA) Fire Protection Handbook
• Concentration (in parts per million [ppm]) x duration (minutes) = 35,000 is likely to be dangerous
• The flammability range of carbon monoxide is from 12.5 to 74 percent
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Catastrophe Potential (1 of 2)
• Lateral extension of truss
• May support a balcony that is the only exit for occupants
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Catastrophe Potential (2 of 2)
• Truss passes through outside wall
• Only firestopping is typically gypsum “buttered” in place with cement
• Firestopping is penetrated by lighting circuits
• Stairway exits with truss voids
• If involved in fire, can collapse before the occupants have escaped
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Automatic Sprinklers and Truss Voids
• NFPA 13R
• Purpose to prevent flashover and save lives
• Code describes partial systems that should not be expected to provide full fire suppression
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Lightweight Wooden I-Beams
• Wooden beam
• Sawn beam
• Steel I-beam
• Sawn wooden beam
• Wooden I-beams
• Web of the I-beam
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Truss Frame (1 of 2)
• The roof and floor trusses and studs are tied into a unitized frame
• The small dimension lumber will burn faster than larger solid lumber
• Unsupported spans in trussed structures are subject to total collapse
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Truss Frame (2 of 2)
• Loss of a stud could precipitate the collapse of the integral roof or floor truss
• Design assumes that every truss member will remain in its assigned position under load
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Firestopping
• Fire spreads through hidden voids
• No tested standards for wood firestopping or draftstopping
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Differences: Firestopping and Draftstopping
• Both limit the spread of fire
• Firestops typically limit vertical movement
• Draftstops limit horizontal movement
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Types of Firestopping
• Inherent firestopping results from normal building construction
• Legal firestopping meets the requirements of a code
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Effectiveness of Firestopping (1 of 2)
• The lack of firestopping in one stud channel is sufficient to transmit fire
• In older houses, sides of the chimney-like stud channels are combustible
• Lack of firestopping is particularly critical in balloon-frame buildings
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Effectiveness of Firestopping (2 of 2)
• Not all building inspectors are familiar with the basic gas law:
• If the temperature rises and the volume remains the same, the pressure rises
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Cutoff Ends of Joists
• Used as firestopping
• No seal because the wood “cut out” creates a space
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West and East Coasts
• Uniformly unsatisfactory practices
• Sheet of gypsum board “buttered” into place
• Thin plywood or flake board poorly fitted
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Removal of Firestops
• Removed for the installation of such items as heat ducts, electrical cables, sprinkler systems, and central vacuum cleaner systems
• Unlikely to be replaced
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Draftstopping and Truss Floors
• Truss voids in each floor
• Truss proponents argue that firestopping will mitigate the problem
• Severe backdraft explosion can provide a significant collapse
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Protecting Wood from Ignition
• Sobering News
• Most fires are fought by fire fighters standing on, in, or under a combustible structure
• Encasing wood in concrete promotes decay
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Impregnation
• Wood can be impregnated in a variety of way: pressure- or chemically-treated.
• Wood cannot be made fireproof or noncombustible
• It can be made fire retardant
• Impregnated wood is not noncombustible wood
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Pressure Treatment
• Can reduce wood’s flame spread
• Pressure treatment can reduce the hazard of wood construction
• Treated wood will burn, although at a slower rate
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Underwriters Laboratories, Inc. (UL)
• Provides various classifications of treated lumber
• Consult “Lumber, Treated (BPW)”
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Chemicals
• Previously used ones impregnated the wood but leached out and corroded metal connectors
• New products cite noncorrosive materials
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Bad Practices
• Fire retardant plywood (FRT), one sheet wide, on both sides of a firewall
• Plywood delaminates with fire exposure
• Plywood treated with certain chemicals decays from heat
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Surface Coatings
• Intumescent coatings swell up when heated
• People spread it thinner than recommended
• Applying to existing installations leaves the unexposed surface untreated
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Dangerous Treated Wood
• Pressure treated plywood and lumber treated has been used for basement walls
• Widely used for exterior structures
• Still combustible and might cause a collapse
• Fumes are toxic
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Plywood
• Plywood was seen as answer to wood’s lack of shear strength
• Plywood exposed to fire delaminates
• Plywood can be impregnated to render it fire retardant
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Spliced Timbers
• Are joined by metal connectors to transfer loads
• Acts as a single member
• Heated metal connections can destroy the wood and the timber may fail
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Laminated Timbers
• Plank-like sections of nominal two-inch boards are glued under pressure
• Burn like solid heavy timbers and do not delaminate like plywood
• Laminated wooden beams are not the same thing as wooden I-beams
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Paper Wrapping
• Laminated timbers are shipped in a protective paper wrapper
• Paper is hemp-reinforced and coated with a bituminous moisture repellant
• Paper ignites readily and has a high flame spread
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Planks
• Fabricated by gluing three boards together with the center board protruding on one side and indented on the other
• A sample ignited burned like a solid piece of wood
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Finger Joints
• Made by cutting a series of long points into the end of each piece
• The joints are glued together
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Chipboard
• Wood chips are often glued together to make flat sheets
• Sometimes used for the floors of mobile homes
• Some is water soluble
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Flitch Plate Girders
• A composite of a steel plate or plywood sandwiched between two beams
• Failure of the connection between the wood and steel could cause failure
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Wood and Plastic Roof Panels
• Large plywood roof panels with a gypsum board interior surface sandwiched a thick plastic foam core to create panels
• Panel used as a roof support
• Dangerous to vent such a roof
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Sheathing
• Covering applied to the studs or framing of a structure
• Exterior surface covers the sheathing.
• Many old houses were built without sheathing
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Low-Density Black Fiberboard
• Moisture and vermin proof
• Carries the warning, “Combustible. May burn or smolder if ignited.”
• Plumber’s torch
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Plywood and Gypsum
• Both are used for sheathing
• Combined with brick veneer surfacing and gypsum interior surface, gypsum is used to provide rated fire-resistive exterior walls
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Foamed Plastic
• Also used for sheathing
• May or may not be flame-inhibited
• In a fire, may degrade and give off noxious fumes
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Siding (1 of 2)
• Novelty siding
• Batten
• Plywood siding
• Shingle and shake
• Asbestos cement shingle
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Siding (2 of 2)
• Asphalt felt siding
• Vinyl siding
• Metal siding
• Corrugated metal siding
• Stucco
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Brick Veneer
• Is laid up from the foundation in one wythe
• Such a wall is unstable because it is thin
• Galvanized steel anchors are nailed to the studs
• Pyrolytic decomposition can be an issue
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Stone Veneer
• Natural or artificial stone and cast concrete
• Permastone is one trade name
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Wood Shingle Roofing (1 of 2)
• Fire hazard
• Some of the greatest fire disasters in history have been due to the spread of fire by wood shingle roofs
• The conflagration hazard presented by wood shingles is a serious consideration
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Wood Shingle Roofing (2 of 2)
• A Strong Comeback
• Many areas have wood-shingled roofs.
• Permitted wherever frame buildings are permitted
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Testing Laboratories
• Rate wood shingles in accordance to NFPA 256
• Elements considered include flame exposure, spread of flame, and resistance to burning
• Roofing materials are classified as A, B, or C
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Tile Roofs
• Heavy tile roof
• Collapse of thousands of pounds of tile
• Truss-supported tile failure
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Imitation Timber
• Watch for the following:
• Unprotected steel beams or columns boxed in wood
• Unprotected steel encased in plaster
• False wood beams
• Polyurethane imitation wood beams
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Making Wood Construction Safe
• Wood is a uniquely renewable resource
• Unfortunately it is combustible
• Combustibility can be dealt with only by complete automatic sprinkler protection
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Summary (1 of 2)
• Type V construction has six types of wood frame structures
• In a balloon-frame building, fire can spread through all the interconnected spaces from cellar to attic and across the ceiling
• Firestopping is often required by code to be installed to prevent the spread of fire
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Summary (2 of 2)
• Trusses and specially treated or constructed wood materials create unique challenges for fire protection
• Wood cannot be made fireproof or noncombustible
• Combustibility can be dealt with only by complete automatic sprinkler protection
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