Ch 06 Wood Frame Construction

6 Wood Frame

Construction

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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