9 Noncombustible

Construction

Page 197

Objectives (1 of 2)

• Understand the difference between noncombustible and fire-resistive construction

• Identify the different types of steel building components and their characteristics

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Objectives (2 of 2)

• Describe different types of steel structural systems

• Describe the hazards of a metal deck roof fire

• Understand the hazards of high fire loads in unprotected steel structures and ways to improve the situation

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Introduction• Noncombustible and fire resistive

construction

• Differ in the level of fire resistance assigned to the structural frame, walls, floors, and roof

• Noncombustible construction has little fire resistance

• Fire resistive construction has moderate to heavy fire resistance

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Noncombustible Construction

• Allowable area and height is much less than fire resistive construction

• Maximum height is 12 stories

• Fire resistive can have unlimited height

• Fire-resistive construction can use steel for its framing system

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Steel

• Modulus of elasticity about 29 million pounds per square inch (psi)

• High tensile strength and shear strength

• Strong but lightweight members have little inherent fire resistance

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Fire Characteristics of Steel

• Substantial elongation

• Above 1300°F, steel members may fail

• Cold-drawn steel will fail at about 800°F

• Steel transmits heat readily

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Unwarranted Assumptions

• False belief in steel’s “fireproofness”

• Need to set priorities

• Heat absorbed by contents or structural elements is the most important heat

• Heat being evolved from contents that are burning is of secondary importance

• Heat leaving the structure—let it go

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Water on Hot Steel

• Water is the fire department’s heat removal medium

• Myth: Water should not be thrown on heated steel

• Cooling effect of water draws steel back to its original dimensions

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Definitions: Steel Construction Members (1 of 3)

• Angles

• Bars

• Box columns

• Box girders

• Channels

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Definitions: Steel Construction Members (2 of 3)

• I-beams

• Plates

• Purlins

• Rolled or built-up members

• Spandrel girders

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Definitions: Steel Construction Members (3 of 3)

• Tees

• Tubes

• Weight

• Wide-flange shapes

• Zees

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Steel as a Construction Material (1 of 2)

• Makes it possible to erect tall buildings

• Has consistency in structural characteristics

• Can be connected to other structural elements

• Used for fire escapes

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Steel as a Construction Material (2 of 2)

• Provides the tensile strength that concrete lacks

• Used in concrete flooring systems

• Used to repair failures in concrete buildings

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Steel Buildings

• Used in peaked roofs

• Bar joists span the main trusses to support a flat roof

• Steel is almost universally unprotected.

• Buildings often can only be classified as noncombustible

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Protected Noncombustible Sprinklered Construction

• Found occasionally

• Major structural elements have fire resistance

• Building itself is not fire resistive

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Rigid Frames

• Column is narrow at the base and tapers to its widest point at the top

• Girder is also tapered

• Wide haunch resists the outward thrust of the roof

• Clear spans of about 100 feet

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Steel-Framed Buildings

• Many are prefabricated

• Butler Company is a prominent manufacturer

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Huge Spans

• Span collapses can be sudden and tragic

• Adjacent bents are tied together

• Tying the steel units together creates dependencies between torsional or eccentric loads

• The higher the resistance to wind load, the more likely a progressive collapse

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Developing Wide-Span Trusses

• Designs may push the limits of steel

• Hasty field changes or errors in construction can have catastrophic consequences

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Deep Parallel-Chord Trusses

• Floor beams in hospitals

• Interstitial space

• Such voids should not be used for storage or maintenance

• Automatic sprinklers should be required

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Heavy Parallel-Chord Trusses

• Have been used as transfer beams

• Often hidden in partition walls

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Trussed Arches

• Arch of a steel arch bridge is often a truss

• Is a compression structure

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Walls of Steel-Framed Buildings

• Wall composition varies

• Metals, cement-asbestos board, masonry, concrete, and reinforced plastics found

• Wall insulation and coatings also factors

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Cement-Asbestos Board

• Noncombustible and is often used for friable construction

• Friable construction is used where an explosion is a possibility

• Will break away readily and relieve pressure

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Glass-Fiber Reinforced Plastics

• Noncombustible

• Resinous binder most often used with it is flammable

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Aluminum

• Noncombustible, but has a low melting point

• Has little mass per unit of area, so it disintegrates rapidly in a fire

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Precast Prestressed Concrete Panels

• Usually erected in large sections

• Collapse is hazardous to fire fighters

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Masonry Walls

• Often used for walls for unprotected steel-framed buildings

• Made of concrete block or a composite

• Usually only curtain walls

• Important to analyze the effect of the expansion of the steel frame on the wall

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Galvanized Steel Walls

• Used when heat conservation is not important

• Asphalt asbestos protected metal (AAPM)

• Robertson Protected Metal (RPM) is one proprietary name

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Metal Panels

• Prefabricated metal panels in a sandwich construction

• Plastics are often used with metal panels

• Insulation, vapor seal, or adhesive in the panels may be combustible

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Polyurethane Insulating Panels

• Protected by gypsum board and stainless-steel sheathing

• If a cutting torch is later used, a smoky, destructive fire may result

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Aluminum Sandwich Panels

• Can be made with foamed polyurethane

• Some are listed by Underwriters Laboratories (UL) Inc. for low flame spread ratings

• Smoke-developed ratings may be quite high

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Failure of the Closure of the Wall Panel to the Floor Slab

• Design of panel walls

• Method of installation

• Degradation of insulation

• Expansion of metal under fire conditions

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High-Rise Framing

• Steel once stood unchallenged as a method for high-rise buildings

• Concrete now is finding more use

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Builders’ Hesitation

• Brick, stone, and terra cotta added to framed buildings

• Goal was to reduce the apparent or perceived height of the building

• Didn’t openly discuss use of steel-frames

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Tilt-Slab Hazards

• Walls braced with tormentors or braces until the roof secured

• If the roof is being lost in the fire, beware of wall collapse

• If heavy smoke is present, the sprinklers are not controlling the fire

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Steel-Framed Buildings Under Construction

• Wind forces must be resisted, because the building is not fully connected

• Braces may not be properly installed

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Plastic Design in Steel Construction

• Connections are built to transfer loads beyond the column

• Beams are lighter and columns are smaller than they would be otherwise

• The lighter the steel, the less fire resistance

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More on the Fire Characteristics of Steel

• Conducts heat

• Elongates as temperature increases

• Loses strength at high temperatures

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Steel Conducts Heat

• Steel transmits heat

• Tin ceilings can transmit fire

• The conductivity of steel can be a factor in spreading fires

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Ships

• Practice of using ships as buildings is growing

• Ships have steel walls known as bulkheads

• Welding operations are performed without concern for heat transmission

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