9 Noncombustible Construction Page 197. Objectives (1 of 2) Understand the difference between...
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Transcript of 9 Noncombustible Construction Page 197. Objectives (1 of 2) Understand the difference between...
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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