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Dunn > Chapter 10-Lightweight Steel Roof Collapse > Flashcards

Chapter 10-Lightweight Steel Roof Collapse Flashcards

1
Q

Steel does NOT melt during a fire (requires more than 2400 deg F to melt steel)

A

Steel will start to DISTORT /LOSE STRENGTH at 1,000-1,100 deg F.

heated steel expands, bends, twists , SAGS(not snap), BUCKLES

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2
Q

HEATED STEEL can :

A
  • expand and PUSH OUT MASONRY WALLS
  • BUCKLE and collapse a floor
  • warp and twist
  • sag /bend

-can cause a BUILDING COLLAPSE

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3
Q

Steel has GREATER tensile/compressive strength than wood/masonry

A

Stell can be bent and twisted into shapes to INCREASE its strength ,,,this is called SHAPED STEEL –greater load capacity and less material

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4
Q

Steel BAr joists:

A

long steel bar BENT at 90 degree angle and welded to angle irons (chords) at top and bottom of the bar bends

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5
Q

Steel C beams:

A

16” wide/20 ft long strip of 3/8 inch sheet steel bent into the shape of “C” to create 20 ft roof or floor beam

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6
Q

Fire will reach?

A

1,000 degrees- 5 minutes
1,300 degrees- 10 minutes
1,700 degrees- 1 hour

THIN STEEL (open web bar joists and C-beams) will reach CRITICAL TEMPERATURE OF 1,110 DEGREES MUCH FASTER than thick solid steel I beam sections

-LARGE SOLID STEEL can ABSORB MUCH MORE HEAT and thus takes more time to reach critical temp.

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7
Q

ASIDE from HEAT OF FIRE….critical failure temperature of STEEL depends on:

A
  • LOAD it carries
  • dimensions of the steel and SPAN of beam
  • fire PROTECTION insulating steel surface
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8
Q

There are 3 types of FIRE PROTECTION:

A
  1. CONCRET ENCASEMENT -BEST METHOD for FF
    -1-2 INCHES is effective
    Drawback- SPALLING- causes concrete chunks to fall when steel is heated
    -Spalling- is expansion of MOISTURE in concrete
  2. MEMBRANE CEILING- FIRE RETARDING ceiling between occupancy and steel
    - NOT as effective as concrete
    - SUSPENDED CEILING SYSTEM of LIGHTWEIGHT steel frame—1 hour fire retarded ceiling panels (not properly fastened, missing panels, not tight, openings for utilities)
    - membrane ceilings can collapse
    - MORE serious than spalling—ENTIRE FRAMING CAN COLLAPSE-falls like NET with FRAMING AND HANGER STRAPS entangling FF’s (this is hazard-framing, NOT PANELS)
  3. SPRAY ON FIRE RETARDING -MOST POPULAR, LEAST EFFECTIVE
    - 1-3 Hours depending on thickness, but rarely protects for that time frame
    - applied over entire steel surface/girder/beams
    - steel not prepared properly
    - workers do NOT apply evenly
    - over paint primer- reduces by 1/3 to 1/2

FDNy has PROHIBITED the use of open web steel joists in NON residential high rise buildings until appropriate fireproofing standards are developed

SFRM:
adhesion
consistency
thickness

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9
Q

STEEL open web bar joists are extremely HAZARDOUS for 2 reasons:

A
  • CHANGE IN JOIST SPACING

- FAILURE CHARACTERISTICS of unprotected steel(collapse 5-10 minutes)

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10
Q

Serious fire in UNOCCUPIED BUILDING, that has roof supported by UNPROTECTED LIGHTWEIGHT STEEL BAR JOISTS or C-BEAMS….if fire is so sever that roof stability is in question:

A
  • too dangerous to send FF’S ON roof for ventilation and for interior attack with a hoseline
  • IC should consider HORIZONTAL VENT and Exterior attack (defensive)

1 story NON COMBUSTIBLE building using lightweight steel bar joist:

  • LARGE RECTANGULAR WINDOW
  • effective for venting smoke
  • located high where heat accumulates
  • horizontal length greater than vertical depth of window area
  • venting these windows faster and SAFER than roof cut
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11
Q

Important design difference between WOOD JOIST and Steel bar and C-beam is ?

A

SPACING OF JOISTS

Open web- up to 8 feet apart, this spacing creates dangers:

  • outline of roof deck may not be directly above steel bar joists, CUT ROOF can bend or hinge down
  • FF’S, ONE FOOT PLACE INSIDE roof cut, could FALL BELOW
  • Wide space allows FF to completely fall through
  • blade of power saw can cut TOP CHORD COMPLETELY IN TWO
  • roof deck near initial vent hole is unstable
  • edge of roof can bend downward and drop a member through the roof
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12
Q

Four FACTORS determining failure of UNPROTECTED STEEL during fire:

A
  1. TEMPERATURE of fire
  2. LOAD STRESS
  3. STEEL THICKNESS
  4. SIZE OF FIRE
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13
Q

UNPROTECTED STEEL has ________so a steel building can be destroyed by a fire?

A

NO FIRE RESISTANCE

-early collapse potential

At 1100 degrees , still loses almost half(40%) of its load carrying capacity

the TEMPERATURE WITHIN THE STEEL ITSELF must be raised to 1100 degrees (not surrounding heat/flame)

  • there will be a time lag, because steel is goo conductor of heat
  • steel can also expand
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14
Q

50 foot long steel beam that is heated uniformly over its length from 72 degrees F to 972 degrees F , average increase is ?

A

3 feet 9 inches

  • within 1st five minutes , the temp. of fire will rise to 1100 degrees
  • at 10 minutes , 1300 degrees
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15
Q

Greater the supported load, the FASTER structural member can fail..

A

IN MODERN non combustible buildings, roofs are not designed to support same load as floors below

-roofs are designed to support only weight of rain/snow…NOT PEOPLE

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16
Q

IN OLDER BRICK AND JOIST BUILDINGS…Flat roofs can often support the same as floor load

A

IN non -combustible open web joists or C-joists:

  1. either the bar joists used in roof are spaced FARTHER APART THAN FLOOR JOIST
  2. or STEEL of smaller dimension (roof joists, smaller than floor joists)
  • roof deck is also load factor
  • heavier roof deck, faster collapse
17
Q

2 common ROOF DECKS: used above fluted steel deck of a bar joist roof or C-beam support system:

A
  1. lightweight insulation
  2. precast concrete plank–heavier

-HEAVIER CONCRETE DECK may REDUCE safe operating time on top or below UNPROTECTED STEEL

18
Q

THICK STEEL has GREATER RESISTANCE than lightweight section

A
  • absorb more heat and longer time for failure
  • heat sink (capacity to absorb heat from a fire ant its ability to conduct and transfer the heat away from flame contact–to cool interior regions of steel)
  • heat sink can lengthen time of collapse
  • there is LITTLE HEAT SINK in lightweight steel structural supports
19
Q

Fire Size:

A
  • small fire with large steel —steel will absorb and transfer it away to cooler parts …this fire could burn for some time before failure
  • LARGE FIRE area —heated to critical failure quickly
  • Flash Fire- more rapidly
20
Q

Steel is NON COMBUSTIBLE, BUT NOT FIRE RESISTIVE …

A

Steel will not add fuel, but will COLLAPSE

  • heat destroys LOAD BEARING CAPABILITIES OF STEEL
  • to make steel fire resistive you must protect it by covering with insulation or separating with MEMBRANE CEILING
  • UNPROTECTED STEEL BAR JOISTS AND C BEAMS ARE NOT FIRE RESISTIVE and COLLAPSE WITHIN 5-10 MINUTES

Dunn (5 decks)

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