Chapter 10-Lightweight Steel Roof Collapse Flashcards
Steel does NOT melt during a fire (requires more than 2400 deg F to melt steel)
Steel will start to DISTORT /LOSE STRENGTH at 1,000-1,100 deg F.
heated steel expands, bends, twists , SAGS(not snap), BUCKLES
HEATED STEEL can :
- expand and PUSH OUT MASONRY WALLS
- BUCKLE and collapse a floor
- warp and twist
- sag /bend
-can cause a BUILDING COLLAPSE
Steel has GREATER tensile/compressive strength than wood/masonry
Stell can be bent and twisted into shapes to INCREASE its strength ,,,this is called SHAPED STEEL –greater load capacity and less material
Steel BAr joists:
long steel bar BENT at 90 degree angle and welded to angle irons (chords) at top and bottom of the bar bends
Steel C beams:
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
Fire will reach?
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.
ASIDE from HEAT OF FIRE….critical failure temperature of STEEL depends on:
- LOAD it carries
- dimensions of the steel and SPAN of beam
- fire PROTECTION insulating steel surface
There are 3 types of FIRE PROTECTION:
- 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 - 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) - 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
STEEL open web bar joists are extremely HAZARDOUS for 2 reasons:
- CHANGE IN JOIST SPACING
- FAILURE CHARACTERISTICS of unprotected steel(collapse 5-10 minutes)
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:
- 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
Important design difference between WOOD JOIST and Steel bar and C-beam is ?
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
Four FACTORS determining failure of UNPROTECTED STEEL during fire:
- TEMPERATURE of fire
- LOAD STRESS
- STEEL THICKNESS
- SIZE OF FIRE
UNPROTECTED STEEL has ________so a steel building can be destroyed by a fire?
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
50 foot long steel beam that is heated uniformly over its length from 72 degrees F to 972 degrees F , average increase is ?
3 feet 9 inches
- within 1st five minutes , the temp. of fire will rise to 1100 degrees
- at 10 minutes , 1300 degrees
Greater the supported load, the FASTER structural member can fail..
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
IN OLDER BRICK AND JOIST BUILDINGS…Flat roofs can often support the same as floor load
IN non -combustible open web joists or C-joists:
- either the bar joists used in roof are spaced FARTHER APART THAN FLOOR JOIST
- or STEEL of smaller dimension (roof joists, smaller than floor joists)
- roof deck is also load factor
- heavier roof deck, faster collapse
2 common ROOF DECKS: used above fluted steel deck of a bar joist roof or C-beam support system:
- lightweight insulation
- precast concrete plank–heavier
-HEAVIER CONCRETE DECK may REDUCE safe operating time on top or below UNPROTECTED STEEL
THICK STEEL has GREATER RESISTANCE than lightweight section
- 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
Fire Size:
- 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
Steel is NON COMBUSTIBLE, BUT NOT FIRE RESISTIVE …
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
