Chapter 9...Flat Roof Collapse Flashcards
Today—Roof construction is mostly lightweight materials:
- Steel Bar joist
- Wood parallel lightweight truss
- Wood I-beams
- metal C-beams
Past experience of working on SOLID Wood beam roofs does NOT APPLY to Lightweight construction
Lightweight construction, like UNPROTECTED STEEL BAR JOIST can fail in 5-10 MINUTES
-Lightweight system MUST be in dispatch system (CIDS)
To determine risk of roof collapse, we need to know 3 facts about roof construction:
- TYPE OF roof support system(unprotected steel bar joist, C-beams, etc.)
- Method used to CONNECT ENDS OF ROOF SUPPORTS ( simple or fixed)
- Built up structure above main roof supports?( inverted/raised/rain roof,) or is roof deck nailed DIRECTLY TO ROOF SUPPORT SYSTEM
FLAT ROOF SUPPORT-OPEN WEB STEEL BAR JOIST:
- metal trusses with TOP/BOTTOM CHORDS equal in SIZE AND PARALLEL
- fluted metal steel deck welded to top chord…above steel deck will be several inches of INSULATING FIBER…topped by THINNER layer of TAR/GRAVEL MIXTURE
- Steel bar joists may be: unprotected/spray on fireproofing/fire retarded with membrane panel ceiling (membrane panel only provides protection, if installed correctly–no panels removed/damaged/improperly set)
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Fire penetrating these ceilings (MEMBRANE PANEL) will collapse the roof?
RAPIDLY
-membrane ceilings will NOT resist effects of fire well
-renovated taxpayers give no indication of their steel bar joists systems from EXTERIOR or the ROOF
It is almost IMPOSSIBLE to identify Steel Bar Joists Roof system, unless?
- ROOF being CUT–METAL SPARKS
2. FF’s below discover METAL TRUSSES after opening membrane ceiling
An EXPOSED steel bar joist may collapse after ONLY?
5-10 MINUTES of fire exposure
When a FIRE BUILDING has a Steel bar joist supporting the roof, HORIZONTAL VENTILATION OF___________is preferred to vertical roof ventilation ?
WINDOWS and DOORS
LIGHTWEIGHT WOOD TRUSS:
- another DANGEROUS ROOF
- fail within 5-10 MINUTES of fire exposure
Lightweight wood truss is widely used in ?
PRIVATE DWELLINGS
and
STORES
SOLID WOOD JOIST ROOF:
- most common in OLDER buildings
- GREATEST STABILITY(compared to Lightweight roof joists)
- SPACED CLOSE TOGETHER(if one is weakened, other assumes roof load)
- DOES NOT collapse suddenly, it will give WARNING (light weight does not)–because wood joist roof has a ROOF DECK, NAILED TO THE TOP OF JOIST AND COVERED WITH ROOFING PAPER AND TAR-
THE ROOF DECK will _________first and serve as warning?
SOFTEN
-An FF walking on a weakened roof deck , may plunge his foot through the roof deck and because of a CLOSELY SPACED SYSTEM , in MOST CASES, can grad onto a BEAM and avoid falling (not the case with lightweight, space FARTHER APART)
Wood joist is FAR SUPERIOR in withstanding high temperatures for longer periods before it fails (light weight does NOT…5-10 MINUTES)
Restrained Roof Beams (Brick cavity):
- SOLID WOOD BEAMS placed in brick and cemented
- BEAM can support more weight near its ENCASED END and not as much at its UNSUPPORTED CENTER)
- restrained in brick cavitY, WOOD BEAMS ends will sometimes continue to provide support to a roof near the supporting brick wall, even when center of roof is weakened , this may provide support as CANTILEVER BEAM
for this reason: FF’S walk CLOSE TO PARAPET WALL on a FIRE WEAKENED ROOF (Lad 3 mentions walk towards front wall, because of shafts towards rear)
Unrestrained Roof Beams (corbel shelf /fire cut roof beams)
- -LESS STABLE than brick cavity
- beams rests on GIRDER or brick
- beams may rotate off their resting position, as center of beam burns away
- MOST H-TYPE apartment ROOF JOISTS are resting on corbel shelf
Raised or Inverted roof has intervening space in which roof deck is resting on a SMALLER wood framework, built over MAIN ROOF SUPPORTS
Fire entering this space will FIRST damage smaller wood framework
- roof MAY seem unsafe (sag), yet MAIN ROOF BEAMS will be structurally sound
- softening of an inverted roof, may cause members to evacuate prematurely
if members operating on ROOF KNOW that it is only inverted (springy) roof supports that are weakening, and that main supports are STABLE roof operations can be COMPLETED
- SMALLER framework supporting roof deck can burn away and collapse
- this framework is 2x4 and spaced FARTHER APART than roof beams ….LESS CHANce MEMBER WILL BE STEPPING ON ROOF JOIST ..both feet could plunge through and FF could be WEDGE in roof decking (different from solid wood joist roof, where FF can grab beam)
