Chapter 8-Timber Truss Roof Collapse Flashcards
(26 cards)
A timber truss roof can be built in a variety of shapes?
a. inclined plane truss (gable shaped roof)
b. parallel Chord truss (flat roof)
c. bowstring truss (arch roof)
Most common shape of Timber truss roof is?
BOWSTRING TRUSS ROOF
-entire truss section may fail as soon as the smallest web member weakens
During Fire conditions, the truss is only as strong as its?
WEAKEST MEMBER
-the strength and stability of truss depends upon the smallest web member
Truss construction is…?
MOST DANGEROUS roof condition
- known to collapse during EARLY STAGES of fire
- often cause subsequent collapse of the front masonry enclosure wall of the building
Why do front walls collapse more OFTEN than rear walls?
rear walls are more stable and have few openings
front walls have MORE OPENINGS-less able to resist any movement of sloping roof joists
openings weaken load bearing capabilities and permit fire spread
High decorative parapet walls can…?
CONCEAL roof shapes
FF discovering any type of Truss , shall immediately notify ?
INCIDENT COMMANDER
Large space created by the concave arched underside of the bowstring truss roof acts as a ?
HEAT SINK- allows the flow of large amounts of heat/smoke upward from the fire floor below
a serious fire in a building with a bowstring truss roof might allow firefighters to enter the structure, walk upright to the rear of the occupancy and become trapped under a collapsing roof
3 size up indicators of a truss roof in a building?
a. large open space without columns, indicates long span roof support, such as truss
b. bowstring truss roof only- MOUNDED roof shape
c. certain occupancies frequently use truss construction- supermarkets, bowling alley, garages, theaters, places of worship, auto dealerships, armories, piers
Fire strategy for Timber Trusses…
- timber truss roof supports are often visible from the floor level, because often, NO CEILING EXISTS…this absence of ceiling greatly assists firefighters because:
- it allows identification of trusses from inside
- gives interior hose stream an open path to extinguish fire in timber truss roof quickly
What is more dangerous…truss roof concealed by ceiling or without ceiling?
CONCEALED is much more dangerous
TIMBER TRUSS concealed strategy ?
NO EXTINGUISHMENT FROM BELOW THE BURNING TRUSS THROUGH A TRAP DOOR IN CEILING
NO EXTINGUISHING A FIRE IN A TRUSS SPACE FROM ABOVE
-when fire extends to a timber truss attic space, prepare for a DEFENSIVE OUTSIDE ATTACK and protect exposures
Timber Truss NOT concealed by a ceiling?
- first engine attack fire directly with large diameter hose line
- powerful stream, capable of reaching 50 feet , will be needed for upper portion of trusses
-if first stream does not control flames within the FIRST FEW SECONDS OF WATER DISCHARGE, and fire will increase, interior fire fighting should be discontinued
if 1st line is successful- then stretch 2nd line to back up (large diameter hose)- this line should sweep underside of roof, if necessary
If web members are enclosed …trusses will appear like curtain boards ?
if collapse of this compartmented roof occurs, timber truss roofs will fail one at a time
if web members are UNENCLOSED, on one side…not covered with fire retardant ?
entire truss roof will fail at once, with all timber trusses failing at the same time
Timber Truss Roof Vent?
-if roof stability is ASSURED and roof venting is needed- FF’s can sometimes open the skylights or scuttle covers quickly and leave roof immediately
Roof Vent: using vent opening in the sloping front or rear portion of the roof…
if SMALL fire exists-overheated motor/electric wiring in bow string truss roof structure and concealed by ceiling -access ladder can’t be found….quick access can be obtained by:
making a roof cut opening at the SLOPING ENDS of the roof deck
- hose line can be directed into a truss roof area through this roof cut opening, if the small normal access opening cannot be located
- to quickly determine if a fire has spread to the roof space of a bowstring truss, a triangular cut may be made in the sloping front or rear portion of the roof from an aerial platform
Truss identification will only be observed from the roof shape when the truss is ?
BOWSTRING TRUSS ROOF
-parallel chord or gable truss roof will not be identified by the roof shape
FF’s ordered to evacuate should retreat in a direction ?
PERPENDICULAR to the roof trusses
same goes for members inside the building
FF’s operating initial hoseline should be?
- positioned behind a truss section, next to the one involved in fire
- reach of a powerful stream will allow them to be out of the collapse zone of a single truss
Truss is discovered?
notify INCIDENT COMMANDER
Places of worship?
-often most unstable structures
TOWER and STEEPLE most unstable
Church fire?
- large fires/lack of partitions
- as soon as flames are knocked down, walls and attics should be checked
- if 2 lines do not extinguish, withdraw to defensive position
Rose Window?
- PRIMARY ventilation at front of church or temple
- large stained glass window
- where first TL should operate
- vent entire window
- notify IC if unbreakable glass is encountered
- LCS through Rose Window may extinguish fire at the upper peak near ceiling
