Ch. 21 Structural Collapse Flashcards
Structural collapse is ________ on the fireground.
One of the most feared occurrences
Large percentage of multiple-casualty incidents
Resistance to collapse per classes of construction:
Most resistant to least resistant Class 1 fireproof Class 4 heavy timber Class 3 ordinary/standard brick and wood joist Class 5 wood-frame Class 2 noncombustible
Which class of construction has:
- up to 4 hr fire resistance
- skeletal framework, poured concrete or steel I-beams
- collapse is usually rather localized
- load being supported generally remains in place
- supposed to provide resistance to collapse to allow time for evacuation and fire control
Class 1 (greatest resistance to collapse)
1-4-3-5-2
Most severe collapse hazard in Class 1 bldgs is:
Poured-in-place concrete bldgs under construction
- fire involves wooden formwork
- pancake collapse
Second best construction type, as far as resistance to collapse is concerned:
Class 4, Heavy timber
- generally 12 x 12 columns and brick walls
- manual FFing long since shifted to exterior operations before danger of collapse
- exception is bldg has been scene of numerous repeated fires in same area
- advanced fires, apparatus removed (1 1/2 times collapse zone)
Third most collapse resistance:
Class 3, ordinary or standard brick and wood joist
- more prone to burn through than collapse
- flr/roof sheathing burn through long before floor joists fail
- FFs could be working underneath, unaware due to hung ceilings or smoke
- distinct danger=high floor loads and concentrated loads =roof-mounted AC
Fourth category in order or resistance to collapse is:
Class 5, wood frame
- dimensional lumber more prone to deck burn through
- only standard construction~doesn’t apply to lightweight truss-construction
Least resistant to collapse:
Class 2
- large quantities of unprotected steel
- steel expands –> 100 ft expands 9.5 in. at 1000F
- at 1500F - lose its strength, will drop its load or twist/sag
- steel cooled, contract back to 100 ft while retaining distorted shape
- steel would shrink, end of beams no longer resting on original supports, collapse occurs
Hierarchy of structural components:
BCGBF
- Bearing walls
- Columns
- Girders (support beams)
- Beams and joists
- Floor or roof decking
Protected steel or concrete supporting system resist _____ collapse rather well.
These are called _______ since most of their weight is carried in the frame or skeleton.
Total
Framed structures
Collapses that occur in framed structures are usually ________.
Very localized
only area between two supporting members
Structures with most of weight on bearing walls?
UNFRAMED structures
failure of vertical member more serious than horizontal
Wooden structural elements burn through at a rate of about:
1 in. for every 45 minutes of open-burning time
-fire attack standard 1 1/2 in. joist from TWO SIDES, burn through in less than 45 minutes
One of the most potentially dangerous situations is when columns support a ______ that carries the weights of upper-floor walls and floor joists when a __________ is removed to expand.
Girder
Ground floor wall
As little as ______ exerted laterally on a brick wall can topple it like a house of cards.
1 psi
Any location where _____ or _____ are stored demand tactics shift towards cautious, defensive operations.
Flammable gas
Explosives
Biggest offender at fire operations concerning overloading of floors and roofs?
Runoff from hose streams
- also heavy loads - plumbing supplies, snow, etc. with no fire
One factor FFs have direct control over (cause of collapse) is:
Cutting of structural members during overhaul
If forced to operate in area where structural weakness is suspected:
Use methods and tools that produce little or no impact load or vibration
- use circular instead of axe or reciprocating
- can be external sources
- watch for elevated platforms
Examples of cause of collapse, miscellaneous
- high wind
- hose streams
- age of bldg
Indicators of collapse
12 of them
- each indicator separately might not raise SUFFICIENT ALARM
- when viewed as whole, signs bldg is in danger
Number One indicator-occupancy by problem business
Plumbing supply, appliance dealership, printing shops (P’s)
Type of occupancy ALONE not reason to withdraw
-should trigger CAREFUL EXAMINATION for signs of collapse
Of all possible warning signs of collapse, only one more serious than truss construction
Structure that contains Explosives (rare and usually over before we arrive)
Number 2 collapse indicator
THE fire problem of 21st century
Fire in truss construction
-particle board I-beams deserve same technique
