Transcript Truss Construction and Fire!

Truss Construction and Fire!
A review of building construction,
truss systems, and how they apply
to fire conditions and operations.
Typical lightweight truss construction.
What are trusses?
A truss is made up of structural members joined together to form a
ridged frame work of a structure.
Most commonly they are in the shape of a triangle or a series of
triangles.
Some may be in the shape of rectangles.
Engineered to be lighter, cheaper and stronger.
From a firefighter’s viewpoint they are lighter, cheaper and weaker.
Fire Fighters are being injured and killed when fire
damaged roof and floor truss systems collapse,
sometimes without warning.
Over 60% of today’s structures use truss systems.
Truss material is lighter and less expensive.
Trusses can be found in roof and floor systems.
Trusses allow for larger openings in areas of a structure.
Trusses are easier and safer to handle during construction.
Trusses can be designed to carry expected loads.
Triangular Trusses
Are the most common trusses used in single-family dwellings.
Triangular trusses provide a peaked roof.
Triangular Trusses
Scissor Trusses
Are common in construction with cathedral ceilings. They are
often found in churches.
Scissor Trusses
Parallel Chord Trusses
Provide a flat roof or floor.
The top and bottom chords are parallel.
They are commonly used in single-family dwellings, row houses,
apartment buildings, and smaller office buildings.
Parallel Chord Trusses
Bowstring Trusses
Get their name from the curved shape of the top chord.
Parapet walls may hide the curved roofline on large commercial
buildings.
Bowstring Trusses
Exterior view of a
bowstring truss building.
Interior view of a bowstring
truss building.
Types of Trusses
Heavy Timber Floor and Roof Trusses
Light timber Floor and Roof trusses
Most common!
Metal Floor and Roof Truss systems
Note: The same trusses found in roof construction can also be
found in floor construction.
Heavy Timber Truss Systems
Usually used to create large clear areas like Cathedral Ceilings.
Bolts are commonly used to connect truss members.
Most common form of construction in commercial structures
pre 1960.
Heavy Timber Example
Cables used as web members
Bolts used in fastener plates
Heavy Timber Examples
Metal Truss Systems
Much stronger than Timber.
Most common in commercial structures.
Can be found in combination with other materials.
May use cable in the truss web.
Light Weight Steel Example
Light Weight Steel Example
Open Web Steel Bar Joist
Evaluate the need for a roof
vent.
Unstable area around the
cut.
Sacrifice 10 feet or more,
and make a safer cut farther
away from the danger/collapse
point.
Once the hole is cut, vacate
the roof as soon as possible.
Steel will expand and twist.
Steel will push out the walls.
Open Web Steel Bar Joist
May be spaced widely apart
8ft.
Span large distances up to
60ft.
Non combustible but
susceptible to heat.
Can fail in 5 to 10 minutes.
Heavy Steel Example
Light Weight Truss Systems
Made from 2x4, 2x6, and 2x8 sized lumber.
Found mostly in single and multi family dwellings.
Use the lightest weight lumber necessary to support the
anticipated load.
Gusset plates, phenol resorcinol glue, or nails are used to
connect truss system members.
Parallel Chord Lightweight Wood Trusses
Extremely Dangerous.
2 X 4 Nominal Lumber.
Metal gusset plates connecting trusses.
Used in both roof and floor construction.
Rapid catastrophic failure: No Warning Signs.
Parallel Chord Lightweight Wood Trusses
Used both for roof and
floor construction.
Light Weight Timber Example
Exposed Truss with insulation
Notice Electrical Wires and AC Vent
Gusset plates
Example of Gusset plates
Metal tooth plate
connectors like those
shown are used
extensively in parallel and
pitched chord trusses.
The multi-tooth plates
are embedded into the
wood fiber using
hydraulic presses.
Note that the typical lumber joint may have up to a 1/8” gap between
the ends in roof truss assemblies per building codes.
Example of Gusset plates
Example of Gusset plates
Note the
position of
the gusset
plates.
Example of Gusset plates
Note the gaps between the wood ends.
Example of Gusset plates
Note the
opening for the
ridge vent.
Built in
ventilation.
Light Weight Truss Systems
Things we do not know
Lumber Quality Issues
Structural Performance Related
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High Moisture Content (Reduces Connector Strength)
Knots & Other Lumber Characteristics (Reduces Plate Holding
Strength)
Lumber Thickness Variation (Reduces Plate Holding Strength)
Insects / Many different types of insects attack & destroy wood
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Subterranean Termites
Carpenter Ants
Factors Affecting Wood Strength: / Environmental
Characteristics
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Temperature
Load Duration
Chemicals
Decay Fungi
Insects
Manufacturing a Truss
Trusses are
manufactured on large
horizontal tables called jigs.
Roller pressure is
applied to each plate to
assure the teeth are
properly embedded in the
wood.
Metal Connector Plates After a Fire Test
It is commonly alleged
that metal connector plates in
trusses fail by curling away
from wood due to heat in a fire.
In fact, the curling occurs
due to tension forces pulling on the
metal connector
plates.
Metal Connector Plates After a Fire
After a Fire
Glue-Laminated Timbers Used in Roof Truss
Glulam is produced in laminating plants by gluing together
layers of sawn lumber to form large cross-section timbers that
retain the traditional look of wood along with engineered
strength.
Glulam is fabricated using individual pieces of nominally 1- to
2-inch thick, kiln-dried lumber, laminated together under
controlled conditions of temperature and pressure, to form
large timber sections.
Glulam can be fabricated in almost any straight or arched
configuration for long spans.
Glue-Laminated Timbers Used in Roof Truss
These large cross-section dimension
members can be used almost anywhere, and
typically are installed as floor or roof beams,
headers over doors and windows, rimboard
around the edge of a foundation, or as studs
in wall framing.
Finger-jointed lumber
Finger-jointed lumber is made up of
short pieces with the ends machined
in a finger profile and glued together.
The glue used in finger-jointed
trusses is phenol resorcinol. Auto
ignition temperature is said to be 1130
deg. F.
The Auto ignition of wood is 520
deg. F. to 880 deg. F.
A finger-joint can have up to 90% of
the tensile strength of clear wood and
exhibit similar behavior.
New Trusses Without Gusset Plates
New style wood trusses are commercially available that no
longer use the metal “gusset plates”.
They are now using 2 X 4 “Finger Jointed” lumber held together
by the application of glue where the webs meet the chords.
New Trusses Without Gusset Plates
New Trusses Without Gusset Plates
New Trusses Without Gusset Plates
New Trusses Without Gusset Plates
New Trusses Without Gusset Plates
Note the finger-jointed lumber
New Trusses Without Gusset Plates
A new term for the fire service –
“Truss-loft”
Once sealed it is hidden.
Open end to end, front to rear.
Electrical, plumbing, HVAC.
Rapid fire spread with early collapse inevitable.
Truss-loft
Fire can travel unrestricted once in a truss loft.
Heavy Timber Truss in Fire Conditions
Heavy timber trusses are spaced several feet apart—much further
apart than lightweight trusses, which may be spaced on 16 or 24
inch centers. Roof ventilation on a heavy timber truss roof may
leave the fire fighter standing on several feet of unsupported roof
board.
The metal connectors or pins holding the heavy truss system
together can fail before the wooden timber fails.
A collapsing heavy wooden timber truss roof system can cause
the subsequent collapse of the building's front, rear, or side
masonry walls.
Light Timber Trusses in Fire Conditions
Loose or lost gusset plates can lead to tensile forces
pulling the truss apart.
Alterations to individual trusses or to the building. Trusses
are often cut or altered to accommodate plumbing, wiring,
ventilation ducts, and other fixtures.
Truss systems may be overstressed by heavy suspended
ceilings, or other objects suspended below the truss such
as air conditioning units; ventilation systems; material
storage; or other loads within or above the roof system that
the truss was not designed to bear.
Steel Truss in Fire Conditions
Cold-drawn steel cables can totally fail at 800º F.
At temperatures above 1,000º F, the expanding steel in bar joist
trusses can exert lateral thrust forces on surrounding masonry
walls sufficient to cause their collapse. Higher temperatures can
lead to failure of the steel itself.
Expansion within metal trusses may also cause the
bottom chord to buckle and fail, resulting in downward thrust and
collapse of the roof or floor.
Signage for truss-type construction
The State of New York implemented legislation that requires all
new commercial and industrial structures that utilize “truss-type
construction” to have a sign warning emergency personnel that
the building was built with trusses. The bill defined “truss-type
construction” to include all fabricated components made of wood
and steel.
Floor
Floor / Roof
Effective: DECEMBER 29, 2004
Roof
TRUSS IDENTIFICATION SIGN
LOCATIONS
Exterior building entrance & exit doors
Exterior roof access doors to a stairway
Not more than 12 inches from the fire department hose
connections
Things the department should
consider
Conduct pre-incident planning and inspections to identify
structures that contain truss construction.
Whenever possible, inspect buildings during the construction
phase to help assess the different types of construction,
materials, etc.
Do not cut steel or poured concrete roofs. It may not be worth
allowing your firefighters on these increasingly unstable roofs.
Command Considerations with Truss
Construction
Ensure that the incident commander conducts an initial size-up
and risk assessment of the incident scene before beginning
interior fire-fighting operations.
Consider using a thermal imaging camera as part of the size-up
operation to aid in locating fires in concealed spaces.
Continually conduct size-up from the time the alarm is received
until the fire is under control.
Pay close attention to the conditions of the structure, monitor
the roof.
Plan for search and rescue operations before an emergency
occurs in case a fire fighter becomes trapped.
Command Considerations with Truss
Construction
Ensure that fire fighters performing fire-fighting
operations under or above trusses are evacuated as
soon as it is determined that the trusses are
exposed to fire,
(not according to a time limit).
Establish a collapse zone when operating outside a
burning building, since truss roof collapses can
push out on the walls, causing a secondary collapse
of the exterior walls.
The collapse zone should be equal to the height of
the building plus allowance for scattering debris.
Three scenarios can cause fatalities and
injuries at fires involving truss systems.
Operating above a burning roof or floor truss, firefighters may
fall into a fire as the sheathing or the truss system collapses
below them.
Operating below the roof or floor inside a building with burning
truss floor or roof structures, the trusses may collapse onto the
firefighters.
Operating outside a building with burning trusses, the floor or
roof trusses may collapse causing an internal or external wall
collapse.
Firefighter’s dealing with truss
construction
Make sure the command officer is aware of the interior fire
conditions.
Use a defensive fire-fighting strategy once burning of truss
members is identified.
Expect imminent collapse once lightweight truss roofs or floors
are involved in a fire.
If possible, avoid cutting the truss chords when cutting holes
for roof ventilation.
Firefighter’s dealing with truss
construction
Avoid roof areas loaded with heavy objects.
Be aware of alternative exit routes at all times when working
above or below a truss.
Immediately open ceilings and other concealed spaces
whenever a fire is suspected in a truss system.
Be aware of the possibility of flashover or back draft when
opening concealed spaces.
Firefighter’s dealing with truss
construction
When opening ceilings or other concealed spaces, have a
charged hose line ready.
Expect the unexpected.
New buildings, new tactics
The building is our enemy.
Take a hook into every room.
Pull the ceiling before entering the room too far, use the safety of the
doorway.
Check the roof stability.
10 minute time frame for control of the fire.
Summary – Truss Systems & Fire
Remember today’s trusses are designed to carry expected
loads. These loads do not include the firefighter or firefighter’s
in full turn out gear plus equipment.
Inspect new buildings in your district before the framing
structure is covered up.
Always try to be aware of an additional means of egress and
hope you don’t need it.
If you have lightweight truss roofs or floors involved in a fire
expect imminent collapse. GET OUT
Summary – Truss Systems & Fire
When fire involves a building with a lightweight steel truss
system, consider keeping all firefighters off the building and
floors, even if this means reverting to a defensive firefighting
tactic. One firefighter is worth much more than any building.
Expect the unexpected.
Good luck and be safe.
Presentation by: TRK