Truss Construction and Fire! A review of building construction, truss systems, and how they apply to fire conditions and operations. Typical lightweight truss construction.
Truss Construction and Fire! A review of building construction, truss systems, and how they apply to fire conditions and operations. Typical lightweight.
Mar 28, 2015
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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
Bolts used in fastener platesCables used as web members
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
Gusset plates
Exposed Truss with insulationNotice Electrical Wires and AC Vent
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
Structural Performance Related• 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• Subterranean Termites• Carpenter Ants
Factors Affecting Wood Strength: / Environmental Characteristics
• Temperature• Load Duration• Chemicals• Decay Fungi• Insects
Lumber Quality Issues
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 allegedthat metal connector plates in trusses fail by curling awayfrom wood due to heat in a fire.
In fact, the curling occursdue to tension forces pulling on the metal connectorplates.
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 furtherapart than lightweight trusses, which may be spaced on 16 or 24inch centers. Roof ventilation on a heavy timber truss roof mayleave the fire fighter standing on several feet of unsupported roofboard.
The metal connectors or pins holding the heavy truss systemtogether can fail before the wooden timber fails.
A collapsing heavy wooden timber truss roof system can causethe subsequent collapse of the building's front, rear, or sidemasonry walls.
Light Timber Trusses in Fire Conditions
Loose or lost gusset plates can lead to tensile forcespulling the truss apart.
Alterations to individual trusses or to the building. Trussesare often cut or altered to accommodate plumbing, wiring, ventilation ducts, and other fixtures.
Truss systems may be overstressed by heavy suspendedceilings, or other objects suspended below the truss suchas air conditioning units; ventilation systems; materialstorage; or other loads within or above the roof system thatthe 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 joisttrusses can exert lateral thrust forces on surrounding masonrywalls sufficient to cause their collapse. Higher temperatures canlead to failure of the steel itself.
Expansion within metal trusses may also cause the bottom chord to buckle and fail, resulting in downward thrust andcollapse 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 Roof
Effective: DECEMBER 29, 2004
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
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