Truss Construction and Fire!

1
Truss Construction and Fire!
A review of building construction, truss systems,
and how they apply to fire conditions and
operations.
Typical lightweight truss construction.
2
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 firefighters viewpoint they are lighter,
  cheaper and weaker.

3
Fire Fighters are being injured and killed when
fire damaged roof and floor truss systems
collapse, sometimes without warning.

• Over 60 of todays 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.

4
Triangular Trusses

• Are the most common trusses used in
  single-family dwellings.
• Triangular trusses provide a peaked roof.

5
Triangular Trusses
6
Scissor Trusses

• Are common in construction with cathedral
  ceilings. They are often found in churches.

7
Scissor Trusses
8
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.

9
Parallel Chord Trusses
10
Bowstring Trusses

• Get their name from the curved shape of the top
  chord.
• Parapet walls may hide the curved roofline on
  large commercial buildings.

11
Bowstring Trusses

• Exterior view of a bowstring truss building.

• Interior view of a bowstring truss building.

12
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.

13
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.

14
Heavy Timber Example
Bolts used in fastener plates
Cables used as web members
15
Heavy Timber Examples
16
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.

17
Light Weight Steel Example
18
Light Weight Steel Example
19
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.

20
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.

21
Heavy Steel Example
22
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.

23
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.

24
Parallel Chord Lightweight Wood Trusses

• Used both for roof and floor construction.

25
Light Weight Timber Example
Exposed Truss with insulation Notice Electrical
Wires and AC Vent
Gusset plates
26
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.

27
Example of Gusset plates
28
Example of Gusset plates

• Note the position of the gusset plates.

29
Example of Gusset plates

• Note the gaps between the wood ends.

30
Example of Gusset plates

• Note the opening for the ridge vent.
• Built in ventilation.

31
Light Weight Truss Systems
32
Things we do not know
Lumber Quality Issues

• 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

33
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.

34
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.

35
Metal Connector Plates After a Fire
36
After a Fire
37
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.

38
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.

39
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.

40
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.

41
New Trusses Without Gusset Plates
42
New Trusses Without Gusset Plates
43
New Trusses Without Gusset Plates
44
New Trusses Without Gusset Plates
45
New Trusses Without Gusset Plates
Note the finger-jointed lumber
46
New Trusses Without Gusset Plates
47
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.

48
Truss-loft

• Fire can travel unrestricted once in a truss
  loft.

49
Heavy Timber Truss in Fire Conditions

• Heavy timber trusses are spaced several feet
  apartmuch 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.

50
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.

51
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.

52
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
53
(No Transcript)
54
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

55
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.

56
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.

57
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.

58
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.

59
Firefighters 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.

60
Firefighters 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.

61
Firefighters dealing with truss construction

• When opening ceilings or other concealed spaces,
  have a charged hose line ready.
• Expect the unexpected.

62
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.

63
Summary Truss Systems Fire

• Remember todays trusses are designed to carry
  expected loads. These loads do not include the
  firefighter or firefighters 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 dont need it.
• If you have lightweight truss roofs or floors
  involved in a fire expect imminent collapse. GET
  OUT

64
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