Flashcards in Private Dwellings Ch. 6 Lightweight Construction Deck (31):
In the absence of increased mass, the strength of a lightweight system is obtained through the interconnection of multiple structural components providing support in ___________ and _______.
Fill in the blanks
Compression and Tension
Furthermore, the decrease in size and mass of the individual structural components will impact the stability of all other structural components; if one fails, the others may fail as well.
In any building construction, the critical area subject to failure as a result of fire is where?
The point of connection.
For fires in “traditional” private dwellings built of ordinary/wood frame construction, early collapse is not a primary consideration. At these types of buildings, depending on the duration and intensity of fire, the size of the structural components, and the type of construction involved, the amount of time before collapse occurs is generally estimated to be an ____. However, when there is no ceiling to provide protection, therefore exposing structural components, failure can occur within __ minutes.
Fill in the blanks
In private dwellings constructed with lightweight materials, when fire extends from the building’s contents to its structural components, collapse may occur within __to __ minutes. Collapse in lightweight structures can occur suddenly, and with little or no warning signs. Collapse is always the primary consideration in buildings constructed with lightweight materials.
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Lightweight construction often provides for the creation of large rooms. Areas unsupported by columns with spans greater than how many feet are generally an indication of lightweight construction?
25’ are generally an indication that lightweight construction is present. The collapse potential in a “wide-open” room of this type is greater than in a compartmented area that has interior walls.
General Construction Deficiencies:
Inadequate fire stopping
Improperly installed metal gang nails
Out of plumb structural components
Cracked or damaged truss components
Unprotected wood floor joists
What is a trussloft?
A trussloft is an enclosed area between floors and ceiling where open truss construction is found.
What is the most common type of peaked roof found in lightweight construction?
The most common type of peaked roof found in lightweight construction is the open-web lightweight wood truss. When a lightweight wood truss peaked roof is exposed to sufficient fire, the rafters (top chord of the truss), the roof decking, and ceiling joists (bottom chord of the truss) can be expected to collapse as one complete unit into the structure, exposing firefighters to falling and burning debris.
Fire-stopping (draft stopping) – installed above the ceiling is limited, resulting in an open space which could encompass the entire area of the building. The NYC Building Code requires lightweight constructed spaces between the ceiling and the floor above or the ceiling and roof above to be divided into approximately equal areas of ____ sq. ft. or less unless the building is equipped with an automatic sprinkler system.
Fill in the blank
This is also true in ordinary construction when there is a detached ceiling (drop ceiling) in new construction.
Lightweight Parallel Chord Wood Truss
This type of support system is made up of 2x3” or 2x4” wooden web and chord members connected with sheet metal gusset plates (also referred to as gang nails). How many inches does these connections penetrate the wooden truss member?
Since these connections only penetrate 1/4” - 1/2” into the wooden truss member, the gang nails are the weakest point of the support system. When exposed to fire and/or high heat, failure at the point of connection should be expected before failure of the structural wood components.
When lightweight wood trusses are exposed to fire, high heat, or prolonged exposure to water, the gang nails may loosen and fail, causing the entire span of that particular truss section to fail. The surface-to-mass ratio of the wood trusses provides an abundant fuel source, and the air supply in the concealed truss void allows for rapid horizontal extension. Due to the open-web characteristic of the truss system, fire which has entered a ceiling (trussloft) or roof may likely affect what truss supports?
All truss supports on that level, creating the potential for a large-scale collapse of the area supported by the affected trusses.
Another type of open-web wood joist uses finger joints and glue to connect the web and the chords. The finger joints are approximately ___ deep; and, similar to parallel cord wood truss without the metal gusset plate, these trusses can be expected to fail rapidly when exposed to fire and/or high heat.
Fill in the blank
Laminated Wood I-Beams
This type of support system is primarily comprised of 1/2” oriented strand board (OSB) or 3/8”- 1/2” plywood web members, and 2x3” or 2x4” wood flanges. The web is fitted and glued into a routed slot in the top and bottom flanges. In some cases, they may span lengths of over how many feet?
The strength of the beam is a function of the mass of the flange and the depth of the web. These beams are usually connected to load bearing walls with sheet metal joist hangers.
This type of support system uses lightweight cold-formed steel joists to support the floors and/or roof. Although these joists are designed to replicate the dimensions of a 2x10” wood joist, they are engineered with a significant decrease in mass (1/8” thick web) and are extremely susceptible to the effects of fire. In order to stabilize the building, the joists must be braced with what to prevent them from twisting?
Strapping and/or blocking to prevent them from twisting.
A heavy fire and smoke condition on a floor or in the cellar accompanied by little or no smoke condition on the floor or floors above may be an indication of what type of floor?
A concrete or gypsum floor poured over corrugated steel (Q-decking) supported by C-Joists. The additional dead load may lead to early floor collapse; caution should be used at fires in buildings with this type of construction.
This type of truss system is similar in design to a lightweight parallel chord wood truss, but comprised of two types of materials: wood and steel. A formed sheet metal web is attached to 2x3” or 2x4” wood chords with nails or sheet metal gang nails. It is commercially known as what
A “Space Joist”.
Under fire conditions, the composite truss will react the same as other lightweight (wood or metal) trusses, they will lose strength and fail rapidly.
It is critical that all firefighting units are made aware that a lightweight support system is present. When lightweight construction is suspected, an immediate examination of what voids shall be conducted?
The ceiling voids shall be conducted as soon as conditions permit. If any type of lightweight system is found at an operation, an immediate notification to the IC is required.
The IC shall ensure all members on scene are made aware of the presence of lightweight construction. Once discovered, this information shall also be transmitted to whom?
The borough dispatcher in the preliminary and/or progress reports so that it can be relayed to responding units.
Once the fire enters the concealed space containing lightweight trusses or joists, it can rapidly travel to remote locations. The IC shall immediately be notified. All members shall be directed not to enter the fire area or areas where?
Directly above the fire until the IC determines the risk of a planned coordinated interior attack. This is especially true with the open-web design of lightweight parallel chord wood and composite trusses. Additionally, when heated gases build up in the concealed spaces of the trussloft and attic/cockloft, there is a much greater potential for a backdraft to occur.
A Thermal Imaging Camera (TIC) should be used to assist in detecting fire and heat in concealed spaces. When inspection holes are made, the TIC should be used to determine if any heat is present in and around what components of the lightweight construction?
Metal gusset plates and C-Joists.
Units operating above the fire should use caution, since the TIC may not always detect the intensity of the fire below. In addition, it may not detect heat or fire where what is used?
Double - 5/8” plasterboard is used.
Unless the fire is minor, or confined to a small area, the primary emphasis for a fire in a lightweight building under construction is that of what type of attack?
An exterior attack. Exterior streams should be positioned and operated from safe areas outside the collapse zone.
The presence of smoke pushing at the floor line on the outside of a building may be an indication of a fire in a what?
TRUSSLOFT. When this warning sign is evident at a private dwelling fire, it is an indication that the probability of collapse is significantly increased.
The positioning of the 1st hoseline for a quick knock-down of a contents fire that has not extended to the structural components is of paramount importance. Where should the 2nd and if needed the 3rd hoseline be positioned?
4.2.1 & 4.2.2
A 2nd hoseline must be positioned to back up the 1st line. When the 2nd line is needed to address a potential life hazard or is directed by the IC to a location remote from the 1st hoseline, a 3rd hoseline must be positioned as a back-up line.
The IC shall special call what for a structural fire in a building constructed of lightweight materials?
The IC shall special call an additional engine company for a structural fire in a building constructed of lightweight materials.
When can we cut the roof of a bldg. constructed of lightweight materials?
Under no circumstances shall the roof be cut in any peaked or flat roof building of lightweight construction. Cutting the roof and pushing down the ceiling could expose the cockloft area to additional heat and fire from the top floor. In addition, a saw cut could sever a structural member causing the failure of one or more trusses/joists.
When the fire is on the top floor and there is clear indication that the fire has not entered the cockloft, roof operations shall be limited to what operations?
Checking the rear and sides of the building.
Venting the top floor windows, and skylights if present
Inspecting HVAC duct vents for unusual heat.
All members shall be removed from the roof upon completion of these duties.
Before proceeding above the fire floor, members should determine what items?
Location, and extent of the fire.
Presence of a life hazard (known or suspected).
Position and progress of the 1st and 2nd hoselines.
Key size-up indicators for the IC include what items?
Extent of the fire. (light, medium, heavy)
Location of the fire (top floor, cellar, void spaces, etc.)
Volume of smoke and fire (light, medium, or heavy)
Smoke action (pushing, twisting, or rolling under pressure)
Why should the IC must be immediately notified of the presence of “Colliers’ mansion-type” conditions?
Water from hoselines absorbed into the large amounts of debris will substantially increase the weight on the floor. Since the built-in safety factor
normally found in traditional, ordinary/wood frame construction is not provided for in lightweight construction, the increased live load may potentially cause a collapse without warning.