PD ch6 Flashcards
JD 1.4 In the absence of increased mass, the strength of a lightweight system is obtained through the interconnection of multiple structural components providing support in 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.
JD 1.5 In any building construction, the critical area subject to failure as a result of fire is the point of connection.
1.6 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 hour. However, when there is no ceiling to provide protection, therefore exposing structural components, failure can occur within 20 minutes.
- TEST** JD
- In private dwellings constructed with lightweight materials, when fire extends from the building’s contents to its structural components, collapse may occur within 5 to 10 minutes. (Figure 1) 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.
Experience has shown that structural collapses have occurred in buildings where the magnitude of fire was relatively small. Therefore, positioning the 1st hoseline for early extinguishment or control of a contents fire that has not extended to the structural components is of paramount importance.
JD 1.8 Lightweight construction often provides for the creation of large rooms. Areas unsupported by columns with spans greater than 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.
- 9 The most important factor to a safe and successful operation at these types of buildings is the identification of the presence of lightweight construction. It is critical that any member becoming aware of lightweight construction notify the Incident Commander (IC).
- 10 When lightweight components are EXPOSED to fire or high heat, the IC shall be immediately notified. It may be necessary to remove members or suspend interior operations until further assessment of structural stability and collapse potential can be made.
*new urgent any time a change will severly impact an operation or safety of FF trasmit. P=problem nature U-units L-location
1.12 Lightweight buildings under construction, within a unit’s administrative area, should be the subject of company familiarization drills. These drills should be used to obtain critical information for entry into eCIDS.
2.2.2 Exterior walls – Various types of construction materials are used (e.g. wood, stone or brick veneer; stucco, sheet metal). The heavier type of exterior finishing products (stone or brick veneer) may be anchored to the structure with metal straps or wire mesh. These walls may potentially fail as one complete unit when exposed to fire and high heat.
JD Finished ceilings – May consist of 5/8” or 1/2” plasterboard. A trussloft is an enclosed area between floors and ceiling where open truss construction is found; however ceilings may not be present in basement/cellars. In some cases, the structure is built over a crawl space or cellar with an unprotected first floor support system.
*can have backdraft in trussloft
JD Roofs – Lightweight wood truss roofs may be flat or peaked. A large, open cockloft/attic is commonly found in either design.
2.2.5. A The most common type of peaked roof found in lightweight construction is the open-web lightweight wood truss. In an ordinary peaked roof structure with dimensional lumber, the collapse of the roof rafters and roof deck may be supported by the ceiling beams below, thereby protecting firefighters operating on that floor. 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.
JD
-Roof decks found on both peaked and flat roofs are constructed with 3/8” or 1/2” plywood instead of the 1-inch wood sheathing used in ordinary construction. Plywood burns rapidly, and will fail at a faster rate. Fires originating in, or extending to, the cockloft/attic space will quickly vent through the roof.
-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 500 sq. ft. or less unless the building is equipped with an automatic sprinkler system.
JD
-In some larger private dwellings, the plasterboard ceiling may be installed several inches below the lightweight supporting members to provide for the running of utilities. This type of installation is more prevalent in cellars with wood I-beam and metal C-joist construction than the open web truss due to the lack of horizontal openings to run utilities. The result is a large open area between the ceiling and the joists, creating the potential for rapid fire spread and extension.
JD
-Decks/Balconies – Outside decks and balconies are often supported by lightweight joists extending through the exterior wall. Any cantilevered deck/balcony should be considered a serious collapse potential.
JD
-TYPES OF LIGHTWEIGHT SYSTEMS
3.1 There are many different types of lightweight systems in use today. This bulletin will address the four most common types of lightweight systems that may be encountered in private dwellings:
Lightweight parallel chord truss Laminated wood I-beams
Metal C-joists
Composite truss
JD
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). 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.
3.2.1 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 space (cockloft/attic) may likely affect all truss supports on that level, creating the potential for a large-scale collapse of the area supported by the affected trusses.
JD
Another type of open-web wood joist uses finger joints and glue to connect the web and the chords. The finger joints are approximately 3⁄4” 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.
JD
- Laminated Wood I-Beams
3. 3.1 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 60 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.
3. 3.2 These I-beams are pre-engineered, and may come with openings in the web to accommodate utilities. In other instances, builders will bore holes in the web for plumbing, electrical lines, and HVAC ductwork, thus compromising its strength. A fire-retardant material sprayed on the beam will tend to dry out the wood over time, making it brittle and further reducing its strength. Once the ignition temperature has been reached during a fire, a laminated wood I-beam will burn rapidly. The sheet metal brackets holding the beams in place may also fail when exposed to fire or high heat.
JD
Metal C-Joists
3.4.1 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 strapping and/or blocking to prevent them from twisting.
3.4.2 Metal C-Joists may come with pre-drilled holes for plumbing and electrical lines. (Figure 8) A newer type of C-Joist, with design characteristics similar to lightweight parallel chord wood truss, has larger openings that allows for the easier running of utilities, and the potential for fire and heat spread is significantly greater. When exposed to fire and/or high heat, these lightweight steel joists will lose strength and fail rapidly..
JD
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 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.
cr. brannican
blue wave several ft deep hit deck from under …
JD
-Composite Truss
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 a “Space Joist”.
3.5.1 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 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.
JD
-4.1.3 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 the borough dispatcher in the PRELIMINARY AND/OR PROGRESS REPORTS so that it can be relayed to responding units.
(fire)4.1.4 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 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.
JD
-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 an exterior attack. Exterior streams should be positioned and operated from safe areas outside the collapse zone.
-When fire is found in a vertical void, the cockloft/attic space should be promptly checked for extension within the limits of safety.
JD
-A serious fire inside the building may affect the outside deck/balcony supporting members, causing it to fail without warning. When there is any doubt as to its stability, members should not operate on or below the outside deck/balcony.
-The presence of SMOKE pushing at the floor line on the outside of a building may be an indication of a fire in a trussloft. When this warning sign is evident at a private dwelling fire, it is an indication that the probability of collapse is significantly increased.(in branican also)
JD
-During overhaul, only the minimum number of firefighters necessary to complete the task should be used. Any floor or roof support system that has been heavily damaged due to fire should be hydraulically overhauled from a safe area using the reach of the hose stream. Members should not operate directly above or below the structurally damaged area, including the entire length of the joist.
- Engine Company Operations
4. 2.1 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.
- *TEST** JD
- 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 an ADDITIONAL ENGINE company for a STRUCTURAL FIRE in a building CONSTRUCTED OF LIGHTWEIGHT MATERIALS.,,
4.2.3 When the fire is of such magnitude that it cannot be quickly knocked down with the two hoselines, then an outside operation must be considered. The operating personnel inside the building must be limited so that quick egress is not impeded.
A serious problem may occur when radiant heat and fire exposes and weakens the floor decking on the floor supporting members. This condition is often discovered when engine company members moving into the immediate fire area suddenly plunge through the floor deck. When the spacing between the light weight floor supports is 24” apart, the potential exists for a larger opening to be formed for members to fall through to the floor below. Caution should be used when entering the immediate fire area until the integrity of the floor deck can be verified.
JD
- 4.3.1 When lightweight construction is suspected, the first arriving ladder company shall make an inspection hole in the ceiling from a safe area in order to determine the type of support system present (i.e. for a fire on the 2nd floor, an inspection hole can be made on the 1st floor). This critical task shall be done immediately upon arrival. The IC must be notified of the results of this examination.
4. 3.2 When fire is located in the cellar or the first floor and conditions prevent visual identification of the type of construction, an examination could be made in a similar attached exposure. This inspection shall be assigned by the IC to an available unit.
JD
-Before entering any room that was involved in fire, one or more inspection holes shall be made in the ceiling to check for fire extension. The ladder company officer shall ensure that a charged line is in position before any additional openings are made. The IC must be notified of the results of this examination.
