Private Dwellings Ch. 6 Lightweight Construction Flashcards
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
1.4
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?
1.5
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
1.6
hour
20
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.
Fill in the blanks
1.7
5-10
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?
1.8
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 Knowledge
Lightweight Construction
General Construction Deficiencies:
2.1.2 Inadequate fire stopping Improperly installed metal gang nails Inadequate bracing Out of plumb structural components Cracked or damaged truss components Unauthorized alterations Unprotected wood floor joists
What is a trussloft?
2.2.3
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?
2.2.5. A
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
2.2.6. A
500
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?
3.2
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?
3.2.1
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
3.2.2
3/4”
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?
3.3.1
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.
Metal C-Joists
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?
3.4.1
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?
3.4.3
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.