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Type V Construction
The next type of construction that we are going to talk about is Type V, or wood frame construction. This is the most commonly used type of construction in most jurisdictions. This type of construction is typically associated with residential occupancies, namely single-family dwellings, but many commercial buildings are now built using wood frame construction.
Years ago, wood frame meant real dimensional lumber. A 2×4 was really a two inch by four inch piece of wood. Roof systems were stick built with rafters of dimensional lumber that were connected with a ridge beam. We forget that this traditional type of construction is wood frame. Balloon frame is also a type of wood frame.
We have just gotten so ingrained that wood frame mean light weight, or now commonly known as “low mass” construction. This “low mass” construction uses engineered products that makes construction faster and cheaper to build. Although these components like engineered I-joists and roof truss systems are very strong for the engineered loads, they fail miserable during fire conditions.
We just need to remember the hazards and myths of this type of construction. One is that if one truss fails they all fail. I like to ask the classes that I teach if any of them has seen a house constructed with wood truss systems with a part of the roof burnt off and the rest of it still intact? The answer is always yes.
I am not saying that these truss systems are safe, quite the contrary. All I am saying is that we need to keep our firefighters educated about these systems. I know there are some chiefs and instructors upset with some of this, but we can operate on and under these roofs with some careful size-up and thoughtful tactics. One thing I have learned in the fire service; never use the words never or always. There is always a circumstance or situation that will challenge both.
Now, characteristics about this type of construction. There are void spaces everywhere and these components use a lot of glue to help keep them together. These buildings are getting bigger on the residential side and this construction type is very popular for many commercial buildings like fast food joints, restaurants and strip malls.
Something we need to keep in mind also is that some of these wood frame buildings look like masonry or brick, or type 3 buildings. These are just veneers and we need to understand the challenges dangers associated with that. Masonry and brick veneers can easily collapse of the foundation and still kill or seriously injure firefighters.
Pay attention to your area and be familiar with the buildings you may have to operate in. Know the different characteristics of the construction types and the challenges each one poses for us operationally.
Train hard and stay safe.
Replies to This Discussion
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Permalink Reply by Jack/dt on
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Jason,
This PDF http://www.sbcmag.info/Archive/2003/jun/0306%20Publishers%20Message... has some interesting information on metal gusset failure and other issues regarding lightweight construction.
As I indicated in your Type IV Construction post -http://www.firefighternation.com/forum/topics/type-iv-construction?... the data/information I could find indicates that the glue used in lightweight construction doesn't fail when exposed to flame/heat.
http://www.nrc-cnrc.gc.ca/obj/irc/doc/pubs/tt/tt1131.pdf
One question I always have with regards to lightweight construction discussions is the mention of voids. I'm not aware that there are anymore voids that were found in legacy construction. Attics, while may be considered 'voids' are a typical aspect of residential construction. Unless one is building double walls for sound insulation, the typical wall, interior or exterior, legacy or lightweight are still constructed in similar manner with no increase in void spaces. Walls and ceilings once dry walled create void spaces between the joists or studs.
I've only once ever seen what I would call a true void space, this was in a large SFD (50,000ft sq - no typo) and the living room was stepped up on a low knee wall to create a large void space between to accommodate electronic components for the home sound system. The ceiling beneath was a finished room.
In some of the larger (5000sq ft) 2 1/2 story SFD's in my area, the "attic" space is carved up into living space, for office, den or sleeping quarters. I have seen where in the spaces, doors in the knee wall open into attic space that surrounds the living space.
Those spaces around plumbing components that rise up through the floor plates (or through joists), for supply lines and the stack/soil pipe as well as for running wires up to the floors are voids and could allow for smoke and fire extension. In my town the building code requires all those voids to be sealed with a fireproof (red) caulk.
If I beam floor joists are used one of the methods of installation is to use hot dipped galvanized steel joist hangers. These hangers react the same way as the metal gusset plates on trusses. That being said, there is some question as to how quickly these connecting components actually fail. They will fail with sufficient exposure to heat or flame, and more quickly than conventional framing methods but if they are protected by dry wall then the time to fail is extended by the time to fail of the drywall.
You make an interesting point about the fallacy of if-one-roof-truss-fails-they-all-fail. A truss system is engineered to support a particular roof load, that of all the structural members, sheathing and shingles as well as expected snow loads. Remove one or two trusses and the roof will not collapse but it will be structurally impaired. Over some period of time the roof would begin to sag where the truss(es) failed (removed) and would weaken the overall structure. But this would be over time. In a fire in an attic multiple trusses could be effected, with those trusses failing and collapsing but it doesn't imply that all the rest of the trusses would then likewise fail. The roof does begin to lose its lateral rigidity though and adjacent trusses could then fail/fall laterally, effecting a domino affect. However, a fire that starts or gets into the attic can burn unnoticed for a long period of time, which can allow the heat/fire to weaken many if not all of the trusses, under such circumstances one can predict a catastrophic roof failure.
Certainly attic fires (point of ignition) are not all that rare but neither are they all that common. Extension into the attic space and impingement on the framing is almost always a result of the failure of the drywall ceiling beneath, from a fire started in someplace other than the attic. However, in many new homes mechanicals are installed in the attic; air handlers, some heating/cooling components and even water storage tanks may be located there. It's these additional loads (that may or may not have been engineered for) that can now pose an additional hazard.
One other *hazard* I've seen in these large McMansions is multiple staircases. In one home, there were three. One main, 'grand staircase', a lesser, secondary one off of the kitchen and the third leading from the second floor to the third, attic living space. All were open staircases and communicated directly from floor to floor.
http://www.lbfdtraining.com/Pages/buildingconstruction/woodframe.html
This site has some good information - http://www.gp.com/build/product.aspx?pid=1390
The attached PDF by Georgia Pacific has some great information and details on lightweight construction. - Attachments:
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127520-1.pdf, 3.1 MB
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Permalink Reply by Jason Hoevelmann on
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Jack/dt,
Great points and yes, I agree about the roofs and the role that the connectors play. The post was one I did after a conversation with another chief about operations. You have some really good information in your post and I hope everyone takes a look at it. In regards to the Mcmansions, that is for another post, but it is something I have been teaching and instructing on in regards to how we approach these. These are basically commercial buildings that pose as homes. : )
Thanks again for all of your comments and hopefully people are learning from them.
Jason
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Jason,
The fire service (IMO) seems to be stuck in a loop of conventional wisdom, i.e. that lightweight (low mass) construction now has "void space" issues. In any article I've read on the subject, invariably the first issue is rapid burn through/collapse and the second is about the dangers of voids. Yet as one reads through, the voids mentioned are almost exclusively attic and basement. Unless I'm mistaken, we've had both for nearly as far back as we've been living in houses.
In my parents house, the basement was unfinished which means that the floor joists are all exposed. Probably with few exceptions this was the norm. Likewise attics were continuous, open from gable end to gable end. Both of these *voids* have always allowed for quick and robust fires yet today they appear to be a "new" threat.
One of the new hazards that keeps showing up now is the use of spray on expansive foam insulation and it's part in hotter fires. I'm not disputing that (although I haven't found anything yet as to exactly how flammable it is) but, think of its application: Expansive Foam Insulation, especially when used in the attic to reduce heat loss through the roof, is eliminating the need for soffit and ridge vents. As a result, a home without ventilated soffits has eliminated the earlier exposure routes of flames auto ventilating through windows and being drafted directly INTO the attic. Yet I have not seen that particular issue addressed anywhere. Makes me wonder if the fire service as a whole has just switched one set of blinders for another.
When you consider that new home construction is tighter, draft infiltration significantly reduced and chases required to be caulked, routes for smoke and flame extension has been significantly reduced. Concurrent with that are more stringent overall building codes, especially with regard to wiring. The overall affect appears to be far fewer structure fires, albeit those that do ignite in fact burn hotter and faster. But nonetheless one could deduce that the reduction of home fires is a direct result of new homes being built better.
A comparison one might make is the difference between a 1960 Chevy and a comparable 2011 Chevy. The old, "legacy" style manufactured used heavier steel and more robust construction techniques, i.e. they were built like tanks. But lacking seat/shoulder belts, crumple zones, collapsible steering columns and air bags, the fatality rate was significantly higher. In the new cars today one can survive with little or no injury a crash that would have killed instantly 50 years ago. Yet instead we focus on hardened steel reinforcement bars, hybrid and alternative fuel issues, kind of like how we focus on time-to-fail joists/trusses and void spaces without accepting that the new homes are, overall, more safe.
I haven't gathered the facts but, I would bet that floor collapses kill or injure more firefighters than roof collapses mainly because more fires start in basements than in attics and as we've been taught, working above the fire is the most dangerous place to be.
Personally I'm more concerned working in an older, legacy built house because the building codes were more lax and homeowners modified and added on without regard to proper (at the time) building standards. But all things being equal my bigger concern with the newer homes (at least in my area) is their size. In a 1950-60's home you typically had around 1200-1400sq ft to search. In today's McMansions, 1200sq ft is just the living room. Greater chance for disorientation and extending in beyond the limits of your air supply.
Ultimately it IS about education. Low Mass construction is here to stay (and will only increase, probably with even lower mass). Understanding how a building is built is essential to understanding how a building burns. You don't have to be a fireman to be a carpenter, but helps to be a carpenter if you're a fireman.
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Permalink Reply by Ben Waller on
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One of the places that you can find truss voids in modern Type V construction that you don't see in legacy construction are floor truss voids.
I have such a void in the 2nd floor level of my house. You can also find them on the 1st floor of engineered lightweight construction homes that have basements.
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Permalink Reply by Wade Seely on
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The one place I have seen voids with light weight constructon is in the new modular built homes. These homes are constructed in pieces off site and then put together like legos. Where you end up havingthe void spaces is between floors you can have a 2 foot open void throughout the entire house. This coupled with the glue that is used can lead to faster fire spread.
Here is a link to an article on it from fire engineering
http://www.fireengineering.com/index/articles/display/361520/articl...
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Ben,
You've piqued my curiosity; what is the nature of the void space in your 2nd floor? Has the floor been raised to meet a new addition? Is this new construction or renovation?
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New construction, circa 2003. The entire 2nd floor assembly is made from parallel-chord wood trusses with metal gussets. The void opening is approximately 6 inches high and extends from Side A to Side C. All of the 2nd-floor plumbing and most of the electrical run through the openings in the trusses.
These truss floors are very common on 2nd (and higher) floors in multistory new SFDs in the southeast. We have literally hundreds of them in my area.
We don't have a lot of truss floors for the 1st floor in these - basements are rare here due to the sandy soil and high water table. Most houses are either built on slabs. The 1st floors that do have trusses are usually elevated over a crawl space or soft (open) 1/2 first floor.
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I haven't seen these used...yet. I agree, significant void space for extension.
Clearly not much to be done about it but, does local code call for all entry points into the floor void to be fire caulked?
Curious; what's the truss spacing, 24 o.c.? Decking; 3/4 cdx, osb? Is dry wall attached directly to the bottom chord or resilient channels used?
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Truss spacing is 18" OC. The ceiling is sheet rock mounted to RCs, then taped and mudded, then blown popcorn ceiling cover applied.
The decking is 3/4" plywood.
The wall headers and truss ends are tied together with hurricane tie-downs to meet the local storm code.
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Your code require fire caulking entry/exit points?
D/F on P/T, is that a cantilevered deck?
So what color palate did you choose ;-)
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The photo isn't from my house - it's just representative of the hurricane ties.
The vertical fire entry points must be caulked, but there is HVAC ducting and electrical that can either start or transfer heat/fire in the void spaces, even when the code is met.
The color palate - pale green siding with forest green doors and shutters, white trim, deck posts, and gutters, and green/black speckled shingles.
The mailbox is black on a white 4" x 4" post assembly. We grew jasmine over the mailbox - it's kind of plain. :-)
You can spot the house from way down the street - it's the only one in the neighborhood with the Fire SUV in the driveway.
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