I’m getting ready to put a second story on my cape and the architect has speced 12 inch i joists with 3 1/2 in flanges for a span of 20 ft and change. I’m wondering if this will result in a sufficiently stable floor or if it will be a little bouncy. Would rather move up to 14 inch deep if it will mean a sturdier floor. Any help is appreciated.
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I did a thread on Floor Vibration that is worth reading over.
I don't like spanning 11 7/8" I-joists more than 18'. But I'm pretty conservative on floor design.
The wide flanges add to the strength, but it is still marginal for vibration
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According to Weyerhausers span tables http://www.ilevel.com/literature/TJ-4001.pdf
what your designer is spec'ing will span 23'8" at L/480. You could stick with the 11 7/8 joist and go to a 360 series TJI 2 5/16 top and bottom cord joist and still span 20'11" at L/480. Depending on how big your floor is that you are doing.....I think I would consider going 12 inch centers if you are that concerned about stiffening up the floor.
Definately bookmark that weyerhauser website. There is a wealth of information there!!!! I find myself there on a regular basis.
Sam
if yopu will take a look at Boss Hog's links in the vibration thread, you will see there is a difference between the load/span relationship dealt with in codes and tables VS the vibration issue which is what was asked here. Loads may be satisfied with fairly small I-joists, but over a long span like this, vibrations can still make a safe floor quite uncomfortable.
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Look at the cost difference on moving up to the 14" I-joists. Probably not much and almost definately cheaper than running them at 12" OC or doubling every other joist. If you don't have a problem with rising the floor 2" then it's probably well worth doing and may actually save money if it allows you to go with a smaller flange.
The engineer will specify something that will work but may not always specify the least expensive thing that will work and provide optimal stiffness. Probably the architect had a preference for a thinner floor. Many do like to keep stairs as shallow as possible, tighten up the windows on the facade, etc... It's all valid but you may end up paying a premium for those two inches and end up with a bouncier floor.
We use 16" OJ-2000's whenever we can, even though they have bad noise transmission characteristics, largely because I'm a plumber and an energy nerd who likes to have all the ductwork and plumbing in the conditioned space. While your at you may want to consider 14" floor trusses and make your plumber, electrician, and HVAC guys happy.
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"You cannot work hard enough to make up for a sloppy estimate."
Speaking of floor trusses.
What is the cost comparison between floor trusses vs I-joist vs dimensional? And where to you source trusses?
In my area a 2X10X14 is $.87/lf, a 2X12X14 is $1.09/LF and an 11 7/8" LPI 20plus is $1.85/lf. Of course cost per LF can't be used as you need to figure on cost per SQUARE foot as engineered lumber can be spaced further apart then dimensional lumber for the same deflection criteria.
Don't know about floor trusses as I've never used them.
Runnerguy
Edited 9/1/2007 5:01 pm ET by runnerguy
Edited 9/1/2007 5:05 pm ET by runnerguy
One little trick to use when designing a floor with I-joists is to use the Canadian span charts if available.
Their code requires a 225 lb pt load at midspan to account for vibration.
http://www.ilevel.com/literature/TJ-4500.pdf
Edited 9/1/2007 5:36 pm by MiCrazy
Edited 9/1/2007 6:41 pm by MiCrazy
The rule of thumb for floor system costs is:
dimensional: cheapest$I-joist: medium $Floor trusses: most $Trim Joist floor trusses: even more $ (just to throw ya a curved ball) :-)
That's just a rule of thumb though. On the houses I'm building now I wanted a 14" floor system because of the spans required (up to around 19'), and to get a happy medium between fitting mechanicals in there and not too many steps. I actually ended up getting the 14" floor trusses cheaper than I could get 14" I-joists once I shopped around. I think the floors preform very well. Dimensional lumber really was not an option because of the required spans and the need to put mechanicals inside the floor system.
Often times there is NO straight forward cost comparison because we use thefloor trusses so often to eliminate a post and beam situation to do what can't be done with solid lumber. Then there are the labour savings that can come into play too.
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
I think Matt has it right but you need to look at the system as a whole. We're working on a design right now that has a number of different variables on the second floor. If we go with dimansional we'll be looking at adding an I-beam so the joists can run the short dimension. The I-beam and dimensional would still be cheaper than the floor trusses (esp if we go with the trimmable and steel-free OJ 2000's I prefer) but that would not allow me to have the HVAC supplys in the edge of the loft like I want. We could do it with 16" I-joists and get one supply only and no steel beam. Or we could decide to run load bearing columns down through the ends of the kitchen island and do the whole thing with dimensional lumber, cool the house with mini-splits and the heck with the duct work. (the main floor is a solar/rinnai heated radiant slab.
In general the lumber is cheapest but you need to think about the system as a whole including HVAC and plumbing considerations.
Hope this is helpful
M------------------
"You cannot work hard enough to make up for a sloppy estimate."
>> cool the house with mini-splits and the heck with the duct work <<
Mini-splits???????
<< Mini-splits??????? >>
Ductless HVAC units very high efficiency now available
We used to call them "Sanyo's" they are now made by Amana and Mitsubishi amoung others at more competitive prices than when they were new. They can be useful for dehumidification and spot cooling in super efficient homes with very small cooling loads. Some versions are available in a verticle PTAC- style configuration that can be ducted to as many as four supply registers. For very small and efficient homes they may well be all you need.
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"You cannot work hard enough to make up for a sloppy estimate."
Edited 9/2/2007 6:28 pm ET by ShelterNerd
Sorry it took so long, my computer is on the fritz. Thanks for all the advice. I talked to the architect and told him I want 14 inch deep I joists for a more solid floor. Thanks again.
As Piffen said, having a floor to code and having a floor that doesn't bounce are two different things.
I Joists are cheap. Better to buy a few more and have a solid floor.
Runner Guy
You need to consider your steps when planning for deeper joists. It could push you to needing another riser & tread which is something that you definitely need to know about...
take a look at this thread: http://forums.taunton.com/tp-breaktime/messages?msg=94227.1