*
I’m having a new 2-story house built, which I designed, and have specified BCI I-joists, 11-7/8″ deep x 2-5/16″ wide, 16″ o.c. Max span is 13 feet. As you can tell, this will make a really stiff floor. It seems that most new houses around here use the 9-1/2″ deep x 1-1/2″ wide joist, 16″ o.c. According to BCI’s literature, one should use blocking below any load-bearing walls running perpendicular to the joists. Since I’ve oversized the I-joists by 2 or 3 steps, is the blocking still necessary? Do I need to worry about racking?
Discussion Forum
Discussion Forum
Up Next
Video Shorts
Featured Story
Listeners write in about haunted pipes and building-science tomes, and they ask questions about roof venting and roof leaks.
Featured Video
Highlights
"I have learned so much thanks to the searchable articles on the FHB website. I can confidently say that I expect to be a life-long subscriber." - M.K.
Replies
*
Hi Chris,
Down here in lovely California we use a lot of I-joists manufactured by Trus-Joist MacMillan. I usually run a Microllam board all the way around the perimeter of the floor. Then we back it up with a regular joist on parallel runs and blocking in between the perpendicular ends. It's overkill for sure but it doesn't matter then whether or not the wall is load bearing or not. If you really want to go all out use 1-1/8 tongue and groove plywood for the floor.
You can ask my clients, I really have a knack for spending their money.
Marshall Winn
*Chris,I also use Truu joist macmillian. They mnufacture a timberstrand rimjoist that is made for this. even with the large flanges the 1/2 osb in the middle is what cannot take the crush. Your local rep should be able to tell you about bci products and how to use them. Macmillian's distributor around here will come onsite and make sure all is well.I always use at least 12" joists with 1 3/4 flanges. What youare doing sounds like more than overkill it is ridiculous. I have spanned 18' with these on 16" centers and still get a soild floor. 12" joists are overkill for for 13 foot. The money could be better spent elsewhere (maybe more insulation). I dont have a problem with an overbuilt structure but I think it is a waste of resourses. Just my 2 cents. Make sure the plumber and hvac gus dont screw those joists up. this is the most common problem. Good luck with your house.Rick Tuk
*Chris - Oversizing I-joists doesn't eliminatre the need for blocking at load bearing walls. The blocking is needed to prevent crushing of the I-joists when load bearing walls stack on top of each other. Rick is correct in saying that you're overkilling things, and could use the money elsewhere. You might consider going to 19.2" spacing, which would still give you a stiff floor but reduce material costs and labor. Also, make sure you're using adequate supporting beams. Spending a ton of money on I-joists and using an undersized support beam doesn't make much sense.
*In that zone between maximum span and owl-killer use of wood, do y'all have a rule of thumb for choosing joist size? For example, pick a joist capable of 120% of actual span?Don't judge Chris too quickly -- he may have elephantine relatives over for barn dances or something...
*I think the required blocking under load bearing walls is to prevent the joists from rolling under the load. Even if you secure the ends with blocking, joists or beams will roll in the center as they start to fail. That is also why we used to have to put midspan blocking, or bridging, in many joist runs. I agree with previous posters that what you describe seems overdesigned for the span, but it doesn't eliminate the need for blocking under load bearing walls. But I'm no engineer, just a "rule of thumb" guy. - jb
*Rolling is still an issue ... crushing is the additional and more threatening problem. I noticed in researching these that sometimes the blocking is just plywood attached to the sides of thge webbing, such as where the I-joist is used as a rafter.
*Are you talkin' about "crush blocks"? When the weight is mid span, like he said, how is crushing an issue unless there is a wall beneath the bearing point? Are you sayin' the bottom chord will stay straight, and the top chord will deflect, causing the web to be crushed? Seems more likely to me that the entire joist would either sag or roll. - jb
*Ditto, ditto, ditto...Flange/web size and the need for crush blocking are independent issues. Good response on all three points, Ron.One small consideration when using 19.2 spacing is insulation. Blown-in will obviously work, but if using batts, we do a combo of 16" and 24" batts. Slice the 24" batt down to 20", frition-fit the 20" piece into joist bay. Take the 4" cutoff, combine it with a 16" batt to go into the next bay. A bit of labor, but not as bad as it sounds.
*L/720 for kitchens (to handle weight of islands over mid-joist span, cabinetry concerns, etc. Also baths if jacuzzi/cast iron weight is a concern, even in "entertainment-type rooms" where high loads will occur from Y2K parties, etc.Or, L/480 in these same areas, and doubling up joists under these areas. Depends on customer, budget, floor-depth design, etc.In my area, L/360 (trampoline) is code minimum. Most builders consider L/480 as minimum.Remember, TJI's are stong, but they can flex or suffer from the tympany drum syndrome.I've seen inspectors that still don't understand TJI's. As a result, they occasionally try to muddle up installations until the are "re-educated".All TJI installations should be designed by the comapany and installed per specs. You can specify your concerns when sending in the plans and these concerns should be addressed by the CAD person. I have had to hunt around and send the plans to several different CAD folk, though. Some enter accurate measurements and you get a tight design, some round off an 18'6" span to 15' and the TJI spec can be, let's say, slightly off? Garbage in, garbage out...
*"Web stiffener" (or crush block) was the word I was looking for -- such as where an I-joist is placed in a hanger (attached pic). The OSB just doesn't do point loads, for obvious reasons.You're right Jim -- in my mind I see continuous load-bearing in stacked walls or other load-bearing elements. Maybe he's not doing this? Sounds like a problem for engineering.Here, because the joist is so tall and flimsy from a twisting perspective, I see why you'd be concerned about rolling and block. It's not a strength problem per se. In other, simpler, words, the oversizing actually increases the need for blocks.
*Crushing tends to be an issue when the TJI is sandwiched between a plate/mud sill/beam below and a plate above, typically in a load-bearing wall. Usually it's only a factor in the first floor of a two-story structure, or in the first and second floors of a three-story structure, etc.Always exceptions to the rule...
*I haven't been around much new tract framing for several years, but when I was, in the late 70s, we used to use that metal cross bridging between joists mid span all the time. I could never figure out how it did much until an engineer explained to me about horizontal members rolling when they get overloaded. I thought he was askin' about blocking underneath a wall perpendicular to the joists, say mid span or so. I agree that crush blocks are important when you stack one wall over another creating compression on the web. Still learnin', that's for sure. - jb
*Thanks for the feedback. I am using an LVL rim joist (1-1/4"). The particular situation I'm concerned about is a load-bearing wall stacked over a crawlspace beam. I'm also concerned about rolling the joists, as their total span is almost 40 feet. Max clear span is 13'6". I do have a big kitchen island, as well as a wife who loves to do aerobics in the middle of a long joist span. Thus, I chose the joist size based on an L/960 span table "for the discriminating purchaser" that Boise Cascade provides on their website, and a 15' span (which I later reduced). Sheathing is 3/4" T&G CDX plywood, glued & screwed. I did consider 1-1/8" plywood, but my builder said it was too heavy to work with and would double the labor cost for the sheathing. I think that a 3/4" subfloor will be fine, especially since most of the first level will be finished with 3/4" oak hardwood flooring.Now, a new problem to worry about...RAIN. Yep, it's that time of year in Seattle. Of course, the seasonal rain just started the day my first lumber package arrived onsite!
*Definition: "seasonal rain" in Seattle is the rain that doesn't stop until Memorial Day.
*4th of July.
*Andrew,As for joist size I have to say I am very unscientific about it. I can tell you off the top of my head what size you need in diminisonal lumber. My Tji dealer is great, he can tell me how a floor will feel compared to a wood floor. I do look at the span charts, Never take one that will just do the job. I also consider plumbing and hvac. They are not supposed to affect proformance but why take a chance. My accounting background forces me to make a cost benefit decision. What do you get for what price? Roll that all together and flip a coin.Rick Tuk
*Generally I've been going up just one size. But these are the short spans characteristic of old houses, 6'-12'. With modern long spans I would be more conservative -- and actually would try to discourage the long span itself if I could! Chris, I don't think you're risking much at all if you've held spans to 13' or less -- invest your money in other measures.Anyone like floor trusses?
*I'm gonna give them a look for an old farm house I'm gonna reframe. I have to replace all the 2nd floor joists and I like the idea that I can get a top chord bearing truss. Have you price compared them with TJIs? - jb
*Is this the sort of blocking info you meant, Chris? Looks like just squash blocks, and they say in red no midspan blocking with BCI's neeed. (I'm looking at these things too.) Or are there different rules for the big ones?They do appear to expect stiffeners on rafters over the plate ... where a birdsmouth has been cut. http://www.bcewp.com/southspecguide5b.htm
*Yes, that's where I found the info. BC's webpage is one of the best in the engineered lumber industry.
*Yes, although I wish they'd repeat everything in the nice graphics in text form. I wondered if I might miss something unless I examined every square inch.Is my different interpretation of the graphics (from yours) mistaken? That is, blocking is only needed where load-bearing walls are stacked?
*Down here in Thurston County, the plans reviewer just wants you to produce a printout from the manufacturer to get approval. I guess they design the system for you, and the county just takes them at their word that it is gonna work. Kind of odd, huh? - jb
*Jim, The plans I get are approved by Macmilians engineers. If we cut or drill anything I have to show the inspector where it says I can do this or have the rep look at it and give me a letter saying it is all right.Rick Tuk
*How do you anticipate everything the trades will do (besides hold a gun to their heads)? Or are you just limited as to flanges and can otherwise follow the standard cutout schedules?