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Got a call from an old friend, a good builder in Jersey whose houses I used to trim. He’s building his own house, using I-joists for the first time. He used 9 1/4-in. GP I-joists, 16-in. OC, spanning 15 ft. 5 in., with 3/4 T&G screwed and glued plywood subfloors. He’s at the drywall stage, but doesn’t want to proceed because he’s worried about the floor’s deflection. He’s measured that at 1/4 in. midspan, with a one-man live load above. L/360 at that span is 1/2 in., so the deflection may not be the problem as much as the floor’s bounciness, which he describes as outrageous. Does anybody have either comments or solutions? Will drywall stiffen the floor? What will?
Thanks,
Andy
Replies
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Glue & screw plywood to the bottom or the sides....or enter the young ones in the next trampoline olympics in Greece.
near the stream,
aj
*first... sounds like a 30 ft. house.. so he's maxed out on his spans...hindsite is 20/20..hhere's what LP says (same basic product as Gp )....for stiffer floors use the L/480 deflection tablesglue..change to thicker decking (7/8)bridging , blocking, furring the ceiling below, all will improve performance...the span tables for 40 PSF (15 PSF dead load) show 15'4 inch span @ l/480also .. LP has 4 grades of joists thier lightest has a 1.5 inch flange..the next has a 1.75 inch flange.. then a 2.25 inch flange.. then the biggest is 3.5 inch flangein the 9.5 depth.. he may have either the 1.5 or the 1.75... and that makes a difference in the stiffness......bottom line.. at this stage..furr the ceiling below and a quick call to the mfr.. will yield some specific suggestions..b but hey, whadda i no ?
*Andy- I've been using these products going on ten years now- If he has excessive deflection it is a supplier or manufacturer problem unless he didn't frame per their plan. As long as he provided a plan to them showing loads present and future they should be accurate.The floors bounce like hell till loaded...They only design what you say you need...
*Andy - Short of deepening the member or shortening the span (most effective but most intrusive fixes) My approach would be as follows:(1) If he cares about the light load 1/4" deflection (probably mostly dead load), shore the joists from underneath with a temporary frame wall.(2) Cut some 3/4" plywood approximately 24" x 9 1/4" and glue/screw to the sides of the I-joists at mid-span.(3) Install solid blocking between joists, ending on the plywood (centerline).(4) Remove temporary shoring.Drywall won't do much of anything.Jeff
*Andy,
View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle
*Maxed out on the joists is an understatement. Those floors will have some deflection even after the floor is loaded above. Andy, you state this is your friend's first time use with I-joists. Did he talk to other builders? The guys I know would have steered him up one level on the product. Those tables tell you what you can safely get by with; the tables should not be the word on what you should build with.
*Joe...Did ya cheat and read my post first?....That's OK....As ya filled in all the details except the glue you recommend and how to screw all together when the web is not very thick for a screw base...But alas I do agree that done your way even with PL-400 as quick as a man can move would probably dampen the bounce...It will not add any strength though or help with the deflection (just my armchair opinion.)The manufacturers recommend attaching plywood to the flanges when a tile floor is to be layed above...Adds strength to fight deflection and will bounce less...And the idea of sheathing the whole underside of the floor would be hurricane, earthquake and Fusco tantrum stomping proof....near the stream,aj
*Your friend should have had the floor engineered for L/480. That means 11 7/8 TJI Pro 250s. I'm with Doug, they only design what you tell them. I always use 11 7/8 TJIs with great results and stiff floors and I always have mine engineered by TJI pros. I would check with the engineer for a fix before I started nailing or screewing plywood to the flange side. If this is an acceptable fix, they will have a design and nailing schedule for it. To many nails in the side of a flange can damage the member. He's in far enough already so get professional advice before proceeding.
*Are you familiar with the term "egitt"? It's Irish.
View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle
*Andy, I agree completely with kcoyner. That is what we do. We never ever use those size joists on a span like that. We always spend the extra money to stiffen the floor, especially since we use so much tile. Basically, he screwed up big time and will have to spend more money to help correct the problem than he would have spent on heavier joists. A lesson to remember for others planning on using these joists in an upcoming project!
*Hey Andy- coming back to this- everything everybody is suggesting will stiffen this floor. It will also most likely void any manufacturer warranty- if there is an issue with this floor work thru the supplier to resolve it.Just an opinion- been there done that
*He shooda deflected them at the driveway entrance with a flag...
*He shoulda used 2x10's!blue
*doug....the suggestions would void warranties?...How did you arrive at that?....Just decide to post it?near the stream...aj
*My point on attaching to top and bottom flange was to maximize depth, which minimizes deflection, and to connect top and bottom flanges reducing stress on the web and eliminating the tendency for upper (compression) and lower (tension) flanges to want to move differently. The relevant formula for the 'strength' of any beam (section modulus, moment of inertia, etc.) is bd3/12 (the depth is cubed) meaning that effective beam strength is maximized through beam i depthmore than thickness. The deflection formula puts moment of inertia in the denominator (384 EI) so to minimize deflection you want to maximize effective beam depth. Every inch of depth counts, and with full-height plywood (9 1/2" vs. 6 1/2") you make a difference. Extending the plywood to cover the middle third of the span (roughly 5 feet) is a good idea, anything more than that is probably overkill.Warranties? Well OK it's a point. But living with a trampoline is, too. 2 x 10's? lower E so more deflection than with I-joists. 2 x 10's around here are really spotty for quality.Jeff
*Thanks for all the input, guys. He's got the supplier and the manufacturer involved already, as well as his architect. FWIW, according the manufacturer's info, he isn't even close to maxing out the span for these joists. I'd suggested sheathing the bottom of the joists, perhaps even jacking 1/4-in. of camber into the midspan first. Since his mechanicals are already in, that's the easier solution. Next time, I bet he overdesigns.Andy
*TJI's do have the tendency to resonate...the "trampoline" or "drum" effect. This does go away or is reduced once a bit of load is placed upon the floor. Still, 9.25's? Designing to L/360?Having the mechanicals already in is the killer...that in itself blows most of these side-applied solutions out of the water.A bottom skin will help reduce some of the resonant "trampolining," and I fear the best bet at this time will be after all the mechanicals are in will be to structurally skin the mid-span of the bottoms of the first floor platform. If more support is needed, installing a mid-span beam w associated lally columns down in the basement would help. Like others, though, I'd listen to the what solutions the joist reps offer and counter with the ideas mentioned in previous postings.Tough lesson.
*Andy - Are these joists multiple spans over a center beam ? That can affect the "feel" of a floor. When we have floor trusses passing over a beam, we have the builder cut the top chord over the center bearing so the 2 sides act as separate spans. I'd definitely talk to the I-joist manufacturer before doing that, though. Also, an underdesigned beam can contribute to "bounciness". What kind of beam is supporting the I-joists ? and what's the post spacing ?
*Jeff,
View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle
*Andy,After reading this last night, I watched a PBS show about bridges. Watching it spurred an idea...To make a strong long span they would put criss-cross braces between the parallel beams. Even half of that criss-cross works well, and is certainly better than nothing at all.I would suggest that you cut plywood braces, maybe 4 inches wide. Glue and screw them to the outsides of the flanges in that half-cross pattern.
*Luka...that's a great illustration...the problem with your solution is in the strength of the joint...It would work if a welded fillet but needs to be whole sheets as suggested above so as to spread the load out so the glued and fastened line of attachment would indeed be able to transfer the forces...near the stream,ajnice pic though.
*Ron, yes, these are continuous joists that pass over a bearing wall for the second floor, and over some sort of girder (probably an over-specced steel I-beam, knowing my friend)for the first floor. Andy
*Andy - Web stiffeners are required at intermediate bearing points, period, per manuf. std. details.Joe - My use of bd3/ anything was to demonstrate the value of full depth. The 2 pieces of plywood look like an extra 2 x 10 (2 x 3/4" x 9 1/2") at the center of the span, where max. moment and deflection occur.I didn't understand your last sentence, sorry.Jeff
*Jeff,
View Image © 1999-2000"The first step towards vice is to shroud innocent actions in mystery, and whoever likes to conceal something sooner or later has reason to conceal it." Aristotle
*Jeff, I didn't ask about the web stiffeners, but I know this fellow well enough to be certain that the joists were installed to spec, and that he followed the manufacturer's sizing advice. That's one reason why the bounciness in the floor is such a disturbing issue to him.BTW folks, thanks for all of your advice and comments.Andy
*Andy...It was mentioned above but the bounce does go away often once the home is done and the furniture is brought in...But as you have heard here the trick is to upsize, as the manufacturers are competing on price (marketing dept and sales control the company!...and the least doable system costs closer to standard framing)near the stream,aj
*Another I-Joist issue: When running mechanicals i throughthe joists, take pains to isolate ductwork from contact with the joist webs. This can cause very strange noises during the heating season when the ducts expand and contract.
*Jeff: Up here in the cold country us smart ones have that problem solved. We use wood stoves.
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Got a call from an old friend, a good builder in Jersey whose houses I used to trim. He's building his own house, using I-joists for the first time. He used 9 1/4-in. GP I-joists, 16-in. OC, spanning 15 ft. 5 in., with 3/4 T&G screwed and glued plywood subfloors. He's at the drywall stage, but doesn't want to proceed because he's worried about the floor's deflection. He's measured that at 1/4 in. midspan, with a one-man live load above. L/360 at that span is 1/2 in., so the deflection may not be the problem as much as the floor's bounciness, which he describes as outrageous. Does anybody have either comments or solutions? Will drywall stiffen the floor? What will?
Thanks,
Andy