Hi.
Sorry if I might be recycling an old topic here……..
I wanted to ask for advice on floor trusses. I’m in the process of coming up with a design for a workshop. It will be 2 story, 24ft x 40 ft and built on a slab. In order to span the full 24 foot width, a truss manufacturer has specified a truss design for the upper floor. I believe it used 13inch deep web-style trusses on 19.2 inch centres, but I don’t have the exact figures on hand. My question is this:
No doubt the suggested truss was based on ‘minimum code’ requirements. I am concerned with the floor being somewhat bouncy and I am trying to assess what might be a reasonable ‘upgrade’ in truss design to give a floor that will feel truly solid. I have not walked on a wooden trussed floor of a similar span so I don’t have a “feel” for how the quoted trusses might perform. Any experiences out there on a similar application?
The second floor will initially be for general storage, but may well become living space a few years down the road so comfort and minimal bounce under foot will be a big plus. I’m figuring that a few more dollars now might safe a lot of grief later.
Any other suggestions for stiffening the floor would be much appreciated.
Thanks,
Dave
Replies
Ask your truss manufacturer what the live load and dead load specs are for the trusses and what the local code is. If they are the minimum you will deffinately have a bouncy floor. One thting that can be done to help is when you put walls in the upper floor, glue the plates down and screw them down. screw off the sheetrock every 6-8" at the bottom plate and this will help. It wont stop the floor from being bouncy but will help. Even if you go larger with the size of truss you wont have a "solid feeling floor". You just wont have as much bounce.
Thanks for the suggestions. I'll check with the manufacturer to see what load figures were used.
As you say, the addition of interior walls will no doubt help the situation but it'll likely be a while before they go in.Dave
The load design and deflection allowed would probably be fine for storage and they can give you those fiugures easily.But you question the issue of bounce.Vibrations are not a standard design issue that is adressed typically. boss hog here does adress it frequently and he has some good lionks and advice. Your title will atract him to comment. I'm thinking myself that for minimal vibration a 16" deep truss would be right. You might even get similar load ratings with a deeper truss laid out at 24" OC to save money and have better resistance to vibrations or bounce.
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Like Piffin said. In my house I used 12" deep spanning 38' with a support wall in mid span. They are black diamond spacing (19.2) and one of my fat felines running across the room causes something like the old suspension bridge bounce. That is in a 33 x 19 room with almost all of the heavy stuff around the perimeter. Maybe if I moved the weight to the center it would stabilize it.
If I were to do it again I would have dug down one more block and used 16" deep.
BossHog has a mini PHD thesis on floor vibration here somewhere.
Everything you need to know & more.
Joe H
I'm also doing a 24' wide workshop on a slab. At first I considered TJIs for the upper floor but at least locally I would have to use 2 layers of rock on the lid. That may apply everywhere, not sure. When I checked out the vibration using Boss Hog's I-joist vibration calculator, the extra layer of rock really put the vibration into the red zone and I would have had to go another size larger to solve that.
My solution is to use a 10x4 1/4"-wall tube steel as a center girder (flush framed so the ceiling has no beam showing) and 2x10 joists with 1x strapping or furring on the bottom. Stiffer than anything I could come up with in I joists that aren't 16" deep, and a lot cheaper too. Span is about 23'.
I got a couple of girder options in parallams and glulams but there is more deflection, they're more expensive, and they're 12" tall.
Anyway, if you use Boss's vibration calcular don't forget to add for the extra rock on the bottom if that will be required.
Dave,
Three years ago I framed a garege with living space above it that had a 25' span but we use 3-1/2" x 14" I-joists @ 12" centers and the floor was solid.
Most floor trusses I install require a continuous "strongback" most of the time its a 2x6 running through the webs of the truss and splitting all seams half way onto a truss. This is run perpindicular to the trusses. Just put 14" floor trusses in 3 identical houses 20' span still had alot of bounce to them. So bigger truss sounds in order for you, maybe 2x8 strongback nailed off at each truss and, most important do not use 3/4 T&G OSB for the floor deck anything over 16" centers for that stuff on the floor is too much.
Piffin is right that your floor trusses should be 16" deep. (IMHO)
Stilletto is also correct that strongbacks are recommended, and will help reduce vibration.
JoeH is right that I have an extensive post on floor vibration. (But I wouldn't call it a "mini thesis")
.
Looks like I'm not really needed in this thread. (sniff)
Except to add that here's a link to the thread on floor vibration:
http://forums.taunton.com/n/mb/message.asp?webtag=tp-breaktime&msg=21010.1
Your concerns about webbed floor truss deflection for a 24' span are well founded. Houses I have built have shown me this first hand. The one in particular I'm thinking of I believe was a 26" span, but definitely had some issues.
First, the comments above about vibration are very important.
Secondly though, floor systems are normally engineered for deflection in terms of values like L/360, L/480, L/600 etc. L/360 is often the code minimum required value that is used. For example L/360, 24' span means that on the total length (24' or 288") the floor can deflect length divided by 360 (L/360) or 288/360 = .8" To me, > 3/4" of movement is not acceptable. L/480 or L/600 are more in line with what I read your expectations to be.
To complicate it a bit more a value of L/360 is most often referring to live load only, but a more accurate value would be the total load, which, again for a code minimum floor might be something like L/240 which would allow 1" of sag! (Boss Hog taught me this).
So, in summary you need to consider deflection and vibration design values.
Edited 2/1/2006 7:29 am ET by Matt
Thanks very much for all of the responses/suggestions. It looks like I need to do a bit of homework here. It'll likely be a day or so before I'll be able to speak with the truss company to confirm what their design basis was. Had a bit of a blizzard here in Nova Scotia last night...lots of closures/cancellations.......today's lookin' like Shovelling Day.Thanks again,
Dave
Well, I haven't managed to get through to anyone at the truss company yet but the guy at the building supply place (who did the quote) indicated that the trusses quoted were to the 'minimum' spec. So it's already been a huge benefit getting comments from you all.I'm supposed to be meeting with a prospective builder over the weekend so I'm hoping he'll have some good input too. I'm not likely to start the job 'til late summer so I have a bit of time to hammer out a few details....but not much time.Will be back in touch after the meeting....likely with more questions than answers.Thanks again,
Dave
Well, I met with a builder on the weekend and he agreed that 16" deep floor trusses would be the way to go.
Should have an estimate from him in a couple weeks, so it should be interesting.
The main question I still have about this building concerns the siding. I'm trying to mimic an old train station that used to be around here so I've been trying to find out as much as I can about wooden clapboards (prefinished preferred)that are available these days. (Yup, I'm a woodoholic....God help me). I posed a clapboard question on another thread in this forum but there's been no feedback yet. Maybe that's an omen.
Anyway, thanks again for all the feedback!Dave