Vibrating Floor Over Post/Truss/Beam
Hi Gang! I’m a first timer here….an old guy, and Fine Homebuilding subscriber for a few years. I’m having trouble with vibration in floors with parallel cord trusses. This is a new house, custom built from scratch. Here’s some details:
It’s been going on about 2 1/2 years and getting a little worse…working with the contractor who helped me, the truss maker, and the carpenter/framer, after an independent engineering review, to solve the problem. This is a two story house, with most of the house on a slab, but about 1,000 sq ft over steel post, glue-lam beams, and engineered parallel cord trusses that cover the equivalent of a walk-out basement. House on a slope, built on limestone and caliche, “walk-out” is common in Colorado, N. Virginia, etc…but fairly rare here outside of San Antonio, Texas.
Trusses are stressed right for weight, 19 1/2 inches center, about 25 ft long, lower hung on ledgers and load bearing perimeter walls. Deck is 1 ” all weather, with backer board screwed down, and liquid nailed, then covered with travertine. One large room is about 25 ft square, and the adjacent one is about 21 ft square, and the other is smaller. Nothing sags. No stress cracks. But, walking across the rooms causes everything in cabinets and on tables, etc to vibrate and rattle.
Carpenter forgot strongbacks at first. Access still available underneath. Installed 2 x 6 s every eight feet. Reduced vibration, but not gone. Six months later it is back and worse.
Any other suggestions before we empty this basement to put in many more strongbacks?
Any diagnostics to run? Tricks? Things to look for? “Finger Pointing” going on now!!
Thanks, Jerry G.
Replies
Boss Hog is our resident truss guru. I will do a shout out to him in the next post.
[email protected]
Your expertise is needed.
[email protected]
what's the depth of the trusses? Got any pictures?
John,These are 18 " deep cross-webbed trusses, with 60 psf rating, and the correct "arc" still there to show there is no overloading going on.
There is no support beneath the trusses, but the 18 x 5 1/3 glue-lam beams and steel posts form perimeters of support around all of the rooms. The trusses do not have double thick top or bottom chords.Thanks,Jerry G
http://forums.taunton.com/tp-breaktime/messages/?msg=110545.1
You can start here, then try that useless search thingee and try "floor vibration" with bosshog as the poster.
Just guessing, but your trusses aren't deep enough.
Joe H
Edited 9/27/2008 10:58 pm by JoeH
Thanks Joe,Still reading Boss's reference. These are 18 " deep, cross-webbed trusses, with 60 psf rating, and still with the correct "arc" to show that they are not overloaded. They do not have double thick chords on top or bottom. The spaces are free-standing where the trusses are, with the 5 1/3 x 18 glue-lam beams and steel posts around the perimeters of the rooms.Thanks,Jerry G
Lotsa questions -
Are they wood webbed trusses, or I-joists?
Most importantly - How deep are they?
What do you mean by "lower hung on ledgers"?
As John said - Any pictures? That would help a lot.
.
If you already have strong backs at 8' or so, adding more will not help.
.
Here's a thread on floor vibration that should help you understand what's happening.
http://forums.taunton.com/n/mb/message.asp?webtag=tp-breaktime&msg=21010.1
Thanks Boss,They are cross hatch, with std metal plates at all intersections.
They are 18" deep. They are 60 psf rated. They have not lost their
"arc". An article I read said if they are not hung by the top cord,
but are hung with metal brackets to a ledger, or sitting on top of
beams, walls, etc, they are called bottom hung (may have the term wrong). If bottom hung, one article said the strongbacks go on the bottom cord of the trusses.An old Fine Home Building article said the brackets can be loose
on the ledger. Another said that parallel cord trusses are not designed to stop vibration.These floor spaces are free-standing, with the glue-lam beams, and the steel posts around the whole perimeter of the rooms....so there is a big glue-lam between each room for major support.Since these babies are so long, should the cords have been double thickness? It ain't gonna fall down..but "when mama ain't happy, no one's happy"...Thanks, Jerry G
An 18" deep wood webbed truss should have no problem spanning 25'. And there's no reason to think that the chords should be doubled.Just had a thought - The strongbacks you put in should be installed in the vertical position, so they're 5.5" tall. If they're laid flat they're all but worthless. Here's a picture:http://www.sbcindustry.com/images/pubs/large/sbr-t.jpg?PHPSESSID=o6fii47062ahv26b36ppm3l8b3.In the floor vibration research articles I've read, they say that beams can contribute to the vibration of floor systems. Since it sounds like your floor trusses hang on beams on both ends, I wonder if that's where your problem is. (But I admit, I'm just guessing)On this web page it talks about that very thing: (About half way down the page)http://www.gostructural.com/article.asp?id=1818There are a lot of calcs there that show how floor members and beams interact. An engineer could use that formula to consider if the beams are where your problem lies.Or you could stick a temporary jack post under the mid span of one of the beams and see what that did. If it made a difference in the "feel" of the floor system, then you would know you hit on the source of the problem.I'd still like to see some pictures, if you can come up with them.
If you want a place in the sun, you have to put up with a few blisters.
Boss,
What's the details for attaching the strongback other than on edge? Nail tyep/length? Glue? Piffin screws?
The usual recommended attachment method is three 10D nails.
Only the winners decide what were war crimes [Gary Willis]
Installed 2 x 6 s every eight feet. Reduced vibration, but not gone. Six months later it is back and worse.
Just nailed on? Glue and screw (or bolts) when you repair strongbacks like the deck was done.
From "and getting a little worse" sounds like stuff has worked loose and original "liquid glue" has hardened and cracked.
Thanks. Have been dreading anything linked to the backerboard. Forgot
to mention that the 25 ft square room and 21 ft square rooms have no
beams within the spans...the big glue-lams are between the rooms, with
trusses hung on ends, parallel to the big beams.Jerry G
Junkhound,My kind of name!!! Resembles my big room underneath my vibrating floors!!!!!Will check all of the strongback contact points...were nailed with guns and mucho mucho nails per contact point, but, who knows? Have also read that the hanger brackets that tie one end of the trusses to the big ledger bolted into the vertical slab wall might be ever-so-slightly loose.Thanks,Jerry G
Boss just mentioned what I would do. If the experts haven't solved this in six months, I'd just start experimenting the old fashioned way. I'd go to Home Depot and pick up $50 worth of lumber and start shoring up different parts of the assembly till I found the culprit. Trial and error might not be highbrow, but it works!
Boss has some good questions,
What deflection was the floor rated for? 360 or 480
My guess is the problem is the 25' or the 21 foot span. Even with the higher deflection of 480 if the trusses run 25 feet you are looking at 5/8 of an inch of deflection. If the floor is rated for 360, then the deflection is over 3/4 inch. If the span is the 21 feet then you can subtract a little from those numbers but not much. IMHO you need to add some support under the floor at mid span, i.e a beam, either free spanning, or with post's to take the load off the floor joist and to cut the deflection in half, The beam must be sized correctly maybe a second engineer? But not having any clue as to what your trusses were designed for and if they can handle a mid support beam I'm just guessing.
My free advice is worth excatly what you pay for it
Good luck
rob