Vibration of Wood Floor Sysytems 4

[connect2]

Hi All,
Is anybody aware of limits/criteria for acceptable vibration of wood floor systems? This particular residential floor is composed of 20' long proprietary wood floor trusses with an additional inch and a half of lightweight concrete topping. A review of codes and standards gives lots of comments about the need to satisfy/look into vibration control but not much info about what constitutes acceptable/unacceptable vibration and nor how to calculate as in found in AISC-11 Floor Vibrations due to Human Activity. It would seem that traditional deflection requirements of l/480 for a UDL or point load don't apply in the case of this particular floor. The Manufacturer has written all over the shop drawing system that their floor joist/system has been designed for code required vibration control but now that the floor and topping is installed it is noticeably 'Bouncy and Lively' to use the Owners performance standard/criteria.

 

[structSU10]

I know a number of wood design programs made by the TJI and LVL manufacturers have some algorithms to determine vibration, but they are generally based on collected data, not hard math. I have also seen a publication on wood floor vibrations by the applied technology council:

https://www.atcouncil.org/component/mijoshop/product/205-minimizing-floor-vibration

It covers wood, and is at least a start (some in my office question its accuracy/validity)

APA may also have some literature, but not sure on that.

 

[campbelltc]

You may want to contact a guy named Dr Stacy Worley. He is now with John Deere, but I believe he did masters work on proprietary wood joist deflections/vibrations when I knew him at Penn State. He may be able to guide you.

 

[XR250]

Ugh, that is why I avoid long span floor systems, when possible. I push 2x10's and steel beams.

 

[CELinOttawa]

There has been a great deal of work in wood vibration. Some of the best work has been undertaken by David E. Allen at NCR here in Ottawa. Also, there are limits in the current National Building Code of Canada based on Onysko (1998).

Here is a good summary from the internets:

Canadian, Danish and Norwegian Instructions for Maximum Deflection under a 1.0 kN Point Load
In Canada Onysko (1998) has proposed that vibration of wooden floors should be controlled by limiting the vertical
deflection U under a static 1.0 kN point load according to the equation:
U_max = 0.6 + 2.5 * exp[-0.6 * (L-2)] ≤ 2mm (4) <-- Original reference retained; See name of paper below
in which
L = span length. This criterion is much stricter than the 1.5 mm criterion given in EC5.

From:

http://timber.ce.wsu.edu/Resources/papers/P19.pdf

Note that a simplified version of this formulation has been codified here, but I prefer the longer version. Thanks to the code, I have to run both. I haven't found any cases where I felt the code version was more accurate.

Applications: ALL wood based floor systems, including engineered lumber "I" joists and steel webs. It is a stiffness technique, so applies to all timber based floors since the modulus comes from the timber and the shape determines the moment of inertia component of EI.

 

[manstrom]

We design buildings using 18" deep 24' long prefab wood trusses all day long. By all day long, I mean 10,000s of townhouses and multifamly units. It is a pretty time tested and standard construction. Gypcrete and 2x6 strongbacks will stiffen up the floor pretty good. Non-bearing walls both above and below will help too.

You do have to keep in mind that wood is not the same as a concrete floor. Both in vibration characteristics and cost. It's a trade off, but often a worthwhile one.

One thing that the vibration equations neglect is dead load dampening. I am no expert on the subject, but I would think that dead load will dampen and mitigate vibration issues.

 

[connect2]

CEL,
CSA 0.86-09 and previous for that matter in A.4.5.5 Floor Vibration states basically that the UDL and point load criteria should be used with caution and applicable to sawn lumber joist floors and does not adequately address other floor system types and that a more refined analysis should be considered.... to paraphrase...
I'll take a look at the paper you attached although by title 'Stressed Skin....', another beast altogether, perhaps in the references? so thanks.
But yes Onysko and the NBC span tables are of course conventional lumber floor systems... 2" x whatever.

 

[CELinOttawa]

You misunderstand - There is no UDL in the original work by Onysko. The simplified version is only good for sawn lumber, and I have found even that limitation lacking.

The O86 reference is telling you to not use the simplified version. Onysko isn't perfect for any type of joist, but does do a very good job of avoiding vibration based serviceability failures in most.

If you can provide a better reference which is more specific to engineered joists, I'd very much appreciate the reference and would switch. Until then, I'll happily keep using Onysko as it has served me well so far and I know of nothing better to date.

 

[CELinOttawa]

Here's a summary from Dr. Allen (back in the day). It has evolved somewhat from here, but this should help you see what I'm talking about... Please read the light framed construction section and beyond in particular.

https://www.nrc-cnrc.gc.ca/ctu-sc/ctu_sc_n22

 

[KootK]

For what it's worth, I used to be a metal plate connected wood truss designer.

To add another reference to the mix, this text also covers wood vibration: Structural Wood Design - A Practice-Oriented Approach Using the ASD Method.

I second Manstrom's comments. 20' isn't a long span for standard depth wood trusses by any means. I've done a lot in the 26-28' range successfully. And tinkering with the strongback arrangement is a great place to start as far as a remedy goes.

I've been called out to numerous sites where vibrations have been reported in wood flooring systems. In the vast majority of cases, the drywall hasn't yet been installed on the ceiling below and the interior, non-bearing walls haven't been constructed. Installation of those elements usually eliminates the problem. Is there any chance that those elements haven't yet been installed on your project Connect 2?

For all of the research and fancy math, vibration issues are just damn hard to predict. In a wood truss, shear deflection will factor into the stiffness. And the shear deflection is affected by local flexibility at the metal plate connections. And that , to the best of my knowledge, is not considered in standard pre-fab wood truss deflection computations.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[CELinOttawa]

You're bang on Kootk... I have done a lot more vibration work than most structural engineers in consulting practices, and I have found shear deflection is quite important.

Note that repairing problem floors can be as simple as checking to see that sufficient screws were installed in floor and ceiling... Basically all residential floors are stress skin panels from a vib standpoint.

 

[KootK]

CEL's stressed skin comment got me thinking: has the flooring been glued to the trusses in this application?

As a point of comparison, CANAM doesn't like to use floor slabs less than 5-6" thick for vibration sensitive occupancies. The reason being that, if vibration is an issue, it's hard to make up ground in a steel joist with stiffness being the only parameter at play. If the steel guys have trouble with a 4" concrete floor, it's gotta be pretty tough for the wood truss guys too. Of course, that's no excuse for supplying a product that doesn't get the job done.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[CELinOttawa]

Low stiffness, hight weight floors are notoriously lively. And very, very hard to calm down. The first mode is easily addressed through damping (not dampening, by the way), but dead load is less effective on second mode and INVERSELY effective on third and higher. In high weight, low stiffness floors you can be dealing with a second or third mode issue specifically because of the high weight to stiffness ratio. Those problems are frequently unaided by added load, and can in fact quite often get worse with increased dead loads.

That comment does not address added load through partitions and other standing obstructions with secondary connections. If you can introduce load with any even little bit of secondary stiffness (fastenings, hinging effects, friction, etc, etc), you will still help calm the floor down.

 

[woodman88]

Here are some links about this.

http://timber.ce.wsu.edu/Resources/papers/4-6-4.pdf

http://www.alpeng.com/images/stories/pdfs/dolan.pdf

http://www.anthonyforest.com/pdfs/A...ng-Floor-Vibration-by-Design-and-Retrofit.pdf

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[connect2]

Thanks all for input. Thanks CEL and woodman88 for references. Woodman88 the article on strong-backs is of great interest.

No the home is not finished and as an example the basement ceiling is not yet dry walled. In any case a trip to site is planned with the floor truss manufacturer to observe the vibration and see if the installation is as planned and detailed etc. Time to go see what was actually built.

CELinOttawa, interested in your comments on vibration modes and when and when not adding mass might actually be detrimental. My curiosity was raised when I read in the Manufacturers Evaluation Report '..in cases where concrete topping is applied ... the current vibration criteria may not address all occupant performance expectations., but will need to get caught up a bit. Curiously the Manufacturer says in his literature that '..directly applied ceiling material with or without bottom flange strapping, does not significantly affect the floor performance.' Comments anybody about this? Almost everybody above seems to feel that the ceiling has an important role to play in damping vibration and improving floor performance. The Manufacturer appears to disagree.

 

[XR250]

A problem I see on a regular basis with long span trussed floors is long-term deflection. Seems the metal plate connections creep over time.
There has been some research to back this up (can't remember where a saw it)
Also, when the contractor puts a huge stack of drywall on them, they can permanently sag.
I have seen the same problems with i-joist floors under Kitchens where there are high, sustained dead loads. Much more observed deflection than calculated.

 

[KootK]

I'd be inclined to trust the manufacturer regarding the ceiling. Their opinion may be based on testing and/or extensive experience.

With vibration stuff, you're trying to alter the natural frequency of vibration of the system to get it into a suitable band. And there are only so many parameters to play with: stiffness, mass, damping. With something like the ceiling sheathing, you'll be adding mass but then also perhaps increasing stiffness as a result of unintended composite action.

Way back when, I authored a little technical bulletin on construction loading for wood trusses: Link. Overloading trusses in that way is very common occurrence. And the parameter most affected is stiffness. All the plate teeth get pulled part way out and the lumber beneath them deforms excessively.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[CELinOttawa]

Access to your link is listed as disabled there Kootk...

 

[jayrod12]

Kootk your link is dead? Or at least you haven't provided access..

 

[KootK]

Try this: Link. Don't get too excited though; it's not a very exciting read.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.