Concrete Topping on GLT Floor for Vibration - Composite Action 1

[KootK]

I'm looking to use concrete topping on a GLT/CLT floor system with the goal of improving vibration performance. And I'm wondering about the degree composite action required. Is it the case that:

1) I shouldn't bother with composite action but, rather, just utilize the extra mass OR;

2) I should employee some means of making the system truly composite OR;

3) As with steel systems, I can count on sufficient composite behavior at serviceability load levels even if I've taken no steps to ensure positive shear connection for composite behavior?

I'm discussing this with a CLT supplier concurrently but would like to source some supplier independent information as well for my own edification. Trust but verify.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jayrod12]

I'd personally be incline to go with 1). I would expect the requirements to make it truly composite may be uneconomical. Did you have an idea how you were thinking of making the connection?

Provided you get a non-planed top surface, I would expect the typical adhesion between the concrete and the wood to give you some sort of appreciable service level connection between the two. But I would have no idea how to quantify that.

 

[JoshPlumSE]

Structure magazine had an article about vibration on wood floor systems a few years ago. You might look up that article. If I remember correctly it was mostly just a re-arranged set of the AISC DG-11 formulas. But, there could be more that I'm not remembering.

Also, the author was a professor in the northwest somewhere. He might be a good person to contact about these questions. I'll see if I can locate an on-line reference to that article.

 

[JoshPlumSE]

Couldn't find the article. I think it was the (now defunct?) Structural Engineer magazine. But, I believe I found a reference to the authors. Not sure if it helps or not.

Daniel Dolan, Ph.D., P.E., is Professor of Civil Engineering, Wood Materials &
Engineering Laboratory, Washington State University, Pullman, WA 99164.

Frank Woeste, Ph.D., P.E., is Professor Emeritus, Virginia Tech University,
Blacksburg, VA 24061.

CLT may be a little different. But, maybe these authors have looked at it as well.

 

[JoshPlumSE]

There is a Structure magazine article on CLT that has some coverage of Vibrations though.

[URL unfurl="true"]http://www.structuremag.org/wp-content/uploads/2016/05/C-StrucDesign-Breneman-Jun161.pdf[/url]

This references another 2013 article about a CLT project that used DG-11 designed by SOM. So, the authors of that article might have some more insight into your question.

They also have reference to Hamm et all 2010 which specifically talks about topping slabs.

 

[Tmoose]

"the goal of improving vibration performance."

Is this an existing installation ? Residential ?

If so, what are the vibration issues ?

If not, what vibration issues do you anticipate ?

 

[KootK]

@Josh: thank you kindly for all the info. It may indeed come down to contacting the authors of some of those papers.

Did you have an idea how you were thinking of making the connection?

Screws came to mind but I hadn't given it much thought yet. Like you, I'd like to avoid having to make a real composite connection for economic reasons.

Is this an existing installation ? Residential ? If so, what are the vibration issues ? If not, what vibration issues do you anticipate ?

It's a new office facility with the anticipated issue being footfall vibrations annoying computer jockeys.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Ingenuity]

Screws came to mind but I hadn't given it much thought yet.

Should you go the composite route, this paper may be of relevance: TESTS ON SHEAR CONNECTIONS IN PREFABRICATED COMPOSITE CROSS-LAMINATED-TIMBER AND CONCRETE ELEMENTS by Jacquier and Girhammar, 2012.

The authors report on an experimental test program on different shear connectors to be used in floor elements where prefabricated concrete beams are to be connected on top of cross-laminated timber (CLT) panels. Not exactly your situation.

I don't have a link, but if interested I will upload.

 

[molibden]

I can give you an advice from practice. In Europe we have used CLT for quite some time now. Me personally never used composite concrete-CLT slab but certainly would if I there were long spans in an office or residential building with little to zero damping. Also you need an owner who is willing to cover the extra cost (which is debatable because certainly there are cases where it is actually cheaper to use composite slab).

For composite action I would use the HBV system from TiComTec or screws made for this application, like VB screws from SFS Intec (also Wuerth has them and probably many others).

In Europe the standard slab with CLT would be like this:
[ul]
[li]floor finish[/li]
[li]floating cement screed 50-80 mm[/li]
[li]PE foil[/li]
[li]sound insulation 30-80 mm[/li]
[li]CLT slab[/li]
[li]visible timber slab or ceiling cover[/li]
[/ul]

Floating screed is not connected to the slab in any way, but it adds mass and provides damping around 4%, based on research by Hamm, Richter and Winter. This research has led it's way to the National Annex of Eurocode 5 in Austria and I think the method presented will be soon adopted by the Eurocodes. Many european producers of CLT have their own software to design CLT slabs, also to control vibration based on this research.

A lot depends on support conditions and internal walls. If there are none internal walls, little furniture, long spans, simply supported, there is a big chance of problems with floor vibration. I would be very careful because recently I had a case like this, where the designer designed everything to code to a point I would say he overdesigned, but didn't check vibrations. The floor vibrates so hard they are considering supporting the slab mid-span and send the check to the designer. For long spans and high demands I would definately look into ribbed CLT slab to boost stiffness.

 

[dik]

and, obviously, check to see that what you end up with has the dynamic characteristics you are looking wanting. Can you use a plywood or something that will help tie the system together so it is more composite? adhesive and fasteners?

Dik

 

[KootK]

I don't have a link, but if interested I will upload.

Thanks for pointing me to that. I found the paper easily enough I think: Link.

I can give you an advice from practice. In Europe we have used CLT for quite some time now.

Thanks for taking the time to post such a detailed response molibden. It's a great help. Sounds like non-comp is probably the way to go. I'd never even heard of ribbed CLT prior to this post. Bloody cool.

Can you use a plywood or something that will help tie the system together so it is more composite? adhesive and fasteners?

It will indeed have glued and screwed plywood. I've been getting mixed messages as to whether or not the plywood is necessary for the thing to be useful as a diaphragm. Anybody have any insight to share on that front? That seems to be the common approach but, with some interconnection between GLT panels, it's hard to imagine that not being the stiffer path for any applied lateral loads.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[molibden]

KootK, it is my pleasure if I can help and give something back to the community.
You can do a ribbed CLT slab also on-site with gluing and screwing timber glulams to bottom or top of the slab. If there is glue and under right conditions one can presume rigid connection, if not then you rely on screws (perpendicular or under inclination) and consider shear slip with connection stiffness.

For CLT diaphragm you can make all joints work. I like half-lapped joints because it's simple and more rigid vertical direction than with plywood. Just make sure you have sufficient distance from slab edge to screws and sufficient number of screws. Most common we put self-tapping screws diameter 8mm @ 250-300mm.

 

[KootK]

Beauty. Whenever someone goes to the trouble of posting graphics for me, I just assume an incurred personal debt of 0.25 billable hours min.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.