Timber concrete composite deck - natural frequency

[Arnuga]

Greetings!
I am designing a timber concrete composite (TCC) desk/floor - a thin concrete plate should be poured on existing timber beams, connected with TCC screws. The existing beams are parallel to each other and run in the x direction.
The static analysis was made on a T-section (a timber beam as the web, concrete plate as the flange - where the flange width is the distance between the beams). The design was made “by hand” according to Eurocode 2 and 5 taking the slip between the two surfaces into account. As a result I got the bending stiffness in the x direction (direction of the beams).
Now I would like to determine its first natural frequency with modal analysis - the standards require at least 8 Hz.
I wonder if the following model (in Sap2000) of the deck could give good results: frame objects with the bending stiffness of the T-sections are positioned in the x direction with appropriate restraints. Over them the thin concrete plate is placed – but for this area object the bending stiffness modifier in the x direction is set close to zero, say 0.01. In that way I hope, I get the bending stiffness in x directions as it was calculated by hand, and in y of the concrete plate.
I would be grateful for any comments or suggestions.

 

[msquared48]

Just curious here, but what is driving you to use this type of construction?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Arnuga]

The house is quite old (73 years), made of masonry without any RC vertical links. So the reconstruction shouldn’t drastically change the weight of the deck – using the TCC system the weight stays almost the same (basically we change the sand filling for concrete). On the other hand the thin concrete plate acts as a stiff diaphragm connecting the walls and improving the behavior of the structure in case of an earthquake.

 

[ishvaaag]

If you were able to ensure full composite elastic behaviour between the wood web and concrete head you might use some equivalent stiffness and simply apply some formula in Pilkey's or Roark's.

But maybe that the modifier is so low is simply an alternate statement of the difficulty of properly fastening the RC plate to the web, particularly because the thickness of the plate.

Anyway I don't think it necessarily impossible, just not cheap; quite likely a header plate heavily screwed to the web might work, and come with welded shear ties enough to pass the forces. This however may call for a thickness you are not considering to use, and full composite action even if mechanically ensured is not likely to be entirely attained.

 

[rscassar]

Just a question, is this form of construction (Timber Concrete Composite), is it a recommended form of construction.

How many TCC screws are required to ensure full composite action and what size timber are you using.

My recommendations would be the typical timber beam with plywood floor which can be used as a diaphragm.

 

[dcarr82775]

If you have a bending stiffness, you could always estimate something by hand and to see if you are close to the limit. I would think given a reasonable sized room the dominant mode will be along the joists, not the deck between the joists.

 

[ishvaaag]

A related paper

http://www.doiserbia.nb.rs/img/doi/0354-4605/2006/0354-46050602127S.pdf

 

[Ron]

I would expect the natural frequency to be less than 8 Hz...probably less than 5.