Asked to design a new floor over an existing floor 4

[Structintern3]

So I've been asked to do a new floor over an existing floor and have a couple of questions for you guys.


1. When you design a floor over an existing one, do you just treat the new floor as completely separate and span the new joists and such or do you rest them on top of existing? I'm thinking design the new floor as separate because it does add weight.

2. How do you know where you need blocking? You need it at panel edges right? Are the panel sheets normally 48" wide or? So do you just call it out at 48" o.c. blocking lines?

Thank you!!

 

[SWComposites]

Why is the new floor needed?
Is it just new floor coverings? or complete new joists, coverings, etc.?

 

[Structintern3]

@SWComposites

Basically to raise the existing floor height, we'll have new joists and all. The bottom of the new joists are flush with the top of the old floor

 

[KootK]

So this is a new wood floor over an existing wood floor?

How much of the existing floor will be overbuilt in this way?

You can rest on the exist or span over it depending on the situation. If you span over it, keep an eye out for details that would result in the upper floor loading the lower in spite of your efforts to isolate the two.

Sheathing is typically 4'x8'. That doesn't mean that the blocking needs spaced that way though. You may not need interior diaphragm blocking at all unless you intend to use the overbuilt floor as a high demand shear diaphragm etc.

Some sketches of your proposed condition may net you higher quality answers here.

 

[KootK]

The bottom of the new joists are flush with the top of the old floor

That condition will make it very difficult to structurally isolate the new floor from the old.

 

[JAE]

Unless you do a full analysis and design check of the existing floor framing, I would suggest that you NOT rest a new floor on top of an existing floor.

With wood construction (which is what you are suggesting it is) you would be probably adding 10 to 15 psf dead weight to the existing floor.
The existing floor may have an original dead load of 10 to 15 psf and a live load of 40 to 100 psf depending on what the space was used for and how it was originally designed.

With that - your added dead load could increase the total design load from 13% to 30% of the original. The IBC only allows a 5% increase without fully verifying the capacity.

So as KootK suggests, setting your new joists on the existing floor would either:
1. Simply add load to the original floor - ASSUMING the new framing isn't really "spanning" but simply "resting on" the original floor....OR
2. Add strength and capacity to the overall system where each floor would support a share of the total load based on their relative stiffnesses - ASSUMING you properly frame the new floor to span from supports that can take the additional load.

 

[Structintern3]

@KootK @JAE Thank you for the answers, I think it's a good idea for us to lift the new floor a bit above the existing floor so that we aren't loading the existing floor at all. Do you guys know of a good resource explaining floor framing(sheathing types/when blocking is needed?)

 

[udraft]

quote- "The bottom of the new joists are flush with the top of the old floor"
It would be desegned as separate

 

[KootK]

Do you guys know of a good resource explaining floor framing(sheathing types/when blocking is needed?)

Check this out to get a feel for the technology and report back if you feel that you need more advanced stuff: Link. Obviously, from the other side of cyberspace, it's difficult for us to know what level you're starting from.

 

[JLNJ]

Remember that the forces go where the stiffness is. Forces don't "ignore" the floor below just because you are ignoring it. If the floors are in contact, they will share the load. The manner in which they share it (that is to say the distribution of the forces) may or may not be what you want it to be.

 

[RontheRedneck]

My concern with this situation is more practical that structural. Seems to me the top floor joists would deflect and hit or rub against the lower one, and could cause some odd noises. I think they should be separated a short distance.