Basement wall with timber diaphragm

[Eng_zc]

Hi,

I have situation where I either use timber diaphragm as restraint for my basement walls (option 1) or design as cantilever basement walls (option 2).
Basement is 3m high and I am getting about 20kN/m lateral load at top which will apply to the timber floor if option 1 is used. I have found similar topic been discussed here and I tend to not go for option 1 unless someone can tell me this will work...
For option 2, my question is, how do I make sure that no compression will go to my timber floor. What kind of connection do I need here? Something similar to slotted holes?
Thanks.

 

[Eng_zc]

Some areas are like this.

 

[jerseyshore]

I'm not sure what the construction process for this will look like, but a lot of times it comes down to what's going to be built when.

If you're shoring up the backfill and then building the floor afterwards, it can't hurt to design it as a cantilever. Otherwise you are relying on them to temporarily shore it until the floor and diaphragm are constructed.

If you're building a new structure entirely and it will be excavated behind the wall, there's no problem using the wood floor to brace the top of a basement wall. 99.999% of houses in the US with basements are built that way. Excavate, build wall, build floor, backfill.

 

[Eng_zc]

Thanks jerseyshore,
It is a new structure. Piles and shoring walls will be constructed first and then timber floor comes after.
I know there were discussions about using the timber floor as basement walls here previously. The main concern is the axial force applied to timber floor joists or sheathing. (

https://www.eng-tips.com/viewthread.cfm?qid=221373)

So instead of using timber floor as restraint, I am thinking of designing the basement walls as permanent cantilever walls, assuming no axial forces go to timber floor. Just wondering how details should be done to insure this.

Also, since the floor joist will be designed by contractor, I also want to avoid complicating the design (especially the connection to sheathing) by providing them the axial forces.

 

[Eng_zc]

Do you mind if I ask you for a complete example of how the diaphragm is analysed and designed?

 

[jerseyshore]

The NDS is not only the wood code in the US, but is also pretty to easy to work off of. The SDPWS book has plenty of info about diaphragm design. Also, most of the NDS webinars are on Youtube so I'm sure you could find their lateral/ diaphragm one there with a bunch of good examples.

 

[XR250]

Is that service load? 'cause that is like 1360 plf in my world. Don't know how you would ever get that thru a timber diaphragm.

 

[DoubleStud]

Add some counterforts to your design and don't rely on the floor diaphragm. You can reduce that load and count on the basement wall to span horizontally as well.

 

[Eng_zc]

Thanks jerseyshore I will have a look.

 

[Eng_zc]

That's why I am concerning. I would expect axial force will go to floor joists

But that means it is a cantilever wall system, which is the option 2. I still don't know how the above details can be done.

 

[jayrod12]

Is this basement substantially deeper than most home basements? Since option 1 is done all the time for standard basements, perhaps there's a reason for the substantial load in your case, or perhaps there's an analysis thing that is resulting in larger than standard numbers at that connection.

How tall of a basement? What sort of backfill? Is 12 kPa (250 psf) surcharge expected (where I practice this is the load considered for a fire truck), or is it higher?