16' or 8' or 4' Retaining Wall?

[Apache1]

Hi everyone, I'm designing a foundation wall (see picture below). Curious how you all would design a wall like this. Is it an 8' tall retaining wall? Is it a 4' tall retaining wall? Depending how I design this I will get wildly different results for the rebar in the wall and the footing size.

This is an approximately 25' long by 13' wide structure with full height concrete walls with wood-framed trusses bearing on the top. What I'd like to do is make sure the front and back of the wall are backfilled at the same time and then consider this a 4' tall retained height to get a 2' wide footing at the bottom.

What do you think?

 

[jhnblgr]

Bring the footing up to slab height, insulation under slab to protect from frost if unheated. Backfill will 3/4” clean stone at a 45 degree slope back and install drainage. Slab needs to be installed prior to backfill and don’t drive equipment around the building prior to roof install. Roof will provide later restrain and will have significant leverage. Check footing design and throw a jog into the wall to provide additional strength. All likely overkill.

 

[jayrod12]

He may need the depth for frost. Where I practice the frost is routinely over 6 feet. 8 foot basements are the norm.

This wall has the loads on it that are there. The roof diaphragm would provide support for the top of the wall.

 

[sdz]

[B]jhnblgr[/B] has a good suggestion to bring the footing up to slab level.
Can the roof provide lateral restraint? What if roof need to be removed? I would tend to not rely on the roof providing restraint and allow some movement at the top of the wall.
What about drainage, exterior loads, property boundary?
One possible arrangement is shown below. ​

 

[BridgeSmith]

Is it an 8' tall retaining wall? Is it a 4' tall retaining wall?

Neither. It's a 12' tall non-gravity cantilever retaining wall with 4' of embedment. It would typically be anaylzed as such, similar to how a sheet pile wall is designed/analyzed. The 4' of soil inside the foundation and the friction between the soil and the footing provide the resisting forces. The 8' of soil on the outside and the wind load on the 8' of wall above are the driving forces.

However, if the top of the wall is restrained by the roof system, then you have the equivalent of a tie-back anchor, which can be included as a restraint or ignored.

 

[JAE]

I wouldn't even call it a cantilever retaining wall - the roof is there to laterally take out any horizontal loading from the soils, assuming you can design the diaphragm to transfer the loads properly and also to be stiff enough.

 

[jayrod12]

Not counting on the roof, assuming it has plywood, is being overly conservative in my mind.

What if roof need to be removed

What if? You can't plan for future removal of structure in every project, or else you must be panicking all the time. What if someone busted out the slab adjacent the wall to put in a new sump pit? I've seen that all the time but don't plan for it at the original design stage.

 

[brucesilver]

Hi everyone, I'm designing a foundation wall (see picture below). Curious how you all would design a wall like this. Is it an 8' tall retaining wall? Is it a 4' tall retaining wall? Depending how I design this I will get wildly different results for the rebar in the wall and the footing size.

This is an approximately 25' long by 13' wide structure with full height concrete walls with wood-framed trusses bearing on the top. What I'd like to do is make sure the front and back of the wall are backfilled at the same time and then consider this a 4' tall retained height to get a 2' wide footing at the bottom.

What do you think?

If you're backfilling both sides at the same time and keeping the grade level even, then yeah, I'd treat it as a 4' retained height since there's no real lateral load differential above that. Just make sure the backfill stays balanced during construction. I've done similar setups and used 2' footings with vertical bars spaced for 4' retain height without issues. If there's any chance of unbalanced fill though, I'd be more conservative.

 

[Apache1]

Hi all, thank you for your feedback. Yes, the roof will be sheathed with 5/8" plywood and can take the load out to the side foundation walls running perpendicular to the wall in my picture. I was wondering if I should take that into consideration. Sounds like most of you would, although sdz brought up the potential for the roof being removed, which....yeah sure that could happen I guess.

 

[JAE]

If the roof was to be removed then there would most likely be a structural engineer verifying that it could be removed (at least there should be).
But why in the world would the roof of a building be removed except in some large windstorm? I've heard of walls being removed for additions, doors, etc. but not whole roofs.

 

[XR250]

If the roof was to be removed then there would most likely be a structural engineer verifying that it could be removed (at least there should be).
But why in the world would the roof of a building be removed except in some large windstorm? I've heard of walls being removed for additions, doors, etc. but not whole roofs.

I completely agree with you but I question how many engineers (at least in my neck of the woods) would even consider that the roof was bracing the wall.(edited - against soil loads) Many may just assume it was constructed as a retaining wall.

 

[BridgeSmith]

I completely agree with you but I question how many engineers (at least in my neck of the woods) would even consider that the roof was bracing the wall. Many may just assume it was constructed as a retaining wall.

The good ones in my area would check to see if the stability was ok without being restrained at the top. If it was, then reinforce it as necessary and move on. If not, then specify the connection to the roof framing and account for that restraint in the design.

I don't think it's reasonable anticipate someone removing a substantial portion of the roof sheathing in the future without consulting an engineer. If I'm not mistaken, walls being braced by the roof system is a standard assumption for building design, residential or otherwise.

 

[phamENG]

The good ones in my area would check to see if the stability was ok without being restrained at the top.

I think the issue is that, 35+ years from now when this is being done, the drawings will no longer exist and the footing will be buried. So there's nothing to check without doing some major investigative work, which the homeowner will be unwilling to pay for.

I go into these assuming everything was built based on the most minimalist IRC details imaginable and work up from there. But I'm familiar with residential construction. If they hire a firm that dabbles in residential and hands it off to a junior engineer who has only ever touched mid-rise buildings...they'll approach it from a maximalist, engineered perspective and likely assume there's a retaining wall there.

But I agree - roof removal is not a reasonable design consideration. If that ever comes, it's reasonable to assume it will be replaced. The minutiae of how that probably yet to be born engineer will or will not do his/her due diligence or that contractor handles means and methods is waayyyy beyond the scope of any reasonable design you'll conduct today.

 

[Ron247]

I would not worry about what to call the system, I would use the word "miscollected" if I had to name it. You are still left with a free-body diagram (FBD) that can change its stripes during construction or 10 years after. Think each phase out, apply the appropriate FBD for that phase and design something that meets the reasonable possibilities. Allow for surcharge loads next to the wall, unpredictable backfill operations and other issues need to be considered.

Also, adding the roof later will not reduce stresses already present from earlier loadings, so those reasonable stresses must be included in the roof addition portion. It has already been mentioned about later excavations. Your original sketch already shows the lowest 4' of wall having unbalance soil loads since one side has a depth of 8' wile the other has a depth of 4'. Accurate FBDs should be you best guide. Or basically, the results of each FBD in order of construction or "life" needs to be considered on the later FBD.