Basement Walls

[twinnell]

Has anyone every seen or designed a house that has a half walkout basement with the non retaining portion of the basement being wood framed and the rest being CMU? So half the foundation is crawlspace and half is basement, all of CMU. Then you have the wall with the exterior door out of the basement, which is wood studs. Is this common. I have only seen all CMU or all concrete, never a mixture.

 

[registeredpe]

Greetings:

Yes,this is common, but the wall must be designed as a fully cantilevered(unrestrained)wall as opposed to a basement wall(restrained)because you essentially have a "hinge" condition at the point where the wall transitions from masonry to wood frame construction.

Best regards, registeredpe in AZ

 

[concretemasonry]

If you have entire walls built out of masonry and one walk-out wall of wood frame, your basement walls can be conventional walls (plain or reinfoeced, depending on conditions) meeting the requirements of IRC. - Especially if you a have a floor/joist system with blocking properly connected to the CMU walls. If you are dealing with a notmal configuration. This is common approved construction in mosr areas and can even be empirical or design by tables. The walkout wall can be conventional wood frame construction.

If you have a very unusual configiration with an open vault from basement to roof, or something similsr, you can go through the gynmastics of assumptions, rotation, deflection, inspections and create a truely unique design.

If you are talking about a wall that is masonry on the lower portion and frame above, you will have a masonry wall that is a canitlever and may end up being thicker (up to 12") and reinforced. This is why you often seen a masonry wall with continuity from footing to floor system, since it is cheaper and the interior wall finishing can be easier.

 

[registeredpe]

Greetings:

Concretemasonry, this is true, provided that the floor diaphragm can resist lateral soil and wind loads, which in my experience has usually not been the case-for typical basement ceiling heights-therefore, it is highly recommended that the wall be designed as a fully cantilevered retaining wall as opposed to a full height basement(restrained)wall.

Best regards, registeredpe in AZ

 

[twinnell]

registeredpe, are you saying that you feel all basement walls should be designed as cantilever walls since they are retaining soil and the framed wall above is resisting wind load?

There is nothing unique about the house. Grade slopes from front to back about 8 feet total, so the back wall will be completely exposed. I would have used cmu for the entire basement level, but someone mentioned doing the back basement wall out of timber.

I don't think there would be a need to design the cmu as a cantilever. It is braced at the top by the floor diaphram and it falls within the height limits of the IRC....I think.. I will have to double check.

 

[registeredpe]

Greetings:

twinnell, In this particular scenario, yes indeed, the wall would need to resist a triangular lateral soil load for that portion below grade as well as a uniform wind load for the small portion of the masonry wall exposed to the wind and a concentrated point load at the top of the masonry wall resulting from the tributary area of the wood framed wall above.

Best regards, registeredpe in AZ

 

[twinnell]

I agree, but this particular house falls within the prescriptive requirements of the IRC, thus no engineering is required. Now some people don't trust the IRC and engineer it anyway.

 

[concretemasonry]

twinnell -

You are right about a conventional basement not being a real technical engineering challenge since there are proven, acceptable methods in the IRC to build a very common structure.

I have fought with the concept for about 40 years, especially when we wrote the codes. I discovered you can make a basement foundation wall as complicated as you choose. Once you start making complicating assumptions assumptions you begin to get the analysis beyond the comprehension of project that can be quite simple. - Once it is a cantilever (or propped cantilever), you fall into the trap of footing rotation, etc. Then you realize that the actual construction loads may be different and possibly more severe.

In my 40 years experience, it is fun to get theoretical and philosophical about a very common structure and invent a new wheel, but there are better things to spend valuable engineering talent on since the customer (owner, builder, developer) does not appreciate the cost and complications. Often this is where many customers get a questionable opinion of the value of a professional engineer.

Dick

 

[dcredskins]

I agree with you guys. Such kind of wall is called ENGLISH BASEMENT condition. You design CMU portion of basement wall as cantilever wall for soil lateral pressure and UDL load along the edge of the cantilever due to wind load from stud wall portion of basement. If you are concerned about the wind design of the house, the top of CMU wall will be the base of wood frame.

 

[registeredpe]

Greetings again:

twinnell, in my neck of the woods we abide by the IRC/IBC, however the City of Phoenix also has an amendment which requires any earth retaining structure taller than 3'-4" (measured from top of footing to top of retained soil)must be engineered. Not only that, but for this type of structure it just makes prudent sense to have it engineered.

Best regards, registeredpe in AZ

 

[registeredpe]

Greetings again:

twinnell, I left out something regarding your post about the floor diaphragm resisting the lateral load. If the masonry wall only goes up a partial way then transitions to wood frame (as I understand your case to be), then as I mentioned earlier, you have a "hinge" condition at this transition since there is no way to develop a moment connection between the masonry & wood frame. Therefore, the lateral loads would never even get to the diaphragm, let alone be able to resist the shear per foot, so there is no other recourse in this case than to design as a cantilevered retaining wall.

Thanks again for listening(reading), registeredpe in AZ

 

[twinnell]

The transition to wood is not vertical. The cmu will extend to the floor framing. The transition to wood is a horizontal transition. There will be a vertical joint in the basement wall; one side of the joint is cmu and the other is wood. The wood will not retain any soil; only the cmu.

Picture a box. 3 sides will be cmu and one side wood. The cmu wall opposite the wood wall will retain full soil. The two adjacent cmu walls will have sloping grade. The wood wall has full exposure to wind and will not retain any soil.

 

[registeredpe]

Greetings:

twinnell, I misunderstood your original post. I agree that the walkout portion of the basement along the non-retaining side can be built out of wood frame construction, provided that the plywood/osb shear walls can safely resist shearing and overturning forces. However, the wall opposite this said frame wall must be designed as a fully cantilevered retaining wall because, as I mentioned earlier, I doubt that the floor diaphragm can resist the high lateral loads from the retained earth. Secondly, by designing as a cantilevered retaining wall you do not need to shore up the wall prior to backfilling and compacting. now if the basement was retaining on all four sides then all four sides can be designed as a basement(restrained)wall since the forces in all directions cancel each other out.

Best regards, registeredpe in AZ