Basement Costruction

[Steel2Steel]

I posted this under the geotech section by accident. Please don't scold for reposting...........

A friend of mine is about to build a house. He has a basement that is fully buried on one side, about half of the side walls are buried, and the rear is completely exposed. I have a couple of questions.

1. What construction is better, CMU or Concrete? Obviously the Concrete is more expensive but as far as waterproofing goes what would be the better choice. He is near a lake so the water table is likely higher than on other sites.

2. Once the wall becomes completely exposed, say the rear wall, should you switch to a timber wall for economical reasons?

 

[msquared48]

I posted my answer to the second question in the Geotech section.

Mike McCann
McCann Engineering

 

[twinnell]

defrazier,

see my post started today.

 

[MichiganPE]

I recommend concrete, and designing the front wall as a cantilevered retaining wall since there is no lateral soil pressure on the back side of the house to counteract the soil pushing on the front wall of the house.

 

[twinnell]

MichiganPE, Are you trying to say that if the front wall was not a cantilever retaining wall then the load from the soil would travel from the wall, into the floor diaphram, and then into the rear wall, and since there isn't any soil behind the rear wall, the walls would rack?

I am not sure if this is what your saying, but the load from the soil will enter the floor diaphram and then extend out to the adjacent shear walls. The load on the front wall will have negligable effect on the rear wall. In fact, the only effect would be displacement due to diaphram bending. So you have to make sure your floor diaphram can resist the soil and wind load from the front wall.

 

[jike]

If you are gioing to use the diaphragm to resist the wall load, make sure that the load can be transfered to and from all elements in the load path from the anchor bolts and sill plate to the connection of the framing to the sill plateand the diaphragm.

 

[concretemasonry]

Just look at the ACI 530 and the IRC and determine if the structure can be built according to the requirements.

If not, then make practical, realistic assumptions and not simplifying assumptions for individual walls. Recognize the parameters of the walls (h/t, fixity at ends, rotation of the strip footings and the loading effects of a complying floor/diaphragm). - Use Roark if necessary and have found there very helpful for many walls.

All to often, it is too easy to jump in and design a basement, which can be compared to reinventing the wheel. Recognize constructability and historic performance!!!! - This comment is based on 35 years experience as a registed engineer with deep involvement in residential construction, standards and codes. Too often, well-educated engineers design residential structures they are not not really qualified to do because they are not really involved and use practices/standards that are more applicable to different types of structures.

I have seen an engineer design a 20' long, 8' high 12" basement wall in good soil as a pure cantilever retaining wall instead of taking time to look at some of the accepted code/standards design tables. The result is a costly, complicated project that gives engineers a bad reputation.

 

[MikeE55]

Defrazie, I think the concrete is a better choice for waterproofing than cmu. You have fewer joints, so less potential paths for a leak. I would recommend your friend have the wall contractor place ACI minimum steel in the wall, since some residential codes allow less. I have seen many shrinkage cracks in residential construction due to bars being spaced too far apart.