affect of partial concrete wall in seismic activity

[jonx]

I am in the 3rd world, in an earthquake zone, in need of building a small house. There are no engineers to be found ( I am computational not structural/geotechnical) and absolutely no building standards. The 'local builder guys' have just enough education to read a tape measure. To try to make up for the lack of proper engineering, I'm adopting the IBC standards however I am considering a small variation and need some professional feedback.

The project is a small (700 sq ft) one story house with an attic, in a tropical area (i.e. humid, heavy rain, termites, ants, etc) combined with low/infrequent seismic activity but the looming possibility of 'a big one' (6.x +). The house is to be wood framed on top of a monolithic reinforced foundation(i.e. foundation bolts, etc) adopting the IBC recommendations.

Where I need some feedback is with the idea of making the external walls by combining a short 2' concrete wall at the base with the wood frame on top of it (i.e. instead of the standard 8' wood framed exterior wall). The concrete wall would be reinforced and tied to the foundation. The reasoning behind this mini-concrete wall is to provide better termite resistance and avoid problems with the humidity. My concern and question is, in the event of an earthquake, is this partial concrete wall a bad idea?

One on hand, I can picture the concrete wall making the structure stronger. On the other hand, with uplift, the extra weight of the concrete wall will be thrown into the roof and may overload the structure (not to mention a hinge point in the middle of the wall). I'm not sure what the net affect would be and would appreciate some feedback. If I'm playing with fire or potentially creating more problems than its solves, I'll just go back to normal wood construction.

 

[ishvaaag]

On this question, I recently saw an article of Blodgett giving some advice on welds: don't design with the heart. It was a call to analyze. There are certainly many aspects that are difficult to include in common design, such the behaviour of some particular foundation embedded in the ground subject to an unknown earthquake. Just putting higher the wood structure will augment the lateral forces (whip effect), but if on a mat, you can be assured at least that no deformability of the foundation proper will enter the scene. With walls one can't be as sure and the waves may impel irregularly the foundation against the superstructure, for which everything needs be prepared for survival. In sort, as in any case, in the design you make choices; codes limit your choices in order to keep safety. By not following procedures you can't be sure of your standard of safety.

So one can't ascertain properly the virtues of one or other election without examining the structural design itself, and even then, introducing as you do an earthquake variable you would only know if you'll have extant the particulars required by the code. So your concerns may be well stated, you may have a better structure for ordinary condition with the wall, but, how frequently a damage-level earthquake is expected?

 

[Archie264]

Do you have the resources to make the walls reinforced concrete all the way up to the roof? If so, and if they're well anchored to wall foundations below grade, that might provide the most robust solution.

Wood walls can be made to work, especially if there are intermediate lateral shear walls, but with the conditions you describe they would probably be eaten by termites in short order. Masonry walls could be made to work, either reinforced or laterally braced.

And a hybrid solution might be possible. That is, concrete or masonry knee walls with timber bearing walls built above them, provided they're laterally braced.

I'll be curious to learn how this turns out.

 

[msquared48]

The short concrete stemwall is used all the time in my area and I am in a high seismic zone.

I would not worry at all about any lateral force from that short section of wall affecting the roof diaphragm, particularly with a roof area of only 700 square feet. Any lateral force will go directly to the ground. No worries.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[jonx]

Thank you very much for the feedback. It is appreciated more than you know.

ishvaaag, sound advice. Historically, a major earthquake happens on average every 30 year. In between there are random events.

Archie264 I hear you about concrete and its benefits but unfortunately its not an option. The soil conditions under the foundation represent largely an unknown (*really* don't like that) so the lighter the building the better. Shear walls, yes, are a must and will be located on each corner.

msquared48, when you say "stem wall" are we talking the same thing? A short concrete wall 'built on top of the foundation above the floor' and not the normal stem wall with the floor built on top? If it is commonly used in your region and it is a code, it sounds like a viable, proven option. You wouldn't happen to have a link (or name) of the building codes used in the region?

 

[msquared48]

It is common, to prevent water intrusion and rot, to extend the foundation wall, or stem wall, above the level of the floor if there is locally earth against it. Otherwiwe, the floor joists rest on top of the concrete stem wall with 6" mimium clearance between the bottom of the sill plate and the finish grade. The floor joists dqan also be hung off the sill plate on top of the stem wall. Either way sorks.

No particular IBC code reference other than the required clearances of wood to earth for the joists, beams, columns and exterior wall.

FYI, the concrete stem wall that rests on a continuous exterior strip footing, almost always exists in a residential structure where the frost depth is 12" or greater and there is a crawlpace. Very common method of construction.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com