Return to index: [Subject] [Thread] [Date] [Author]

Re: Hillside foundation question.

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Robert
 
Wow!  A lay person who asks intelligent questions before launching into a tirade over the design!
 
I am in Canada, so my response will be "motherhood" at best, but here goes.
 
Ask these same pointed questions of your engineer.  It may cost you a few dollars to go "around the mulberry bush" a few times, but if your engineer is really interested in giving you a good solution, he will bear with you.  Engineers (me too) sometimes get a bit frayed at the edges if we sense that the only reason for the questioning is to reduce the cost.  I sense here that you wish to be educated (?) as why the design is the way it is.  I keep reminding myself that there is no one correct solution to a problem, but possibly many "appropriate" ones.
 
I suggest that you keep on asking questions, but you may have to accept answers you don't want.  If you are still unsatisfied, your engineer may be willing to call in a second opinion.  Having said that, a concept/design review by a 3rd party goes a long way to possibly identifying these type of issues.  Whichever path you take, be prepared to pay something for the journey.  The up-front costs may, however, result in the economical design you are wishing.

Thor Tandy P.Eng
Victoria, BC
Canada
vicpeng(--nospam--at)telus.net
----- Original Message -----
Sent: Thursday, June 16, 2005 3:23 PM
Subject: Hillside foundation question.

Let me first say that I am not a structural engineer and I apologize for the
length of this post. I am a property owner that is trying to build a
hillside home, in Southern California, without "breaking the bank". I came
across this list as I was researching hillside foundations and have been
following it for a few days and reading through some of the archives. You
have a great wealth of information and expertise here. Although I am capable
of learning and doing many things, I am fully aware that a hillside
foundation design is well outside of my expertise. Given enough time and
training (and not to mention the legal ability to stamp my plans) I would
love to engineer my own house, but that's not going to happen (at least not
in a timely manner and without ending up divorced).

I have a couple of questions regarding hillside foundations. I designed this
house myself and tried to simplify the design to make it as economical as
possible (like aligning upper and lower story walls and windows, providing
plenty of shear wall space, etc). I have done a lot of research in various
areas and am trying to work my way through a very steep learning curve.
After interviewing a few recommended structural engineers, I hired one and
he has been working on the foundation design (I am self-contracting the
construction). We have a great working relationship and he has made some
good additional recommendations to cut costs. I want to make sure I do not
offend him by questioning his design.

Where I am stuck is on the basic design of the foundation. The engineer has
come up with a system composing of 24" grade beams and 10-12 24" friction
piles located at the corners and main interior walls. I would not even
question this design if it were not for a) that many piles sounds like too
many, and b) Arthur Levin's book "Hillside Building: Design and
Construction" implies that many engineers over design hillside foundations
and use too many friction piles. I have read through Levin's book numerous
times over the last few years in anticipation of building this house.
Specifically, Levin says "Grade beams can span and cantilever farther than
most engineers expect". I realize the book is over simplified and hardly
touches on the actual mathematical engineering of a design, but with the
increased costs of steel and concrete, I am trying to be as efficient as
possible.

What I am looking for are some generalizations regarding grade beam spans
and the number of friction piles. I realize that no one here can know enough
of the details of my project to make any firm recommendations. In Levin's
book he talks of designing 30'x50' hillside buildings with 4 piles by
increasing the size of the grade beams and cantilevering them. He also talks
about a design that spec'd 24 piles, but only needed six. I understand these
are general examples not relating to the specifics of my design. As a lay
person, am I wrong to second guess my engineer's preliminary design? I know
I am only basing my opinion on Levin's book, which may be totally outdated
(second edition, 1999). Perhaps codes have changed significantly since then.
I only want to end up with reasonable foundation costs so I can afford to
build the rest of the house. I could easily live with six friction piles.

Project details:
2 story hillside home composed of two appx. 30'x20' sections (the second one
angled and intersecting the first one by about 25 degrees to match the angle
of the hillside). Floor area about 1300 sq ft per floor. The building pad is
relatively flat (+/- 4'). No special loads except for some cantilevered
decks on the downhill side. (The engineer is recommending a concrete floor
on the first level with a cantilevered deck to help mitigate any fire
hazard.)

Hillside height 135', average slope angle 45 degrees.
Friction piles along the top of the hill will need to be 40' deep to meet
the H/3 requirements (40' to daylight). The back row of piles would be about
20' deep since they are set back about 20' from the slope edge. The engineer
said that without the H/3 requirement the front piles would be about 32'
deep.

Soils Report:
Soil consists of competent, granitic bedrock (decomposed granite) starting
at about 1.5' in depth and is considered suitable for building.
Seismic zone: 4, Soil profile type: Sc
Recommended to use friction piles, minimum 24" diameter. Minimum 10' into
bedrock and minimum of H/3 (not to exceed 40')
Skin friction of piles: 700psf

Since I have to have these 40' deep friction piles, it seems like I should
get something back for that additional depth. Am I wrong for thinking that a
larger, more rigid, grade beam design with fewer friction piles, might be a
more efficient way to go, rather than designing a simpler plan of a friction
pile at every major wall? The current design has the piles about 15' apart.
Is it horribly complex to design a foundation where the piles do not
necessarily line up with the building shear walls? How did Levin design
30'x50' foundations with only 4 piles? He says "Use 4 piles wherever
possible. Grade beams are relatively easy to construct, can be increased in
depth to span farther, and can be cantilevered a considerable distance".

I'm sorry this is so long, but would appreciate any input I can get. If this
is the wrong place to ask a question like this then just say so and I will
leave you all alone.  :)

Robert


******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
*
*   This email was sent to you via Structural Engineers
*   Association of Southern California (SEAOSC) server. To
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you
*   send to the list is public domain and may be re-posted
*   without your permission. Make sure you visit our web
*   site at: http://www.seaint.org
******* ****** ****** ****** ******* ****** ****** ********