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
----- Original Message -----
Sent: Thursday, June 16, 2005 3:23
Subject: Hillside foundation
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
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
house myself and tried to simplify the design to make it as
possible (like aligning upper and lower story walls and
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
After interviewing a few recommended structural engineers,
I hired one and
he has been working on the foundation design (I am
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.
am stuck is on the basic design of the foundation. The engineer has
with a system composing of 24" grade beams and 10-12 24" friction
located at the corners and main interior walls. I would not even
this design if it were not for a) that many piles sounds like too
b) Arthur Levin's book "Hillside Building: Design and
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.
says "Grade beams can span and cantilever farther than
expect". I realize the book is over simplified and hardly
touches on the
actual mathematical engineering of a design, but with the
of steel and concrete, I am trying to be as efficient
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.
story hillside home composed of two appx. 30'x20' sections (the second
angled and intersecting the first one by about 25 degrees to match the
of the hillside). Floor area about 1300 sq ft per floor. The building
relatively flat (+/- 4'). No special loads except for some
decks on the downhill side. (The engineer is recommending a
on the first level with a cantilevered deck to help mitigate
Hillside height 135', average slope angle 45
Friction piles along the top of the hill will need to be 40' deep
the H/3 requirements (40' to daylight). The back row of piles would
20' deep since they are set back about 20' from the slope edge.
said that without the H/3 requirement the front piles would be
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:
Recommended to use friction piles, minimum 24" diameter. Minimum 10'
bedrock and minimum of H/3 (not to exceed 40')
Skin friction of
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.
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