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RE: Hillside foundation question.

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Robert,

Ten to twelve friction piles for a hillside foundation in Los Angeles
for a typical hillside floor plan is not unusual, and does not
necessarily sound excessive. The layout of the piles usually is dictated
by the geometry of the architectural plan and how it relates to the
topography of the hill. A 20x20 box on a moderate slope would be no
problem to do with 4 friction piles. You have a more complex floor
plan... 2600 sf in two sections and the sections intersect at 25 degree
angles.

I have not reviewed Mr. Levin's book. I do not wish to disrespect Mr.
Levin's opinions and his experiences, which he has placed in his book.
Perhaps Mr. Levin used some rather stout friction piles to support his
deep grade beam system. But four piles on a hillside provides little or
no redundancy to redistribute loads, which is important in seismic
situations, and  cracked friction pile + grade beam footings are
extremely expensive to repair. Constructability is an issue with heavy
footings that need to be drilled and placed on a steep lot, usually with
a narrow street for access, and poor surface soils. Deep grade beams
mean heavier rebar sizes, longer rebar splices, and more critical
inspection. You may also need to upgrade the concrete to a higher
strength mix. Bottom line is you're getting a lot more expertise out of
the professionals (structural and geo) that you hired than a $30 book,
no matter how good the book is.

The usual grade beam span I run into for residential construction is
about 15 to 20 feet. I have occasionally used a 30 foot span. The
friction piles have ranged from as small as an 18" diameter, to the
largest I believe was 36 inches. The contractor complained about the 36
inch piles. Grade beams have ranged from 12 to 18 inches wide and from
24 to 42 inches deep, depending on the grade beam span. Bedrock has
usually been found about 8 to 15 feet below the slope surface, and in
your case you mention that the bedrock horizon is a bit shallower,
starting at 1.5 ft. However, the bedrock horizon probably dips down
quite a bit by the time you get downslope, if it is like many of the
jobs I have seen. With your soil report recommending 10ft into bedrock,
and 24" piles, I would expect that you would need at least 20 ft depth
on the downslope side, and if you have an average 1:1 downslope, the
daylight to foundation issues will increase that further. I think a 40
ft friction pile with a 24" diameter is reaching rather slender
proportions, and where soils are poor, I would opt for the largest
diameter that the contractor can handle There's always a tradeoff.

For example, interior shear walls will require anchorage into concrete
and will require a continuos footing. This means a grade beam underneath
them. By aligning as many shear resisting elements as you mention that
you already have, you have already taken important steps to economize
the design. Minimizing the need for intermediate grade beams is a good
choice in economizing the design.  I have yet to have a client that did
what you did, I usually see an architectural plan that has offset shear
walls, misaligned vertical support elements, and generous window
openings in the walls. I won't get into the design merit of that
approach, but it does lead to higher structural costs on account of the
detailing, the additional elements, and the contractor's slower pace of
construction, and inevitable field questions.

Offhand, it sounds like your structural engineer knows what he's doing.
He appears to be following the soils report recommendations, which is a
wise thing to do, both from a technical and a legal liability
perspective. Explain what you're trying to achieve (material usage
reduction to achieve lower construction cost) and he'll know where and
how to optimize the structure. Remember that the labor cost is a
significant portion of construction cost, and elements that take longer,
or require greater skills will cost as much as simpler elements that can
be done by general labor.

Best regards
T Honles, SE, PE, California

-----Original Message-----
From: junk01(--nospam--at)sgds.com [mailto:junk01(--nospam--at)sgds.com]
Sent: Thursday, June 16, 2005 3:23 PM
To: seaint(--nospam--at)seaint.org
Subject: Hillside foundation question.

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.



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