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RE: Hillside foundation question.

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Robert,

15 feet is a typical/max span for a 2ft deep grade beam.

You have hard rock. Drilling that deep will be tough. Trenching for long
grade beams (deeper ones) will be tough as well.

Not having read the book or knowing anything about itm, but if it's on a
hillside, I would take it that the grade beams (or portions) are
elevated? If so, then with concrete beams, you could probably span about
25 feet without getting too huge of a beam. Then you need a moment
connection at the interface to the pier if it's up in the air. That gets
messy for thin grade beams. I would think you would need something like
a 16" wide grade beam at least for the detailing requirements to even
have a chance. Not to mention Rectangular Beam to Circular Pier and all
the ties.

The daylight thing is a tough one. You want a pier where the grade beams
intersect and you need them at your span maximums.

There is also the RHO factor/redundancy factor in the code. It makes you
not rely on a single or few elements to take the brunt of the force.
It's rarely checked at the foundation level because usually something
above would trigger it first. What you are trying to do sounds like it
could drive the RHO factor to 1.5 causing a 50% increase in the
foundation forces (and all forces in the lateral system of the house as
well). If you are relying on say 4 piers and they have equal loads, you
have 1 element taking 25% of the seismic load. That may/probably will
set RHO to 1.5

My recommendations are to make the superstructure as light as possible
to reduce the seismic loads. Concentrate your resistance where your
piers are the most efficient (i.e. best daylight plane) and tie the
weaker locations to the good spots with grade beams/braces/ whatever.

A lot of homes in my area call for drilled piers about 8-12 feet deep
typically on hillsides into bedrock. Often times, the drill rigs hit
refusal before this and the on-site Geotech okay's it.


hth
Good luck,
-gm

-----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.

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


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