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FW: Footing Design Questions (L/W ratio) for an Addition....

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If your building is configured to allow this, try a strap footing from the
footing next to the existing building over to the first inboard footing.
Design it for the overturning moment and then check both footings for the
added (or subtracted) vertical forces.  You'll also have to check the amount
of flexibility this adds to the system.

Hope this helps.

Curt La Count
Jacobs Engineering
Portland, OR

-----Original Message-----
From: Daniel Boltz [mailto:dboltz(--nospam--at)]
Sent: Friday, October 29, 2004 6:18 AM
To: seaint(--nospam--at)
Subject: Footing Design Questions (L/W ratio) for an Addition....

This A/E firm prepares construction documents for quite a few additions to
educational facilities.  I try in every way to keep the addition off the
existing structure so that I don't have to get into analyzing the existing
building.  I normally hold the column centerline 2'-6" off the outside face
of the existing building.  This usually leads to an argument with the
architects similar to using moment frames vs. braced frames.  (Am I the only
engineer that has architects locating windows everywhere?) 

In my current project, I have a cafeteria addition that is approximately 50'
wide x 100' long, windows on the 3 sides which are exposed.  I have moment
frames along all four sides.  The problem I have is that I needed to fix the
bases of these moment frames to limit drift to H/480.  Geotech gave 2000 psf
for an allowable soil bearing pressure.  My questions are as follows:

1. If I locate the new foundations along the existing foundation, I end up
with a 4' wide x 15' long footing (supports a column in the center), mainly
due to the large moment introduced into the footing.  The design of this
footing is fairly simple, however the L/W ratio is 3.75.  Is there a L/W
ratio limit set by the IBC, or any code for that matter?

2. If I were to underpin the existing 24" wide x 200' long spread footing, my
frame/column footings would become about 7' wide x 10' long.  However, I'm
not quite sure how I would go about underpinning the existing footing.
Normally, we underpin in 24" wide sections.  Does it sound reasonable to push
the underpin width to 36", pour 18" leaving an 18" void which would allow for
a #3 or #4 bar splice parallel to the direction of the existing spread
footing?  I would continue this sequence of excavating 18", place/splice
rebar, and pour 18" until I'm left with a 7' wide footing that is 10' long?
Another option would be to provide a mechanical connector for each rebar
rather than push the underpin length.  I'm just not sure what would be the
best way to approach this one. 

3. Budget wise, would it be economical to look at a different type of
foundation along the existing building such as auger-cast piles or
micro-piles with pile caps?  


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