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

• To: <seaint(--nospam--at)seaint.org>
• Subject: Footing Design Questions (L/W ratio) for an Addition....
• From: "Daniel Boltz" <dboltz(--nospam--at)1st.net>
• Date: Fri, 29 Oct 2004 09:18:01 -0400

```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?

Thanks.
--Dan

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