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RE: PEMB's and lateral deflection

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Rich,

Lateral resistance of spread footing can be developed by simply placing them
a deeper below grade.

You get both the shear at the base, through friction or adhesion, and the
lateral passive resistance of the soil.
Also, with clay soil, you can use some additional resistance from the
lateral shear planes.  You have to be careful when you combine these since,
in reality, they will develop at different strains and rates.  Deflection
can be a real problem.  Your shear resistance won't show up until you get
about 1/2 inch movement, that's why it's rarely used.  However, the base
adhesion and lateral passive show up at much lower strains.  So I don't
usually add them all together.

Although a key has been recommended, I always have had problems with getting
these constructed correctly.

Check with your Geotech and see what values he give you.  He may want to run
some actual triaxial tests to get good C and Phi values.

Just a thought.


Arvel



-----Original Message-----
From: Rich Lewis [mailto:seaint02(--nospam--at)lewisengineering.com]
Sent: Tuesday, June 07, 2005 4:21 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: PEMB's and lateral deflection


Thanks Bill for the information.  Problem is, this is not a drilled pier
project.  It is a simple spread footing.  I wish I had piers.

Rich


-----Original Message-----
From: Polhemus, Bill [mailto:bill.polhemus(--nospam--at)tyson.com]
Sent: Tuesday, June 07, 2005 3:19 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: PEMB's and lateral deflection

-----Original Message-----
From: Jordan Truesdell, PE [mailto:seaint1(--nospam--at)truesdellengineering.com]
Sent: Tuesday, June 07, 2005 3:01 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: PEMB's and lateral deflection

It is feasible/practical to lower the footing and add a key to resist
the horizontal thrust?

Rich Lewis wrote:

>In typical PEMB's I use hairpins or tie rods to tie the
>2 ends of the rigid frames.  Since this is 475' long this doesn't seem
>practical.  I see it as 2 options: 1) design the exterior footing for
the
>overturning force.  This will make it a large footing.  2) tie the base
of
>the frame to the first interior column, at one end 150' and the other
end
>120'.  Both have their advantages and disadvantages.
>
>Which method have you typically used for large horizontal thrusts on
long
>span rigid frames?

-----/Original Message-----

What we TYPICALLY do for PEMBs in Texas is have a large grade beam at
the edge, both to transmit wall loads to the soil and to provide
resistance against thrust. There are drilled shaft footings at the
columns, sitting underneath the edge beam. Normally I don't
dowel-connect the pier to the grade beam, but in this case I'd surely
make an exception (and check the interface for direct shear/shear
friction--actually, you might find you don't need the dowels after all).

I haven't done a foundation as large as the one Rich describes, but I
would think this sort of thing would scale reasonably well. Hairpins
plus edge-beam plus drilled shafts with or without dowels across the
interface.


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