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Re: UBC 1808.2.2

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Paul,

Bless you Paul, this is good info indeed! I've searched our library and do
not have the commentary (91, 94 or 97). May I impose upon you to fax it to
me please?
805-525-4958
Thank you, thank you, thank you.

Regards,

Mark

----- Original Message -----
From: "Paul Feather" <pfeather(--nospam--at)SE-Solutions.net>
To: <seaint(--nospam--at)seaint.org>
Sent: Tuesday, February 12, 2002 5:42 PM
Subject: Re: UBC 1808.2.2


> Mark,
>
> In the "Handbook to the Uniform Building Code, an illustrative
commentary",
> published in 1991 by ICBO, the following is stated:
>
> Section 2909(b) (now 1808.2, unchanged) "The limitations placed on these
> piles and the conservative allowable stresses are due to the reasons
> discussed in section 2908, where possible uncertainties may be
encountered.
> These may be water infiltration into the bore hole which could weaken the
> concrete, or granular soil constituents may slough off and restrict the
> cross section.  Although the code requires that care be exercised in the
> placement of concrete in this type of pile, these uncertainties still
> exist."
>
> Section 2908(k) (now 1807.11)  "In the case of cast-in-place concrete
piles
> cast against earth, intrusion of soil into the concrete of the pile can
> create the uncertainties which call for conservatism.  However, it may be
> determined on the basis of a foundation investigation that these problems
do
> not exist for the particular circumstances (paraphrased for time), and as
a
> result, an increase in allowable stresses may be justified."
>
> Taken on a case by case basis, if the geotech is ready to assure you and
the
> building official in writing that these uncertainties cannot occur at a
> given site, you would be justified in using typical concrete stresses.
>
> Paul Feather
>
> ----- Original Message -----
> From: "Mark Baker" <shake4bake(--nospam--at)earthlink.net>
> To: <seaint(--nospam--at)seaint.org>
> Sent: Tuesday, February 12, 2002 4:54 PM
> Subject: Re: UBC 1808.2.2
>
>
> > Michael (and others who responded to this post),
> >
> > Thanks for responding, I have been slaying other dragons of late and am
> now
> > coming back to this topic.
> >
> > First, allow me to lay down some more info:
> >
> > A large portion of our work consists of underpinning residential
> structures
> > experiencing distress through foundation settlement. In many cases we
are
> > dealing with hillside conditions in which soil creep is a factor
(combined
> > axial and bending).
> >
> > Our underpinning designs typically consist of new concrete grade beams
> cast
> > adjacent to and attached to existing foundations, supported by drilled,
> cast
> > in place concrete caissons. Caissons are usually 24", 30" or 36" dia.
> > depending on conditions.
> >
> > Section 1808.2.2 of the 97 UBC requires limiting concrete compressive
> stress
> > to .33f'c and steel compressive stress to .34fy or 25500 psi. My primary
> > question is, where does this requirement come from and what is its'
basis.
> >
> > I have had many discussions with many colleagues and found no clear
> > reasoning. Some have suggested it is a "left over" from allowable stress
> > design. Others have reasoned code authors want to limit stresses in
shaft
> > due to uncertainties associated with construction quality of caissons.
> >
> > Chapter 21 of ACI 318-99 has requirements for dcip piles, some of which
> are
> > similar to UBC but no limitation of compressive stresses.
> >
> > As you can imagine (and as Michael hit on in his response) foundation
> repair
> > is expensive. Caissons are expensive, in our case beyond even what
Michael
> > likely imagined since he was thinking new construction when he
responded.
> > Caissons are also THE BEST SOLUTION to these type of subsidence
problems.
> > There are lots of other solutions out there. We have on multiple
occasions
> > gone back onto jobs after someone elses non caisson solution did not
> perform
> > and installed caissons (doing it the way it should have been done in the
> > first place).
> >
> > Due to expense we try to limit the quantity of caissons on a given job.
> This
> > results in maximizing the benefit of axial and bending capacities of 24,
> 30
> > or 36" dia. caissons. Limiting compressive stresses per 1808.2.2 simply
> > results in more caissons (add more caissons, decrease individual caisson
> > load) and the expense soars. Expense can very quickly get to the point
> where
> > the repair is not economically feasible.
> >
> > Now, to respond to Michaels post (I'm not picking on Michael)
> >
> > Caisson construction is continuously inspected by a registered deputy.
No,
> > he can't watch the flow of concrete around the cage but he does ensure
> shaft
> > depth, concrete mix, delivered steel etc.
> >
> > Concrete to the bottom: Concrete is delivered into the shaft via a
tremie
> > extending to the bottom of the hole, no free fall.
> >
> > Reinf. cover: Flowability is a must and mix designs specified to ensure
> it.
> >
> > Caving: Depends on soil strata, shafts can be cased or drilling fluid
> used.
> >
> > Soil content: These projects simply don't happen without competent soils
> > engineers, geologists and engineering geologists involvement. Sulfates
is
> > just one of many things these professionals evaluate.
> >
> > Unbraced column length/base fixity: An enormous amount of research has
> been
> > conducted by Dr. Lymon Reese and many others on the non linear response
of
> > soil to lateral loading. You would be amazed at how accurately pile
> > deflections, point of maximum moment/shear can be calculated.
> >
> > Point bearing vs. skin friction: 99% of our projects utilize skin
friction
> > for developing axial loads. This always includes the effects of downdrag
> > from the "loonshit" (I like that term and henceforth will use it
often:)).
> >
> > In the end, my problem with 1808.2.2 is that it can kill a project.
> > Considering the type of caisson work we are talking about (repair) the
> > expense associated with it, the level of quality control, extent of
> > preliminary soil exploration, all the things mentioned above... why has
> the
> > compressive stress been limited? Is it because of the fear that caissons
> get
> > put in the ground with none of the quality control measures or design
care
> > stated above or something else?
> >
> > I must quit, I have said more than intended when I started.
> >
> > Regards,
> >
> > Mark
> >
> >
> >
> >
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