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Re: UBC 1808.2.2

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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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