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Re: Fixed or pinned support

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Alex:

There's still some missing data, such as the anchor rods and plate material
and the vertical load on the column. Nevertheless, I did some runs with our
program (ASDIP 3.5) assuming A307 rods, A36 steel, f'c=3 ksi, and several
levels of axial load. The following is a tabulation of my results:

Axial Load     Moment
20 kips            20 k-ft
40 kips            30 k-ft
60 kips            39 k-ft
80 kips            48 k-ft
100 kips          52 k-ft

Javier Encinas
ASDIP Structural Software
www.asdipsoft.com

----- Original Message -----
From: Nacionales Alex <acnacionales(--nospam--at)yahoo.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Thursday, May 03, 2001 1:11 PM
Subject: RE: Fixed or pinned support


> There seems to be varying opinions so I am providing
> more data.
>
> The baseplate is 18"x18"x1" thick with 4-1" (25mm)x24"
> anchor bolts placed 2.5" from the edge of the side
> parallel to the web and 4" from the side parallel to
> the flange. The column is W14x109.
>
> The building has two rows of columns spaced at 13.5'.
> The column rows are 30' apart.
>
> The base plates sits on 20"x20" RC pedestal with
> 12-#7(20mm) RSB and with a 8'x20" thick continuous
> footing and also large RC grade beams(tiebeams)
> 16"x39" on the transverse span and 12"x24" on the
> longitudinal span .
> The foundation is very stiff because the soil below is
> loose sand with allow. bearing of 1500 psf
>
>
> <<You haven't said how or why you are checking the
> structure.>>
>
> I am the owner's project engineer for construction and
> also a structural designer.
>
>    << Basically, in your case the column base supports
> are partially
> fixed.
> Most of  the computer programs do offer to model
> partial fixidity of
> connections. Such as  "Partial Moment Release" option
> stated after
> Member
> Release Specification, stating upto what %age the
> joint may be
> considered
> fixed. Such option is available in STAAD, I am sure
> other computer
> programs
> do have a similar option too.
>      Anyhow, considering the safety of the structure,
> when in doubt, it
> is
> safer to assume the joint to be pinned.
>    C. M. Iqbal >>
>
> I agree. I know the connection can resist bending
> moments from the column and can be considered
> partially fixed but the software i am using does not
> permit partially fixed connections. What I did was
> assume that the supports are hinged or pinned in
> modelling the frame and checking adequacy of columns
> and beams.
> The base plate can be checked for moments and axial
> load from the frame assuming fixed support.
>
> This method is safer but I will also check
> if the connection above can safely assumed to be fully
> fixed when checking the columns and beams.
>
> Thanks a lot to those who responded.
>
> Alex C. Nacionales
>
>
>
>
>
>
>
>
> --- Charlie Carter <carter(--nospam--at)aiscmail.com> wrote:
> > >The framing in the calculation shows fixed supports
> > >but the detail in the plan shows the supports are
> > >steel baseplates with 4 25mm-anchor bolts.
> > >The columns are directly welded to the baseplates.
> > >It is my opinion that this kind of connection is a
> > >pinned type connection and should be treated
> > >as such in the calcs.
> >
> > I'm assuming you are describing a base plate that
> > has four anchor rods with
> > each one placed near a corner of the base plate. I
> > would design this base as
> > a pinned base, unless the base plate were thick
> > enough to justify a fixed
> > condition -- and it would have to be very thick in
> > most cases. Run a few
> > numbers on the plate for its stiffness in bending
> > and you'll see that a
> > large thickness is usually required. As an
> > alternative to the thicker plate,
> > you might consider building a bolt box (sometimes
> > called a "boot" in the
> > Eastern states) off the column flanges as a more
> > direct load path to the
> > anchor rods.
> >
> > With the new OSHA safety rules for steel erection,
> > four-rod column bases are
> > destined to become the norm, especially for pinned
> > column bases. See a
> > summary of those regulations here:
> >
> >
> >
> http://www.aisc.org/documents.asp?mode=docdetail&doc=246
> >
> > Putting four rods at the column base makes erection
> > safer since the column
> > has better flexural strength at it's base to resist
> > the wind in the
> > cantilevered condition it sees during erection --
> > providing the foundation
> > is similarly adequate. The corresponding rotation
> > that results at that
> > flexural strength is not usually of significance
> > during the erection phase.
> >
> > On the design side, for similar base-plate
> > thicknesses, two-rod and four-rod
> > bases will have about the same rotational
> > performance. In either case, there
> > is usually a significant moment that can be
> > developed between the
> > compression under the base plate and the tension in
> > the anchor rods.
> > However, there is also very good rotational
> > flexibility provided by usual
> > plate thicknesses in flexure.
> >
> > Charlie
> >
>
>
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