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RE: Fixed or pinned support

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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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