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RE: Fixed or pinned support

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Title: RE: Fixed or pinned support

>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