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Re: STEEL: Connecting HSS to HSS for Moment

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Bill,

This sounds like a traffic signal structure manufactured by a company named
Ameron for which I worked in my youth. I believe a company called Valmont
makes them also. While I was involved on the periphery with these structures
I think I may have kept a copy of calcs for one of these structures that the
senior engineer prepared. I'll look at office tomorrow.



Mark

----- Original Message -----
From: Bill Polhemus <bpolhem(--nospam--at)swbell.net>
To: <seaint(--nospam--at)seaint.org>; <structx(--nospam--at)polhemus.cc>
Sent: Friday, August 04, 2000 7:29 PM
Subject: STEEL: Connecting HSS to HSS for Moment


> I have been checking some existing plane-frame type structures that are
used as
> traffic sign supports. They are rather simple: Two HSS (Structural Tube)
columns
> with two HSS beams in an over-and-under arrangement connecting them, and
two
> vertical HSS members at third points going from the top of the lower beam
to the
> bottom of the upper. That's it, very simple, like so:
>
>
>                 |           HSS 8x6x3/8       |
>                 +---------+---------+---------+
>                 |         |         |         |
>                 |         |         |         |
>                 |         |HSS 8x6x |HSS 8x6x |
>                 |         |    3/8  |    3/8  |
>                 +---------+---------+---------+
>                 |           HSS 8x6x3/8       |
>                 |                             |
>                 |                             |
>                 |                             |
>                 |                             |
>                 | HSS 16x12x3/8               | HSS 16x12x3/8
>                 |                             |
>                 |                             |
>                 |                             |
>                 |                             |
>                ---                           ---
>
> Also, keep in mind that the frame members are arranged such that the MINOR
axis
> of bending is coming out of your computer screen toward you; this so that
> (obviously) the major axis is available for the "cantilever" bending that
is
> going to result from the wind load on the sign that is mounted from the
beams.
>
> Now, the problem:
>
> The connections of the beams to the columns are "stepped" of course, as
shown in
> AWS D1.1-96 Figure 2.14(L). This means that one limit state of the
structure
> would be the local buckling or (more particularly) distortion of the face
of the
> column from the bending of the beam. To counter this somewhat, the
original
> drawings show a detail wherein vertical plates are welded inside of the
column
> to "continue" the vertical "web" faces of the beam through the column.
>
> To make matters more complicated, of course, there is a rectangular
opening in
> the face of the column, slightly smaller than the beam cross-section,
located
> such that the axis of the beam passes through the center of the opening.
This is
> to allow wiring to be threaded through the column and into the beam,
ultimately
> to exit the tubular beam at various locations through a threaded coupling
> screwed into a round opening in the beam.
>
> There are also smaller rectangular openings in the top and bottom flanges,
> respectively, of the bottom and top beams where the verticals attach, for
the
> same purpose.
>
> I am concerned about these details for the following reasons, and would
like to
> get comments:
>
> 1. I fear that the vertical web plates inside the column won't be readily
> conducive to proper welding. At best, I think that only a fillet weld one
side
> of the plate (the side toward the axis of the tube) will be possible,
since the
> space between the plate and the face of the column parallel to the beams
will be
> only about 3 1/2" or so, not to mention the dimension from the weld area
along
> the column to the splice at which the welder will access the work.
>
> 2. What about lamellar tearing in the column walls where the beam webs
outside
> and "web extension plates" inside the column meet? I realize that this
might not
> be a sure thing since the web extension plates are fillet welded, likely
on one
> side as I mentioned above, but this detail certainly seems to be very
> constrained.
>
> 3. How do these rectangular openings affect the situation? Even assuming
the web
> extension plates are competent, there is still got to be a transfer of
force
> from the upper and lower "flanges" of the HSS 8x6 beam directly into the
wall.
> The flange force profile will SURELY not be linear from say the edge to
the
> center of the flange. What is that profile? What is the limiting factor?
This is
> surely NOT a "rigid" joint, but rather semi-rigid. How to account for this
in
> modeling.
>
> 4. How do the openings in the beam, even with the threaded couplings to
> "reinforce" them, affect the beam capacity? How do the rectangular
openings in
> the columns at the beam connections, and in the beams where the short
verticals
> attach, affect the capacities of column and beam?
>
> I am probably making this out to be more complicated than it is, but I'm
worried
> that I don't have enough information to make an intelligent model and
subsequent
> design. I love learning new things, but NOT on a deadline!
>
> This looks like the kind of investigation that would make a nice thesis
for an
> M.S. candidate!
>
> Comments, please.
>
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