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STEEL: Connecting HSS to HSS for Moment

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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

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

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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