Meredith,

Considering a box frame for a horizontal seis is certainly a good
idea. In fact, the frame for the famous Sprengnether long period
horizontal as used in the WWNSS, is such a design if you look at
it properly. The layout is that of a large cubic box cut in half
diagonally through opposite vertical corners. Then the remaining two
sides are cut at an angle from near the top of the remaining top
corner to near the base. Thus the remaining corner is the support
mast, with the diagonal cut across the base being the front of the
horizontal seismometer.

For the Sprengnether seis, two large aluminum castings make up this
construction, one for the base, and another for the support mast with
the diagonal braces. The boom, mass, magnets, etc are mounted inside.
The taught wire supports are fastened to the top front and base rear
of the upright (with the end of the boom forking to bypass it).

There is a drawing of it on my web site.
http://www.eas.slu.edu/People/STMorrissey/index.html
stmmisc.html" PSN INFO ... SLU Seismic Network
(abstracts.html" Abstract of paper for 2000 AGU meeting: has been updated)

Finding the ideal cubic box made of thick wall aluminum or stainless
steel could be a problem. I would think that 16" on a side would be a 
minimum, making the front diagonal 22.6", and a consideration for the
size of the pier. However, a two piece construction of a thick triangular 
base and a bolted on corner angle would work. The corner could be made with
a piece of 1/8" aluminum sheet bent in the middle after cutting the
diagonal sides, and joined to the base with T or J bolts. Someone with 
a Tig/Mig welder could make it a one-piece design. Bending the angle to
less than 90 degrees would allow for a narrower profile. I think that the 
massive side angle braces would allow 1/8" aluminum to be used for the 
base if a heavy beam (like 2"w x 1/2" thick Al bar) was run across the 
front between the lower front of the side angles (where the leveling 
screws would be installed.

Regards,
Sean-Thomas
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