PSN-L Email List Message
Subject: Re: Reinforced concrete base for Lehman instrument
From: beezaur beezaur@..........
Date: Sat, 08 Mar 2003 10:53:59 -0800
> temperature changes. I would be most surprised if 1:1 sand and cement
> was not strong enough in practice, . . .
> Have you considered making a T base frame out of say 1.5" black
> steel angle? . . .
> Chris Chapman
I think you are right about the grout mix being strong on its own. It
isn't a trick to get regular concrete to 6000 psi compressive strength
(garden variety being 2000-3000 psi), and in fact the stone is what
usually breaks first in the really high strength stuff. I still worry
about brittleness - this device will have to survive a couple of moves
in the next few years. It would be interesting to see if my school will
test a cylinder or three before I graduate. How much water is typically
Since rebar is a problem in concrete for the base, I assume I don't have
to ask about pouring the pier over 8 ft R/C pilings to better couple
with more stable ground at depth. It sounds to me like the soil/piling
interface would have the same noise problems as rebar/concrete. But
please correct me if that technique has proven reliable.
Steel or aluminum is "plan B" for the base of the instrument. The
support for the boom will be a metal tripod, bolted to the base. I
would like to keep the natural (resonant) frequency of that structure
very high, kHz if possible. I want to put my pickup and damping
hardware on the same base as the boom, hence the 40 in length. I hadn't
expected a "T" to get me there, but the design you describe may well be
stiff enough, especially with channel or square structural tubing.
Another thought: has anyone considered using carbon arrow shafts or
kevlar/spectra bowstring for boom components? They are pretty cheap
now, and their strength-to-weight ratios give them very good vibration
characteristics, and some bowstring absolutely does not creep.
A day without math is like a day without sunshine.
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