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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? . . .
>       Regards,
>       Chris Chapman


Thank you.

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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