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RE: Are You Sure?

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Posted by tom@digitalmoon.net on September 20, 2000 00:43:41 UTC

hi rick, your comment caused me to check my calcs, and i found an error. the natural frequency of vibration of the pier is about 12 cycles/second rather than 1.7 as i originally reported. i only referenced the lower threshold of human hearing as a reference point, to try to give a person an intuitive feel as to what a low vibration frequency is like. i noted same to brett directly.
i note that you have great misgivings about the soundness of this design. well, i just happen to have a 10 foot piece of 6 inch schedule 40 pipe down in the lower part of my yard that i was going to do something with some day. this evening when i read your note, i thought i would go down and talk to it for a minute to reassure my self of its brothers ability to do the job. he says no sweat. and i believe him, he is an impressive hunk of steel, about 190 pounds himself. mentally i stood him up and put a 10 inch schmidt cass. on top. absolutely no problem. also. i thought about trying to make him vibrate in a bed of sand. no way. anyway, i think brett will like his setup when it is finished. by the way, i noted somewhere that you talked about putting rebar inside the pipe. absolutely not necessary, the pipe has a cross sectional area of over 5.5 square inches, a nominal yield strength in tension of close to 100 tons, enough to lift several D9 cats. fill the pipe with sand or concrete, and be done with it. by the way somebody mentioned that the concrete should be a good mix for strength. if readymix bags are used, mix them so that the mix is stiff, not runny. it should not drool off your shovel. if you mix it from scratch with cement/sand/gravel, since one yard is the amount, the super conservative guy would use a 5 sack of cement per yard mix, and mix it so that it does not drool. if bret uses readymix bags, 80 pounds each, it will take about 40 bags. just got cut off, time to stop, tom

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