huangyhg

air discharge stack - design
davenz,
i can't answer all of your questions, but i can offer some feedback on the structural design. in order to do some rough calculations, here are some assumptions i made:
mat'l: a-36 carbon steel
corrosion allow: 1.5 mm
height: 20 m
dia: 1 m
support: fixed at base and free standing
wind: asce 7-98, 100 mph, exp. c, cat. iii
based upon these assumptions, the stack can be 6 mm thick. some local reinforcement may be required at the blower inlet. the stack would have a critical wind speed of 12.4 m/s, which could cause large deflections up to 0.7 m. to minimize these deflections you could use a vibration damper.
regards,
chris
6 mm? please take another look at this!
1/4 inch wall or 6 mm is very common on small stacks such as this one. if you expect large corrosion, then perhaps a heavier wall is required.
please elaborate on why you feel 6 mm is inadequate.
regards,
chris
chris,
i have to admit that i didn't run the calculations and may have stuck my slide rule in my mouth. however, it seemed to me that a 3' diameter, 1/4" thick steel "pipe" 65' long standing upright in the air and subject to 2'-4" (.7 m) deflection was iffy (.7/20 = .0035 which = 1/28 when i'm use to seeing structural deflections limited to 1/180, 1/240, 1/360). sometimes intuition is not all that great, i'll run the numbers.
i may not have been clear on my original post. the deflection of 0.7 m is due to wind induced vibration (vortex shedding), and is totally unacceptable. that is why i suggested that a vibration damper be used, which would bring the deflection down to an acceptable level.
for the along wind loads, the deflection that i calculated is only about 0.085 m (3.3"). this is assuming that my wind load assumptions are realistic for this location.
thanks for helping me clarify this point.
thanks,
chris
i am designing a similar chimney.
the 0.7m. due to vortex shedding occurs perpendicular to the wind direction, correct? will vibration occur in the direction of the wind? i can't find anything in wind loading codes for this effect
thanks,
richard
ricellis,
yes, the vibrations at the critical wind speed occur across the wind direction (cross-wind response), and can arise at a critical velocity a lot less than your maximum design velocity. (critical velocity is a function primarily of the diameter of the stack and its natural frequency.)
vibration in the along-wind direction will also occur, and is best quantified by conducting a dynamic wind response analysis (gust factor method). most wind codes have some sort of dynamic model which should be used for the along-wind design of wind-sensitive structures, such as tall chimneys.
you can get some really good documents on chimney design from cicind - the international committee on industrial chimneys -

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