Pole fatigue calc

  • Thread starter Fat Ryan
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  • #1
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I was asked by the CE to do a fatigue calc for a speaker pole going up in key west. im using AASHTO standard spec for structural supports for highway signs, luminaires, and traffic signals. he said to assume a Vc of 45mph, an importance factor of 1.0 (from category I-lights in the cantilevered section) to be conservative. he also said to use the detail category E' for the CAFL, which results in a CAFL of 2.6ksi. all the specs from the pole and speaker set are from another spreadsheet we have here. its a 50' 16-sided tapered steel pole. to give you an idea, the diameter ranges from 21.2 to 14.2 inches with an X-section area ranging from 16.5 to 11 in^2. the speakers mount atop the pole and weigh in at 440lbs.

ive attached my spreadsheet with the calc. ive never done a fatigure calc and this AASHTO spec is god awful at explaining what to do. im getting a required loads that are like 50,000 times higher than the CALF lol. im guessing the "conservative" 45mph wind speed would play a large role considering its squared in the pressure equation. and the drag coefficient i have for the speaker seems awfully high. but i also dont really even know what im doing in the first place. im assuming the critical section is at the poles base (by the fillet weld to the base plate). It seems that'd likely be where fatigue cracks would form in my mind.

so am i doing something wrong here? perhaps im missing some key theory...
 

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  • #2
One thing you are doing wrong, self weight is a static load and cannot induce fatigue damage.

It is not possible to verify the equations in your spreadsheet, but you might be working with the wrong units.
 
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