Vortex shedding of tall tower

[farzam]

Dear All,
I have some tall towers(h=70m, OD=7.8m) which i applied vortex shedding to design the tower and the anchor bolts.Recently our client infromed us which the towers will be installed and erected in site for a long period(6 months) without any attached accessories such as platform. so i checked the bolts for erection condition with applying the vortex shedding and i found some anchor bolts are failed. Please let me know, is it required to check the vortex shedding for erection condition?and if i have to check may i increase the allowable stress?

Thanks in advance

 

[TomBarsh]

There seem to be two issues: direct loading and fatigue.

Vortex shedding is not typically a long term condition, as lasting continuously for 6 months. But then again, if you have a relatively low critical wind speed then you may get a large number of load cycles acting on the vessel and you could get fatigue failure of the bolts. Applying a preload on the bolts will reduce the stress range that will occur, and will help the fatigue life.

The bigger problem is the direct loading. If based on the smooth surface you obtain a high-enough direct load that the bolts are overstressed...then that is a problem. 6 months is a long time for that vessel to be standing with the bolts potentially subject to overloading.

What is your critical wind speed?

Are you combining the load due to vortex shedding at critical wind speed with the direct wind at design wind speed? ...this should not be done, be sure to combine the vortex shedding load and the direct wind load both acting at the critical wind speed.

 

[TomBarsh]

Let me add, if your critical wind speed is very high, then consider what is the probability of that wind speed occurring over the next 6 months?

And to induce vortex shedding, the wind must be present at the critical wind speed for sufficient duration to induce the vibration. The higher the critical wind speed, the less likely this may be.