Conveyor tower - dynamic model required?

[SeanMD]

I'm designing pile fdns for a large conveyor system (150'-175' high towers by others). Soil site class F, high seismic area. In reviewing ASCE 7, I am not sure if the towers need to be designed with dynamic modeling, or can ELF be used? The period is less than 3.5Ts, but given that the towers are tall with the mass concentrated at the top, would that be a vertical irregulartiy, and is ELF appropriate? It would be considered a "non-building structure", so it doesnt exactly have stories. Given the language in Ch. 15C, I am thinking a dynamic analysis will be required.

And if a dynamic model is used for the steel design, does the fdn need designed with a dynamic model also? Or can the max reactions from the towers be used with a typical pile fdn design?

Anyone do conveyor towers in high seismic areas before?

 

[cessna98j]

While I haven't had much experience with site class F, I've designed conveyor transfer towers on offshore pile foundations before. In that case, we performed dynamic analysis of the tower, piles and pile cap together as one structure since the dynamic behavior of the tower itself would be different in assuming a rigid foundation.

Even if this is an onshore foundation with piles, your pile foundation would still alter the dynamic properties of the structure as a whole since the depth to pile fixity is going to be several feet below the surface.

In short, I think you're right in assuming that dynamic analysis is the best way to go, and I would say the most accurate dynamic model is going to include both the tower and your foundation.