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Part 6 of EC8 deals with tall and slender industrial structures. It covers concrete, steel and masonry chimneys, braced and unbraced steel towers (including tank supports), guyed steel masts and electric transmission towers. The superstructures are often governed by wind loads, even in areas of high seismicity; as noted in Annex F for electric transmission towers, it is the foundations which more often give rise to seismic failure, through soil failure due to liquefaction or other causes.
Comments on all aspects of Part 6 are invited; those of particular interest to the Moderator of this section are as follows:
- Given the fact that wind often dominates superstructure design, what degree of analytical sophistication for seismic effects is warranted?
- In particular, Clause 3.1 (1) requires rotational as well as translational ground motions to be accounted for in ‘tall structures in regions of high seismicity’. A note proposes the threshold at which rotation needs to be considered (this is subject to amendment in national annexes) and Annex A proposes a rotational response spectrum. Are these requirements appropriate and rational? Does the rotational response spectrum have any field data to support it? Note that base rotations due to soil flexibility can be accounted for without including rotational ground motions, by introducing soil rotational flexibility at the base of the structure.
- Is the guidance on SSI effects in tall slender structures appropriate and sufficient? Is specific guidance on foundation design needed?
- How appropriate and justified are the q factors proposed for concrete and steel chimneys and for steel unbraced and braced (figure 7.1) towers? How appropriate are the proposed damping ratios? The differences between the unclad industrial structures covered by Part 6, and the heavily clad building structures of Part 1 are relevant here.
- Is the textbook like advice in Annex F for electric transmission towers useful and appropriate? Should similar guidance be added for guyed masts?
- How does Part 6 compare with US guidance for similar structures, in terms of the cost and complexity of the structures that result?
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