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Subject: Direction of Seismic Loading

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I agree but to clarify.

100% + 30% is for when a line of lateral resistance is eschewed or not perpendicular, or parallel, to any other one or more lines of lateral resistance. This causes the orthoganal lines of analysis to not aligned with the principle axis.

The added 30% is maximum possible for all arrangements of eschewed lines of resistance. An analysis will usually result in substantially less design demands because eschewed lines of resistance are usually a minor player.

In a circular plan any direction is the principle axis, the maximum design demand. The 30% component is not needed.

To push along a principle axis causes no perpendicular deflection.

What If the mass is not symmetrical or the center of mass is not at the center? Torsional momentum with a concentric center of mass could cause lateral movement perpendicular to the applied force. Most likely insignificant unless the frame is extremely flexible or the mass eccentricity is extreme.

If Nukes require 100% 40% for all cases... apples and oranges, what is the design force for what return period? Lets assume its the same issue of principle axis. The extra 10% might be because of the goofy approaches for eschewed elements. Requiring that the combination be used at all times is probably due to the lack of confidence that designers will acknowledge an eschewed principle axis. Do not worry though its not an over-kill. I have found high Nuke requirements are usually buffered, in the details, with load reductions or increased ultimate strengths, diminishing the original increase of analysis.

David



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