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# PGA reduction factor on wall inertia?

## PGA reduction factor on wall inertia?

(OP)
Hi all, I can't seem to find a definitive answer in texts, I've been looking through AASHTO, FHWA/NCHRP reports and USACE guidelines.

For a concrete block gravity retaining wall, I've accepted that a 50% reduction to Kh is acceptable and that 1-2" of movement that this correlates to is okay. However I'm unsure if this reduction should only apply to the seismic earth pressures, or can it also be applied to the wall inertia force?

Thanks.

### RE: PGA reduction factor on wall inertia?

calky117 - When you refer to concrete block gravity wall, what type of wall are you talking about? Small segmental wall units? Large concrete blocks?

If you look in AASHTO LRFD, Chapter 11, beginning of chapter, you can find what you are looking for in general terms. There is a big difference in the seismic section between 2012 AASHTO vs. earlier versions as it was re-written.

The current thought is kh = 0.5As (As is the site adjusted peak ground acceleration) is fine for retaining walls that can freely displace and is applied to both the earth pressure and inertial force. The 2012 AASHTO code goes a little further and also suggests that one can use 50% of either the Pae (seismic earth pressure) or the Pir (inertial) in combination to arrive at the highest total load (however, Pae@50% can not be less than Pa-static). This is based on the logic that seismic earth pressure and inertial forces are not a maximum levels at the same time during an event. AASHTO also suggests a load factor of 1.0 be used on both components which is more aggressive than the previous AASHTO code.

The AASHTO commentary explains the logic behind this but I think they have may have gone a little too far and do not think it applies well to large block gravity systems which typically fail in overturning. Concrete walls and MSE walls are much more forgiving when overloaded and do not fall like a stack of blocks would. Just my personal observation as most people would think AASHTO is the most conservative code and it is not anymore with regard to seismic analysis of retaining walls.

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