CMU Sound wall and Fences

[Kris the Engineer]

Hello All,
At my firm we occasionally design CMU sound walls and retaining walls and the seismic values have changes drastically.

There was no specific callout for walls in the ASCE 7-10, however there was an article in Structures magazine that said that it would be added and gave a value of R=3.

When the ASCE 7-16 was published they added the fences to chapter 15 and have it a value of R=1.25. Does anyone know why the value changed so drastically from what was previously used? This is reeking havoc on my fence systems and just seems ridiculous as I have not been seeing any issues with CMU sound walls falling over during earthquakes.

Does anyone have any information as to why this changed or if there is something I am missing in the code that might lower the seismic design values to something similar to the previous code?

 

[bones206]

I have no specific insight, but I can take a guess... A cantilever fence or wall can't really maintain stability after developing plastic hinges, so it needs to be designed to remain in the elastic range. An R value of 3 implies a system that can absorb energy in the inelastic range. An R value of 1.25 implies elastic behavior.

Fences also have a ρ factor of 2.5, which penalizes the lack of structural redundancy.