Friday, February 2, 2018

We should all hug (or at least plant) more trees

An article in cites the work of Sébastien Guenneau (and others) of the Fresnel Institute in Marseille, France and says that “buildings in the future could be isolated from earthquakes by being placed behind rows of trees.”  Apparently, according to the article:
(Photo from linked article)

“…certain seismic waves, known as Love waves, could be diverted away from the Earth’s surface as they pass through a forest containing trees of a certain height. The forest acts like a metamaterial – an artificial structure usually used to steer electromagnetic radiation around objects.

“Best known for their use as invisibility cloaks, metamaterials are made from large arrays of tiny resonators that manipulate light and other electromagnetic waves in unnatural ways. In recent years, however, the mathematics underlying metamaterials have also been applied to other kinds of radiation, including seismic waves. The idea here is to use arrays of suitably-sized objects either below or above ground – holes or posts of some kind – to divert seismic waves around vulnerable buildings.”

Guenneau says that, in practical application, trees would have to be roughly 10-15 meters tall to resonate with horizontal Love waves.  Protecting a building from the vertical “Rayleigh” waves, however, would require trees some 5 times this tall (that would take decades to mature) making it less practical. That said, scientists say that by using trees to prevent horizontal shaking and conventional techniques to “guard against vertical motion, forests could halve the work of civil engineers.”

Much more research is needed to better understand the behavior of these waves and the potential real world applications.  The science, however, is fascinating.  And the potential benefits are worth continued investigation.  

The bottom line is that mature trees are good.  And worth protecting for a variety of reasons.

This occurred in September, but I just received notice of it via an e-mail blast from FEMA.  The National Earthquake Hazards Reduction Program (NEHRP) issued a new set of Recommended Simplified Provisions for Seismic Design Category (SDC) B Buildings.  For a look at Category B, see Map here

Not all portions of the country are affected, but the new document should be a helpful reference for all in A and B zones looking to improve resilience in the face of such events.  (I happen to live in a Category B area.)  And of course those in Categories C through E will have much more rigorous requirements.  Here’s the link to the new document. 

The introduction to the report includes this statement:

"For Seismic Design Category (SDC) B, which designates a low seismic hazard region, structural engineers still need to complete a full seismic design process to meet the building code requirements.  Seismic design is necessary because earthquakes are a hazard with long return periods and large uncertainties, and the sudden occurrence of earthquakes in SDC B regions, such as the 2011 Mineral, Virginia earthquake, can cause significant damage or collapse if buildings are not properly designed for seismic resistance.  The recommended simplified seismic design provisions described in this FEMA NEHRP document aim to assist structural designers in meeting building code requirements for ordinary SDC B buildings without wading through the full, complex seismic design process in ASCE/SEI 7.”

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