Issue link: https://read.dmtmag.com/i/63983
SERVICE & SET-UP All Shook Up BYGEORGE PORTER The title of this article is of course referring to one of Elvis's hit records. "The King" is from Memphis TN. I wonder if he got the idea for the song from the fact that his home town is on one of the greatest earthquake zones in the United States. In many high and moderate seismic risk areas, earthquakes pose a tremendous threat to lifeline services, such as power, water, and in- frastructure systems. Nowhere is there a clearer example of such a threat than in the City of Memphis, Tennessee. The City of Memphis is located within the impact area of the New Madrid fault system. The Center for Earthquake Research and Infor- mation at the University of Memphis has re- ported a 40% to 60% probability of a New Madrid Seismic Zone earthquake in the magni- tude of 6.0 to 6.3 within the next 15 years. Therefore, it is only amatter of time before life- lines in the City ofMemphis experience the im- pact of a significant earthquake. In recognition of the risk posed to lifelines in the New Madrid area, the Memphis Light, Gas, andWater Division has initiated a seismic retrofit project to protect its DavisWater Pump- ing Station (located in Southeast Memphis), and to enhance the survivability of the connec- tions between the water distribution lines in one-third of the city's production wells. The seismic retrofit of theDavisWater Pumping Sta- tion will involve the strengthening of supporting structures and tying together of components so that they will vibrate as a unit during an earth- quake. These same principles apply to our homes. When one part (house half) wants to go one way and another part wants to go another way, troubles are sure to follow. MAY 2012 14 THE JOURNAL Allow me to say right up front thatManufac- tured Housing (HUD Code) is absolutely the safest standard home to be in during an earth- quake. The steel frame is a major factor in the "unitizing" of the structure. Plus, we test ever one of our homes for earthquake before we even install it, the test is called delivery! If it can make it to the lot through potholes in the road and whatever else is in the way, an earthquake is not a big deal. About nine years ago NFPA published a Manufactured Home Installation Standard called NFPA 225. There were several hereto- fore unheard of "standards" in the document and the entire industry considers it interesting but not practical. It has since been "shelved" but it is still around. One of those "standards" was a section called "Seismic Design Criteria" and it gave in great detail all the different "Seismic De- sign Categories." (SDC) The details are cov- ered in just FIVE pages! That is a lot of information about something nobody had even discussed before. They listed 25 States that "re- quire" SDC's. In Tennessee alone there are 39 counties that are in the zones listed. Who ever heard of earthquakes in Tennessee? California sure, but Tennessee, this has to be a mistake! Ah, the human condition and our inability to remember history! InMay of 1811 there was an earthquake with a magnitude of 7.9. (The great San Frisco Earthquake of 1906 was smaller, 7.7) It was followed by twomore; they relocated the Mississippi River and killed 200 people. It was centered in New Madrid Mis- souri, about 125 miles from Memphis and it caused the church bells to ring in BostonMass. Imagine if you will, how many "fur trappers" were living in the area then and how many peo- ple are now. So, we need to talk about the problem. Happily the problem is already solved except for that "human thing." Our friends in California have been having events fairly steadily and that enables us to keep it in our conscious minds out there. They require earthquake systems on everything in the zone. So what is an earthquake system? Well, there seems to be two directions that we can go here. One is a system called "base Isolation" and the other trend is to reinforce the structure so that it can't separate. Base isolation is ap- pearing to be the best way to keep the effects of the event to a minimum. The principal is that you have big footings right on top of a surface base that will allow the ground to move beneath them with out transferring the forces to the home. The inertia of the home will tend to keep it in one location. 'Ever see the snatching of a tablecloth out fromunder a set of dinner dishes? Same idea, the ground goes crazy but the home doesn't. Interestingly the adding of an earthquake component to our homes is just another layer of requirements, all the old ones still apply. For instance, suppose you had a home next to a river (flood plane) in wind zone 1, with a 30 lb roof load, with thermal zone 2, and an earth- quake zone SDC-Do . You would have to tie it down as required by FEMA for flood, com- plete with flood piers, also satisfy the require- ments of WZ 1, have footings appropriate to a 30 lb roof load and install a base isolation system for earthquakes appropriate to the SDC require- ments. Problem, how can the base be isolated if the anchors are on the home? Or, how can the home be tied down properly if the foundation must be able to shift. Then \ 15