Earthquakes and MCM

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turboblown
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Earthquakes and MCM

Postby turboblown » Sun Jan 23, 2011 9:15 am

Several of us were haveing a discussion (well, arguement really)yesterday about this subject and were hoping that some of our friends on the west coast could answer it better.

How well has the post & beam and other types of common building methods and architechture that are used in typical SoCal MCM construction held up during earthquakes? It seems that this type of dwelling would survive better than "typical" construction since it is not only based on a common structural frame (when post & beam), but also since this design seems like it would offer more flex when it is necessary.
Another point that I also had is that many of these structures were engineered- not just slapped together like common construcion so the loads and other factors came into play during their design.

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Postby scowsa » Sun Jan 23, 2011 12:07 pm

Local reports after a few quakes in my time here, -- including the '89 "World Series quake" when I lived in Palo Alto -- tend to focus on three key factors, beyond the obvious fault-line location:
1. The nature of the ground, which is why that '89 quake affected a lot of houses in SF's Marina district which was developed on filled-in shoreline. Soil testing and possible compaction prior to building is now common for new builds
2. The nature of the walls, with masonry or adobe being particularly vulnerable as they cannot flex while timber-frame or steel-frame can. In Palo Alto I can remember larger framed houses being fine but their brick chimney falling through the roof. Our timber-frame PA house was three story and flexed fine.
3. How the house is tied to the foundation, where we had older timber-frame houses literally shift off their poured footings. Code now has them bolted to the footings and older houses can be retrofitted.

I cannot remember other house construction factors being highlighted that suggest post and beam has additional benefits.
scowsa

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Stephen
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Postby Stephen » Sun Jan 23, 2011 4:49 pm

Okay...as a kid obsessed with earthquakes I'll take my stab at this. I live in a post and beam home and have done a fair amount of thinking about this.

All things being equal (same soil, shaking, etc) MCMs are a mixed-bag versus traditionally framed homes of the same era. Here are some of the points and things I've considered:

1. Method of failure -- Traditionally-framed homes almost rely on complexity and interconnectedness to add strength. MCM have very clear, concentrated loads. This means you always know what's going on, but singular points of failure can be catastrophic.

2. Shear Strength -- In CA, shear strength wasn't really on the lay builder's radar until the Northridge earthquake in 1993. Nobody's designs for the 50s were that great, but at least many MCM designers were "shear aware" and showed they were thinking about while traditionally-framed homes of the era had no consideration.

3. Single Story -- The larger the weight held farther from the base of the building, the more pendulum action you have. MCMs have lighter roofs and rarely a second story. In general, there's a lot less mass moving around and that puts less strain on connection points.

4. Connection Points -- Sadly, these are the weakest areas of MCMs. Even as early as the 1960s a lot more engineering went into the post and beam connection points. Architects and builders saw this. Early Cliff Mays use butt joints between ridge beam sections. Later models use much stronger keyed joints.

5. Loading -- Nearly all of a post and beam's load is downward, to the Earth. Traditionally-framed homes (trussed roofs) present a large side load to the walls at all times. Shifting can result in significant structural damage that may not be visible apparent, at first. Additionally, portions of the structure can end up tensioned in ways they were never designed. This can lead to eventual failure later on.

Generally-speaking, in a post and beam structure the loaded structure is all visible to the naked eye. If the house looks okay, it probably is. With traditionally-framed structures it's much harder to determine when things go wrong as significant damage can occur without it looking "that bad."
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Postby Mod' i-fy » Mon Jan 24, 2011 1:37 pm

I often wondered about this as well. I walked through the whole house and examined all beams and posts etc and to my eye everything looks good considering all the earthquakes the house experienced since it was built in 59'. I suppose too it helps that the house was built mostly on rock mass on the side of a hill, not too much flexing.
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Postby Joe » Tue Jan 25, 2011 6:19 pm


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Postby Slim and Gabby » Tue Jan 25, 2011 8:06 pm

Our House is on a slab, and in 2003 we had a 6.5, that knocked the crud outta our town, but as far as I know out little house rode it out exceptionally well.
I think quite a few people are unaware of the New Madrid Fault system:

http://en.wikipedia.org/wiki/New_Madrid_Seismic_Zone

It's not just the West Coast that has EQ's: yes they're more frequent here, we live on the Pacific Ring of Fire, but when the New Madrid fails, holy smokes, it fails! Between Dec. 1811 and Feb. 1812 the area had three months of exceptionally large EQ's, in fact, it's believed the largest in North America's recorded history, ranging anywhere between 7.0 & 8.6! So fierce it reeked utter havoc on Mississippi River making appear to run backward!
When I lived in San Jose, CA we had the 1989 Loma Prieta EQ that was a 6.9 (surface-wave magnitude 7.1). The Spanish Style house we lived in built in 1925 had very little damage, except for the chimney that shifted about 1.5-inches.
Just my two cents,
Slim
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turboblown
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Postby turboblown » Thu Jan 27, 2011 7:25 am

We've had quakes here in PA, but nothing like the west coast has seen. We just had one this past spring....it was like a 2.X Any geo-failures around here are usually man-made from under mining of the land. I have a mine shaft that sits directly 147 feet below by house.

You all answered my question and seem to agree with what I assumed would really be.


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