Residential Earthquake Engineering
An interesting framing feature of this fifty+year old SoCal tract home is the way it’s joists, carriers and piers are arranged.
The foundation is reinforced concrete. There are piers under every interior wall, load bearing or not, at about four foot intervals. The joists and the joist carriers are all 2X4 doug fir, the carriers tripled.
The crawl space is very shallow under this house, barely enough room for an average sized male body to slide under the carriers.
When I first noticed this odd arrangement, I concluded that the reasoning behind this method was the economy of using only 2X4s for the entire frame, including the rafters, headers…everything.
But later I recalled that my dad, an insurance underwriter when he bought the house, had told me that the foundation was engineered to be better able to withstand earthquakes.
It’s held true, during two major earthquakes in this part of Los Angeles since the house was built, that the homes in this tract have gone undamaged, while similar homes built on slabs in the same area have suffered badly.
So, is anyone familiar with residential earthquake engineering ideas from the the 1950s? Does this multiple pier method fit?
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Except for the stacked 2x4's, that "odd arrangement" is found on every CA house I've seen that has a crawl space. Basements are very rare here and (with the exception of Eichler homes built in the 50's) slab-on-grade is actually fairly new.
My first CA house (built in '61) had 4x6 fir stringers with 2x6 joists and a 1-1/8" T&G ply subfloor. This house (built in '79) is the same except it has 3/4" ply for the subfloor.
I suspect that this design was more about cost effeciency than seismic engineering although earthquakes are certainly part of the equation. In the '50's and '60's most roofs were shakes on skip sheathing so 2x4 rafters were fine. We sure don't have to worry about snow loading. - lol
This house has settled a bit over the last several years, however. When we bought it, I easily crawled all around the crawl space installing phone lines and stereo speaker wiring. A few scoops with a folding shovel gave me plenty of room to get under the heating ductwork and sewer lines.
A few years ago, I had to go back under the house to install cable TV wiring and had much less clearance than before. I say the house settled, but SWMBO blames it on the fact that my jeans are two sizes bigger. - lol
This house has settled a bit over the last several years, however.
Hahaha. When I read that I chuckled, thinking, "I wonder if his house has settled the same way this one has?" Then you fessed up and I knew we were on the same page.
I guess we'd have to know more about the post war economy and the state of the doug fir forests at that time, to make a better judgement about the economics involved in the choices they made on framing lumber.
What I can say for certain is that every stick in his house is 2X4. So that means that the entire tract was built the same way.
The rafter framing doesn't look very substantial but it has served quite well.
I still can't draw the conclusion that all that 2X4 was strictly an economy measure.
The number of piers, the board feet of 2X4 used in the many carriers and the additional labor involved in building the floors that way seem to indicate that it would've been more economical to use larger dimension joists with far fewer supporting members.
Therefore I'm left with the obvious conclusion, that they were using this multiple support engineering to help absorb or dissipate earth quake shocks.
BTW, modern slab engineering is an evolution of the breed but houses built on slabs go back many years in SoCal, at least to the post war era. I have memories of numerous adult conversations in our home on the relative merits of slabs vs. raised foundations, during the 1950's.
As it has turned out, the homes built on slabs over old lake beds and ancient giant washes off the very tall SoCal Sierra mountains have been the ones which have suffered the most earth quake damage.
This house, built on solid clay, near the Santa Monica mountains, which are actually just a 500' high ridge along the coast, has never suffered more than a few minor cracks in the plaster and the stucco, all of them cosmetic.
So...I guess I'll have to call around, see what kind of answers I can find from local engineers, architects, etc. I'll let you know if anything interesting comes of that.
Edited 8/16/2008 12:08 pm by Hudson Valley Carpenter
IIRC their is some merit to the pier and post when it comes to earthquakes in some soils conditions. The shorter spans between bearing points allow the building to flex and roll with the quake "wave" as it passes beneath the structure. A slab lacks that ability and therefore it snaps and causes a sudden and large force to be transferred to the structure above.
What I remember comes from reading about traditional Japanese building styles. they used boulders for support and it allowed the entire structure to literally roll was the theory. The problem with the Japanese structures lies in the heavy clay tile roofs, the buildings are literally top heavy IIRC.
They can't get your Goat if you don't tell them where it is hidden.
The shorter spans between bearing points allow the building to flex and roll with the quake "wave" as it passes beneath the structure. A slab lacks that ability and therefore it snaps and causes a sudden and large force to be transferred to the structure above.
That's a more complete explanation of what I remember my dad telling me. His work as an insurance underwriter gave him ready access to current engineering theory and practice. He told me that was an important factor in buying this house, in this location.
The North Ridge quake, centered only five miles from here, shook this whole neighborhood very badly but caused no structural damage and very few plaster cracks.
Like I said in my earlier post, the stringers I've seen have been 4x6. When I've seen stacked 2x4's, it was obviously a repair or remodel effort.
Doug fir was (is?) common in the west. The forests in the Pacific Northwest provided huge amounts of old growth framing wood. I've seen 20's and 30's era houses framed with some of the most beautiful VG fir you can imagine. Yes, I said framed with VG fir!! The stuff gets so hard that on one job I had to pre-drill everything just so I could drive the nails.
We also have several Victorian homes that are full of old growth redwood. Once upon a time the Santa Cruz Mountains were covered with the stuff. These days, possessing a chain saw within a mile of an old growth redwood tree can get you some severe jail time. - lol
Dave,
I'm real familiar with doug fir, my favorite framing material. I've also used it as exposed rafters and floor beams, unfinished and finished. I can be very impressive when it's colors and grain are brought out with analine dye stain.
The question I have is whether or not doug fir was significantly more expensive in dimensions larger than 2X4, during the post war years.
I'll be doing further research to discover the answer to that question.
Larry Haun wrote an interesting article on SoCal tractwork in the 50's some time ago in FHB. It would be interesting to hear from someone who was around at the time if seismic design were much of a concern in that era, or if the builder of the tract your home is in got a great deal on 2x4's.
If I recall correctly, the seismic event that got the ball rolling to increased seismic safety ,so to speak was the San Fernando quake of 71 where a number of hospitals were seriously damaged.
There's been a major earthquake in SoCal, about every twenty years. Each of them has caused more damage than the previous one, mainly because of the population growth in the area.
As a result of these events, increasing attention has been paid to controlling damage in all types of construction, each time they have occurred.
The San Fernando quake caused a lot of damage to Olive View Hospital, which was subsequently rebuilt to much safer standards.
Until fairly recently, building heights were limited by code. Exhaustive engineering studies has improved the sustainability of taller office buildings so there are now many of them in the downtown LA area as well as other localities in the city.
I plan to make some calls this coming week, to see what I can learn about the type construction that was employed under this tract's houses. I'll gladly share whatever comes to light, in this thread.
Edited 8/16/2008 8:53 pm by Hudson Valley Carpenter
PanicMechanic mentioned Larry Haun. I remember an article he did in FHB about 5 years ago on framing techniques. He was stressing the advantages of gang cutting, and the illustrations of the house showed why. The framing was an intricate web of nothing but but 2"x4"s perched on closely spaced interior piers - just as you describe. Either Larry never changed his style or it didn't die out in the '50's after all.