[Greenbuilding] rigid or flexible? are there any lessons from the Japan earthquake for fastening residential structures to their foundations?

JOHN SALMEN terrain at shaw.ca
Sun Jun 5 14:36:42 CDT 2011


It is a good question as they say that earthquakes don't kill people -
buildings do. North American construction is built on 2x practice and we
feel good because we are told that 2x buildings survive earthquakes because
they have inherent elasticity. Unfortunately that presumption is so rampant
that it affects code, common sense and reality and we impose the supposed
elasticity of wood construction on layouts and floorplans that can't support
it. I see subdivisions with buildings that have poor building shape and then
are so perforated with unsupported openings (windows, grand entrances, huge
garage openings, etc) that they generally start to fail (lateral shifts,
popped windows, etc.) even without an earthquake - and in an earthquake the
ground floor is so soft the 2nd floors just collapse on it. 

 

The issue is not so much how well they were built (as best practice in the
assembly details) as that they are designed for failure. Building plans are
not typically subject to engineering review and if they are as a 'stock'
plan - are often significantly modified and compromised without any one
really paying attention (other than proper header size or joist sizing).  

 

When we talk about flexibility or stiffness we should really be talking
about ductility and brittleness - as there is a range of ductility in all
materials and how we design and join things together depends on that. A 2x
building is really a combination of stiff and flexible elements. A pencil is
flexible but becomes brittle and breaks when its ductility is exceeded.  A
shear wall is a good example of a stiff element in a building. A knee wall
should be built as a shear wall, etc.

 

I wouldn't say that simply bolting a woodframed building to a stiff
foundation is state of the art for earthquake design but it is really all we
have outside of complex dampening - so we might as well make that connection
work well - but realistically it is the rest of the building from the roof
down that needs to be equally well connected and also the foundation needs
to have a good ground connection. 

 

As for the building foundation connection. We have concrete (stiff and
strong but brittle) - we have steel bolts in the concrete (stiff but ductile
- meaning it can bend and move before breaking) and we have the wood plate
(ductile but can become brittle in compression or shear). Where is the
failure going to occur? Typically even if the concrete cracks there is
sufficient area to resist pullout or more commonly shear, especially if the
bolts are deep enough. A 5/8 steel bolt has a lot of ductility for bending
before breakage. A 2x wood plate is likely to become compressed and brittle
against the washer area and fracture before a bolt will break or pull out of
the concrete or the washer itself will fail in combination by bending.  So a
proper holdown connection to a foundation is really dependent upon the
surface area of the washer(s).  The use of a large square washer (not
common) is probably more useful than doubling the number of connections. 

 

So the state of the art for foundation connections is actually making the
common connection actually work. I use shearwalls and actual holdowns (hud)
type and often use bolt connections that extend to top plates. I also employ
an engineer to ensure that a design will work. I think employing engineers
is the most progressive thing we can do to both use materials effectively
and be conserving - as well as provide us with documented buildings which
improve the knowledgebase as well as experience base for trades to base best
practice on. 

 

From: greenbuilding-bounces at lists.bioenergylists.org
[mailto:greenbuilding-bounces at lists.bioenergylists.org] On Behalf Of Alan
Abrams
Sent: June-05-11 10:33 AM
To: Green Building
Subject: Re: [Greenbuilding] rigid or flexible? are there any lessons from
the Japan earthquake for fastening residential structures to their
foundations?

 

Yoshihiro Takishita describes how the massive, timberframed "gassho zukuri"
houses of Japan withstood tremors and storms--in which "the roof may become
deformed or shift slightly, but...It resettles naturally, since the tenons
are forced back into their mortises by the weight of the roof itself."*
Major beams were often selected from twisted and crooked trunks, to take
advantage of natural curves that were employed to enhance stability.  This
observation, however, hardly seems applicable to stud and joist
construction.

Nevertheless, Hugh Newell Jacobsen, an architect with a long and carefully
considered career in residential design, commented on how many vintage homes
in South Carolina withstood Hurricane Hugo, while new ones "literally
exploded." 

"That is primarily because corners and rafters are just butt nailed, with
little to hold them except good intentions.  There's no diagonal, no
strapping, nothing to firmly hold.  And even with the technical design of
plywood skin to reduce torque and rack, you can't wrap corners with it."**
[emphasis added]

By the same token, I recall photos of Galveston following the recent
devastating hurricane--showing near complete destruction of block after
block of residential neighborhoods, punctuated by the occasional surviving
house--which uniformly looked like construction subsequent to adoption of
wind resistant building codes.  This tends to validate Jacobsen, in that
connections between roof, floor, and wall assemblies had been taken into
consideration, resolving the essential flaw he identified.

All this to say that there is no place for an intermediate position--that if
we are building plywood skinned platform framed buildings--that we need to
follow all the way through with best practices or engineering for our
connections.  

AA
 
*Japanese Country Style: Putting New Life Into Old Houses/Kodansha
**The Wood Book 1990/Hatton Brown


Alan Abrams
Abrams Design Build LLC
A sustainable approach to beautiful space

6411 Orchard Avenue Suite 102
Takoma Park, MD 20912
office  301-270-NET- ZERO (301-270-6380)  
fax      301-270-1466      
cell     202-437-8583
 <mailto:alan at abramsdesignbuild.com> alan at abramsdesignbuild.com
 <http://www.abramsdesignbuild.com/> www.abramsdesignbuild.com




On Sun, Jun 5, 2011 at 12:31 PM, Kat <molasses at q.com> wrote:

I'd stick to the code.  It was revised/became more stringent after the Kobe
earthquake and will probably change again in a few years based on analysis
of the most recent earthquake.

I've heard that ancient Chinese structures survived earthquakes because they
were built without fasteners of any kind - my (non-engineer) understanding
of that is the jiggling distributed itself throughout the structure at the
joints, which were sort of lincoln-logged together.  I think the key there
is NO fasteners, though.  None.  I'm certain that if you build a house the
way we do now (all full of nails) and neglect to attach it to the foundation
and we get a big earthquake you'll end up with a house that slides off the
foundation.

-Kat

 

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