Tag: redundancy

Home Earthquake Vulnerabilities: Weak Cripple Walls

This FEMA photo, taken after the 1994 Northridge Earthquake, shows what can happen to a house with a weak cripple wall.

The most well-known earthquake vulnerability with houses seems to be inadequate foundation anchorage, as discussed in the previous post.

The twin sibling of an “unbolted” house is a weak cripple wall, which we will discuss here. These two seismic weaknesses often occur together in older houses. They are also the two primary earthquake weaknesses that seismic retrofit contractors address, and the main reasons that these type of contractors exist.

Weak cripple walls can cause even greater seismic damage to a house than weak foundation anchorage, as the house can fall a few feet- whatever the height of the cripple wall was- and also shift laterally. This vulnerability is therefore typically more dangerous and more significant economically. If the seismic damage of a house sliding off its foundation doesn’t require a complete demo and rebuild, a weak cripple wall almost certainly will.

What is a cripple wall?

A cripple wall is the wood-framed wall between the foundation and first floor of a house. It typically follows the exterior foundation. Not all houses have cripple walls; some bear directly on a concrete stem wall or footing. Other houses bear directly on a slab-on-grade foundation system, although this is uncommon in the Pacific Northwest.

Houses with crawl spaces often have a cripple wall concealing the crawl space around the perimeter of the house and filling the space between the first floor and the foundation. If you have an old house with an elevated porch and no basement, you likely have a weak cripple wall.

Houses with basements often have a concrete (or brick) foundation wall with the first floor framing sitting directly on the wall. There is no cripple wall in this situation, although it is fairly common to have a portion of the house with a crawl space and cripple wall, and a portion of the house over a basement with no cripple wall. Some basement walls only extend part of the way up to the first floor and have a cripple above them.

The damage shown to the house above is the result of a strong earthquake (Northridge, 1994) combined with a weak cripple wall. It may appear that lack of stability at the porch caused this damage, but the porch collapsed because of the weak cripple wall. Notice the red placard, which indicates that the home is considered too unsafe to enter, even for the homeowners to retrieve their belongings.

Why are cripple walls so prone to seismic damage?

Seismic retrofit work in wood-framed houses should primarily focus on the base of the house between the foundation and the first floor. Severe seismic damage is much more likely here, in general, than above the first floor.

There are two main reasons for this:

  1. Seismic forces (and damage) increase with weight. Weight is greatest at the base of the house. Buildings and their components react to earthquake accelerations roughly proportionally to their weight. Also, where the house is closest to the seismic accelerations occurring in the ground, it “feels” them the most.
  2. A house almost always has virtually no seismic redundancy below the first floor level. “Redundancy” is an earthquake engineering concept that indicates a structure has more potential seismic load paths than it needs- from the upper levels of the structure to the ground. Earthquake forces in buildings are resisted by the stiffest elements- which happen to be the walls in typical wood-framed house construction. Above the first floor, houses often have multiple interior walls that inherently draw seismic forces into them during an earthquake due to their stiffness. This means the seismic demand of any individual wall above the first floor is usually considerably less than that of the cripple walls below the first floor. Even if a house has few interior walls, the drywall or plaster sheathing adds redundancy. Exterior siding does, also. Although these non-structural wall coverings are often brittle with minimal strength, they can, and do, have some capacity to resist seismic forces (potentially even after cracks form).

Cripple walls are forced to bear the entirety of the earthquake forces exerted on a house, without the help of any other stiff elements- such as interior walls.

This cross section of a typical house shows the importance of strengthening a weak cripple wall for seismic forces, as described above.

What not to do with a weak cripple wall

Some contractors- and even some engineers- have attempted to improve the seismic resistance of houses by installing post caps at the interior posts in a crawl space or basement. This does virtually nothing to seismically strengthen a house.

I believe this error may be more common in the Pacific Northwest than in California, where seismic retrofitting is more mainstream.

The strap and post cap shown above were done in the name of a “seismic retrofit” in Portland. The strap is incorrectly installed and serves no purpose anyway. The post cap shown at the bottom of the photo doesn’t hurt anything- but is typically insignificant in reducing earthquake risk.

If you own a house that has been “retrofitted” in this manner, it’s possible the contractor didn’t understand the science behind seismic retrofitting. Make sure the cripple walls have been strengthened, including every part of the load path. It wouldn’t hurt to have a seismic engineer or qualified retrofit contractor look at the “retrofit” that was done to make sure the “retrofit” doesn’t need to be retrofitted.

Seismic forces are drawn into the stiffest elements of a structure, as explained previously. In a house with cripple walls, the cripple walls are by far the stiffest elements between the first floor and the foundation. The cripple walls have stiffness many orders of magnitude greater than the interior posts. The first floor “diaphragm” (engineering term) also acts as a single structural element that has a stiffness many orders of magnitude greater than the interior posts.

Seismic forces that manage to get into the interior walls above the first floor will hit the stiffness of the first floor diaphragm and move to the exterior cripple walls, essentially bypassing the interior posts below the first floor.

If a house has weak cripple walls and rigid interior post caps, the posts would still fail in an earthquake when the cripple walls fail, as they are far weaker than they need to be to resist strong seismic forces.

If you are looking for redundancy with the cripple walls, or a “belt and suspenders”, so to speak, to resist seismic forces, then overkill the cripple walls by strengthening more than necessary with increased length and/or increased nailing, instead of focusing on wood posts. I typically take this approach. At least one of the seismic contractors I regularly work with takes this approach, also.

A seismic retrofit in a house with weak cripple walls should strengthen the cripple walls themselves, not the interior posts.

One last comment on this topic: post caps aren’t a bad thing. It’s generally a good idea for beam-to-post connections to have more than a nail or two connecting them together. There are also some instances where post caps can decrease seismic risk. But even in those instances, they wouldn’t be the top priority in a seismic retrofit.

A detail from the FEMA plan set indicating how to strengthen a weak cripple wall. The FEMA plan set is a prescriptive set of drawings intended to help contractors or homeowners retrofit a house without engineering.

How can a cripple wall be adequately strengthened?

Plywood sheathing is the go-to method for strengthening a weak cripple wall. Plywood shear walls have been rigorously tested by the American Plywood Association and are typically used as the primary seismic force resisting system in modern wood-framed construction. Modern wood-framed buildings do well in earthquakes in general.

Well-built plywood shear walls (i.e. plywood-sheathed cripple walls) are strong, ductile, and somewhat flexible, which makes them an excellent method to resist earthquake forces.

In contrast to properly installed plywood shear walls, old cripple walls are often constructed of 1x planks. Nailing may be minimal. These walls were simply intended to close the gap between the exterior of the house and the crawl space, while providing nominal stability to the house. They weren’t designed to resist strong earthquake forces. The wood may have lost strength due to rot, the nails may be rusting, and the connections can be brittle (“brittle” in the context of seismic resistance means sudden failure).

Some important components (shown in the FEMA plan set detail above) of a modern plywood shear wall are:

  • 1/2″ or greater thickness “Structural I” grade plywood
  • Tight nailing pattern around all edges of each plywood sheet, or “panel”
  • Nails installed at least 3/8″ from the edges of the framing members
  • Double studs, if required, are adequately nailed together
  • Nailing at intermediate studs does not exceed 12″
  • Seismic load path is addressed above the plywood sheathing
  • Foundation anchorage is strengthened from the mudsill to the foundation

While old houses, say, pre- 1940’s, have 1x planks for cripple wall sheathing, houses built from the 50’s to 90’s sometimes have weak cripple walls for different reasons. Many are sheathed with lower grade plywood, don’t have all panel edges blocked, are stapled instead of nailed, and occasionally have very poor nailing where a contractor repeatedly missed studs with a nail gun.

Other sheathing types, such as T1-11 siding, are weaker than plywood but likely stronger than 1x planks.

Hillside home cripple walls can be seismically weak, even in newer houses. See my post on hillside home structural problems for more on this topic.

Plywood shear walls were added in the garage of this split-level house built in the ’60’s. The exterior sheathing was questionable, and it was “cheap insurance” for the homeowner to have the walls strengthened.

Often, but not always, plywood sheathing is added to a weak cripple wall from the crawl space. This is often the least expensive and easiest way to strengthen a cripple wall, since the existing siding and sheathing doesn’t have to be removed.

Correct installation of plywood on a weak cripple wall is important and can be tricky. The seismic load path must be carefully followed from the first floor to the foundation. If one component of the load path is missed or inadequately strengthened, the cripple wall has a weak link and may not perform as intended during an earthquake.

Conditions vary from house to house, but a common seismic load path from the first floor to the foundation is as follows:

  • Through the first floor sheathing into the rim joist via existing nails
  • From the rim joist into the existing double 2x top plate via shear transfer ties
  • From the double top plate to the new interior plywood sheathing via nails
  • From the plywood sheathing to the existing mudsill via nails
  • From the mudsill to the existing foundation via new screw anchors

This typical load path can be followed in the sketch below.

This section indicates a common way to strengthen a weak cripple wall from inside the crawl space.

Houses with a strengthened cripple wall may perform better than seismically retrofitted homes without a cripple wall

Another benefit of plywood cripple walls is they have a great structural response to earthquakes. “Structural response” is another earthquake engineering term that refers to how a building responds to seismic loads.

Think of a building like shocks on a vehicle. As an earthquake hits a building, it responds in one direction and then rebounds back. Flexibility in a building can be good for earthquakes, similarly to the flexibility of shocks on a car. If the shocks are too tight, the vehicle feels the impact of a bump more, as do the passengers. Some flexibility in the shocks dissipates energy, as does some flexibility in a building.

Plywood shear walls have a good deal of flexibility as well as strength. This makes them a great system for resisting seismic forces, if installed correctly. A house with a strengthened cripple wall, in addition to being strong enough to resist earthquake forces, may actually experience less non-structural damage in an earthquake than the neighbor’s house that is bolted to its foundation without a cripple wall.

I believe a strengthened cripple wall is similar to “base isolation”, in that the house has a type of seismic damping system.

This makes strengthening a weak cripple wall possibly the best value in seismic retrofitting, in that it brings a home from a potential total economic loss, with some life safety risk, to a home that would likely perform well in an earthquake.

This house had unreinforced brick “cripple walls” between the top of the concrete basement walls and the first floor framing. Since URM (brick) is notoriously bad for earthquakes, the homeowner chose an upgrade that replaced the brick with plywood-sheathed cripple walls. The wood framing is new and the plywood was installed from the exterior in this case. Note the shear transfer ties at the top of the double top plate and the screw anchors with 3″x3″ plate washers through the mudsill into the concrete.

For more information about seismic risk assessments and retrofitting, please see the Cascadia Risk Solutions website.

How to Cure Earthquake Anxiety

The reality of a potential impending large earthquake in the Pacific Northwest has made its way to the general public in the last few years.  When the New Yorker article “The Really Big One” came out in the summer of 2015, many of us were shocked and genuinely scared.  After some initial shock wore off, people reacted in a number of ways ranging from buying earthquake insurance to storing up food and water to just living with a new level of anxiety they hadn’t known before (the latter probably being the most common reaction).  I recall talking to an insurance agent soon afterward and he told me that it was crazy how many people had rushed to buy earthquake insurance recently.  He didn’t miss a beat transitioning to sales mode in his next statement, which was something like, “you should consider buying some also to protect your family.”

But what should we actually do with this information that is new to many of us? And, to address the title of this blog post, how do we deal with “earthquake anxiety”?

Consider this goofy story as an anxiety illustration:  I was recently in a restroom at Starbucks and the lock on the door didn’t give me any evidence that it worked. This mattered because the door opened to a view of the coffee line and cash register. I played with the lock and couldn’t gain any confidence in it.  It made almost no sound when I pressed it, and it didn’t seem to even change position. Opening and closing the door a couple of times didn’t verify it’s functionality, except for a small, barely audible click. I reluctantly used the restroom anyway, but not without some anxiety.

That’s already too much information with the personal restroom story.  But here’s the point about anxiety: it is caused by feeling like you have no control over a potential problem.  The solution, I believe, is a combination of taking purposeful action that addresses the problem and letting go of control over the things you don’t have control over.

Consider how much lower my anxiety in the restroom would have been if the door had a deadbolt that made a loud noise as it was being engaged and it visibly could be seen sliding into place through the crack in the door. In the same way, adequate earthquake preparedness steps give you confidence that you have addressed the problem and will likely survive “the really big one”, and even thrive in the aftermath and be able to help others.

I just mentioned that the solution to earthquake anxiety is to take purposeful action that addresses the problem and to let go of control over things you don’t have control over.  The first step involves practical actions, many of which could be accomplished by anyone living in the Pacific Northwest.  The second is more related to mental health, psychology, etc, which I’m not an expert on (but see Matthew 6:25-34 if you are inclined as I am to address this through the lens of faith).

I will dive into some specifics in further posts, mainly related to actions we can take, but for now I want to point out that we can only do what we can do.  We can’t control when an earthquake (or any future high stress event) will come, but we can make a huge difference in our resilience when it does.  A head-in-the-sand approach is irrational and will not make anxiety go away.  Also, resigning to the overwhelm of tasks ahead is not a good approach, either: “I’m not going to do anything because there would be too much to do”, or “it will happen if it happens” are examples of this kind of thinking.

The work ahead in earthquake resilience can indeed be overwhelming, but even just a first few steps can make a big difference on an individual or family level than not being prepared at all.  A good preparedness plan will add redundancy to whatever plans you put in place.  This is a topic, that, if you plan to live in the northwest for a long time, is very important.  What should your next steps be in 2017 to make yourself and your family more earthquake resilient? If you have no ideas, stay tuned here…