This is a difficult decision. You want your retrofit done right and it is natural to think a seismic retrofit engineer is the way to go. After all, seismic retrofit engineers design construction projects every day. They will make sure your house is retrofitted correctly, right? I mean, these people are scientists and will apply the latest science of seismic retrofitting to my house. My house will survive for sure. Think again!
The California Building Code is woefully inadequate. It only has five sentences that address seismic retrofitting. In summary, it says: “You can retrofit your house however you want to. It is not our job to tell you how to do it, if it will be effective, or who you should hire.” This leaves much to the imagination of the engineer or contractor. This is why retrofit designs, and therefore costs, can be so wildly different.
Making Things Up
There are some seismic retrofit guidelines, but they have so many problems they are hardly usable. These guidelines are archaic, contain hardware that has not existed for decades, and are based on inaccurate engineering. Standard Plan A is one of the most recent guidelines and was written to replace the older guidelines. It offers inferior retrofits at higher cost, and once you understand its flaws, you would never use it to retrofit your house.
There is no retrofit building code for seismic retrofit engineers and contractors to refer to for information. For this reason retrofit professionals have no choice but to come to their own conclusions and “make things up.”
The author of this article was a member of the Standard Plan A development committee (which is also hopelessly inadequate). In this capacity he sent out a questionnaire to several seismic retrofit engineers with decades of experience. Most of them lived in Los Angeles and had seen damage from the Northridge Earthquake first hand. Please look at question number 1. Notice there is unanimous agreement that only bolts attached to plywood do anything. Notice the retrofit guidelines mentioned above call for bolting the bottom of the house to the foundation everywhere, even though bolts won’t do anything without plywood bracing. In spite of this, 80% of the engineered designs I see recommend unnecessary bolts at great expense to the homeowner.
How Do I Find a Good Seismic Retrofit Engineer (contractor)?
At a minimum make sure your retrofit engineer understands basic engineering! The basic concepts of seismic retrofit engineering are simple and available on this website. Let’s say a seismic retrofit engineer recommends bracing posts that hold your floor up. As we shall see, if they recommend this, they do not understand basic retrofit engineering or what happens to houses in actual earthquakes.
If they recommend a procedure to prevent the house from jumping up and down, keeps the roof from falling off, or something else that historically has never damaged a house, stop wasting your time and look somewhere else. If you have already had a set of plans drawn up by an engineer, get a second opinion. Take the plans to a structural engineer who specializes in designs for wood-frame houses (houses are consider wood-frame) and see what they think. I recommend you pay them by the hour. The amount you save by doing this might save you thousands of dollars. Remember there is no building code for this and you need to find someone who has thoroughly studied the topic.
A seismic retrofit engineer or contractor should have researched laboratory tests on the performance of wood in earthquake simulations on a regular basis. They should know the results of shear wall tests. It is also important your engineer understands old building codes. Their ability to determine how strong your house already is is also very important.
Most engineers and contractors just don’t have the time to research these things. Whoever you use, see if they can answer at least one of these basic questions.
Retrofit Engineers Need to Be Aware of the Real World
A seismic retrofit contractor or engineer must be familiar with the kinds of damage that have occurred in previous earthquakes. This information can only be discovered by looking at photographs and reading reports of earthquake-damaged buildings. The photographs and slides at the Karl V. Steinbrugge Collection at U.C. Berkeley are the largest collection earthquake damage in the world. You want your engineer or contractor to be familiar with this collection.
You Might Get a Design You Can’t Afford
You may pay an engineer to design a retrofit you cannot afford, especially when they recommend procedures such as the ones discussed below. Rarely does an engineer understand cost and therefore the recommend practically anything if they think it might help no matter how much it costs.
Real World Examples of Seismic Retrofit Engineers and Contractors “Making Things Up”
Below are photos of some metal post connectors. Our first alert something might be wrong here is that this is not recommended by any seismic retrofit codes and guidelines.
What Does the Building Code Say?
Here are three examples of drawings given to contractors to show what the engineer wants done. In the drawing to the left, the red box contains the steel “T” strap post connector we showed you earlier. The blue box shows the post to block connection. The arrows point at the hardware and bolts connecting the post to the concrete lock. The image to the right from a different engineer shows the exact same thing.
The drawing at the bottom requires a new steel reinforced concrete foundation. It is buried in the ground and is 18″ wide and 18″ deep. It also has two types of steel connectors at the top of the post and one steel connector at the bottom. This contraption is so complex it would probably cost $1200 or more to build.
Mudsill Blocking and Retrofit Guidelines
Engineers often recommend wood blocks which connect the house to the foundation, as well as to plywood bracing. Retrofit guidelines limit the number of nails in these blocks to 4 to reduce the risk of splitting them, as shown below. A seismic retrofit engineer should be aware of the potential for splitting.
Many misinformed engineers and contractors recommend using this Nailed Blocking Method. In this example of a construction detail drawn by an engineer, TWELVE large nails are called for in each block, which will certainly split the blocks. An engineer who recommends nailed blocking is NOT the engineer you want to hire.
The Sub-Floor to Joist Connection
Here is another example of an expensive error often seen in engineer’s plans. The detail below tells the contractor to install a piece of steel that has been bent into a right angle called an L90. This is supposed to strengthen the connection between the sub-floor (which is another layer of flooring under the floor you walk on), and the floor joists (the supporting lumber the the sub-floor is nailed to).
The red arrows show where a connection of the sub-floor to a joist is to be made with the L90. The top of the L90 is nailed up into the sub-floor and the other leg is nailed into the side of the joist. Once the L90 is installed in this way the sub-floor to joist connection is complete.
Failures in the Sub-Floor to Joist Connection Never Occur
While houses can fall off of their foundations, it is never the sub-floor to joist connection that is the cause of this. There is not a single case in all the earthquakes in the United States of this connection ever failing. This is based on the author’s personal experience evaluating damage to homes after the 1989 Loma Prieta and subsequent earthquakes, as well as on my interviews with building inspectors and members of the Structural Engineers Assosication. The numerous photographs in the Steinbrugge Collection also confirm this.
What Does the California Existing Building Code Say?
A304.1.3 Floor joists not parallel to foundations.
Existing connections at the top and bottom edges of an existing rim joist or blocking (the sub-floor is nailed to the top edge of the rim joist in the location of the red arrow above). need not be verified in one-story buildings (because it never fails). In multistory buildings, the existing top edge connection need not be verified (same thing, because it never fails, no matter how big and heavy the house is). This is another confirmation that augmenting this connection is a waste of money. However, the bottom edge connection to either the foundation sill plate or the top plate of a cripple wall shall be verified (there is no way to know if this connection is OK, so put in shear transfer ties). .
A304.1.4 Floor joists parallel to foundations.
Existing connections at the top and bottom edges of the end joist need not be verified in one-story buildings. In multistory buildings, the existing top edge connection of the end joist need not be verified (because it never fails); however, the bottom edge connection to either the foundation sill plate or the top plate of a cripple wall shall be verified (there is no way to know if this connection is OK, so put in shear transfer ties).
Moreover, this connection is never reinforced for code compliant homes built directly on top of a known earthquake fault. Nor is this procedure found in any of the national or regional seismic retrofit codes and guidelines.
In contrast, look at another construction detail below. This detail is found in all six seismic retrofit guidelines. The blue arrow points to the fact that no steel sub-floor to joist connection is shown at the joist to sub-floor location. This is because the members of all 6 committees, consisting of some of the finest structural engineers in the country, believed it is not important.
Compare this image to the one with the red arrows and you will see where the images are dissimilar in this single connection. The black arrows point to the bottom and necessary L90 Shear Transfer Tie recommended by all the retrofit guidelines.
Angle Iron Braces
This video discusses another ineffective and untested retrofit method that your contractor or engineer may recommend. It demonstrates why only tested retrofit hardware and methods should be used. Otherwise, what will happen is anyone’s guess.
The information in this video was created after consultations with numerous structural engineers, especially Bay Area-based Thor Matteson, and Josh Kardon Ph.D. Both of these structural engineers told me they would be happy to tell people about them if you want to call them.
Kelly Cobeen was kind enough to do actual calculations regarding their effectiveness. In conclusion she discovered that an Angle Iron Brace has the strength of a 1/4″ bolt. Numerous structural engineers were kind enough to answer questions I had about their efficacy. In addition, Buddy Showalter with the American Wood Council, the largest wood products research center in the world, was willing to give his expert opinion. All of these engineers are highly distinguished in field of wood-frame seismic retrofitting.
The Redundancy Factor
Let’s look at which connections fail in actual earthquakes. In earthquake engineering there is a factor known as the “redundancy factor.” Redundancy factors are those factors in a home’s original construction that protect it during an earthquake. These factors are unique to each home, so they can be are difficult to measure. Therefore engineers, contractors and building codes simply ignore them, and assume there is NO earthquake resistance in the original construction. The house in the photo below is an excellent example of redundancy factors at work.
According to the what the building code tells us about the strength of existing construction materials, the house should have collapsed. The sub-floor to joist connections should have also failed. Because of redundancy factors, neither of these things happened. Although the house fell off its foundation, the walls, roof, and even the windows held together and remained intact. Some engineers would probably recommend expensive reinforcement of the roof and walls, even after they did NOT fail in an actual earthquake.
Of course we need to look at the connections that did fail. First, the floor joist to cripple wall connection failed. This connection is strengthened using Shear Transfer Ties. Second, there was a weak cripple wall. This is strengthened with plywood. Thirdly, the mudsill to foundation connection failed – this connection is strengthened with foundation bolts.
Let’s Do the Numbers
Floors are only 3/4 inch thick. When doing engineering calculations with this connection the floor is called the main member. When we nail an L90 into the main member the L90 is referred to as the steel side member.
When anything is nailed into the 3/4″ floor, the penetration length of the nail can be no greater than the floor main member thickness. Once it exceeds 3/4 inch, the nail drives through the other side and does not penetrate anything.
The penetration length into solid wood determines the strength of the connection. In other words, when we drive nails through the top of the L90 into the 3/4″ inch thick floor main member the nail penetration can only be 3/4 inches because this is the thickness of the floor.
The American Wood Council Online Line Calculator
When we plug variables such as thickness of side member, type of wood, type and thickness of side member into the Online Calculator, we discover that we need a main member (floor) that is thicker than 3/4″ for the L90 to have any value at all. This is one more reason not to consider strengthening this connection.
The information in the red box which says “Try selecting a longer nail, or a thicker main member, or a thinner side member” tells us the problem. In this example, the 3 inch nail size in the chart is already far greater than the 3/4″ floorthrough the sub-floor so no need to go longer, there is no side member thinner than a steel L90, which leads to the conclusion that we need a thicker sub-floor main member which is fixed at 3/4″.