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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 guys are scientists and will apply the latest science of seismic retrofitting to my house.  My house will survive for sure.  Think again!

The following material is fairly technical.

Person with hand on chin looking puzzled

Which engineer should I choose?

The California Building Code is woefully inadequate. It only has five sentences that address seismic retrofitting. In summary, it says: “You can do anything to your house you want to.  Building departments should encourage anything that might help”.  And building departments interpret it that way.   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 one can follow.   Unfortunately,  they are archaic, contain hardware that has not existed for decades, and have flawed engineering.  These are all seismic retrofit engineers and contractors have to help them.  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 where the plywood is located do anything.  Notice the retrofit guidelines mentioned above require bolts away from the plywood and they don’t do anything.  99% of the engineered designs I see do the same thing until I point out to the engineer they are wasting their client’s money.

How do I find a good seismic retrofit engineer?

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 “bolts everywhere”.  If recommends this he does not understand basic engineering.  Check the other things he recommends and make sure they at least pass the common sense test.

If they recommend a procedure to prevent the house from jumping up and down, will keep the roof from falling off, or some other such nonsense stop wasting your money and look somewhere else.  Maybe you have already had a set of plans drawn up by a seismic retrofit 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 know a very good one.  I recommend you pay them by the hour.  It should not cost more that $200.

A seismic retrofit engineer or contractor should research laboratory tests on the performance of wood on a regular basis.   At the same time he know the results of shear wall tests.  It is also important your engineer understands old building codes. His ability to determine how strong your house already is are also very important.  Most seismic retrofit engineers  and seismic retrofit 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 Word

A seismic retrofit contractor or engineer must be familiar with the kinds of damage that has occurred in previous earthquakes.  This information can only be discovered by looking at photographs of earthquake-damaged buildings.  The photographs and slides at the Karl V. Steinbrugge Collection at U.C. Berkeley are the largest collection in the world.  You want your engineer or contractor to be familiar with this collection.   In the final analysis a seismic retrofit engineer should prove he is serious about his job will have taken the time to look at this collection of photographs.

In summary, find a seismic contractor who understands the engineering, or a seismic retrofit engineer who understands practicality.   There is a drawback to using an engineer who understands practicality versus a contractor who understands engineering.   You will pay barain engineer $1,500 for a mediocre set of plans that will be expensive to implement.  You will pay a good seismic retrofit engineer $5,000 to $6,000 for plans that will save on construction costs willcost $5-6,000.  A knowledgeable seismic retrofit contractor will not need plans.  He will know what to do based on his experience and understand of seismic retrofit engineering. This website should help you find a good seismic retrofit engineer and a good contractor.

Real World Examples of Seismic Retrofit Engineers “Making Things Up”

Below is a construction detail from plans drawn by a seismic retrofit engineer.  A construction detail tells a contractor how to build something.  In this case it is the post to beam connection.  Our first alert something might be wrong here is that this is not recommended by any of the seismic retrofit codes and guidelines mentioned above.   For this reason spending money on work like this is a waste.


Photograph of metal T-strap attaching post to girder (beam) also found in plans created by seismic retrofit engineerMetal at base of post attaching post to concrete block under house found in plans from a local seismic retrofit engineer












More Seismic Retrofit Engineer Details

T strap and post connector drawing from seismic retrofit engineer's plansHere are two more examples from plans drawn by two separate seismic retrofit engineers. The red box circumscribes the “T” strap.  The blue box contains the post to block connection.  The one on the right is so complex it would probably cost $1500 or more to build.  To put it differently, you would be spending $1500 for no gain.Very complex drawing from seismic retrofit engineer showing post and beam and post to concrete block connection

Mudsill Blocking and Retrofit Guidelines


What do the Retrofit Guidelines Tell us?

Retrofit  guidelines do not allow more than 4 of these nails is because the blocks can split as shown by the photo below.  A seismic retrofit engineer who has any understanding of practicality should know this.

In addition, the Nailed Blocking Method is the least desirable in terms of overall effectiveness according to definitive research done at the largest shear wall testing laboratory in the world.


 Photo of a Split Block

Photo of split block because of too many nails recommended in drawing above


Generally speaking almost all seismic retrofit engineers and contractors recommend using the Nailed Blocking Method as we shall shortly see.  The construction detail below specifies TWELVE large nails in each block which will certainly split the blocks. In a word an engineer who recommends nailed blocking is NOT the engineer you want to hire.


Another seismic retrofit engineer's drawing showing too many nails in block. The nails will split the wood


The Sub-Floor to Joist Connection

In like manner here is another example of an expensive error often seen in engineer’s plans.   This detail 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), and the floor joists (the lumber under the the sub-floor is nailed to).

This is how it is supposed to work. 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 L90s hardware is to be nailed up into the sub-floor and the other leg is to be nailed into the side of the joist.  Once the L90 is installed in this way the sub-floor to joist connection will be complete.



Failures in the Sub-floor to Joist Connection Never Occur.

It is interesting to note that 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 not to mention what  interviewed building inspectors structural engineers told me. The numerous photographs in the Steinbrugge Collection also confirm this. .

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 please look at this other construction detail below.  This detail is found in all six of these 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.


A sub-floor to joist connection is not in this diagram which is part of all retrofit guidelines



Let’s do the Numbers

Sub-floors are only 3/4 inch thick and when doing engineering calculations with this connection it is called the main member.  When we nail an L90 into the sub-floor main member the L90 is referred to as the steel side member.

When anything is nailed into the 3/4″ sub-floor, the penetration length of the nail can be no greater than the sub-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 sub-floor main member the nail penetration can only be 3/4 inches because this is the thickness of the sub-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 sub-floor side member 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.  The nail size in the chart is already 3 inches long and penetrating through 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″.

American Wood Council Calculator showing minimum penetration of nails needed for them to work

The Redundancy Factor

Let’s look at how this connection fares 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 impact its reaction to an earthquake.  These are the factors that cannot be measured on a calculator or in a laboratory.  This house 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, this house should have collapsed.  The sub-floor to joist connections should have also failed.  This did not happen because of the immeasurable redundancy factors

Of course we need to look at the connections that did fail.  First, the floor joist to cripple wall connection which is strengthened with Shear Transfer Ties.  Second,  weak cripple wall which is strengthened with plywood.  And third the mudsill to foundation connection, which is strengthened with foundation bolts.

House slid off its foundation but sub-floor to joist connections did not fail


 Angle Iron Braces

This video discusses an ineffective and untested retrofit method that your contractor or engineer may recommend.  First of all it proves 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 1/4 a 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 these engineers are highly distinguished in field of wood-frame seismic retrofitting.


Photograph of angle iron installed under house