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Do I Need an Engineer for my Retrofit?

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This is a difficult decision.   You want your retrofit done right and it is natural to think an engineer is the way to go.  After all, engineers design construction projects every day and will make sure your house is retrofitted correctly, right?  Think again!

The following material is fairly technical. If you simply want to know how best to find an engineer click here.

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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 and it is for you to decide whether it can resist earthquakes.”  This leaves much to the imagination of the engineer or contractor. This is why retrofit designs, and therefore costs, can be wildly different.

As a result state, the Structural Engineer’s Association of Norther California and local agencies developed their own seismic retrofit and guidelines which are purely voluntary so that a contractor may or may not use them.  These guidelines are well thought out and make a lot of sense but are also expensive to apply.  This is why engineers and contractors tend to “make things up” because it is always cheaper and therefore encourages clients to use their services.

As a member of the Standard Plan A development committee, the author sent out a questionnaire regarding the use of bolts to structural engineers with decades of experience.  Please look at question number 1 where it is agreed only the bolts where the plywood is located do anything.  Below is a recommendation on a set of plans from a civil engineer.

Engineer note stating bolts should be placed everywhere on house

“(N)” means new.  “All-THREAD” is a type of bolt.  “SPACING is 3′ c-c 1 STORY” means space them 3′ apart on this 1 story house  “EVERYWHERE” means even where there is no plywood.  This costs a ton of money for no benefit because most of the bolts will be placed outside of plywood locations.

How do I find a good engineer?

At a minimum make sure your engineer includes the components found in these guidelines into his/her own design.  If they are including something that is not in these guidelines, it is probably unnecessary and a waste of money.  Take the plans they should have provided you to a local wood frame design structural engineer and ask them.  If you pay them by the hour it should not cost more that $150.  If they determine that any of these essential components is missing, find a different contractor.

A seismic retrofit engineer or contractor should be familiar with laboratory tests on the performance of wood in earthquakes, conversant with testing of the earthquake resistance of old houses, and familiar with shear wall tests.  More importantly, a contractor or engineer must be familiar with the kinds of damage that occurred in previous earthquakes.

This information can only be gleaned 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 of these photographs, and you want your engineer or contractor to be familiar with it.   An understanding of old building codes and the ability to determine how strong your house already is are also very important.  Most engineers  and contractors just don’t have the time or the desire to do this research.  Whoever you use, see if they can answer at least one of  these basic questions. 

In summary, find a contractor who understands the engineering, or an engineer who understands practicality.  It is important that whichever you choose is familiar with the topics mentioned above.  The only drawback to using an engineer who understands practicality versus a contractor who understands engineering is that you will probably need to pay the engineer $1,500 for a mediocre set of plans that will be expensive to implement and ineffective. A good engineer’s plans that will save on construction costs will probably cost $5-6,000.  A knowledgeable contractor will not need plans because he knows what to do already. This website should help you find one.

Real World Examples of Engineers “Making Things Up”

Below is a construction detail which tells a contractor how to build something.  In this case it is the post to beam connection.  Our first alert that something might be wrong here is that it is not recommended by any of the seismic retrofit codes and guidelines mentioned above.


  • The aqua arrow points to a bolt that connects the bottom of a post to a small block of cement called a pier or pier block.Diagram from engineer showing metal T-strap attaching post to girder (beam) and concrete block


  • The blue arrow points at a steel “T” strap that has zero earthquake resistance.


  • The red arrow points to a ST292 hardware that has zero resistance to earthquakes.


  • ALL of these things are deemed unnecessary by all seismic retrofit codes and guidelines.


  • Lastly, the green arrow points at a T912 hardware that does not even exist!  See what I mean about engineers making things up.

Photograph of metal T-strap attaching post to girder (beam)Metal at base of post attaching post to concrete block under houseThis is what these structural modifications recommended by many engineers look like.  The metal “T” strap on the left connects the top of the post to the girder, also known as a beam.  The metal on the right connects the bottom of the post to a block of cement.  These modifications are not recommended by any of the 5 seismic retrofit guidelines.

T strap and post connector drawing from structural engineer
Very complex drawing from engineer showing post and beam and post to concrete block connectionHere are two more examples from plans drawn by two separate 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 $1000 or more to build.  More education is needed for the engineering community so that ineffective and unnecessary work like this is no longer recommended which will make retrofits more affordable

The chart below was created by a local civil engineer who, as shown by the red arrow, specifies 8-12d (12d designates the diameter and length of nail).  This means he wants eight 12d nails to be nailed into 2 x 4 mudsill blocks.  All retrofit guidelines only recommend four because that is all that is necessary.  Once you watch the following video link you will know what a mudsill block is.

Another reason all retrofit codes and guidelines do not allow more than 4 of these nails is because the blocks can split as shown by the photo below

In addition, these nailed blocks are the least desirable, according to definitive research done at the largest shear wall testing laboratory in the world.

Note from engineer recommending excessive nails in blocks for plywood


 Photo of a Split Block

Photo of split block because of too many nails


More than a couple of engineers and contractors recommend using mudsill blocks. Below are two engineered construction details from two separate engineers that illustrate us of mudsill blocks.  The engineered construction detail on the left specifies TWELVE large nails in each block, far exceeding the 4 nails required by all retrofit guidelines, and a certain recipe for split blocks and poor performance.

The construction detail on the right tells the contractor to look at the chart above.  In this case EIGHT nails are required.


Another engineer's drawing showing too many nails in blockdrawing from another engineer showing nailed blocks


The Sub-Floor to Joist Connection

The construction detail below is from a local civil engineer. A civil engineer is different from a structural engineer who has 3 years extra training studying how earthquakes impact a building. The 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 which the sub-floor is nailed to).

The red arrows in this construction detail 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.

Diagram of Simpson StrongTie l90 Sub-Floor to Joist Connection




Failures in the Sub-floor to Joist Connections

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, interviews with structural engineers who evaluated damage to Los Angles homes after the Northridge earthquake, and the numerous photographs available in the archives of the United States Geological Survey, as well the Pacific Earthquake Engineering Institute, which holds the largest collection of earthquake damaged homes in the world.

This is is also why this connection is never reinforced beyond current building code standards for new construction, even for 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 contradiction to the use of this construction detail, please look at this other construction detail below which 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 arrows that point to the bottom L90 where it functions as a shear transfer tie.


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


The Math behind it all.

In theory, if an earthquake is large enough, the exterior stucco walls of a house should collapse because according to the building code stucco walls can only resist 180 pounds per linear foot.  If a 1200 square foot house is struck by a 0.4 ground acceleration the front 24 foot long wall will need to resist 11,040# of earthquake force. This means each linear foot of stucco would need to resist 441# of earthquake force per linear foot, far in excess of 180# per linear foot the stucco walls can in theory resist.  From a theoretical point of view, if anything were to be done the best option would be to increase the strength of the stucco walls. However, this would be very expensive and we shall see, also unnecessary.

The Redundancy Factor

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 this.  Using engineering formulas the roof of this house should have fallen off, the walls should have collapsed, and the sub-floor to joist connections should have failed.  The fact all these connections did not fail is because of the redundancy factors.

The two  things that did fail were the floor joist to cripple wall or the floor joist to mudsill connection which are strengthened with Shear Transfer Ties, and the mudsill to foundation connection, which is done with retrofit foundation bolts or Foundation Anchors.

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


For argument’s sake, let’s assume someone wants to increase the strength of this connection anyway.

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.  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

 Angle Iron Braces

This video discusses an ineffective and untested retrofit method that your contractor or engineer may recommend.  It is always best only to use retrofit hardware and methods that are based on testing.  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.  She discovered that an Angle Iron Brace has the strength of 1/4 a bolt.  In addition 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 whom are highly distinguished structural engineers in the field of wood seismic retrofitting.

In spite of their being practically worthless, angle iron braces are nevertheless very expensive, as shown in this 2017 estimate from a local contractor.

$2,120.00 quote to install angle iron braces


Sample Angle Iron Braces used Extensively in San Francisco Bay Area seismic retrofits


Image of angle iron braces from contractor website

Photograph of angle iron installed under house



Angle iron brace on engineer's plansDiagram from engineer's plans showing angle iron braceHere are some construction details drawn by a civil engineer specifying an angle iron brace.

To the left the red arrows are pointing at locations for the angle braces.  To the right is the construction detail telling the contractor how to install the Angle Iron Brace.




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