Why Is Retrofitting Bolted Houses Still A Good Idea?
Many newer homes built on flat lots (for us, new means built after 1958) such as those found in San Jose, Fremont, Santa Clara County, and much of the Peninsula. Two serious deficiencies: one created by the California Building Code and the other by common construction practices, make retrofitting bolted houses a good idea. One deficiency has been part of the code since its inception in 1927. Over-sized holes, of which contractors are invariably guilty of causing, cause the other deficiency. These deficiencies might cause damage and may make retrofitting bolted houses a good idea.
One problem was because of mudsills splitting when contractors installed bolts in oversized holes.
A house would normally move a few inches or so on the foundation, and the house would still provide shelter once water and gas lines are repaired.
This is a More Serious Problem
Below is an illustration of what it looks like under your house. Floor joists (sometimes called girders) support the floor you walk on. A piece of redwood called the mudsill supports these floor joists. If the floor joists or girders slide on top of the mudsill, the house can be seriously damaged. A serious oversight in the building code does a poor job of making sure this does not happen.
In the current California Building Code page 308 of Table 2304.9.1, column 1, connection 1, it says, “The Joist to Sill (mudsill) or Girder to Sill connection shall be 3 toenails. Toenails go through the girder and into the mudsill from the side. The image below shows a toenail in a 2 x 4.
This means that wherever the Joists or girders sit on top of the mudsill, only 3 nails need to be installed to meet the building code’s legal requirements.
Before nail guns became common after 1978, driving nails was arduous, and contractors would only meet minimum building code standards to increase speed and save their bodies. The result is that houses built before 1978 are especially prone to this serious structural deficiency.
This is how Shear Transfer Ties work.
A large variety of shear transfer ties are available, and a contractor must know which one to use when confronted with every imaginable floor framing configuration. Given many houses were built without a building code, the varieties of floor framing are legion. A contractor must know which one will fit and how strong it is to properly design and retrofit a house.
Without Shear Transfer Ties, this is the damage a house might suffer. They bolted the houses below to the mudsill, but only a few nails attached the floor joists to the mudsill.
Here is another case of the floor sliding, but the mudsill remained in place even though it was minimally bolted to the foundation.
Foundation Bolts and Over-sized Holes
According to the California Building Code, bolts installed in mudsill holes that are more than 1/16 inch larger than the diameter of the bolt are illegal. Virtually all existing bolts we have seen have over-sized holes and violate the code.
Why do Bolted Houses have Oversized Holes?
Contractors invariably drill holes into the mudsill that are much larger than the diameter of the foundation bolt. That way, the contractor can move the mudsill side to side or back and forth. As you can see in the image to the right, when a foundation bolt has over-sized holes, the bolts do not work together when the earthquake hits. Instead, only one bolt works at a time.
So, what should I do if I have over-sized bolt Holes?
Bolts installed in over-sized holes are half as strong as bolts installed in code approved holes. When designing your retrofit, consider these bolts as being only half the strength of properly installed bolts.
The only way to compensate for this is to add more bolts. Most homeowners choose to re-bolt the house because re-bolting costs about the same as bolting it all over again. The cost is all in the labor and if a crew is already under the house the difference between partial bolting and completely new bolting is the same.
Foundation Anchors hold the mudsill to the foundation exactly as a bolt would. The shear transfer ties attach the rim joist and end joists to the mudsill. In new houses, the mudsill is bolted to the foundation before they nail the floor on it so that there is plenty of room and the process is fast. Once the floor you walk on is nailed to the joists it is a time consuming and expensive procedure requiring of high-cost hardware and work in cramped quarters.
Foundation anchors do the same thing as foundation bolts. There are four types of Foundation Anchors and three types of Shear Transfer Ties. The floor configuration determines the specific Foundation Anchor or Shear Transfer Tie hardware used under a house. Given the fact there were no building codes when many of these houses were built, one does not know exactly which hardware will work and how much is needed until work on the house has begun.
A Sound Load Path
A load path is defined as the method by which the lateral forces of an earthquake are transferred into the ground. The colored arrows below correspond to the correspond to the colored arrows in the photo to the right. The retrofit hardware transfers the earthquake force from the floor, through the Shear Transfer Tie, into the Foundation Anchor, into the foundation, and finally into the ground where the earthquake force dissipates. This process is known as the load path.
Below is another Example of a Complete Load Path
Below is a Foundation Anchor next to something called an Angle Brace or Angle Iron. Angle Braces are commonly recommended by contractors and engineers even though their ability to resist earthquakes is extremely limited based on analyses of some very high-powered structural engineers with decades of experience. They were evaluated by the City of Los Angeles when they were developing their retrofit guidelines and rejected as ineffective. One Foundation Anchor is 5 Times Stronger than an Angle Brace.
There are many kinds and types of Foundation Anchors depending on how and when the house was built. Foundation Anchors do the exact same thing as foundation bolts but were not installed at the time of original construction.
The Foundation Anchor below is used when the Foundation Anchor must be Attached to the Top of the Mudsill. This can Happen for Various Reasons.
Yet Another Type of Foundation Anchor
Bolting New Mudsills to the Side of the Foundation
It often happens Foundation Bolting Hardware or Shear Transfers Ties for various reasons. In these cases, custom solutions must be developed. The most common approach is to Bolt a New Mudsill to the Side of the Foundation and Attach the Floor to it.
Sometimes you can’t reach the mudsill because there is no way to reach. To imagine this, go straight up from the concrete until you are at the upper edge of the wood. This is where the mudsill is. In these cases, we bolt a new mudsill to the foundation and attach that to the floor
This is a summary of all the components used when a new mudsill is bolted to the side of the foundation:
The code neglects earthquake resistance
Let’s assume the bolts were installed perfectly, no oversized bolt holes below are an illustration of the foundation of a common rectangular house viewed from above. When the bolts are placed the code-required 6 feet apart, the 25-foot-long foundations will only have 4 bolts. The 50-foot-long foundations will have 8 bolts each.
Earthquake forces strike a house from all sides equally. An earthquake force represented by the green arrow is resisted by a total of 8 bolts, with 4 bolts on each side. The force represented by the red arrow is resisted by 16 bolts, with 8 bolts on each side. This means the green arrow earthquake force will cause the bolts to fail with only half the force the red arrow bolts can resist. In other words, the bolt strength resisting the green arrow force is only half that provided by the bolts resisting the red arrow force. If 4 more bolts are added to the shorter walls that currently have 4 existing bolts, these 8 bolts will give the short walls just as much strength the as long walls, thus making the house twice as earthquake resistant as it was before.
Putting the House back in Balance
Please look at the illustrations below as we again explain how the bolting portion of a house like this can be retrofitted. If we put 4 additional bolts on the top and bottom, we will have 8 bolts on each side and the 25′ walls will now be twice as strong as they were before. The short walls which were half as strong as the long walls are now just as strong. This additional strength has doubled the earthquake resistance of the house. The red lines seen next to the bolts represent Foundation Anchors which do the same thing as bolts.
A Complete Retrofit
At the beginning of this page, we were discussing how on the short walls it is possible that only 3 nails attach the floor framing (joists) to the mudsill. The short black lines represent Shear Transfer Ties which correct this problem. Remember, the long walls already have enough nails connecting the floor to the mudsill, so we don’t need to do anything here.
The red lines in between the bolts represent Foundation Anchor bolting hardware. This house is as prepared as it can be without adding extra bolts everywhere.
Inadequate Floor Joist Connections Specified in Building Code
The current 2016 California Building Code contains a specification that make homes “built to code” vulnerable to earthquakes. Only 3 nails need to be used to connect the floor to the foundation on two sides of the house to meet the requirements of the building code. This is inadequate.
This means wherever the joists or girders sit on top of the mudsill, only 3 nails need to be installed in order to meet the legal requirements of the building code.
Before the use of nail guns became common, driving nails with a hammer was arduous and caused an injury known as “carpenter’s elbow.” Contractors stuck to the 3-nail minimum building code standard as a way to increase speed, avoid injury, and minimize fatigue.
Below is an illustration of what it looks like under your house. Floor joists (or girders) support the floor you walk on. These in turn are supported by a piece of redwood called the mudsill. If the floor joists or girders slide on top of the mudsill, the house can be damaged.
After nail guns were invented, contractors may have chosen to exceed the code, but there is no way to know for sure without taking off the stucco or wood siding. In any event, houses built before 1978, before nail gun use became common, are almost guaranteed to have this problem.
Diagram: To remedy this deficiency in the building code, special hardware called shear transfer ties or framing anchors should be used.
Why Aren’t Building Codes Followed?
Implementation of the Building Code is the job of the contractor. The job of building inspectors is to make sure contractors follow the code. When contractors rush their work in order to save money, seismic provisions in the code may not be implemented properly. Contractors and building inspectors also may not adequately understand the importance of seismic provisions in the code. Both of these issues may lead to poor workmanship and homes that are not adequately strengthened for earthquakes. Unfortunately, during construction, most time-pressed inspectors only have a few minutes to look at a house while it is being built, and deviations from the building code are often missed. The almost universal installation of bolts in over-sized holes, which the code forbids, is a case in point.
A Real-Life Example: Over-sized Bolt Holes
Bolts are a vital component in new construction as well as in a seismic retrofit. Unfortunately, bolts will not be effective in resisting earthquakes because almost universally they are installed in over-sized holes where the diameter of the hole in the wood exceeds the diameter of the bolt. The 2016 California Building Code prohibits installation of bolts into holes exceeding 1/16 of an inch larger than the bolt diameter:
Bolt over-sizing occurs because at the time of inspection the bolts are installed with nuts and washers such that the size of the hole is not visible, and because contractors are unaware of how important this is, and because it is easier for contractors to have over-sized holes so they can adjust the mudsill.