Retrofitting and Bolting of New Houses?
Many newer homes built on flat lots such as those found in San Jose, Fremont, Santa Clara County, and much of the Peninsula may need additional bolting because of two serious deficiencies: one created by the California Building Code and the other by common construction practices. One deficiency has been part of the code since its inception in 1927. The other deficiency is caused by over-sized holes, of which contractors are invariably guilty. These deficiencies are discussed below.
How important is it to retrofit a newer house?
Newer houses on flat lots, even if they are not retrofitted, have a very low incidence of serious structural damage. While working with FEMA the author did not document a single newer bolted home that suffered catastrophic damage. This is not to say damage did not occur. The primary failure was due to mudsills splitting because the bolts were placed in oversized holes, though as shown by the photos below it is possible for the entire floor to slide off the base of the house.
In the case of houses being damaged by oversized holes, a house would normally move an inch or so on the foundation and the house would still be fully habitable once water and gas lines are repaired. All of this is being mentioned to point out that the likelihood of catastrophic damage is close to zero, that you will still have adequate shelter after an earthquake, that if damage does occur it should be readily repairable, and in my opinion this type of retrofit should only be considered if you want to make absolutely sure your house only suffers very minimal damage. Even though no one can say for sure what will happen, this information is based on empirical evidence from previous earthquakes. I hate to talk you out of giving us a job, but you need to know what you are paying for.
Joist and Girder to Mudsill Connection
Below is an illustration of what it looks like under your house. Something called floor joists (sometimes these are called girders) support the floor you walk on. These in turn are supported by a piece of redwood call the mudsill. If the floor joists or girders slide on top of the mudsill the house can be damaged. As stated earlier, a serious oversight in the building code does a poor job of making sure this does not happen
In the currently used 2013 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 nails, nailed from the side of the joist into the sill (toe-nailed).”
Table 2304.9.1 of the 2013 CALIFORNIA BUILDING CODE
Wherever the Joists or girders sit on top of the sill 3 nails need to be installed in order to meet the legal requirements of the building code. Before the advent of nail guns when driving nails was arduous, contractors would only meet building code minimums to increase speed and save their bodies.
This tendency towards inadequate nailing is further exacerbated when nailing was done by hand rather than with pneumatic nails guns. This connection should be retrofitted with shear transfer ties. There are 4 kinds of shear transfer ties we commonly use and we never know which ones will work until we are actually under a house.
Unless careful examination proves otherwise, Shear Transfer Ties, also known as framing anchors, only need to be installed where shown by the black arrow.
Damage due to lack of adequate nailing between floor framing (joists) and mudsill. Shear Transfer Ties would have saved this house.
This kind of damage occurs with light houses with wood siding and with heavier houses with siding. It does not matter
Why Additional Bolting is a Good Idea
The first California building code was published in 1927 and has had numerous editions since that time. This code required 1/2 inch bolts be installed every 6 feet on the foundation perimeter.
This requirement has not changed since 1927 except as of July 1999 plate washers are now required. Bay Area cities adopted this code on a piecemeal basis until in 1958, when it was adopted by the entire state.
The code neglects earthquake resistance
To the right is an illustration of the foundation of a common rectangular house viewed from above. When the bolts are placed the code-required 6 feet apart then the 25-foot-long foundations will only have 4 bolts each. The 50-foot-long foundations will have 8 bolts each.
The Distribution of Earthquake Forces
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. The force represented by the red arrow is resisted by 16 bolts. This means the green arrow earthquake force will cause the bolts to fail with only half the force needed to cause the bolts to fail on the red arrow side. If 4 more bolts are added to the shorter walls that 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.
Additional bolts are provided by adding Foundation Anchors, which work in conjunction with Shear Transfer Ties as shown to the left. Standard bolts are not used because it is impossible for the bolt drilling equipment to fit.
Foundation Bolts and Over-sized Holes
According to the 2013 California Building Code, bolts may not be installed in mudsill holes that are more than 1/16 inch larger than the diameter of the bolt. Virtually all existing bolts have over-sized holes and are in violation of the code.
Why do Bolted Houses have Over-sized Holes?
Contractors invariably drill holes into the mudsill that are much larger than the diameter of the foundation bolt so that the mudsill can be adjusted side to side or back and forth. As you can see in the image below, 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 and the bolts fail sequentially. One of the most common types of damage in the 1989 Loma Prieta and the 1994 Northridge earthquakes was splitting of the mudsills caused by foundation bolts in over-sized holes.
So what should I do if I have over-sized bolt Holes
In practical terms, bolts installed in over-sized holes can be deemed as being half as strong as bolts installed in code approved holes. When designing your retrofit, consider these bolts as being only half strength as properly installed bolts. The only way to compensate for this is to add more bolts using a simple engineering formula.
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.
Sample Seismic Retrofit Design for Houses Built after 1956 on Flat Lots
psf = Pounds per Square Foot
Foundation Anchor —: Attaches Base of House to Foundation.
Shear Transfer Tie |: Attaches Floor Framing to Base of House.
Sample Calculations for a 1000 square foot house
Divide 12,000# by 2 because ½ the force will strike one side and ½ the force will strike the other. House must resist 6000# force on each side.
Bolt Hardware Needed
6000# of force to be resisted each side.
Minus 2000# existing bolt resistance (500# per bolt)
= 4000#: This is the total force the new Foundation Anchors must resist.
One Foundation Anchor can resist 1340# of force (depending on hardware and/or method used)
4000#/1,340# = 2.9 rounded up to 3 Foundation Anchors required each side.
Add 2 Foundation Anchors each side to exceed building code = 5 FA on each side.
Shear Transfer Tie Hardware
Existing joist to mudsill resistance is minimal and counted as zero.
4000# total force to be resisted each side.
Shear Transfer Ties can resist 500# (depending on hardware and/or method used)
4000#/500# = 8 Shear Transfer Ties required each side
Add 6 Shear Transfer Ties to exceed building code = 14 Shear Transfer Ties each side.