Bolting a house is not the same as a seismic retrofit.  Foundation bolts in an earthquake retrofit are only one component in a three part system.  Without the two other parts, plywood and shear transfer ties,  the bolts are literally decorative.  As you can see below, when the earthquake force pushes against the floor it can only make it into the bolts if it follows the “load path” provided by plywood, shear transfer ties, and bolts.   All retrofit strategies are directed towards transferring the force into the bolts using plywood and Shear Transfer Ties, otherwise the house can be severely damaged.  The size of the bolts does not matter. it is only the capacity that counts.

A 1/2-inch bolt with a Mudsill Plate can resist 1340 pounds of earthquake force.

What Do Foundation Bolts Do?

Bolts are designed to keep the base cripple wall from sliding off the foundation.  If the house does not have foundation bolts, the cripple wall may slide on the foundation.  This in turn can break gas lines, cause significant damage to living areas, and pull out electrical wires.

It is important to realize bolts without plywood and shear transfer ties, as illustrated in the drawing above. don’t do anything.

The image to the left shows the lower portion of a cripple wall, which is also the base of a shear wall once the cripple wall has been converted into a shear wall.  The red arrow represents earthquake forces pushing on the mudsill.  If the mudsill moves on the foundation, the entire shear wall will move, taking the house with it.

Types of Retrofit Foundation Bolts

Three different types of foundation bolts depending on the foundation.

1-THIS BOLT IS EPOXIED INOT CONCRETE     2-WEDGE ANCHOR BOLT EXPANDS AT BASE TO LOCK INTO CONCRETE    3-TITEN BOLTS SCREW INTO CONCRETE.

There are a few types of retrofit foundation bolts:  epoxy bolts, wedge anchors, Titens, and  Simpson Strong-Tie UFP IOs.  These all are used to attach the base of the shear wall (the mudsill) to the foundation.  All of them have been tested for their ability to resist earthquakes and are approved by all retrofit guidelines. 

Use Only Tested Foundation Bolts

It is critical that the bolts have been tested in independent research laboratories to make sure they will be effective in your retrofit.  We use industrial bolts Manufactured by Hilti, a Liechtenstein-based company known for producing the highest quality bolts available.   Tested bolt shear values are on page 8, International Code Council’s Evaluation Service Report 2302.

The bolt to wood connection is the part of the foundation bolting system that fails in earthquakes and should be strengthened whenever possible.  The bolt to wood connection can be increased 58%  with  Mudsill  Plates, a product produced by a manufacturer in Los Angeles.

 

Putting Bolts in Old Foundations

Some people wonder if a bolt will work in an old foundation without rebar in it.  Tests have shown that old foundations do just fine.

A 1992 test report, “Foundation Anchorage Systems,” published by the Structural Engineer’s Association of Southern California, came to the following conclusion regarding old foundations without rebar.

“The difference in the strength of the concrete did not appreciably affect the performance of the foundation anchorage systems (Bolts).  Engineered or prescribed methods for retrofitting foundations of good concrete will also work with foundations of weaker concrete (approximately 1500psi).  The predominant failure mechanism in these anchorage connections was the wood sill plate in both the good and weak concrete foundations.” 

This test used bolting systems of all types including epoxy bolts, wedge anchor foundation bolts, and side plates.  Side plates are a type of house bolting hardware suitable for homes with short or no cripple walls.

Foundation Bolts and Deteriorated Foundations

When dealing with clearly deficient foundations like those shown here, the most effective method without replacing them can be found in a portion of this video on old foundations that discusses brick and marginal foundations.
Bolting into brick is possible, but this brick foundation is hopelessFoundation Bolting in this Foundation is a Waste of Time.

What if the Foundation Slides on the Ground?

Most seismic retrofits are installed on older foundations with shallow footings  (this means they are not very deep in the ground or sit on top of the ground), so it is important to understand their performance characteristics.  Some people worry the foundation will slide on the ground.

The answer is: So what?  The house, ground, and foundation will all move together. The problem arises when the ground stops and inertia causes the house to keep moving off of the foundation.  That is why re-enforcing steel (rebar), age,  depth and size of the foundation are usually not a serious concern.

Brick Foundations

Many people think the only way an effective retrofit can be performed is if the brick foundation is replaced.  Based on my research this is not the case. Please see the research I did on brick foundations.

What if the House is Already Bolted?

Existing bolts can rarely be used because they almost always have over-sized bolt holes.  This can weaken a bolt by 59% according to tests done after the 1994 Northridge earthquake.

 

load-path-of-earthquake-forces

                                                                                              ALL EARTHQUAKE FORCES GO INTO THE BOLTS

 

 

Retrofit Foundation Bolts: The Nitty Gritty

Retrofit Foundation Bolts

This article was written in collaboration with Simpson StrongTie.  All bolts and other materials are proprietary to them.  The catalog used has since been superseded, though the values have not changed much.   For wood frame retofit work, Simpson makes a number of different foundation bolts that can be used.

Foundation Bolts: Characteristics and Capacities

This information will only be of interest to engineers and contractors.

½ Inch Simpson StrongTie Wedge-All Anchor

Page 108 of Simpson Catalog C-SAS-2005 rates their 1/2 inch wedge anchors in 2000 psi concrete with a 5 inch concrete edge distance at 1675 pounds in shear (The minimum embedment for this installation is 4 1/2″).  Factoring in the Simpson allowed 1.33 short term loading adjustment factor; the bolts are worth 2,227 pounds in shear.  If the bolts are put through the center of the 2 by 4 blocks as shown on the plan set we end up with a 2 1/4 concrete edge distance.  Interpolating from the load adjustment factor table dealing with concrete edge distances on page 113 we have a load adjustment factor of .725.  The ½ inch wedge anchor can therefore resist of 1650 pounds of shear in the concrete.  The strength of the wood to bolt is 1119 pounds per the 2005 NDS.    The failure mode here will obviously be in the wood-to-bolt connection, not in the concrete.  It is therefore rational for us to endorse use of the ½ inch wedge anchor.

 

5/8-Inch Simpson StrongTie Wedge-All Anchor

The adjustment factor table on page 113 in Simpson Catalog C-SAS-2005 catalog does not recognize a value for 5/8 wedge anchors with concrete edge distances less than 2-1/2 inches. We only have 2-1/4 inches edge distance if the wedge anchor is put through the center of the 2×4 block as shown in the Plan Set.  It is not rational for us to endorse the use of the 5/8 inch wedge anchor.

 

½ Inch Simpson StrongTie ET Epoxy Bolt

The Simpson catalog on page 51 gives us a value of 2,820 pounds of shear for a 1/2 inch bolt using ET epoxy with a 6 3/8 inch concrete edge distance (embedded 4-1/4″).   Factoring in the Simpson allowed 1.33 short term loading adjustment factor allowed by the Simpson catalog allows us 3,760 pounds of shear.  If the bolts are put through the center of the blocks as shown on the plan set we end up with a 2 1/4 concrete edge distance. Interpolating from the load adjustment factor table dealing with concrete edge distances on page 55 we have a load adjustment factor of .28 so that our epoxy bolt can now resist 1053 pounds of shear.   The strength of the wood to bolt connection in 2” closed grain redwood is 1119 pounds per the 2005 NDS  The failure mode therefore will be in the concrete and not in the wood.   It is therfore not rational for us to endorse use of the ½-inch ET epoxy bolt.

 

5/8-Inch ET Epoxy Bolt

A 5/8 inch epoxy bolt installed with ET epoxy and a 7 1/2 inch concrete edge distance has a shear value of 4890 pounds as shown on page 51 (embedded 5″). Multiply this by 1.33 short term loading adjustment factor as allowed by the Simpson catalog and we get a value of 6520 pounds.  If the bolts are put through the center of the blocks as shown on the plan set we end up with a 2 1/4 concrete edge distance.  Interpolating from the load adjustment factor table dealing with concrete edge distances on page 55 we have a load adjustment factor of .205 so that our epoxy bolt can now resist 1336 pounds of shear.  The strength of the wood to bolt connection in 2” closed grain redwood is 1555 pounds per the 2005 NDS.  The failure mode here will be in the wood to bolt connection not in the concrete.  It therefore not rational to endorse use of the 5/8-inch ET epoxy bolt.

½ Inch Simpson StrongTie SET Epoxy Bolt

A 1/2 inch epoxy bolt installed with SET epoxy at 1-3/4 inch concrete edge distance has a shear value of 2125 pounds as shown on page 38 (embedded 4-1/4″). Note these loads are tested at 1-3/4” with shear parallel to the edge of the concrete, so no reduction factors need to be applied.  Multiply this by the 1.33 short term loading adjustment factor as allowed by the Simpson catalog and we get a value of 2833 pounds.  The strength of the wood to bolt connection in 2” closed grain redwood is 1119 pounds per the 2005 NDS.  The failure mode then will be in the wood to bolt connection and not in the concrete.  It therefore rational to endorse use of the ½-inch SET epoxy bolt.

5/8-Inch Simpson StrongTie SET Epoxy Bolt

A 5/8 inch epoxy bolt installed with SET epoxy at 1-3/4 inch concrete edge distance has a shear value of 2215 pounds as shown on page 38 (embedded 5″). Note these loads are tested at 1-3/4” with shear parallel to the edge of the concrete, so no reduction factors need to be applied. In fact, loads will be higher than specified for our condition of 2-1/4” edge distance.  Multiply this by the 1.33 short term loading adjustment factor allowed by the Simpson catalog, and we get a value of 2953 pounds.  The strength of the wood to bolt connection in 2” closed grain redwood is 1555 pounds per the 2005 NDS.  The failure mode then will be in the wood to bolt connection and not in the concrete.  It therefore rational to endorse use of the 5/8-inch ET epoxy bolt.

½ Inch Simpson StrongTie Titen HD Concrete Screw

A 1/2 inch Titen HD screw anchor installed at 1-3/4 inch concrete edge distance has a shear value of 1165 pounds as shown in the table at the top of   page 99.  The minimum embedment depth is 2-3/4 inches.  This accounts for a 6 ½ Titen Hd that goes thru the ¼ inch plate washer, the nominal 2x block, and through the full dimensioned 2x sill.  In this case we get an embedment depth of exactly 2 ¾ inches.   Note these loads are tested at 1-3/4” with shear parallel to the edge of the concrete so no reduction factors need to be applied.  In fact, loads will again be higher than specified for our condition of  2-1/4” edge distance.  Multiply 1165 lbs by the 1.33 short term loading adjustment factor as allowed by the Simpson catalog and we get a value of 1549 pounds.  This value will go up to 2274 pounds if we use an 8 inch Titen HD 1/2-inch Titen HD concrete screw.  The strength of the wood to bolt connection in 2” closed grain redwood is 1119 pounds per the 2005 NDS.  The failure mode then will be in the wood to bolt connection and not in the concrete.  It therefore rational to endorse use of the ½-Titen HD.

5/8-Inch Simpson StrongTie Titen HD Concrete Screw

A 5/8 inch Titen HD screw anchor installed with a  1-3/4 inch concrete edge distance has a shear value of 1205  pounds as shown  in the table at the top of page 99 of the catalog.  The minimum embedment depth is 2-3/4” .  This accounts for a  6 ½ Titen Hd that goes  the ¼ inch plate washer, the nominal 2x block, and  through the full dimensioned 2x sill.  In this case we get an embedment depth of exactly 2 ¾ inches.    Note these loads are tested at 1-3/4” with shear parallel to the edge of the concrete so no reduction factors need to be applied.  Multiply 1205 pounds by the 1.33 short term loading adjustment factor as allowed by the Simpson catalog and we get a value of 1602 pounds.  This value will go up to 2347 pounds if we use an 8 inch Titen HD.  5/8-inch Titen HD concrete screw.  The strength of the wood to bolt connection in 2” closed grain redwood is 1555 pounds per the 2005 NDS.  The failure mode then will be in the wood to bolt connection and not in the concrete.  It therefore rational to endorse use of the 5/8-inch ET epoxy bolt.

5/8” SET epoxy bolt

½” SET epoxy bolt

5/8” Titen HD screw

½” Titen HD screw

½” Wedge-All bolt

5/8” ET epoxy bolt

½” ET epoxy bolt

5/8 Wedge-All bolt                   (No recognized value)

 

 

Retrofit Foundation Bolts Cost

I called a local hardware store for the retail pricing of commonly-used bolt hardware:  Titen HD’s , ½ inch wedge anchors, and the various epoxy bolts.  Prices for these products from lowest price to highest price are as follows:

 

1/2 inch by 7 inch Wedge-All anchor bolt                                                           $1.60

½ inch by 6-1/2 inch Mechanically Galvanized Titen HD                               $3.69

½” SET epoxy bolt -2833 pounds                                                                          $5.25

½ inch by 8 inch Mechanically Galvanized Titen HD                                       $5.29

5/8 inch by 6 1/2 inch Mechanically Galvanized Titen HD                              $5.29

5/8” SET epoxy bolt -2953 pounds                                                                       $5.50

5/8 inch by 8 inch Mechanically Galvanized Titen HD                                    $6.69

 

The primary expense in any retrofit is the cost of the labor.

The Titen HD is a 2-part labor process:

  1. Drilling the hole through the block and sill and into the concrete.
  2. Tightening the bolt with an impact wrench or right angle drill.

The Wedge All is a 3-part labor process:

  1. Drilling the hole through the block and sill into the concrete.
  2. Driving the bolt into the hole with a hammer.
  3. Tightening the bolt by hand with an impact wrench or with an angle drill.   The amount of labor is practically the same as for the Titen HD.  Once the reduced cost of the Wedge-All is factored in, this is the preferred choice for the ½ inch bolt.

Epoxy bolts involve a 5-part labor process:

  1. Drilling the hole through the block and sill into the concrete.
  2. Blowing out the hole with compressed air.
  3. Injecting epoxy into the hole.
  4. Installation of the bolt.
  5. Coming back hours later and tightening the bolt.

Given strength provided versus the material and labor costs for the hardware mentioned above, the bolt hardware can be ranked from the most preferred to the least preferred as follows in terms of overall cost.  All these bolts are suitable in seismic retrofit applications.

1/2 inch by 7 inch Wedge-All

½ inch by 8 inch Mechanically Galvanized Titen HD

5/8 inch by 8 inch Mechanically Galvanized T-2953 pounds

½” SET epoxy bolt

The 5/8-inch Wedge-All retrofit bolt is not permitted by Simpson Strong-Tie because it is not possible to meet edge distance requirements.

Conclusion: The most cost-effective of the ½ inch options is the ½ inch by 7 inch Wedge All.  Next best is the ½ inch by 8 inch Titen HD