This search bar will answer most questions

Homes without Cripple Walls

Seismic Retrofitting of Houses without Cripple Walls

The purpose of this article is to look at retrofitting methods used for bolting homes without cripple walls and, as described at the end of this article,why even bolted homes can greatly benefit from a seismic retrofit.

How does Bolting a Home without Cripple Walls Work?

This an image of what it looks like under your floor when you don’t have cripple walls.  The floor joists, the rim joists, end joists, and the mudsill need to be attached to the foundation so that none of them slide off when the ground shakes. The red arrows represent earthquake forces trying to push the floor off the foundation.

EARTHQUAKE FORCES TRYING TO PUSH HOUSE OFF FOUNDATION

Bolting a Home with Foundation Anchors

Foundation Anchors do the exact same thing as bolts: they attach the bottom of the house called the mudsill to the concrete foundation.  The only difference between Foundation Anchors and bolts is that Foundation Anchors attach the mudsill to the foundation from the side, while standard bolts do it from the top.  This isn’t enough vertical room under the house for bolt drilling equipment to fit.

 

Preventing earthquake damage from an earthquake with retrofit bolting hardware

The earthquake force is trying to slide the mudsill off the foundation but is retrained by the bolts in the Foundation Anchor.  We have bolted the house exactly as if we used standard foundation bolts but have done it from the side of the foundation.

Earthquake trying to push floor off of foundation and resisted by hardware

This is another kind of Foundation Anchor bolt substitute and is used when the edge of the Mudsill is hard to access.   It is the bolts in the Foundation Anchors that do all the work. 

Earthquake force being countered by bolting hardware

In the upper left corner you can see how this seismic retrofit bolting hardware was bent when manufactured so it can fold over and be nailed to the Mudsill.  

Simpson URFP being used to Bolt House to Foundation

This is one of the more common kinds of Foundation Anchors. Many kinds of Foundation Anchors are used for bolting homes and must fit the different ways houses are built

Simpson StrongTie bolting hardware for homes with low clearance

This is the strongest Foundation Anchor made but only works when the concrete and edge of Mudsill are flush or only need a small shim. 

Retrofitting the Floor to Mudsill Connection

SHEAR TRANSFER TIES CREATING A COUNTER FORCE TO THE EARTHQUAKE

Earthquake Forces try and push the floor of the house consisting of End Joists and Rim Joists off the Mudsill.  Shear Transfer Ties, also known as framing anchors, counteract that force.

Simpson Strongtie retrofit hardware being used to counter-act movement of floor

The Floor Joist and Mudsill are joined together with a Shear Transfer Tie.  When the joist tries to slide the Shear Transfer Tie will push on the mudsill.   The Mudsill is immobile because it is bolted to the Foundation so  the Floor Joist will not be able to slide. 

House slid off foundation in the Northridge EarthquakeEven though this house was bolted with a bolt every 2 feet the floor still slid off the foundation because there were only a few nails holding the end joist to the Mudsill.  Shear Trasfer Ties would have prevented this.

This is how the whole Bolting System Works in non-cripple wall Retrofits

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 .  This process is known as the load path.  If any connection in the load path is weak, the retrofit will fail.

EARTHQUAKE FORCES BEING RESISTED BY 2 KINDS OF SIMPSON STRONGTIE SEISMIC RETROFIT HARDWARE

 

ARROWS SHOWING THE 2 AREAS WHERE EARTHQUAKE FAILURES CAN OCCUR

 

 

 

 

 

 

 

 

The Load Path is an important principle to understand and whoever you hire should know what it is and be able to prove to you how his Retrofit will create a rational load path.

A comparison of Tested bolting Hardware and Angle Iron Braces.

Simpson StrongTie UFP10 hardware compared to Angle Iron Brace

Next to this Foundation Anchor is 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 the Analyses or some very distinguished structural engineers with decades of experience.

Angle Braces were also evaluated by the City of Los Angeles when they were developing their retrofit guidelines and rejected as ineffective.   One Foundation Foundation Anchor is 5 Times Stronger than an Angle Brace.

Additional Photos of Foundation Anchor Bolting Systems.

A Simpson StrongTie retrofit hardware working together to resist an earthquake

Here is a house bolted with a Foundation Anchor and a Shear Transfer Tie which will work together when the Earthquake hits.

 

FOUNDATION ANCHOR THAT FOLDS OVER THE MUDSILL

This Foundation Anchor is used when it must be attached to the top of the Mudsill.  This is necessary for Various Reasons.

Seismic Retrofits by Bolting New Mudsills to the Side of the Foundation

Attaching the floor to the foundation with a new mudsill bolted to the foundation and attached to the floor with steelIt often happens Foundation Anchors and Shear Transfers Ties will not work for various reasons.  The primary one being that a pre-made hardware for a certain circumstance does not exist. 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.

Here a new mudsill is bolted to the foundation and the floor is connected to the foundation with plywood

Here a new mudsill is bolted to the side of the foundation and the floor is connected to the bolted mudsill with plywood

FLOOR BOLTED TO SIDE OF FOUNDATION SHOWING RETROFIT COUNTER FORCE

Here is a complete system with a new Mudsill bolted to the side of the foundation.  This along with the Shear Transfer Ties work together to keep the floor from moving.

 

THE VARIOUS TYPES OF SHEAR TRANSFER TIES

There are many types of Shear Transfer Tie hardware with different strengths.  Some are suitable for retrofits and some are not.

 

Retrofitting and Bolting of New Houses?

LOCATION OF FLOOR TO FOUNDATION CONNECTIONS THAT CAN FAIL IN AN EARTHQUAKE

THIS IS WHERE THE FLOOR TO FOUNDATION CONNECTIONS CAN FAIL EVEN ON NEW HOUSES

House bolting is important for both new and old houses.

EVEN THOUGH THIS HOUSE HAS A SOFT STORY PROBLEM IT WILL STILL BENEFIT BY HAVE THE DOWNSTAIRS PORTION OF THE HOME RETROFITTED

 

These homes are often found in newer cities such as San Jose, Fremont, Santa Clara County, and much of the Peninsula.   Tces.

Why Retrofit Newer Bolted Homes?

There 3 factors that can cause a bolted house on a flat lot “built to code” to fail.

  • The first factor is the building code itself. The building code is created by building officials, volunteers, and representatives from different agencies i The Building code is an ever changing document It also has political considerations such as pressures from special interest groups like the well funded National Association of Builders, who lobby to make construction costs as low as possible, often in flagran disregard of public safety.

Seismic Safety Provisions in the California Building Code

In TABLE 2304.1 of the 2016 California Building Code it specifies “The Joist to sill (mudsill) or girder connection shall consist of 3 nails or 3 staples toenailed  (this means nailed from the side). An 8d common is a type of nail.

TABLE SHOWING HOW ONLY 3 NAILS ARE POSSIBLE IN A HOME'S FLOOR JOIST TO MUDSILL CONNECTIONS

TABLE SHOWING HOW ONLY 3 NAILS CONNECTING THE JOISTS TO THE MUDSILL ARE CODE COMPLIANT

What this means is that wherever the Joists or girders sit on top of the mudsill only 3 nails or 3 staples need to be used in order to meet the legal requirements of the building code. Before the advent of nail guns in mid seventies driving nails by hand was arduous and contractors would only meet minimum building code requirements.  The result is that many houses built before 1978 when nail gun use became pervasive may only have 3 nails or staples connection the entire floor to the mudsill.

EARTHQUAKE FORCES TRYING TO PUSH FLOOR OFF FOUNDATION AND DEFICIENCY IN BUILDING CODE

TO COMPENSATE FOR THIS DEFICIENCY SHEAR TRANSFER TIES OR FRAMING ANCHORS ARE USED.

EARTHQUAKE MOVEMENT RESTRAINED BY SHEAR TRANSFER TIES

TO COUNTERACT THIS FORCE SPECIAL HARDWARE CALLED SHEAR TRANSFER TIES OR FRAMING ANCHORS DISCUSSED EARLIER ARE USED.

The second factor is the implementation of the building code, which is the job of the contractor. This step is where the highest probability of deviation from the code exists due to sloppy workmanship when trying to hurry up and save money as well as not understanding the importance of the seismic provisions of the building code.  The almost universal installation of bolts in over-sized holes is a case in point.

The Building Code and Over-sized Bolt Holes

FOUNDATION BOLT IN OVER-SIZED HOLE IN MUDSILL. THIS VIOLATION OF THE BUILDING CODE IS THE RULE RATHER THAN THE EXCEPTION

FOUNDATION BOLT IN OVER-SIZED HOLE IN MUDSILL. THIS VIOLATION OF THE BUILDING CODE IS THE RULE RATHER THAN THE EXCEPTION

Bolts are an important in new construction as well as in a seismic retrofit.    Unfortunately,  most 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 2013 California Building Code does not recognize bolts installed in holes exceeding 1/16 of an inch over the bolt diameter as having any value because this has not been tested.  This information can the found in the National Design Specification which is part of the build code.

Bolt Holes

 

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.

The third factor is the oversight of individual building inspectors. Their job is to make sure contractors follow the code.  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.   Th almost universal presence of homes with over-sized bolt holes, which the code forbids, is a case in point.

In summary, there are three possible points causes of earthquake damage in more bolted homes: a deficient building code, the ignorance of the contractor, and the oversight of the building inspector.

Too much Hardware and you wasted your Money, too little, and the Retrofit can Fail

Your contractor should provide you with a drawing that looks something like this or at least be able to explain how much and what type of hardware you need using a similar thinking process.  Below is a step by step design process that any retrofit contractor or engineer should use.

I have put relevant information in colored boxes so you can find the information more easily on the page.

 

 

 This is the engineering/mathematical and geological background upon which your entire retrofit is built.  It tells you how much earthquake force your house must resist and how many components you need.  In this example the house must resist 0.186 Gs of ground acceleration. This is multiplied  times its weight of 86,664# (pounds) to equal 16,119 pounds of earthquake force striking the house.  Each piece of hardware is designed to resist a certain amount of force and we need enough hardware to resist it.  In this case the retrofit has been designed to resist 17,400 pounds of earthquake force which is slightly more than we actually need.

The blue box tells us how many pounds of earthquake force each of the different type shear transfer ties can resist.  We need to use enough of a certain type, or a combination of several types, to resist 16,119# of force

The green box tells us how many pounds of earthquake resistance  the different types of Foundation Anchors have.   We need to use enough of a certain type, or a combination of several types, of Foundation Anchors to resist 16,119 pounds of force

The purple box tells us how the different components need to be combined to resist the earthquake force.  Let’s assume that on Wall line C we want to resist 4.020# of earthquake force.  In order to do this we need to bolt the mudsill with 3 pieces of FA hardware.  The various shear transfer ties singularly or combined, must resist this same amount of force.  Exactly which hardware to use is determined on site.

At the bottom of the page it says “Capital letters in sketch correspond to “Wall Line” LETTER designate below (typo here, should be above rather than below”

This means if you look at the sketch and find a capital letter along a foundation wall line it corresponds to the hardware listed in one of the columns.  For example the letter D in the sketch means Two FA10 + Four 90L or Five 4LTP or Six A23A or a combination thereof to reach the capacity we desire will be installed at this location. Because no two houses are built alike and we just don’t know which Foundation Anchor or shear transfer tie or combination thereof we will be using until we get under the house.

No Cripple Wall Retrofits: some Background

Many contractors and homeowners believe retrofits for homes without cripple walls are simple.  This is primarily because of the Bay Area’s regional retrofit guideline called Standard Plan A  This guideline specifies one type of bolting hardware called a UFP10 and one type of Shear Transfer Tie hardware known as “L” hardware (both of these are illustrated a few paragraphs down) for homes without cripple walls. The two types of L hardware are the L70 and the L90 which are identical except the L70 is seven inches long, and the L90 is 9 inches long.  The H10 hardware in the table below is for cripple wall retrofits and does not apply to homes without cripple walls.

HARDWARE ALLOWED FOR HOMES WITHOUT CRIPPLE WALLS BY STANDARD PLAN A

HARDWARE ALLOWED FOR HOMES WITHOUT CRIPPLE WALLS BY STANDARD PLAN A

The problem is that existing site conditions often require other types of hardware that are not listed here.

One might be curious as to why Standard Plan A is so restrictive. Standard Plan A requires that a homeowner have plans from an engineer if these 2 pieces of hardware will not work.

As a member of the Standard Plan A development committee I saw first hand the Structural Engineer’s Association of Northern California lobbying that this standard be so inadequate that on many, if not most occasions, one must pay a structural engineer to design the retrofit.  A clear conflict of interest.

Areas Served

San Jose,Sunnyvale, Fremont,Oakland,Berkeley
And Surrounding Areas
408-664-6355
510-548-1111

Member

eeri-logo icc-icon
Structural Engineer's Association Icon

License Info

Works' Comp RWCC64393236
Contractors Lic #558462
Bond #SC6334450
Liability Ins PCA 1045011
Link to Contractor's License Board