Homes without Cripple Walls

Seismic Retrofitting Houses without Cripple Walls

No Cripple Wall Retrofits for Bolted and not Bolted Homes: some Background

The purpose of this article is to explore retrofitting methods used for homes without cripple walls and, as described at the end of this article,why even bolted homes can greatly benefit from a seismic retrofit.  The first section describes no cripple wall retrofit methods that apply to bolt bolted and unbolted homes and the last part of the article only applies to modern bolted homes.

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

Standard Plan A is hopelessly inadequate and many other methods and types of hardware must be used in order to benefit the house at all.

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.

Regardless,   the engineering background behind the guidelines is the most thorough engineering analysis of homes ever written which, remarkably (unless you remember an engineer will charge to do them all over again), is only published on this website.

How No Cripple Wall Retrofits Work

This an image of what it looks like under your floor when you don’t have cripple walls.  These are the names of the structural components under your floor.  The floor joists, the rim joists, end joists, and the mudsill need to be attached to the foundation so that none of them can slide off. The red arrows represent earthquake forces trying to push the floor off the foundation.  The hardware used prevent this sliding are Shear Transfer Ties and Foundation Anchors

EARTHQUAKE FORCES TRYING TO PUSH HOUSE OFF FOUNDATION

               EARTHQUAKE FORCES TRYING TO PUSH HOUSE OFF FOUNDATION

Foundation Anchors hold the mudsill to the foundation exactly as a bolt would.

FOUNDATION ANCHORS COME IN MANY SHAPES AND SIZED AND THE ONE USED DEPENDS ON SITE CONDITIONS

FOUNDATION ANCHORS COME IN MANY SHAPES AND SIZED AND THE ONE USED DEPENDS ON SITE CONDITIONS

 

HERE IS ANOTHER TYPE OF FOUNDATION ANCHORS USED UNDER CERTAIN CIRCUMSTANCES

HERE IS ANOTHER TYPE OF FOUNDATION ANCHORS USED UNDER CERTAIN CIRCUMSTANCES

 

THIS FOUNDATION ANCHOR OFTEN FITS BECAUSE THE CURVE ALLOWS ADJUSTMENT

THIS FOUNDATION ANCHOR OFTEN FITS BECAUSE THE CURVE ALLOWS ADJUSTMENT

Shear Transfer Ties

Earthquake forces try and push the end and rim joists off the mudsill and the Shear Transfer Ties, also known as framing anchors counteract that force.

SHEAR TRANSFER TIES CREATING A COUNTER FORCE TO THE EARTHQUAKE

SHEAR TRANSFER TIES CREATING A COUNTER FORCE TO THE EARTHQUAKE

Below is a photograph of a single L90 Shear Transfer Tie.  It is nailed at the back to the floor framing and at the bottom to the mudsill.   As the earthquake tries to slide the floor joist off the mudsill the Shear Transfer Ties prevent this sliding.

SHEAR TRANSFER TIES ATTACH THE FLOOR JOIST TO THE MUDSILL WITH STEEL AND NAILS

SHEAR TRANSFER TIES ATTACH THE FLOOR JOIST TO THE MUDSILL WITH STEEL AND NAILS

THIS IS ANOTHER TYPE OF SHEAR TRANSFER TIE THAT PREVENTS MOVEMENT OF THE FLOOR ON TOP OF THE MUDSILL

THIS IS ANOTHER TYPE OF SHEAR TRANSFER TIE THAT PREVENTS MOVEMENT OF THE FLOOR ON TOP OF THE MUDSILL

 

 

WWWWWWWWWWW

FLOOR SLID OFF BOLTED FOUNDATION AND NEEDS RETROFIT SHEAR TRANSFER TIES

FLOOR SLID OFF BOLTED FOUNDATION AND NEEDS RETROFIT SHEAR TRANSFER TIES

Here is another case of the floor sliding but the mudsill remained in place even though it was minimally bolted to the foundation

MUDSILLS CAN STAY ON THE FOUNDATION AND THE FLOOR STILL MOVE

MUDSILLS CAN STAY ON THE FOUNDATION AND THE FLOOR STILL MOVE

This is how the Whole System Works: 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 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.  If any connection in the load path is weak, the retrofit will fail.

EARTHQUAKE FORCES BEING RESISTED BY SEISMIC RETROFIT HARDWARE

EARTHQUAKE FORCES BEING RESISTED BY SEISMIC RETROFIT HARDWARE

 

ARROWS MATCH WITH THE IMAGE ABOVE SHOWING HOW EARTHQUAKE FORCES TRANSFER TO THE FOUNDATION

ARROWS MATCH WITH THE IMAGE ABOVE SHOWING HOW EARTHQUAKE FORCES TRANSFER TO THE FOUNDATION

 

 

 

 

 

 

 

 

Below is a Foundation Anhor 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 these Analyses by some very high powered 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.

UPP10 Comparison to Angle Iron Brace

THE ANGLE IRON BRACE ON THE LEFT HAS VERY LITTLE ABILITY TO RESIST EARTHQUAKES IN SPICE OF HOW SUBSTANTIAL IT LOOKS

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.

A FOUNDATION BOLT SUBSTITUTE WHEN THE MUDSILL IS FLUSH WITH THE FOUNDATION

A FOUNDATION BOLT SUBSTITUTE WHEN THE MUDSILL IS FLUSH WITH THE FOUNDATION

The Foundation Anohor Below is used when the Foundation Anchor must be Attached to the Top of the Mudsill.  This can Happen for Various Reasons.

FOUNDATION ANCHOR THAT FOLDS OVER THE MUDSILL

FOUNDATION ANCHOR THAT FOLDS OVER THE MUDSILL

Bolting New Mudsills to the Side of the Foundation

It often happens Foundation Bolting Hardware or Shear Transfers Ties will not work 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.

IN THIS EXAMPLE THE NEW MUDSILL HAS BEEN BOLTED TO THE FOUNDATION AND THEN ATTACHED TO THE NEAREST FLOOR JOIST.  THE PLYWOOD IS NAILED TO THE TOP OF THE MUDSILL AND ALSO INTO THE FLOOR JOIST

FLOOR BOLTED TO SIDE OF FOUNDATION SHOWING RETROFIT COUNTER FORCE

ATTACHING THE FLOOR TO THE FOUNDATION WITH A NEW MUDSILL BOLTED TO THE SIDE OF THE FOUNDATION. BOLTS AND SHEAR TRANSFER TIE COUNTER FORCE

 

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

 

THE VARIOUS TYPES OF SHEAR TRANSFER TIES

THE VARIOUS TYPES OF SHEAR TRANSFER TIES

 

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

 

Many newer homes built on flat lots are bolted. Especially those built after 1958 These homes are often found in newer cities such as those found in San Jose, Fremont, Santa Clara County, and much of the Peninsula.   These homes also benefit from retrofits  because of three possible failure points: one created by the California Building Code and the others by common construction practices.

Why Retrofit Newer Bolted Homes?

The 3 possible failure factors that create failure points in bolted home which were “built to code” at the time of construction and we will be looking at each of these.

  • The first factor is the building code itself. The building code is created by building officials, volunteers, and representatives from different agencies in an effort to make sure houses are built in such a way that they are earthquake resistance.  The Building code is an ever changing document depending on revelations that arise after large earthquakes as well as pressure from interest groups such as the best funded National Association of Builders, who lobby to make construction costs are as low as possible which sometimes includes undermining seismic safety.

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.

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).”  This a serious oversight in the building cod

Table 2304.9.1 of the 2013  CALIFORNIA BUILDING CODE

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

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

What this means is that 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 advent of nail guns when driving nails was arduous, contractors would only meet minimum building code standards to increase speed and save their bodies.  The result is that houses built before 1978 when nail gun use became pervasive can have a serious structural deficiency.

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

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

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.

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