Buildings without Cripple Walls

Seismic Retrofitting of Buildings without Cripple Walls

The purpose of this article is to look at retrofitting methods used for buildings that are close to the ground and do not have cripple walls, and why even bolted buildings can greatly benefit from a seismic retrofit.

The case studies here relate to homes, though the principles apply to any building, no matter how large.

How does Bolting a Home withthis haout 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, the 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.  Foundation Anchors are used when there 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 restrained 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 Transfer 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. Whoever you hire should know what it is, and be able to prove to you how their retrofit plan 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

To the left of the 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 analyses by 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 Anchor is 5 times stronger than an Angle Brace.

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 that Foundation Anchors and Shear Transfers Ties will not work for various reasons.  The primary cause is 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.
 

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.

 

Retrofit Bolted Homes?

 

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

After 1958, virtually every home and addition in the Bay Area was bolted to its foundation.  Here we will take a look at bolted homes on a flat lot because they can still have big problems.

Three things can cause a bolted house on a flat lot “built to code” to be damaged. The first factor is the code. It is made by building officials, contractors, and members of different private and government agencies.  It is an ever-changing document because of the influences of each group, such as pressures from the powerful National Association of Builders, who lobby to make construction costs as low as possible, ignoring public safety.


A Problem with the California Building Code

In TABLE 2304.1 of the 2016 California Building Code requires the joist to sill (mudsill) connection be made with 3 nails, or 3 staples toenailed.  Toenailed mean put at an angle. An 8d common is a type of nail.  The mudsill is the piece of wood that is bolted to the foundation – see the diagram below the table.

Three nails are not enough to keep keep a floor on the bolted mudsill. It can slide off. However, before nail guns came into common use in the mid-seventies, driving nails by hand was hard on the arm and extra nailing cost the contractor money.

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

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 Contractor’s Role

The second factor that can cause a bolted house on a flat lot “built to code” to be damaged is whether the building code is followed. Following the code is the responsibility of the contractor. This step is where the highest probability of deviation from the code exists. Problems arise due to sloppy workmanship when trying to hurry up and save money, as well as from not understanding the importance of the seismic provisions of the code.  The almost universal installation of bolts in over-sized holes is a case in point.

Bolts are critical but most are of them are installed wrong. The 2016 California Building Code does not consider bolts installed in holes that are more than 1/16 of an inch bigger than the bolt itself as having any value, because of potential splitting problems. The code states:

Bolt Holes

Nevertheless, oversized holes are common, because at time of inspection the bolts must have the nuts and washers on them, even though this means the inspector cannot see the bolt holes.  Contractors over-size holes so they can adjust the mudsill front to back and side to side.

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

The Building Inspector’s Role

The third risk factor with a bolted house on a flat lot “built to code” is the building inspector. Their job is to make sure contractors follow all parts of the building code that relate to earthquake safety.  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 near-universal presence of over-sized bolt holes, which the code forbids, is a case in point.

In summary, there are three possible causes of earthquake damage in bolted homes: a deficient building code, the ignorance of the contractor, and the oversight of the building inspector.  Just one of them can create a problem.

Oversized Hole Damage

 

Floor slid off mudsill because of inadequate nailing

House slid off foundation in the Northridge Earthquake

 

 

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