Many homes have cripple walls, which is a short section of wall between the foundation of your house and the floor you walk on. In such homes, a seismic retrofit reinforces the cripple walls, as in the diagram to the right. These homes are very prone to earthquake damage – and fixing the cripple walls can be complex – you are lucky you don’t have one! Because you don’t have cripple walls in your home, we can attach the floor of the house directly to the foundation with steel.
The purpose of this article is to look at retrofitting methods for homes that do not have cripple walls, and why even bolted homes can still benefit from a seismic retrofit.
Damage to a Home without Cripple Walls
How does Bolting a House without Cripple Walls Work?
This an image of what it looks like under your floor when you do not have cripple walls. One must attach the floor joists, the rim joists, the end joists, and the mudsill to the foundation so they don’t slide off the foundation. The red arrows represent earthquake forces trying to push the floor off the foundation.
Bolting a House 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. Limited vertical clearance prevents standard bolt installation. In these cases one must bolt Foundation Anchors to the side of the foundation and side of the mudsill.
The earthquake force trys to push the mudsill off the foundation. The bolts in the Foundation Anchor prevent this from happening. We have bolted the house exactly as if we used standard foundation bolts but have done it from the side of the foundation.
This is another kind of Foundation Anchor.
In the upper left corner you can see how the manufacturer bent this Foundation Anchor so it can fold over and nailed to the top of the Mudsill.
This is one of the more common kinds of Foundation Anchors. Contractors and engineers use many kinds of Foundation Anchors. The precise type depends on what its to ability to fit the existing framing style. Framing styles used before building codes were common have many variations. Each variation requires its own approach.
This is the strongest Foundation Anchor made. A contractor uses it when the concrete and edge of the Mudsill are flush or needs a small shim.
Retrofitting the Floor to Mudsill Connection
Earthquake forces push the end joists and rim joists off the mudsill. Shear Transfer Ties, also known as framing anchors, counteract that force.
Shear transfer ties connect the side of the floor joist to the top of the mudsill with nails. When the floor joist tries to move, the shear transfer tie transfers that sliding force to the mudsill. The mudsill does not move because it is bolted to the foundation.
Even though the mudsill of this house was bolted to the foundation, damage occured because there wasn’t a strong connection between the mudsill and the floor. Shear transfer ties would have created a strong connection between the floor and the mudsill, and thereby prevented the house from sliding off the foundation.
Foundation Anchors and Shear Transfer Ties Work Together
The retrofit hardware transfers the earthquake force from the floor, through the shear transfer ties, to the foundation anchors, into the foundation, and finally into the ground. This transfer of force creates the load path. Retrofits fail if any connection in the load path is weak.
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.
Bolting New Mudsills to the Side of the Foundation
It 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 one must develop custom solutions. The most common approach is to bolt a new mudsill to the side of the foundation and attach the floor to it.
A retrofit contractor bolted a new mudsill to the side of the foundation and connected it to floor with plywood
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 be able to tell you which retrofit guideline or engineering basis he plans to use and provide you with a proposed design drawing that looks something like this. Then match the drawing to the engineering and make sure they are congruent.
Retrofit Bolted Homes?
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. Building officials, contractors, and members of different private and government agencies write the building code. It is an ever-changing document. Each group influence its creation, such as the powerful National Association of Builders. This group lobbies to make construction costs as low as possible, often ignoring public safety.
A Problem with the California Building Code
See TABLE 2304.1 of the 2016 California Building Code below. It requires 3 nails, or 3 staples toenailed to the sill. Toenailed means put in at an angle. An 8d common is a type of nail. The mudsill is the piece of wood 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.
The Contractor’s Role
Contractors often cut corners such that homes “built to code” often have serious weaknesses.
By law the contractor must follow the building code. Code violations occur from code ignorance, from sloppy workmanship, and hurrying to save money. The almost universal installation of bolts in over-sized holes is a case in point.
Oversized Hole Damage
Bolts are critical but most contractors install them incorrectly. Look at this common Building Code violation. The building code does not give bolts in holes over 1/16 of an inch larger than the bolt any value. This is because of splitting problems. The code states:
Nevertheless, oversized holes are common. Building inspectors require all bolts to have to have the nuts and washers on them. Naturally, it is impossible to inspect the sizes of the bolt holes. So why would a contractor want to violate the building code? So they can adjust the mudsill front to back and side to side. Building inspectors don’t want to look at the bolts twice.
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 take only a few minutes to look at a house. For this reason they often miss deviations from the building code. The near-universal presence of over-sized bolt holes, which the code forbids, is a case in point.
There are three factors that will damage a bolted home: A deficient building code, which we can’t do much about. Ignorance of the contractor, and the oversight of the building inspector. Just one of these factors can cause structural damage. This is why your home can still need retrofitting, even if the foundation is already bolted.