Residential Earthquake Retrofits in BC by Shear Seismic Retrofits

Eliminating Key Structural Weaknesses in Houses

There are three significant areas of weakness commonly found in the structure under the first floor homes built on raised foundations. If any one of these fails during an earthquake, a house is likely to suffer serious structural damage or even total collapse. Seismic retrofits are intended to prevent such enormous damage, ensuring your home remains upright on its foundation and safe enough to continue living in.

Exterior view of a house's cripple wall framing structure
* Mudsills are known in Canada as Sill Plates.

A Look Under the House

At right are the perimeter structural elements that support a house.

Weakness #1:  Cripple Walls

See where the cripple wall is located on a house with a basement
Homes can be built over foundations in two general configurations:

1) with the floor joists sitting directly upon the sill plate, or

2) with the floor joists sitting on top of wood framed walls built on top of the foundation wall.  These shorter than usual framed walls, shown in the image at left, are the cripple walls. They typically vary from 2′ to 5′ tall. The number of stairs to the front door is generally a good indication of their height.

While obviously strong enough to bear the gravitational weight of the house, the cripple walls of a great many homes would give way and collapse under the force of even a moderately strong earthquake.

In fact, prior earthquakes in California have shown cripple walls to be, consistently, the first structural component to fail. This is how it happens:

Unbraced cripple walls collapse during earthquake

As you can see above and below, it doesn’t matter how many anchor bolts are installed if the cripple walls are not equally strong.

An unbraced cripple wall collapses from the force of an earthquake despite foundation bolts.

 Weakness #2.  Connection of the Sill Plate to the Concrete

The sill plate is either a 2×4 or 2×6 piece of lumber. If the floor joists are not sitting directly upon it, the sill plate is then the bottom member of a cripple wall. Either way, it needs to be secured substantially so several tons of seismic force vibrating back and forth won’t cause it to shift off the concrete.

This is what can happen during an earthquake if the sill plates are not bolted to the concrete:

House with plywood shear walls that are not bolted to the foundation is shaken off the foundation by earthquake force. A seismic retrofit must include foundation bolts.
Theoretically, all homes built after the release of B.C.’s 1974 Building Code should have 1/2″ bolts every 8 feet. In practice, this is often not the case. There may be less or none at all. (See “Modern Lower Mainland Homes Missing Foundation Bolts”) Often, they were not installed correctly installed well enough to be of much, if any, value in and earthquake.  Here are some examples:

When seismic retrofitting, these existing bolts would be ignored and new ones installed

From left: 1) A nut and square plate washer are called for. This bolt was installed without room for either. Sometimes there are no nails even. 2) Over-drilled hole allows sill to split more easily under force. 3) Sill plate is made much weaker by unnecessary notching. 4) Sill has been notched around bolt probably during sill replacement. This won’t do.

Bolts with small ring washers are more likely to cause sill plates to split under stess.

As of late December 2012, the building code required a minimum of one (1) 1/2″ bolt ever 6 feet or a 5/8″ bolt every 8 feet. Behind shear walls extra bolts are necessary. As well, standard ring washers which tend to cut into the wood have been replaced with heavy 3 inch square plate washers that help prevent the sill plate from splitting under force.


Modern Lower Mainland Homes Missing Foundation Bolts

Unbeknownst to many homeowners, an enormous number of houses built during the 1970s, 1980s and early 1990s on the Lower Mainland have no bolts whatsoever. Despite being specified in the B.C. Building Code since 1974, a particular type of metal strap was said to be approved as an alternative.  Going back 30 years, there is no government record of this approval.  These lightweight “Tidy Ties” cannot be relied on in an earthquake.
Non - seismic foundation straps

Look to see if these straps were used at your foundation.


Weakness #3.  Floor framing is not connected strongly enough to the cripple walls.

Damage can still occur to the house if the cripple walls are braced with plywood and bolted to the foundation if the this lower structure is not secured to the floor joists above. As the earthquake rocks the house back and forth, the cripple wall will remain upright, but the movement will cause the upper structure to slide off the cripple walls, as shown below.



 Solution:   The Load Path to the Foundation

Seismic energy, the “load”, requires a continuous path through which it can be transferred to the foundation and from there into the ground. When there are breaks in the path, in this case in the connections between structural “members”, the energy tons of energy travelling through suddenly piles up at the break with great impact.

Imagine what would happen if  a speed skater or downhill racer suddenly hit a mud patch. The energy of their speed would throw their bodies forward while their feet were stuck at the mud. It’s like that only it’s your house being thrust forward of its foundation.

Besides stiffening the cripple walls, the purpose of a seismic retrofit is to create the continuous path that leads the earthquake’s incredible energy fluidly though to the ground without major damage to the house.  The 3 key weaknesses are not the only ones. A proper retrofit will connect the framing together in a number of other ways to ensure the greatest capacity for energy transfer. You could also call it “increasing the bandwidth”.

The following illustration shows how a seismic retrofit works.

Load path of earthquake forces from floor to foundation

You’ll be relieved to know that we can prevent the worst damage for a fraction of what it would cost to repair it or rebuild if it came off its foundation.