Over-sized Bolt Holes In Sill are the Rule, Not the Exception. Here’s Why
Bolts are important in new construction as well as in a seismic retrofit. In the end, ALL of the earthquake force is concentrated into the bolts. Unfortunately, most bolts will not be as effective in resisting earthquakes as they should be because almost universally, they are installed in over-sized holes. By over-sized we mean the diameter of the hole in the wood exceeds the diameter of the bolt.
To the right is an illustration of why this is a problem.
Section 188.8.131.52 of the California Building Code does not recognize bolts installed in holes exceeding 1/16 of an inch over the bolt diameter as having any value. This information is found in the NDS published by the American Wood Council and is adopted by reference in the California Building Code.
Below is the actual citation from the building code. If you want to know if your bolts are over-sized, find an accessible bolt and take off the nut and washer and see for yourself. 99.9 of the time they are oversized. If you find one that is not oversized, please send it to me as I have never seen it.
Why Do Bolted Houses Have Over-Sized Holes?
Contractors invariably drill holes into the mudsill that are much larger than the diameter of the foundation bolt. This is so the mudsill can be adjusted side to side or back and forth. This is especially common when the foundation is misaligned.
As you can see in the image below, when a foundation bolt has over-sized holes, the bolts do not work together when the earthquake hits. Instead, only one bolt works at a time and the bolts fail sequentially. One of the most common types of damage in the 1989 Loma Prieta and the 1994 Northridge Earthquakes was splitting of the mudsills caused by foundation bolts in oversized holes. Tests done by an independent laboratory in Los Angeles after the 1994 Northridge Earthquake showed oversized holes can weaken a foundation bolt by 50% or more. The newest codes require the use of plate washers to compensate for this almost universal problem.
So, What Should I Do if I Have Over-Sized Bolt Holes?
In practical terms, bolts installed in over-sized holes can be deemed as being 40% as strong as bolts installed in code-approved holes. This statement is contrary to the opinion of the American Wood Council which maintains the position that if a bolt was not installed in the proper sized hole, it has no value. The best thing to do is re-bolt the house. Shear transfer ties are always a must, and it is not all that much extra work to re-bolt the house when the technician is already there.
The Importance of Bearing Plates in Foundation Bolt Performance
House Bolts and Plate Washers can keep the Mudsill from Snapping
Research has shown that plate washers reduce, and in most cases eliminate, cross grain bending. This is a phenomenon where the mudsill splits in two as shown in the drawing and photograph below. This was a common occurrence until plate washers became part of the building code. However, cross grain bending is primarily a problem with tall cripple walls rather than the short cripple walls that are used in most seismic retrofitting.
Tall shear walls pull up on the mudsill when they try and overturn, which is resisted by the bolts. If these bolts have small washers and/or insufficient bolts at the plywood locations (they should be a minimum of 2 feet apart) the mudsill may go into cross grain bending and split. Plate washers hold the mudsill down because of the larger surface touching the surface of the mudsill.
Cross Grain Bending with Round Washer
Once cross grain bending occurs as overturning forces pull up on the mudsill, all functionality of the shear wall is lost.
The other Purpose of Bolts and Bearing Plates
Experiments with plate washers were originally done by the Structural Engineer’s Association of Southern California. They discovered something else useful about plate washers. Bolts bend slightly when the mudsill pushes on them. Plate washers prevented the bolts from digging into and causing the mudsill to split. Below is a drawing made by one of the structural engineers who performed the tests.
This is what he had to say about their experiments:
“The code requirement for bearing plates came out of the observation by the Structural Engineers Existing Building Committee. The mudsill bolting research was done after the City of Los Angeles demolished 30 or so houses. The tests were done after the houses had already been removed. We isolated mudsill pieces so that we could use a jack to apply a horizontal force on the end of a mudsill piece that would result in a shear force to various anchor devices: existing anchor bolts, retrofit anchor bolts of various types, steel side plates, etc. For anchor bolts, we observed the failure mode to result from bending of the anchor bolt cut into the mudsill like a dull knife, splitting the mudsill. When we installed a plate washer on the bolt, the plate would restrain against excessive bending by holding the nut up, in effect applying a resisting moment to the top of the bolt. We got higher ultimate shear capacities that way. “Deflection” is the amount the mudsill gave way when pushing against the bolt.
Los Angeles City then began requiring plate washers on retrofit anchor bolts. Subsequently, other engineers began to consider the advantage of the plates in helping resist cross grain bending. It was thought that a larger plate would be beneficial for this purpose.