If you would like to know more about the brand new state-of-the-art seismic retrofit guideline, there are around 30 educational videos and web pages that you will find interesting. Just type ICC 1300 in the search box in the upper right.
How big does the building code say the earthquake will be?
Here we will be comparing the International Code Council’s seismic retrofit guideline ICC 1300 Vulnerability Based Seismic Assessment and Retrofit of One and Two-Story Dwellings with the other 8 retrofit guidelines to see how it is different, and it is quite a bit different, from these earlier guidelines.
This cost comparison between ICC 1300 and Standard Plan A makes it obvious that these differences have a big impact on cost and public safety.
We must remember there are no actual seismic retrofit building codes. There are only guidelines. A guideline is not a code because a code has the force of law, while a guideline is voluntary. No one can be forced to retrofit their house, they can only be guided to do it properly.
All these guidelines are voluntary you can do as much or as little of it as you like. The greatest benefit of guidelines is that they provide ideas on how best to approach any retrofit. The intent of this flexibility regarding retrofit work goes all the way back to the 1997 Uniform Code of Building Conservation.
To start, let’s get a basic understanding of retrofit engineering as described in this video. As you will see, the anticipated ground acceleration determines how much force a retrofit must withstand. The Standard Plan A anticipates a ground acceleration of 0.186Gs, all the other guidelines except Vulnerability Based Seismic Assessment and Retrofit of One and Two-Story Dwellings anticipate a ground acceleration 0f 0.146 Gs.
The most important difference in these guidelines is the amount of force they are designed to resist. This can be discerned from the amount of cripple wall bracing required for each wall. In this first example, this table spells out the requirements of Appendix Chapter A3 of the California Existing Building Code. This guideline was first published by the Structural Engineer’s Association of Southern California based on damage observed after the Northridge earthquake. The assumption behind this guideline and most of the others is that force levels will be close to what was experienced during the Northridge earthquake. The ground acceleration behind this guideline is 0.146 Gs
Appendix Chapter A3 of the International Existing Building Code
This table illustrates cripple wall bracing requirements in terms of the percentage of wall length. Most single-story houses are 24 feet wide and 48 feet wide. When this percentage of cripple wall system is applied, the 24-foot long cripple walls will have 12 feet of bracing and the 48 foot long walls 24 feet of bracing.
It is an elementary engineering concept that the weakest cripple walls will be the first to fail. In other words, if one wall has 12 feet of bracing and another wall 16 feet of bracing, the wall with 12 feet of bracing will fail first. For this reason, all sides should have the same linear footage of bracing along both the 24-foot long cripple walls and the 48-foot long cripple walls. The cripple wall with the least linear footage of bracing will be the first to fail so adding bracing in excess of this amount is a waste of money.
Notice that this guideline does not make a distinction between stucco houses and houses with wood siding. This is because of their difference in weight. It is probably based on field observations that noted stucco houses perform much better than houses with wood siding. In other words, there was no reason to create a different table for stucco houses because they were actually more resistant to earthquake damage than wood-sided houses. Photographic evidence strongly supports this
For example, a house with a tile roof, stucco exterior, and plaster walls will weigh approximately 60 pounds per linear foot while a house with wood siding, a shingle roof, and sheetrock walls will weigh almost half as much at 36 pounds per linear foot.
Applying These Calculations To A Typical House
A This is the most common size in California. In order to follow the International Building Code, we need to put plywood on 50% of each wall.
Appendix Chapter A3 is a percentage-based system. Assume we have a typical Bay Area bungalow 24′ wide by 48′ long. We will 12 feet bracing on the front and back and 24 feet on the two sides. The 12 feet of bracing will fail much sooner than the 24′ of bracing, and once it fails, the house has suffered catastrophic damage no matter how much bracing is on the long walls. The retrofits should be balanced with an equal amount of bracing on each side to avoid wasted bracing on the two long walls. Ap
Los Angeles Department of Building and Safety Standard Plan Number 1
The retrofit guideline came out of Los Angeles, unsurprisingly, the bracing requirements are exactly the same and
Standard Plan Number 1 is a percentage-based system and is also wasteful
City of Seattle’s Project Impact
Seattle has the same percentage-based bracing requirements. It assumes the same force levels. This force level assumption varied slightly with Standard Plan A, and radically with ICC 1300 Vulnerability Based Seismic Assessment and Retrofit of One and Two-Story Dwellings. Project Impact also requires differing amounts of bracing on each side and therefore
Project Impact is equally wasteful.
have the same wasteful bracing requirements.
is equally wasteful.
is a percentage-based system that is wasteful
is a percentage-based system that is wasteful
A Big Change Came With Standard Plan A
has very different bracing requirements.
Jim Russel, who chaired the Appendix Chapter A3 committee, was also the chair of the Standard Plan A committee and provided the engineering calculations that are the basis of this table. Notice how in this guideline a distinction is made between light and heavy construction. Heavy construction designates any house made with stucco or plaster. Everything else is light construction. I believe this is because he was not aware of empirical evidence support the assumption that stucco houses are considerably more resilient than houses with wood siding.
Standard Plan A does not use a “per linear foot” approach to the bolting such as a “bolt every 6 feet”. It specifies instead exactly how many bolts are required on each wall line such that the bolting capacity matches the plywood capacity. The other guidelines use a “put a bolt every 6 feet approach” no matter how long the wall. Here force levels are determined by both anticipated force levels and house weight. The heavier the house, the more retrofit components it needs.
Let’s look and see how this works. If we apply Jim’s calculation to a 1200 square foot single-story stucco and plaster house it must resist 5,580 pounds of earthquake force on each wall line.
The plywood bracing in Standard Plan A is also rated to resist 380 lbs per linear foot. Standard Plan A requires 20′ linear feet of plywood on each wall line worth 7,600 lbs of resistance. That is the amount of force this guideline anticipates. The resistance of the bolts will also equal 7,600 lbs. With an equal amount of bracing on each side, this is a balanced system.
Standard Plan A is not a wasteful system.
By the way, I was on the Standard Plan A committee and a geologist made sure our force levels were accurate.
This guideline was published after the Napa Earthquake and uses much of Stanard Plan A as its source material. Whereas Standard Plan A violated International Code Council protocol by naming a specific manufacturer’s hardware this guideline did not. In the Table the Type A Connector represents the Simpson URFP, the Type B Connector is the Simpson FRFP, and the Type C Connector is the USP FA6, which is no longer available except from a company in the midwest and is not available from any hardware or builder’s supply store on the west coast.
The most significant differences between it and Standard Plan A are:
- The quantities of plywood vary with the height of the cripple walls. The average height cripple wall is between 2 and 4 feet and the linear footage of the cripple wall bracing more or less matches the quantities found in Standard Plan A. The same applies to the bolts and the cripple wall-to-floor connectors.
- It applies to cripple walls over 4 feet in height.
- It does not name the manufacturer of the hardware.
- It includes the Simpson FRFP (The Type B Connector)
- It includes the USP FA6 (the Type C Connector). This product is not available on the west coast.
With an equal amount of bracing on each side this is a balanced system.
This is not a wasteful system
Now we go to the most recent guideline published in December of 2018. This guideline radically deviates from its predecessors.
ICC 1300: Vulnerability – Based Seismic Assessment and Retrofit of One and Two-Story Dwellings
This changed in the International Code Council’s ICC 1300, where a third medium construction category was added. For cripple wall retrofits the table looks like this.
Let’s compare its table to Stanard Plan A’s table.
Standard Plan A requires 14′ 8″ of bracing for a 1200sf light category house and 7 half-inch bolts on each side.
ICC 1300-2022 Vulnerability Based Seismic Assessment and Retrofit of One and Two-Story Dwellings requires 26’6″ of plywood (TWO 13’3″ panels on each side” on the typical 3′ tall cripple wall and 14 half-inch bolts. There are twice as many retrofit components in P-1100, which will be twice the cost.
This table from ICC 1300 Vulnerability Based Seismic Assessment and Retrofit of One and Two-Story Dwellings is for living areas above a garage. It allows a single length of plywood is allowed that is 19′ long with 13 bolts on each wall line. Tie-downs and a 7′ tall cripple wall are assumed.
For cripple wall retrofits two 12′ panels with tie-downs equaling 24 linear feet of shear wall is required. Why it is 5 feet longer than the living area above a garage panel is unknown.
When you try and apply this system, the bracing extends beyond the end of the available foundation and the 3.7 Gs anticipated ground acceleration demands so many more retrofit components that no one can afford it.
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