You know that you live in earthquake country when you live in the Bay Area, including Marin County. We all know the area’s vulnerabilities, whether we are talking about seismic retrofitting of bridges or our own homes and businesses. No matter where you live in Marin County, in San Anselmo, San Rafael, Larkspur, Mill Valley, Novato, Sausalito, Corte Madera, or Fairfax, you want to keep your house safe. But how do you do it? How do you make sure that your structure will withstand the next earthquake – or the big one? One approach is to get seismic retrofitting for your home. Some call it bolting and bracing. Earthquake preparedness for the house focuses on keeping the house intact and on its foundations. While building codes call for more modern structures to have better earthquake resistance built in from the ground up, you may still find it worth your while to call in earthquake retrofitters to take a look so you are as ready as you can be. Our discussion now will focus on homes, rather than offices or other buildings.
Seeking out Proper Earthquake Retrofitting in Marin County
Finding good earthquake retrofitting can seem like quite a task, for a couple of reasons. First, there is no code in California specifically for earthquake retrofitting that you or a contractor must follow. There is a seismic retrofit code, but it is not mandatory and it is only for simple constructions on flat land. However, there are mandatory building codes. Changes made as part of an earthquake retrofit must conform to those building codes, but there is no specific mandatory code describing earthquake preparations for the house. This is partly because houses come in many different varieties in Marin County, whether in San Anselmo, San Rafael, Novato, Mill Valley, Larkspur, Sausalito, Belvedere, or Tiburon. A single-level building built on the flat is much easier to secure than a multi-level house built on a hillside. This set of circumstances means that any retrofitting you do is done on a purely voluntary basis. One slight caveat to this is that some homeowners find they cannot get an insurance policy for a newly purchased house or renew an insurance policy on an existing house without some evidence of earthquake retrofitting being provided to the insurer. So you may not be retrofitting entirely voluntarily, but you are as far as the state is concerned.
Earthquake retrofitting is also daunting because the contractors who perform it do not have to pass a test that specifically addresses earthquake retrofitting. Yes, you will need a licensed contractor. Anyone who does more than $500 of work on your house must be a licensed contractor. But though California has many different categories for licensed contractors, there is no specific category for earthquake preparedness contractors. Your best choice is to look for contractors who have experience with earthquake retrofitting. You could also consider working with an engineer to draw up plans to retrofit your house. If you do look for an engineer, you should again look for one who specializes in earthquake retrofitting.
You will need a building permit to complete the earthquake retrofit. Building permits are required for work done on the foundation, where much of the earthquake retrofitting occurs. Some counties and areas waive the fees for the permits because there is no code, but most require the usual fees and processes to get a building permit for earthquake retrofitting work. Your earthquake retrofitting company should be able to help you get the permits. You should be aware that a final building inspection is required for the sign-off on the building permit after the project is complete. The building inspector will look at the entire building, not just the area where the earthquake retrofitting is done. Because of this, it’s a good idea to install the required smoke detectors in bedrooms and hallways before the final building inspection.
That’s not to say the state doesn’t care about the effects of earthquakes. In fact, earthquakes in Loma Prieta and Northridge have drawn much study. Based on observations made at houses after those earthquakes, building codes have been updated, and more information is available about features that help a house survive an earthquake better.
A set of plans is key to an effective earthquake retrofit. The plans form the basis for the bid amount. The plans may be drawn up by an architect or engineer. Or, the plans may be drawn up by the contractor. The contractor can draw up the plans when the changes are simple enough that the prescriptive standards can be applied. The plans should be submitted to the local housing authority when the permit is requested, so that the plans can be reviewed. The housing authority decides if the prescriptive standards are enough, or if a set of drawings from an engineer or architect is needed.
What Exactly Does Earthquake Retrofitting my Marin County House Entail?
An earthquake moves the ground under a house, placing forces on the structure. Weak points in the structure can collapse. Chimneys, porches, or other less-well attached parts can separate. The house can even slide off its foundation. Earthquake retrofitting attempts to prevent significant damage to structures during an earthquake. No house, anywhere in Marin County, whether in San Anselmo, San Rafael, Novato, Larkspur, Mill Valley, Dillon Beach, Kentfield, or Marshall, can be made earthquake proof. But retrofitting can mitigate the damage from an earthquake.
The primary forces that cause damage are the horizontal forces. To prevent damage from horizontal movement, materials are used to create a continuous load path that passes the motion load from the house back into the foundation and the ground. The load path runs from the roof, through properly enforced walls, to the floor, and down to the foundations. The load path may not have any breaks in it if it is to transfer the load successfully. The load path must have connections at every transition – from the roof, which connects to the walls, which connect to the floors, which connect to the cripple wall, which connects to the foundation. Only when this load path is complete and intact can the energy pass successfully down the path and to the ground.
So, how can you ensure that your house has a continuous load path? Seismic forces are strongest at the base of your house, and much of the earthquake retrofitting you will find focuses on this area of the house. Of course, this assumes that your roof is in good shape and can withstand the forces of an earthquake. It is axiomatic that any changes you make in the course of earthquake retrofitting must not harm the existing buildings. Buildings can usually withstand vertical movement because all houses are built to resist gravity. The gravity-resisting features include transferring load from the roof to the walls and from the floors to the cripple walls and foundation. (Cripple walls refer to walls that go from the foundation to the first level of the structure.) The earthquake retrofitting must work in concert with some of this existing gravity-resistance system.
Earthquake retrofitting revolves around strengthening and stiffening the walls, and making sure the house is well connected to a sound foundation. To brace the walls, your retrofitter will create shear walls. Shear walls are reinforced walls that have the stiffness and strength needed to resist the forces of an earthquake. Shear wall retrofitting consists of framing, sheathing, fasteners, and hold-downs. These elements are combined to create a structure that can resist the forces caused by an earthquake. A long, low shear wall resists forces better than a tall, narrow shear wall. Shear walls should be installed to form a box and should be equal in length and should be placed symmetrically on the four exterior walls of the building. Shear walls should exist on each level of the structure, including the crawl space. In some cases, you may need interior shear walls, when the exterior walls lack strength and stiffness, or when the dimensions of your shear wall box exceed 3:1. For example, if the width of the building is 15 feet, you would not have to worry about interior shear walls unless the length is greater than 45 feet.
Framing with the appropriate lumber is the first step in making the shear wall. In existing homes, these framing studs sometimes already exist. However, newer building codes may dictate that more studs be added so that they are closer together. Between each stud, a block is placed at the bottom, so that the sheathing can be attached to it. In California, the framing material is assumed to be Douglas fir-larch. If you find that your framing material is redwood or hem-fir, you must make sure that the design of your retrofit accounts for this. Douglas fir-larch is denser, which makes it better able to hold fasteners like nails.
Once the framing material is in place, sheathing material is nailed over it. The building codes allow different sheathing materials, but after the Northridge quakes, it’s been determined that plywood is the best material for resisting earthquakes. Plywood less than four layers thick tended to pull apart, so most often, five-ply plywood is best for earthquake retrofitting. The plywood sheathing material should have ventilation holes in it when installed in the cripple wall.
Plywood must be attached using the proper fasteners. Your contractor should use nails, rather than screws, since nails are easier to install than screws and are better able to conduct seismic energy than screws. There are building code regulations for the types of nails used and what a properly installed nail looks like and how closely the nails must be inserted to each other. You should look to see that nail heads are flush with the surface of the sheathing. The nails should not be too close to the edge of the sheathing. The nail should never cause a split in the sheathing material – split material is weakened material. The shear wall ends where the hold-down device is installed. These hold-downs are bolts or other devices used to hold down the installed sheathing. The length of the shear wall is measured as the distance between the hold-downs. The hold down should always be installed as specified by the manufacturer – never contorted or improperly placed.
This is a general description of shear wall installation in a wooden building. Framing is installed, and plywood is nailed to it. The framing includes not only lumber going up the side of the wall, but also blocks that are installed at the foot of the wall. These blocks are the subject of some discussion. In older houses, there is a short wall, called the pony, or cripple, wall. This wall exists between the ground and the floor of the first level of the house. It is less than a full storey high. This cripple wall sits on top of a piece of wood, called the mudsill. The mudsill is lumber that sits on top of the foundation. The blocks installed as part of the retrofit are used so that the plywood has something to be nailed to at the base of the wall. However, despite some civic authorities recommending attaching the blocks using nails, it’s been noticed that doing so can split the block.
There are some alternate techniques. One uses staples to attach the block to the mudsill. Another technique is to attach the block to the plywood first, then attach the assembled piece to the cripple wall and mudsill. A final technique that is gaining some recognition is the flush-cut mudsill. In this scenario, the mudsill is cut back so that it is flush with the two-by-four studs used to build the house. The plywood is then nailed directly to the freshly cut mudsill. In cases where a shear wall cannot be applied, you may need to get a moment wall. This is a steel-framed structure designed by an architect or engineer.
Getting the Proper Foundations in Marin County
You may also need to do some work on your foundation as part of the retrofit. There are several ways to achieve more sound foundations. You may cap your foundations, create parallel structures, or replace the foundations. If you choose to cap the foundations, a concrete cap is poured over the existing foundation wall. New steel anchor plates and bolts are installed to attach the foundation to the lumber that supports the floor above it. If you plan to replace the foundation, the building is often propped up so that workers can replace the foundations in four-to-six foot sections at a time. A parallel foundation system may also include placing columns under the flooring joists. The columns are often placed next to a wall. The column is better able to resist the earthquake forces. Some of these foundation techniques may require a firm that specializes in foundation work.
A common approach to retrofitting a foundation is to install bolts. In houses without cripple walls, the house is bolted directly to the foundation. In this scenario, the bolts pass through the mudsill (the wood sitting on top of the foundation) and into the foundation. Building codes require bolting. However, bolting may not be sufficient to keep the house from slipping. Bolts are often installed with holes that are bigger than bolt. During an earthquake, the bolts may move in the holes, tearing at the mudsill. To prevent problems, you may install more bolts. You may also install foundation anchors. Foundation anchors also join the mudsill to the foundation material. In some cases, you may also want shear transfer ties. Shear transfer ties prevent the floor joints from sliding off the mudsill. When the floor joints are perpendicular to the mudsill, they are usually properly nailed to the mudsill and no transfer ties are needed. When the floor joints run parallel to the foundation, they may not be sufficiently nailed and may need shear transfer ties.
You may have to decide between using mechanical or adhesive bolts. When the bolts are installed, they may be installed with a washer or with plates, or with a plate washer. If a washer is used, it must be a square washer, as round washers do not perform as well in earthquakes. A mechanical bolt can only be used with concrete, not with masonry. The mechanical anchor must be used in good-quality concrete, since it attaches to the concrete via friction by expanding. An adhesive anchor uses a chemical bond to attach to the concrete. The adhesive bolts usually cost a bit more. With adhesive anchors, it’s very important that the hole be cleaned because the chemical bond needs to be formed with the concrete of the foundation, not any particles left in the hole.
You should be cautious if you see bolts that are countersunk – in most cases, this is unacceptable installation. A countersunk bolt is one that is inset into the material – if the wood is gouged away so that you can reach the bolt and washer, this is generally considered an improper installation.
A Few Special Considerations in Marin County for Earthquake Retrofitting
Keep a few things in mind for your earthquake retrofitting. Houses built after the 1990’s are likely to have the most up-to-date earthquake preparedness, since building codes were updated after the Northridge earthquake. However, you might want to have a check, since not all builders build strictly to code. If your house is a special case, and a surprising number of homes in Marin County qualify as special cases, whether in San Anselmo, San Rafael, Larkspur, Mill Valley, Novato, Kentfield, Marshall, or Dillon Beach, you may need more than a contractor to design your retrofit. Houses on hillsides, multi-storey houses, houses with large openings like garage doors, all may need special plans from an engineer or architect. A good contractor will be upfront with you about when you need to call in additional expertise.
Do not allow plumbers, heating, ventilation, and air conditioning contractors, or others, to make large openings in the shear wall. Openings or cutting into the shear wall will weaken the wall.
This discussion has centered around standard framing methods. Some houses are built using balloon framing. These houses usually require engineer or architect plans to ensure that the seismic retrofit is properly planned.
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