Part 9f: So You Want to Buy a House and Fix It Up? Thirty Suggestions for Survival
[30 suggestions over 10 sub parts, starting with Part 9a. -ed.]
17. Seismic upgrading — the cheapest insurance you may ever buy
Before an earthquake, decisions about seismic upgrade requirements, including financing, are extraordinarily difficult. After the earthquake, every property owner wishes he or she had done more. — Charles Eadie, former Project Manager of the City of Santa Cruz Redevelopment Agency Downtown Recovery Plan
I went through the Loma Prieta quake in San Francisco in ‘89. A portion of the Bay Bridge collapsed, thousands of houses were shaken off of their foundations, and 63 people were killed. I toured a lot of the damaged areas. . . . I saw dozens and dozens of houses in Oakland that were just fine in most regards, except that they had moved laterally a foot or two and fallen off of their foundations. Many of these houses still had all of their windows intact and still had dishes sitting on the shelves. — Jim Katen, Associated Master Inspectors, Portland, Oregon
Note: I wrote this section on seismic upgrading before the recent earthquake hit Christchurch, New Zealand, and the much more massive earthquake and associated tsunami hit northeastern Japan. These recent events don’t really change anything I’ve written here, but they do suggest I may not be some kind of seismic survivalist nut. Earthquakes actually do happen, and they happen with more severity and more frequency in seismically active zones (like the one in which Vancouver is located). The results of the Christchurch earthquake do underline a couple of points I include below: unreinforced masonry buildings are often the most dangerous type of structure in an earthquake, and even if a structure doesn’t collapse in a earthquake, it may be subsequently condemned. An estimated 10,000 houses in Christchurch will have to be demolished. The earthquake in Japan provides evidence of something else: fires and tsunamis caused by earthquakes can be far more destructive than the earthquakes themselves. Seismically upgrading your house won’t do anything to help it withstand a large tsunami. Living on higher ground is the only defense. Even if a house is securely bolted to its foundation, the direct impact of a wall of water will smash the structure into kindling. However, anchor bolts and holddowns might prevent a house from being floated off its foundation if only a moderate amount of water and force were involved — say, farther away from a tsunami’s initial impact zone. Preventing fires caused by ruptured gas mains or exploding electrical transformers is beyond the control of any homeowner, but you can improve the seismic safety of gas appliances in a house by using flexible rather than rigid supply lines, and by strapping gas hot water tanks to the walls.
For the price of a granite countertop, some relatively simple seismic upgrades can save a wood-frame house from tens or hundreds of thousands of dollars of damage in the event of an earthquake. The upgrades might even save the occupants from serious injury or death, although in big earthquakes wood-frame structures don’t usually collapse completely, unlike old brick or unreinforced masonry buildings, which can. For the owners of houses along the seismically active west coast of North America, the issue is more one of establishing at least a basic line of defense against the potential for massive property damage, and increasing the chances that a house will remain habitable, as opposed to being condemned, in the aftermath of a major earthquake.
The bad news, in Vancouver, is that the City only incorporated the seismic requirements of the National Building Code in 1967 — well after much of the housing stock was built — and one- and two-family dwellings were, and still are, exempt from the seismic requirements in the code. For new home construction, the City passes the responsibility to the structural engineer, who must attest to the seismic adequacy of a design. There is no requirement for houses undergoing renovation to be seismically retrofitted, and the current requirements with seismic implications that do exist for new houses are fewer than those in west coast American cities.
The typical risk factors for an older Vancouver house — all of them present in our own house prior to the seismic upgrading we performed — are a house frame that’s not anchor bolted to the concrete foundation walls, corners of the frame that are not secured with holddowns, and cripple walls (the short walls extending from the foundation to the first floor) that lack shear resistance (resistance to lateral forces). The shaking of even a moderate earthquake can cause an unbolted frame to vibrate off its foundation. A larger earthquake can cause a frame that lacks holddowns to tip up as a unit, or the entire structure to slump to the ground in one direction as all the studs in the cripple walls go over like dominoes, imperiling any occupants on the lower level, such as tenants in basement suites. Cripple wall failure is what caused the partial collapse of this house in the 1989 Loma Prieta earthquake (the one that disrupted the World Series in San Francisco). The cripple walls were probably vulnerable because not reinforced against lateral forces, and were further weakened by large openings for two garages, creating what is known as a ‘soft storey’. With the weight of the house above, there’s just too little material in this wall to resist the side-to-side shaking of an earthquake.
Cripple wall failure during Loma Prieta earthquake
The good news is that significantly improving the seismic resistance of a typical Vancouver house isn’t that technically challenging or costly, depending on when you do the work, and assuming there are no problems with the concrete foundation. Incorporating seismic upgrading with a more general renovation is the most cost-effective and least disruptive approach. Anyone considering a major renovation, especially one that includes the lower level of a house, should take the opportunity to seismically upgrade while other renovation activities are ongoing. Unfinished or gutted basements or lower levels are ideal, because the frame is exposed, and installing anchor bolts, holddowns, various other structural connectors, and plywood shear panels is relatively quick, easy, and inexpensive. Five thousand dollars should cover it if a contractor is doing the work, and perhaps two thousand dollars if you do the work yourself. Two or three percent of the cost of a major reno. If the lower level is finished, seismic upgrading will require removing and later replacing at least some of the drywall, which increases the expense and effort somewhat, but not excessively.
Most advanced do-it-yourselfers can perform the work themselves, working from a ‘prescriptive plan set’ — structural engineering drawings that can be adapted to a wide range of houses. I’ve included a link to a City of Seattle web page below that includes a downloadable plan and other guides, and similar plan sets are available for free download from the Los Angeles Department of Building and Safety, and the Association of Bay Area Governments web sites. The City of Vancouver seems to be lagging in this regard, with nothing similar available, that I could find. As the Seattle site states, “These plans help homeowners with qualifying structures obtain the necessary building permit(s) more quickly and easily and eliminate the need to hire a design professional to develop drawings.”
Consulting a structural engineer may still be advisable, especially for houses with anything other than the basic rectangular footprint, or with large openings in the lower level, such as a garage door, double front doors or patio doors, or numerous large windows. The larger and more numerous the openings, the less the lateral resistance. Involving an engineer is also a good idea if you plan to remove interior walls to create larger rooms or an open concept layout — a popular approach to the renovation of older houses, with their numerous small rooms, but one that can reduce the overall seismic resistance of a structure.
Here’s a quick list of the six most basic seismic upgrades for wood-frame houses:
• Anchor bolts
• Other structural connectors
• Plywood shear walls
• Reinforcing or removing masonry chimneys
• Restraints on hot water tanks
And here’s a picture of the cripple walls in one corner of our house, with various structural connectors in place prior to installing the plywood shear panels.
Cripple walls with seismic upgrading
In the aftermath of the Loma Prieta earthquake, a dramatic and accidental case history emerged from Santa Cruz, a city close to the earthquake’s epicenter south of San Francisco. Architect Michael O’Hearn was in the process of seismically upgrading two identical Victorian houses built side by side. He’d finished upgrading one house, and had just begun on the second when the earthquake hit. The upgraded house was virtually undamaged, and cost only $5000 to repair. The second house “came apart in four sections,” came off its foundation, and cost $260,000 to repair. (APA Homeowner’s Guide: Earthquake Safeguards).
One thing I wonder about with Vancouver Specials is how well they’d do in an earthquake. The partially collapsed house in the picture from the Loma Prieta earthquake is very like a Vancouver Special in its design, but with the addition of the under-house double garage, a feature most Specials luckily don’t have — garages or carports are typically located beneath a rear deck — although some do.
Vancouver Specials with under-house garages
Vancouver Specials gained additional headroom on their lower levels by extending their cripple walls to full height. My lay person understanding is that the taller a cripple wall, the more vulnerable it is to the lateral forces of an earthquake, although I haven’t had that particular point confirmed by a structural engineer. A crucial additional factor is the nature of the sheathing on the exterior of a house. Traditional 1×8 shiplap fixed horizontally to cripple wall studs (see the picture of our walls above) provides very little lateral resistance. And an outer layer of stucco or wood siding adds only a negligible amount more. Contrary to some popular opinion I’ve heard, stucco won’t ‘hold everything together’. By contrast, plywood sheathing provides eight times the lateral resistance of horizontally oriented shiplap (Residential Guide to Earthquake Resistance, page 109). It would be interesting to know how 1960s and 1970s Vancouver Specials were sheathed. If it was with plywood, or shiplap oriented diagonally, which provides six times the resistance of horizontal orientation, there’s probably less reason for concern.
One final note. Vancouver homeowners interested in seismic upgrading may find local builders and general contractors reluctant to incorporate the work in a more general renovation. The reluctance could be based on a lack of familiarity with some of the techniques, and anxiety about what this unfamiliar work will do to carefully calibrated schedules, in which all elements and effort requirements are well understood based on numerous previous projects. Homeowner concerns may be pooh-poohed, with terms like ‘overkill’ being tossed out. The homeowner may be required to stand firm.
I read somewhere that competent house builders understand gravity, and the physics of vertical load-bearing, very well, but they may be less well versed with the physics of lateral forces. In the Lower Mainland, the reason for this lack of familiarity is probably that the current building code governing single family houses don’t require much in the way of seismic provisions — exterior panel sheathing and anchor bolts look to be about it, from my observations, a structural engineer’s stamp notwithstanding. I’d suggest that the building codes don’t require much because unlike Seattle, San Francisco, and Los Angeles, Vancouver has yet to experience an earthquake of any significance. From the research I’ve done over the past few years, I’d say that Vancouver lags those other cities in addressing seismic deficiencies. As one example, Seattle has almost finished seismically upgrading its schools, whereas Vancouver has only recently begun. Human beings have an unfortunate tendency to discount the seriousness of a risk if there is no antecedent event in their personal history upon which to draw. The politicians and staff at city halls around the Lower Mainland, and the citizenry they represent and serve, are perhaps a little like homeowners who’ve never experienced a serious real estate downturn or frighteningly high interest rates. Until it happens to you, you can remain in optimistic denial.
[Further resources/reading, general]
APA Homeowner’s Guide: Earthquake Safeguards. Tacoma, WA: APA, The Engineered Wood Association, 1997. (Requires free registration).
Bay Area Seismic Retrofit. Includes a short video with a good visual demonstration of cripple wall vulnerability and seismic retrofit at 4:50 of “Dangers of the Hayward Fault”.
Residential Earthquake Retrofits. Good information posted by Bay Area structural engineer, Thor Matteson. Matteson has also written a book about shear wall construction.
“Vancouver’s Real Earthquake Risk: Fire”, Canadian Underwriter, Apr, 2001. Good general article that discusses more than just post-earthquake fire risk.
“School earthquake-proofing too slow: VSB”, CBC News, 27 Jan, 2011.
“B.C. vulnerable in giant quake: experts”, CBC News, 11 Mar, 2011.
“8,000 Vancouver buildings vulnerable to quakes”, CBC News, 17 Mar, 2011.
“Quakes can rock building standards; how will Vancouver fare?”, The Vancouver Sun, 20 Mar, 2011.
[Further resources/reading, advanced]
Residential Guide to Earthquake Resistance. Ottawa: Canada Mortgage and Housing Corporation, 1998.
Homebuilders’ Guide to Earthquake-Resistant Design and Construction. Washington: Federal Emergency Management Agency, 2006. (Free download).
Standard Earthquake Home Retrofit Plan Set. Seattle Department of Planning and Development, 2008. (Free download of plan set and various retrofit guides).
Coming soon: Part 9g: So You Want to Buy a House and Fix It Up? Thirty Suggestions for Survival – Suggestions 18 – 22.
Part 9 subsections are posted every Tuesday and Friday.
Read them all before you call for the ‘dozer. -ed.