Seismic & Earthquake Issues

Seismic Retrofitting Your Home

Seismic forces on your home

Earthquake forces can affect your home’s integrity in 3 ways [3]

Background: Earthquake Risk in Utah [1, 2]

The Wasatch Fault in Utah runs 240 miles from Southern Idaho (Malad City) to Central Utah (Fayette). It is classified as a normal, vertical motion fault divided into 10 segments. This fault is responsible for the uplifting of our beautiful Wasatch Mountains. The most active segments are located between Brigham City and Nephi, which are associated with the most densely populated areas in Utah. In Salt Lake City, the Fault line runs along 1300 East.

During the past 6000 yrs, a strong earthquake (magnitude greater than 6.5) has occurred approximately once every 350 yrs somewhere along one of the central segments of the Wasatch Fault. The segments that underlie Salt Lake City and Provo produce a large earthquake on average every 1300 yrs. Experts note that the fault is overdue for another major earthquake. “[1].   The last large earthquake experienced in Salt Lake City was more than 163 years ago [2]. The last earthquake associated with this fault in Utah occurred Friday, March 14, 2014 10:03:51 AM MDT 3.5 miles south of Francis, UT UUSS Shake Maps

“Urban areas along the Wasatch Fault lie on the soft lake bed of ancient Lake Bonneville and as such could face severe damage to gas, electric, water, communication and transportation in a major earthquake” [1]. When the major earthquake strikes, at least 42% of all buildings along the Wasatch Front could sustain moderate damage. Hospitals, schools and many unreinforced masonry building (~200,000), including residential homes are particularly vulnerable to violent shaking [1].

Earthquake forces can affect the integrity of your home in three ways [3]

  • Sliding:  house slides off foundation
  • Racking:  Cripple walls buckle and collapse
  • Overturning: House lifts off foundation

What can I do to protect people and my residential home?

Be Prepared for survival [4]
Residents need a minimum of 3 days of Food and water (1 gallon water/day/person). An accessory cooking source is beneficial. Turn off the gas supply (leaking gas causes major fires). Get a dedicated wrench for this purpose and place in a location that is easily accessible. Follow Questar recommendations [5].  Timeline: High density population areas get help first. Residential areas are last.

Assess the Risk of your home [3]

Factor Low Risk High Risk
Footprint Regular, symmetric Multiple additions
Height 1 story, no basement Multiple stories
Profile Simple box Towers, gables, parapets, etc
Foundation Reinforced concrete Rubble, fieldstone
Walls frame Pre and post 1900 masonry
Condition Well maintained deteriorated

Inspection Checklist [4] “the weak links to seismic safety”

  1. Foundation   Signs of settlement of movement. Cracks, sloped floors, leaning walls.  Loose mortar on brick or stone foundations, Deteriorating wood or water damage.  Downspouts dumping water near foundation and/or ground sloping towards foundation
  2. Walls and Columns   Rotted or undersized columns attached to basement floors.  Weak or undersized, unbraced walls or columns in open basement supporting a heavy solid portion of house. Tall, unbraced masonry parapets or gables. Wood studs without structural sheathing supporting the floors or walls above. Cracking in brick walls, spalled or missing mortar between bricks. Cracked, loose, spalled or missing bricks on exterior. Header bricks absent on multiple wythe masonry walls. Large openings in exterior walls. Additions to house not securely attached to hours or pulling away from foundation. Porch columns angled, shifting or unsecure to deck or roof.
  3. Floors and Ceilings   Bridging between joists poorly secured. Floor joists are simply resting on the foundation. Joists are substantially cut away to allow ductwork, wiring, or plumbing installation. Split, twisted, rotted joists.
  4. Roof  Rafters or trusses fastened to the load bearing exterior walls.  Roof decking composed of skip sheathing (gaps between boards) instead of Oriented strand board (OSB) or continuous plywood sheets. Heavy roofing materials such as slate or tiles. Unbraced masonry chimneys, parapet, or gables that are unreinforced or not secured to the roof or ceiling structure. Deteriorated mortar between bricks on the chimney.
  5. Historic and Interior Features   Cracked plaster (more than hairline or seasonal).  Tall furnishings unsecured to the wall. Hanging- or tall-light fixtures that could swing into walls or fall if they swing. Unsecured free-standing water heater to the building structure. Gas supply pipe to water heater is rigid, not flexible. Unsecured cabinet doors that could swing open. Unsecured valuables (knickknacks, computers, TVs) on open shelves.
  6. Site and Building History   Neighboring buildings with chimneys or other site features that could damage your house if they collapse. House damage by previous earthquakes. Heavy, repeated shaking of ground by heavy equipment use. Poor house maintenance over time

Remedies

The primary purpose of earthquake retrofitting is to keep your home from being displaced from its concrete foundation — making the building safer and less prone to major structural damage during an earthquake [3].

 Create a continuous load path to maximize safety first, then maximize saving house integrity second [4-8]

  1. Anchor roof to house URM (unreinforced masonry) walls.  Attach top plates (2 x 8 boards laid horizontally on top of house walls) by installing high-tensile strength helical pins (HELIFIX is one brand) through the 2” x 8” boards into the double-wythe masonry walls. Install two pins (one pin per wythe) to a depth of at least 12 inches into the bricks. Fold over 3-4 inches of remaining pin above the top plate at a right angle and secure with an electrical staple or other anchor. Install a pair of helical pins every 16 inches around the perimeter of the house. (This is in-lieu of the ¾ inch diameter anchor bolts set in epoxy recommended in most guides).  Use OSB or plywood sheathing to tie the ends of all the roof framing members (rafters, ceiling joists, top-and bearing-plates, etc) together at the top of the wall. Use metal framing anchors such as Simpson™ hurricane uplift anchors to further connect the rafters to sheathing). This item may add 20% of roofing cost to the overall project. Hire a general contractor to do this work and coordinate with their roofing sub-contractor
  2. Anchor rafter to rim board.  Attach floor framing to the concrete foundation and double-wythe brick walls with through bolts and steel angles OR use Simpson™ FJA (Floor Joist Anchor) to connect floor joists to foundation. Use wedge anchor bolts or epoxy anchors at concrete. Use epoxy anchors only at brick, block and stone masonry. Space anchors every 4 feet maximum around footprint.
  3. Create shear walls.  Install structural rated OSB or plywood over 2” x 4” wood framing on interior walls and nail at 4“ on the edge of the panels and 6“ in the centers. Connect the shear walls to the exterior masonry walls or concrete foundation walls with anchor bolts or Simpson™ connectors. Install blocking between floor or ceiling joists at the tops of shear walls and attach the shear walls to the blocking.
  4. Anchor chimney to roof with steel brace   Install steel straps around chimneys and tie back to the roof structure with diagonal steel braces [7]
  5. Anchor/brace water heater [8]   Tether gas-fired water heaters to concrete foundation walls or interior stud walls with steel straps or proprietary braces.  Install flexible tube connector to gas line.

Timeline Approach to Retrofitting

FEMA encourages phased approach-take advantage of scheduled house repair or remodeling projects to improve seismic retrofits, one project at a time.   Substantial retrofits may require professional assistance of a licensed engineer, architect or general contractor.  Words to the wise:  Contact several firms. Evaluate their expertise with historic residential structures. Check references. Have a legal, clear contract. Get the required permits.

Earthquake Insurance

Insurance may be expensive. Retrofit your home first because it improves personal safety, then get insurance if desired. Insurance should cover the remaining mortgage on the house and allow some of the rebuilding. Can borrow additional money for rebuilding against the cost of the property lot, since it retains property value post-earthquake.

20% Utah state tax credit for rehabilitation of homes in a National Historic District

Utah state Tax Credit Program gives a 20% state tax credit of your expenses to retrofit, repair, replace infrastructure for a residential home on the National Historic Register. Yalecrest was listed on the NHR in 2007.  Obtain a State tax credit application from Utah Division of State History 

Caveats to the 20% Utah state tax credit program

  1. The building use after rehabilitation must be residential.
  2. The building must be listed on the National Register of Historic Places at the time of application or within three years of the approval of the completed rehabilitation project.
  3. A minimum of $10,000 must be spent on rehabilitation over, at most, a three-year period.
  4. All work must meet appropriate rehabilitation standards; one “wrong” modification voids the entire tax credit.
  5. This application form must be completed before the rehab project is finished—preferably before work is started in order to avoid problems with ineligible work.

For more information, contact Nelson Knight, nwknight@utah.gov, Tax Credit Program, Utah Division of State History, 300 S Rio Grande St (450 West), Salt Lake City, UT 84101, Hours of Operation: Office 8:00 AM-5:00 PM, M-F, 801.245.7244.  Website

References and Sources

Expert Speaker, Don Hartley   Has lived in Yalecrest since 1995. He isaregistered architect with extensive experience in rehabilitating historic buildings. He has been employed as an Historical Architect with Utah Division of State History, Historic Preservation Department, since 1989. He was the project architect for Salt Lake City & County Bldg restoration using the seismic base isolation technology.

Contact information: Don Hartley, dhartley@utah.gov, Historic Preservation, Utah State Division of History, 333 S. Rio Grande St (450 West), Salt Lake City, UT 84101, Hours of operation: 8:00 AM-5:00 PM M-F, Office: 801.245.7240

    1. Wasatch Fault
    2. For Information and reports about Utah’s earthquakes and their hazards from Utah Geological Society 
    3. Bracing for the Big One: Seismic Retrofit of Historical Homes Utah Division of State History
    4. Advice about earthquake preparation. “Putting Down Roots in Earthquake Country” Utah Seismic Safety Commission
    5. Building a continuous load path
    6. Structural upgrades for unreinforced masonry houses: “The Utah Guide for the Seismic Improvement of Unreinforced Masonry Dwellings” Structural Engineers Association of Utah
    7. Earthquake Preparedness and Natural Gas
    8. Retrofitting contractors  Home-Tech, Inc., 333 W Hope Ave, Salt Lake City, UT  84115
      Contact: Michael Mahaffey, T 801.484.9360, F 801.461.0110 Website

Over Four Times As Many Earthquakes Estimated in ‘15 as in ‘79 – How Can We Prepare?

Earthquakes
Earthquakes for the Homeowner – A Guide to Protecting Yourself
Earthquake Preparedness: What Every Child Care Provider Needs to Know
Homeowners’ Guide to Emergency and Disaster Safety
Disaster Recovery Center Locator
Basic Life Support and Emergency CPR

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