Seismic Retrofitting your home Summary by Lynn Kennard Pershing
March 25, 2014 7-8:30 PM AndersonFoothill Library
Speaker: Don Hartley, Utah State Historical Society
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 at
quake.utah.edu/shake/60059347/intensity.html).
“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]
What can I do to protect people and my residential home?
1) 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.
2) Assess the Risk of your home [3]
Factor
Footprint
Height
Profile
Foundation
Walls
Condition
Low Risk
Regular, symmetric
1 story, no basement
Simple box
Reinforced concrete
frame
Well maintained
High Risk
Multiple additions
Multiple stories
Towers, gables, parapets, etc
Rubble, fieldstone
Pre and post 1900 masonry
deteriorated
3) Inspection Checklist [4] “the weak links to seismic safety”
a. 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
b. 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
c. 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.
d. 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
e. 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
f. 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
4) 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]
a. 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
b. 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.
c.
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
d. Anchor chimney to roof with steel brace
Install steel straps around chimneys and tie back to the roof structure with diagonal
steel braces [7]
e. Anchor/brace water heater [8]
Tether gas-fired water heaters to concrete foundation walls or interior stud walls
with steel straps or proprietary braces. See Earthquake Preparedness and Natural
Gas at http://www.questargas.com/brochures/59503.pdf. Install flexible tube
connector to gas line
5. 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.
6. Earthquake Insurance
May be expensive. Retrofitting 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.
7. 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
http://heritage.utah.gov/history/state-tax-credit/attachment/historic-preservationtax-credit-application-april-2013
Caveats
a. The building use after rehabilitation must be residential.
b. 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.
c. A minimum of $10,000 must be spent on rehabilitation over, at most, a three-year
period.
d. All work must meet appropriate rehabilitation standards; one “wrong” modification
voids the entire tax credit.
e. 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.
Contact for more information: 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
http://heritage.utah.gov/history/federal-tax-credit,
References and Sources
Expert Speaker, Don Hartley Has lived in Yalecrest since 1995. He is a registered
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. en.wikipedia.org/wiki/Wasatch_fault
2. For Information and reports about Utah’s earthquakes and their hazards from Utah
Geological Society Web site: http://geology.utah.gov/utahgeo/hazards/eqfault/index.html
3. Earthquake Safety at http://www.earthquakesafety.com/earthquake-retrofitting.html
4. Bracing for the Big One: Seismic Retrofit of Historical Homes. By Utah Division of State
History at http://heritage.utah.gov/hisotory/earthquake
5. Advice about earthquake preparation. “Putting Down Roots in Earthquake Country” by
Utah Seismic Safety Commission at http://ussc.utah.gov/putting_down_roots.html
6. “Building a continuous load path” www.safesstronghome.com/protect/02.asp
7. Structural upgrades for unreinforced masonry houses: “The Utah Guide for the Seismic
Improvement of Unreinforced Masonry Dwellings” by Structural Engineers Association of
Utah at www.seau.org
8. Earthquake Preparedness and Natural Gas at
http://www.questargas.com/brochures/59503.pdf.
9. Retrofitting contractors
Home-Tech, Inc., 333 W Hope Ave, Salt Lake City, UT. 84115
Contact: Michael Mahffey
T 801.484.9360
F 801.461.0110
http://www.hometechincorporated.com