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Revisiting Earthquake Lessons – Masonry Chimneys and Fireplace Surrounds

Thursday, December 20, 2018  

By Brian McDonald, SE

This is one in a series of short articles revisiting lessons learned in past earthquakes. This series is being presented to share past lessons with newer members of our profession who were not there to experience them first hand. Contact Dave McCormick or Kelly Cobeen with your comments and suggestions.

Structure Component:  Masonry Chimneys and Fireplace Surrounds

Lessons: After virtually every significant earthquake, we are reminded that masonry chimneys are the most vulnerable components of conventionally-constructed wood-framed homes.  In North America, newspaper accounts from as far back as the 1660s1 document widespread earthquake damage to masonry chimneys.  Similar damage has been observed after every subsequent significant earthquake, including the 2014 South Napa Earthquake.2 Chimneys built to prescriptive code requirements will fare better than older unreinforced and unanchored chimneys, but even compliant chimneys and fireplace surrounds have been observed to collapse or suffer significant damage even in moderate shaking.  In addition, architectural masonry fireplace surrounds have proven to be earthquake falling hazards.  Modern factory-built chimneys with metal flues enclosed in light framing are least susceptible to damage from ground shaking.

The 1811-1812 New Madrid Earthquake Sequence. The New Madrid Sequence was a procession of earthquakes, M 7.3 to 8.0, whose epicenters were about 100 miles north of Memphis Tennessee, then part of the Louisiana Territory.3 Because of the geology of the central and eastern US, the affected area was large (50,000 square miles).  Masonry chimneys were a common feature of the predominantly wood-frame housing stock, and chimney collapses were reported more than 400 miles from epicenter, and chimney damage more than 6oo miles.4 Reports of chimney damage from regional earthquakes has continued in the subsequent 200 years, including fallen chimneys reported in the epicentral region (between St. Louis and Nashville) after a M5.5 event in 1968.5 

The 1866 Charleston Earthquake. The Charleston Earthquake had an estimated magnitude of M 6.9 to 7.3 and was epicentered about 10 miles north of Charleston, SC, causing more than 60 deaths.6  Like the New Madrid Sequence, the affected region was large (felt as far away as Wisconsin) and structural damage was reported hundreds of miles from the epicenter.  Around Charleston, the housing stock was predominantly wood frame with masonry chimneys.  Collapses of houses was rare (0.5% of the Charleston stock), but chimney collapses were ubiquitous (up to 90% of chimneys in Charleston).7 This pattern – extensive chimney damage concurrent with negligible structural damage to the word framing – continues to this day.  Of note, many/most of the fatalities in Charleston were attributed to masonry walls or chimneys collapsing onto residents fleeing their houses.  A large shock in Charleston in 1895 also caused chimney collapses, but no fatalities, perhaps because it occurred at 4:00 am and residents were not so quick to flee.8  

The 1969 Santa Rosa and 1971 Sylmar Earthquakes: Some of the most comprehensive surveys of earthquake damage to modern chimneys were done by Karl Steinbrugge et al after these 9 with study areas including 425 blocks in Santa Rosa and over 12,000 residential houses in the San Fernando Valley.  Steinbrugge concluded that chimneys that were well constructed and in compliance with prescriptive code provisions of the time performed much better than older, unreinforced and unanchored chimneys.  Chimneys built prior to the code prescriptions were three times more likely to suffer damage.  (Prescriptive code provisions for residential chimneys were introduced into the Uniform Building Code in 1946.)  In addition, one-story chimneys performed much better than 2-story chimneys.  Steinbrugge also reported that chimney damage could vary greatly within a short distance, and concluded chimney damage was sensitive to intensity changes due to underlying geology.  This was also concluded by Booth et al after the 2001 Nisqually Earthquake.10  

The 1994 Northridge Earthquake. The Northridge Earthquake caused extensive damage to chimneys in the San Fernando Valley.  Some estimates are as high as 60,000 chimneys destroyed.11   Fortunately, none resulted in fatalities, and only three hospitalizations were recorded to be associated with chimneys.12   However, the cost of the chimney repairs was quite high.  In an unpublished study by the author, the City of Los Angeles permitted more than 30,000 earthquake repairs for single-family dwellings that included chimney work.  The average valuation of the 29,576 permits that only included repair of earthquake-damaged chimneys was $4100, thus a total cost of over $120 million (over $200 million today) for damaged chimneys.  The total cost of Northridge Earthquake chimney repairs, including work done without a permit or repairs outside the City jurisdiction, was certainly much higher.

The 2000 Yountville and 2014 South Napa Earthquake. These Earthquakes near Napa, CA were of similar magnitude (M 5 and M 6, respectively) and separated by only 12 miles and 14 years.  In a tragic coincidence, serious injuries were suffered by a young child in each earthquake due to the collapse of a fireplace surround.13   The injuries reminded engineers that the falling hazards of masonry chimneys could extend beyond the chimney itself.

Current Status. Masonry chimneys and fireplaces are ubiquitous in the existing stock of single-family homes in the U.S..  Most recent home construction, where air quality regulations restrict construction of new fireplaces, are often built with gas-burning inserts with factory-built, UL-listed metal chimneys. While once a key component to home heating and cooking, in the last several decades, the presence of masonry chimneys are is more due to aesthetics and nostalgia despite the fact that poor earthquake performance of chimneys has been well known for as long as there have been earthquakes and chimneys. Chimney damage is so common that it is routinely used by reconnaissance teams to define isoseismal boundaries of Modified Mercalli Intensity maps.14   Chimney damage in MMI Intensity VII is binned alongside “negligible damage in buildings of good design and construction”.  Though they pose the most significant falling hazard for most homes, the author is aware of no mandatory retrofit requirements.

Since the mid-1940s, the UBC-based (and now IBC- or IRC-based) building codes in western states have contained prescriptive provisions meant to improve the earthquake performance of masonry chimneys.  (Codes remain essentially silent on masonry surrounds, other than IRC interior masonry anchorage requirements.)  Prescriptive requirements for masonry chimneys include nominal reinforcement as well as anchorage to upper-level diaphragms.  However, the expected performance of “modern” masonry chimneys constructed to those provisions, and whether they meet the collapse prevention intent of the Code, remains debated within the engineering and construction profession.  There is little doubt that chimneys built to these prescriptive requirements perform better than older, unreinforced and unanchored chimneys, but collapses have been observed in even relatively moderate shaking.  Debate continues whether such damage is primarily the result of poor (noncompliant) workmanship or whether the current code prescriptions are simply insufficient to meet the intent of modern seismic codes.  Some, including the author, are concerned that the reinforcement results in a rigid-body toppling failure mode, which may be a more dangerous falling hazard for a larger area compared to piecewise crumbling of older, unreinforced chimneys.

Engineering design or evaluation of masonry chimneys is not as straightforward as one might imagine for such a simple element.  A major source of uncertainty is the interaction between the massive, rigid chimney element (founded on a small shallow foundation in questionable soil), and the flexible, light-framed lateral system of a typical single-family residence.  Little or no research has been done on the deformation compatibility of the systems and the resulting strength and ductility requirements imposed on the diaphragm anchors.  In addition, research to better define the seismic fragility of masonry chimney is ongoing.15

The modern alternative to a masonry chimney is a prefabricated metal chimney enclosed in a light frame chase.  This type of construction has exhibited excellent performance in past earthquakes.  Lightweight veneer can give the appearance of brick masonry for the chimney or fireplace surrounds.  The City of Los Angeles has mandated that repairs to badly damaged masonry chimneys include replacement of some or all chimney masonry with factory-built metal flues in framed chases.  Other California jurisdictions have adopted these (or similar) requirements.  An advisory for owners of masonry chimneys was prepared by FEMA following the many observed chimney collapses in the 2014 South Napa Earthquake.16   More recently, FEMA and California Earthquake Authority have published retrofit guidelines for replacing masonry chimneys with factory-made metal chimneys.17   These guidelines incorporate the past lessons regarding higher risks associated with certain geometries such as large interior masonry fireplace surrounds, interior vs exterior chimneys, and one- vs two-story chimneys.

 
Figure 1. Failure of a presumably pre-1946 chimney in El Centro during the 2010 Baja California Earthquake (photo by author).


 
Figure 2. Failure of the top of a post-1946, reinforced chimney during the 2010 Baja California Earthquake The rebar shows where the chimney was prior to the earthquake (photo by author).


Footnotes:

[1] Ebel, J. E. (2011) A new analysis of the magnitude of the February 1663 earthquake at Charlevoix, Quebec, Bulletin of the Seismological Society of America, vol. 101, no. 3, pp. 1024-1038, 2011

[2] FEMA 2015, Repair of Earthquake-Damaged Masonry Fireplace Chimneys, FEMA Document HSFE60-12-D-0242, January 2015.

[3] https://en.wikipedia.org/wiki/1811%E2%80%9312_New_Madrid_earthquakes

[4] Ron Street, John D. Kiefer, and Jerry Raisor (2008) Assessing the Felt Reports of the 1811-12 New Madrid Earthquakes in the Central United States, Kentucky Geological Survey Report of Investigations, Kentucky Geological Survey, Report of Investigations 20 https://doi.org/10.13023/kgs.ri20.12 Series XII.

[5] Nuttli, O. (1982) Damaging Earthquakes of the Central Mississippi Valley, Investigations of the New Madrid, Missouri, Earthquake Region, Geological Survey Professional Paper 123 6-B.

[6] https://en.wikipedia.org/wiki/1886_Charleston_earthquake

[7] John M. Nichols and James E. Beavers (2003) Development and Calibration of an Earthquake Fatality Function, Earthquake Spectra: August 2003, Vol. 19, No. 3, pp. 605-633

[8] ibid

[9] Steinbrugge, Cloud and Scott (1970) The Santa Rosa Earthquakes of October 1, 1969, U.S. Department of Commerce, Rockville, MD and Steinbrugge, Schader, Bigglestone and Weers (1971) San Fernando Earthquake February 9, 1971, Pacific Fire Rating Bureau, San Francisco, CA.

[10] Booth, Wells and Givler (2004) Chimney Damage in the Greater Seattle Area from the Nisqually Earthquake of 28 February 2001, Bulletin of the Seismological Society of America, Vol. 94, No. 3, pp. 1143–1158, June 2004.

[11] USGS (2015) Putting Down Roots in Earthquake Country - Your Handbook for the San Francisco Bay Region, General Information Product 15, 2015, https://pubs.usgs.gov/gip/2005/15/  

[12] Peek-Asa et al, (1998) Fatal And Hospitalized Injuries Resulting from the 1994 Northridge Earthquake, International Journal of Epidemiology, Volume 27, Issue 3, 1 June 1998, Pages 459–465.

[13] https://www.sfgate.com/bayarea/article/Boy-5-Injured-In-Napa-Quake-Returns-Home-2962025.php , and http://www.latimes.com/local/lanow/la-me-ln-quake-near-napa-jolts-bay-area-20140824-story.html

[14] In older versions of the scale toppling of chimneys was included in Intensity VI, but now appears in Intensity VII, perhaps in recognition of higher construction standards.

[15] Maison B and McDonald, B, (2018) Fragility Curves for Residential Masonry Construction, Earthquake Spectra, Earthquake Engineering Research Institute, in publication,  and Krawinkler, H., Osteraas, J.D., McDonald, B.M., and Hunt, J.P., 2012. Development of Damage Fragility Functions for URM Chimneys and Parapets, Proceedings of the 15th World Conference on Earthquake Engineering, Lisbon, Portugal.

[16] https://www.fema.gov/media-library-data/1439241984631-3b4c44f900c8893449327f0e764ef849/FEMAP-1024RA1.pdf

[17] FEMA Prestandard P-1100, in publication