Exploration of Failures of Mass Masonry Structures Subjected to Gas Line Explosions
James Palmer, M Eng, Civil Engineering (Structures); The Pennsylvania State University, 2015

Mass masonry structures are of particular concern when subjected to impulse loads, such as explosions, because of the rigidity of masonry systems and frequent lack of steel reinforcement in older masonry structures. The incidence of structural failures by gas line explosions is frequent enough to warrant significant exploration and research by the Construction, Safety, and Engineering communities, and the intent of the following article is to discuss blast response of masonry systems, including progressive collapse; causes of gas line failures, including regulatory, contractor qualification, and workplace safety; existing and proposed design guidelines for masonry structures subjected to impulse loading; and an in-depth case study at 428 Daly Street, Philadelphia, Pa.

Blast Response of Masonry Structures

Blast response of structures has been of concern to researchers since the advent of the nuclear era in the 1950s, so much so that the United States Army Corps of Engineers produces a Microsoft Excel-based Spreadsheet program called Single-degree-of-freedom Blast Effects Design Spreadsheet (SBEDS) and makes it available to structural engineers. Zapata and Weggel (2008) were able to compare the validity of SBEDS with an experiment from a controlled explosion within a two-story load-bearing mass masonry structure. They found that SBEDS was able to accurately predict the deflection of bearing walls subjected to blast but cannot predict how close to collapse a bearing wall is after loading. Using sound judgment, a structural engineer should be able to predict the total deflection of a masonry structure subjected to a given blast loading when criteria including open area strength reduction, gravity loads, aspect ratio of walls, and material constants are considered. It should be noted that the SBEDS model, although able to produce accurate post-blast analysis, does not necessarily produce good data during blast.

Analysis methods investigated by Zapata and Weggel (2008) to predict collapse include wall mid-height deflection for seismic analysis and brittle flexural response using axial load resistance, or the deflection and the resistance criteria.

The displacement criterion defines percent of collapse limit state as permanent deflection at mid-height divided by failure deflection. The resistance criterion defines percent of collapse limit state as one minus the quotient of the remaining out-of-plane resistance over the maximum out-of-plane resistance (Zapata and Weggel, 2008).

Gas Line Failures

Although generally a safe fuel source when properly installed and maintained, gas line failures have been of particular concern to earthquake-prone regions in which gas lines can cause secondary failures following seismic activity. The ASCE-25 Task Committee (2002) has outlined a number of challenges facing individuals and communities with respect to reducing gas line failures. Figure 1 shows different agencies' responsibilities regarding gas lines.

Figure 1. Agency Responsibility of Gas Lines. Adapted from ASCE-25 (2002) by author

Natural Gas: Dangers

Natural gas has a specific gravity of 0.6, meaning that it is lighter than air. After a gas leak begins, natural gas can be expected to gravitate upward. Not all gas leaks are dangerous, however, since natural gas will only combust at concentrations between 5% and 15% in air, although higher concentrations can be poisonous. Small gas leaks, though failures in themselves, which do not achieve the concentration threshold of 5% pose little explosion danger. Concentrations around 10%, however, pose the greatest potential for explosion (ASCE-25, 2002). In 2010, a 30-inch diameter gas pipeline ruptured in San Bruno, California and released 47.6 million cubic feet of natural gas and caused an explosion which resulted in the destruction of an entire neighborhood Figure 2 shows the damage caused by the explosion in which enough natural gas was leaked to supply 1200 homes for a year (Hersman, 2011).
Pipe-from-Sanbruno-explosion (1).jpg
Figure 2. San Bruno Pipeline Explosion. This file is licensed under the Creative Commons Attribution 2.0 Generic by Bryan

Natural Gas: Improvements

Improvements to the Natural Gas System can be made at both the individual and corporate levels to reduce gas line failures. At the individual residence level, excess flow valves can be installed to attenuate natural gas flow when activated by high flow rate downstream of the device caused by gas pipe rupture or leak. At the corporate level, gas delivery systems should be maintained, especially older systems where age- or outdated materials-related failures can occur. Older systems, instead of the now-industry-standard welded steel or polyethylene pipe, were often constructed of bare steel, cast iron, or copper. Although replacement of these materials is costly, it should be in the best interest of the public to ensure the safety of aging pipe lines (ASCE-25, 2002). At the site of the San Bruno failure, the cause was found to be poor pipe workmanship and upkeep of the 60-year old line. The pipe "was not seamless but had a longitudinal seam, and NTSB's Laboratory testing found differences in the type of pipe in the rupture area" (Chhatre, 2011).
During the San Bruno failure, Pacific Gas and Electric Company (PG&E) spent more than 90 minutes in shutting off the flow of gas and locating the source of the rupture, due to not having a comprehensive reaction plan in the event of a rupture; following the National Transportation Safety Board's (NTSB) investigation of that explosion, NTSB recommended PG&E implement comprehensive safety plans and emergency response criteria (Hersman, 2011).

Analysis and Design Guidelines: Masonry subjected to impulse loads

"Lessons Learned" is the most valuable positive contribution from any failure, no matter how devastating. Building failures highlight a shortcoming in the industry that is eventually fixed through adjustments in analysis, design, and oversight. Recent research has been focused on the study of explosion damage and progressive collapse of structures, especially mass masonry systems. As mentioned above, Zapata and Weggel (2008) applied the SBEDS model to masonry systems subjected to blast loading. More generally, Pape et al (2010) discuss the importance of choosing an economically appropriate yet sufficiently rigorous structural analysis and design system. For example, dynamic analysis with continuous-mass distribution is only feasible for simple structures. Due to the number of uncertainties inherent in complex structures, approximate methods of analysis become more appropriate. In these systems, every individual component of a structure is reducible to a single-degree-of-freedom system. Alternatively, finite element analysis (FEA) can be applied when analyzing structures subjected to blasts, allowing for the decoupling of the fluid dynamic analysis from the structural response (Pape, 2010).

In US building codes, the topic of progressive collapse is rather vague and often omits important considerations in the analysis and design. There are two general design approaches when designing for progressive collapse: Direct Design and Indirect Design. In Direct Design, there are two main methods, the specific local resistance method, and the alternate load path method. Indirect Design relies on minimum strength at connections, continuous reinforcement, and ductility design to provide redundancy in the event of a failure. In specific local resistance, one designs members to support a specific, abnormal loading. The goal in this method is to avoid a total collapse of the system subject to the abnormal load. In alternate load path, one member of the structure is removed (essentially allowing it to fail), and the rest of the structure is analysed for the redistributed loads (Kirkpatrick, 2009). Additional discussion of progressive collapse due to blast loading can be found here.

Although masonry systems have existed for thousands of years, methods of analysis of masonry continue to be refined. Bagi (2014) discusses methods of analysis of masonry when approached from a non-Heyman material assumption. Heyman analysis assumes masonry elements are rigid (incompressible) and have only compressive and shear resistance (lacking tensile strength). These assumptions are commonly made when designing masonry. Bagi's approach to analysis of masonry failures when collapse is the result of non-Heyman reality of material reflects the continued importance of study of this millennia-old material (Bagi, 2014).

Seismic and blast loading response, though not identical, are similar in that they are impulsive, short-time loads which cause structures to behave differently than more gradual loads. for more information on seismic rehabilitation of masonry structures, see Deteriorating Historic Italian Buildings.

Case Study

Daly St Explosion.jpg
Figure 3. Aftermath of explosion at 428 Daly St. Photograph courtesy of station WTXF Fox. Photo is 10-year, internet-only agreement. Expiration: October 20, 2024
On July 29, 2013, an explosion occurred at 428 Daly Street, Philadelphia, Pa, a row house that was under renovation. The blast from the explosion and subsequent collapse of adjoining mass masonry load bearing walls resulted in the complete collapse of 428 Daly Street, along with the partial collapse of adjoining row houses 426 and 430 Daly Street. The explosion hospitalized eight people, including contractor Steven Barrientos, who was in 428 Daly Street at the time of the explosion and who was responsible for the blast (Gambacorta, 2013). Barrientos was contracted to work on tiling in 428 Daly Street. After unsuccessfully relighting the pilot light of a basement water heater, Barrientos likely lit a cigarette that led to the blast (Warner, 2013), though a final statement on the cause of the blast was unavailable from Mayor of Philadelphia Michael Nutter's office.

The collapse, seen in Figure 3, was described by a Philadelphia Fire Chief as "the whole front [of 428 Daly St] collapsed on both sides. . . both adjoining homes are collapsing." Interior walls were also collapsed (8 hurt, 1 Critically. . ., 2013). This report shows that the collapse of the row homes was progressive, beginning at masonry walls closest to the highest pressure of gas explosion near the water heater in the basement and progressing outward as failure of bearing walls overstressed others. In total, firefighters were on Daly Street putting out fires, searching through the collapse, assisting Philadelphia Gas Works with containing the gas leak, and helping evacuate residents for more than ten hours on July 29 (Donath 2013).


While masonry has been used as a building material for quite some time, it is important that engineers and contractors realize that not everything about the material is known. Significant research in recent years has been dedicated to developing a more in-depth understanding of mass masonry response to dynamic or impulse loading like blast from a gas pipeline explosion. Engineers should account for the possibility of progressive collapse and design accordingly, and more definite requirements for progressive collapse mitigation should enter the codes. As analysis tools like the SBEDS model become more rigorous, structural design should follow suit.
Gas pipeline ruptures and leaks can be reduced through a combined effort at various government, private sector, and individual levels. Implementing excess flow valves and disaster relief plans are two ways to achieve this. It is important for technicians to also make the conscious effort to not "get comfortable. . . [and] get way too complacent about safety" (Technician Safety. . . 2000) when working on projects that appear run-of-the-mill. Curbing gas line failures will reduce secondary failures and collapses of structures.
A confluence of failures, including technician and pipe line error, and outdated design leads to failures like the one at 428 Daly Street. Only by learning from these lessons can the industry avoid future failures.


ASCE-25 Task Committee on Earthquake Safety Issues for Gas Systems. (2002). "Improving Natural Gas Safety in Earthquakes." American Society of Civil Engineers/California Seismic Safety Commission.
Technical guide presenting an overview of natural gas distribution systems and methods of reducing natural gas pipeline failures.

Bagi, K. (July 2014). “When Heyman’s Safe Theorem of Rigid Block Systems Fails: Non-Heymanian Collapse Modes of Masonry Structures.” International Journal of Solids and Structures. 15(14). 2696-2705.
A highly technical paper regarding analysis of masonry systems subject to loading or geometry which does not satisfy the Safe Theorem.

Chhatre, R. (August 30, 2011). "Opening Statement: San Bruno Accident Investigation." National Transportation Safety Board. <http://www.ntsb.gov/news/events/2011/san_bruno_ca/presentations/san_bruno_iic_opening_statement.pdf> (October 20, 2014).
Investigator-in-Charge Ravi Chhatre, PE's statement to NTSB's Board members summarizing the investigation into the San Bruno September 2010 gas line explosion.

City of Philadelphia. (July 29, 2013). “City of Philadelphia Provides Update on Rowhome Explosion.” City of Philadelphia’s News and Alerts. <https://cityofphiladelphia.wordpress.com/2013/07/30/city-of-philadelphia-provides-update-on-rowhome-explosion/> (September 29, 2014).
Press release issued by the City of Philadelphia detailing the immediate steps taken by the City following the explosion on Daly Street, including investigations by the Fire Marshall, Dept of Licenses and Inspections, and the Philadelphia Gas Works.

Donath, B. (July 29, 2014). "Report of Fire Alarm." City of Philadelphia Fire Department.
Report by first responders to Daly Street Fire Alarm. Made available by open records request to City of Philadelphia Dept of Licences and Inspections.

Gambacorta, D. (August 4, 2013). “Demo Work Halted on Daly Street Blast Site.” McClatchey – Tribune Business News. <http://search.proquest.com/docview/1417239358?pq-origsite=summon> (September 30, 2014).
Article describing the legal events in the days following the explosion on Daly Street, including an inspection and evidence collection, which caused demolition work to be suspended.

Hersman, D. (October 18, 2011). "Testimony of the Honorable Deborah AP Hersman." National Transportation Safety Board, United States Senate Hearing on Pipeline Safety: Assessing the San Bruno, California Explosion. <http://www.ntsb.gov/news/speeches/hersman/daph111018.html> (October 20, 2014).
Testimony transcription by Chairman of the NTSB before the US Senate regarding the events of the September 9, 2010 gas pipeline explosion in San Bruno, California. Includes findings of NTSB investigation and safety recommendations.

(July 29, 2013). “8 Hurt, 1 Critically, after South Philly Row Homes Collapse.” myfoxphilly.com. <http://www.myfoxphilly.com/story/22956400/building-collapse-reported-in-south-philly> (September 28, 2014).
Article including two unedited fire commissioner video interviews in which a deputy commissioner describes the progressive collapse of three buildings on Daly Street.

Kirkpatrick, S., MacNeill, R., Smith, J., Herrle, K. (April/May 2009). “Methodologies for Progressive Collapse Analysis.” American Society of Civil Engineers: Structures Congress 2009 Proceedings. <http://ascelibrary.org/doi/pdf/10.1061/41031%28341%29124> (October 3, 2014).
Technical proceeding discussion of progressive collapse analysis, including the Specific Local Resistance Method and the Alternate Load Path Method.

Pape, R., Mniszewski, K., Longinow, A., Kenner, M. (May 2010). “Explosion Phenomena and Effects of Explosions on Structures, III: Methods of Analysis (Explosion Damage to Structures) and Example Cases.” American Society of Civil Engineers: Practice Periodical on Structural Design and Construction. <http://ascelibrary.org/doi/pdf/10.1061/%28ASCE%29SC.1943-5576.0000040> (October 3, 2014).
Technical discussion of structural response from air blast of an explosion and methods of analysis, including example case studies.

(September 12, 2000). “Technician Safety: Learn from the Mistakes of Others.” The Air Conditioning Heating Refrigeration News. <http://www.achrnews.com/articles/technician-safety-learn-from-the-mistakes-of-others> (September 29, 2014).
A trade article featuring case studies and lessons learned of HVAC technicians who made unsafe choices on the jobsite, resulting in dangerous circumstances.

Warner, B., Schleifer, T., and Moran, R. (August 1, 2013). “Nutter: House that Exploded had Passed Inspections.” The Inquirer. <http://articles.philly.com/2013-08-01/news/40918999_1_natural-gas-explosion-mayor-nutter-electric-service> (September 29, 2014).
An article discussing Philadelphia Mayor Nutter’s news briefing following the explosion on Daly Street. Nutter identifies Steven Barrientos as the contractor in 428 Daly St at the time of the explosion.

Zapata, B., and Weggel, D. (April 2008). “Collapse Study of an Unreinforced Masonry Bearing Wall Building Subjected to Internal Blast Loading.” American Society of Civil Engineers: Journal of Performance of Constructed Facilites. <http://ascelibrary.org/doi/pdf/10.1061/%28ASCE%290887-3828%282008%2922%3A2%2892%29> (September 29, 2014).
Technical discussion of experiments performed on an existing masonry bearing wall structure subject to blast loading remarkably similar to the Daly Street row houses. Paper includes discussion of the collapse limit state of masonry bearing wall structures.

Additional Reference

"SBEDS Version 4.2: Single-degree-of-freedom Blast Effects Design Spreadsheet." United States Army Corps of Engineers Protective Design Center. <