February 05, 2015
Protecting Our Infrastructure From Hydrocarbon Fires
by Kevin Mueller
The 2013 “Report Card for America’s Infrastructure,” published by the American Society of Civil Engineers (ASCE), rates the current state of our country’s infrastructure as a D+. Anyone who went through the public education system in the United States knows that D+ is barely above failure. The estimated investment for infrastructure alone needed by the year 2020 is $3.6 trillion, just under the entire 2016 United States government budget submitted by President Obama to Congress. While the task may seem daunting considering the required investment comes out to over $11k per capita (based on the United States Census Bureau online population clock), government at all levels and the civil engineering community are working to try and reverse course. As we begin 2015, Hinman looks at America’s oil boom, a specific situation where the physical security of our infrastructure is paramount to ensuring life-safety and fixing the nation’s lack of reliable infrastructure for generations to come.
America’s oil boom has created a substantial increase in energy production since the turn of the century, with over 2 million additional barrels a day being produced. The substantial increase of petrochemical transport along the nation’s vast network of roadways has led to new concerns of accidental explosions and fires.
New bridges, both large iconic structures and small routine highway overpasses, are exposed to an extremely dangerous threat every time a tanker truck goes by. These trucks, routinely carrying over 8000 gallons of combustible fuel, are transported throughout the United States to refineries, processing plants, distribution centers, and neighborhood gas stations. A couple examples that received national attention is the MacArthur Maze Fire of 2007 and the LA 60 Freeway Fire of 2011.
- In 2007 a tanker truck fire underneath the MacArthur Maze in Oakland caused a span above the roadway to collapse, cutting off access to the Bay Bridge for thousands of people for months. The total economic impact on the bay area was estimated to be tens of millions of dollars.
- In 2011, a tanker truck fire closed the 60 freeway and the Paramount Boulevard overpass in Los Angeles during evening rush hour. The steel bridge structure did not collapse during the fire, but was severely damaged and had to be rebuilt.
While these events were nationally publicized, we found that accidental fires are fairly common in the United States. An internal study found that during just the first week of August 2014 nearly 10 tanker truck crashes and fires occurred on America’s highway system. In the state of California alone, two large tanker truck fires occurred directly underneath highway overpasses causing failure of the bridge structure, major disruption of traffic, and tremendous economic loss.
With an already failing infrastructure system and high occurrence of tanker truck fires occurring throughout the United States, what can be done to properly protect structures from failing under extreme temperatures? In traditional construction, structural elements are assigned fire resistant times to prevent the spread of fire and allow egress without compromising life safety. These fire resistant times are based on furnace tests that follow standard time-temperature profiles as found in ASTM E119 for traditional fires or UL 1709 for hydrocarbon fires. The fire resistance rating is based on the time it takes for a specific temperature rise of the structural member. Fire insulation manufacturers test their product and are given fire resistant ratings that correspond to the duration of the ASTM E119/UL 1709 curve protected. For large-scale infrastructure projects or highway overpasses, NFPA 502 does not specify the type of fire, length of exposure, or limiting temperature. Rather, the structure must maintain life safety, mitigate structural damage, prevent progressive structural collapse, and minimize economic impact.
The lack of specified fire resistance ratings for bridges and elevated highways, compared to buildings, highlights the need for a detailed analysis of what is needed to protect the structure from failure during a fire. For this reason, Hinman has developed the in-house program FLaME (Fire Loading and Mitigation Evaluator), using semi-empirical and physics-based calculation techniques to determine the structural strength reduction of bridges, elevated highways, and other exterior structures to realistic hydrocarbon fires.
The four-step methodology consists of:
1) Calculate the fire’s characteristics and geometry
2) Calculate the heat transfer from the fire to the structural elements
3) Calculate the temperature increase of the structural elements
4) Calculate the resulting material and mechanical response of the structural elements
By utilizing characteristics of the actual fire threat (i.e., fuel type, fuel quantity, mass burning rate, rate of combustion, burning footprint) a modified discretized solid flame model can be created to calculate the radiant heat-flux imposed on the structural elements. This imposed heat-flux causes the structural elements to increase in temperature, which causes a reduction in yield strength and stiffness. Using this method, the fire insulation thickness can be based on the structural performance of the members, not simply the defined temperature rise from a standard fire exposure.
Taking a performance based approach for the calculation of fire protection insulation results in either cost savings for the client (i.e., taxpayers) or a more resilient design with additional insulation beyond what the UL 1709 standard test would require. This is just one more example of how Hinman is protecting what matters, and leading the way in revitalizing our infrastructure for the next generation.