Designing Steel Bridges against Fire Hazard- Juniper Publishers
Juniper Publishers- Journal of Civil Engineering
Abstract
Bridge fires can lead to significant economic and
public losses; however, no provisions are specified for fire resistance
of bridge structural members in current codes and standards. This paper
presents an approach for mitigating fire hazard in steel bridges. The
approach comprises of two steps, estimating fire risk in a bridge, and
then developing strategies for enhancing fire resistance of a critical
bridge. An analytical procedure is employed to derive a fire-based
importance factor to estimate the vulnerability of a bridge to fire, and
then a sequential finite element analysis is carried out to develop
strategies for mitigating fire hazard in that bridge. The validity and
applicability of this approach is demonstrated through a case study on a
steel bridge that experienced fire recently.
Keywords: Fire hazard; Bridges; Fire; Mitigation strategies; Finite element analysis Introduction
Fire represents a significant hazard to built
infrastructure and can induce significant damage during the service life
of a structure [1].
Thu fire hazard in buildings is minimized through provisions of active
and fire protection systems, as prescribed in codes and standards. These
provisions may not be applicable for bridges due to major differences
in key factors such as fire severity, member characteristics and design
objectives. Unfortunately, there are no specific requirements in codes
and standards for designing bridges to withstand fire hazard. This is
due to the common assumptions that probability of fire occurring on a
bridge is rare and hence it is not practical to design all bridges for
fire hazard. Further, only few of these bridge fires grow into larger
size fires that can affect the structural members of a bridge. Still,
bridge fires are a growing concern due to rapid urbanization and
increased transportation of flammable materials. Such bridge fires are
characterized by high intensity burning with peak temperatures reaching
as high as 1100 °C within few minutes of ignition [2].
Such intense fires are referred to as hydrocarbon
fires and can cause significant structural damage, as well as large
economic and public losses specifically to steel bridges [2,3].
In current practice, steel is widely used in bridge construction due to
its high strength and ductility properties, ease of installation and
cost considerations. However, due to its high thermal conductivity, low
specific heat, and lower sectional (and thermal) mass of steel,
temperature rises rapidly in fire- exposed steel members. Since strength
and modulus properties of steel are highly sensitive to
elevated-temperatures, rapid rise in steel temperature causes fast
degradation in strength and modulus properties of steel. Hence, steel
structural members exhibit lower fire resistance than concrete members
which experience slower rise in cross-sectional temperature (due to its
low thermal conductivity and high specific heat) as well as slower loss
of strength and modulus properties of concrete with elevated
temperatures. Thus, such damage is amplified in steel girders, more so
than that in concrete piers or abutments [2,3].
In fact, recent fire incidents have shown that bridge
steel girders can fail and collapse in 20-30min of fire exposure which
leaves little room for firefighting activities given that average
response time of firefighters is 15-20 minutes [4-7].
It can be inferred from above discussion that there is a need to
develop practical approaches for evaluating vulnerability of critical
“steel” bridges to fire hazard. This paper presents a novel approach
that can be used for mitigating fire hazards in bridges.
Proposed Approach for Mitigating Fire Hazard in Bridges
The proposed approach for mitigating fire hazard in a
bridge comprises of two main steps. In the first step, the risk of fire
hazard to a given bridge is quantified analytically through the
application of a fire-based importance factor derived in an earlier
study [2].
If the fire risk for the bridge under consideration is high, then
relevant strategies for mitigating fire hazard in that bridge is
developed. In the second step, structural members of the selected bridge
are analyzed through a nonlinear finite element analysis to evaluate
inherent fire resistance of structural system. If the structural members
do not have sufficient fire resistance then relevant strategies to
enhance fire resistance are developed through series of analysis. These
steps along with number of sub-steps are illustrated through a flow
chart in Figure 1.
Identifying critical bridges
As part of the first step, the magnitude of fire risk
to a bridge is to be quantified. This can be done by calculating a fire
based importance factor for a given bridge [2,3].
Relevant data on characteristics of the selected bridge in terms of
degree of vulnerability of a bridge to fire, the critical nature of a
bridge from the point of traffic functionality, and fire mitigation
strategies adopted for that bridge is to be collected and analyzed. For
instance, such characteristics may include material type, structural
configuration etc. Each of these characteristics is assigned an
ascending numerical (from 1 to 5) where the largest value indicates the
highest risk to fire hazard. These weight age factors, which indicate
significance of an influencing factor to fire performance of a bridge,
are assigned based on engineering judgment and recommendations in
previous studies. Using this data, a fire based importance factor for
selected bridge is determined through an approach recently proposed in
literature [2].
Based on the value of this fire-based importance factor, the fire risk
associated with bridges is grouped under four risk grades. These risk
grades are defined as low, medium, high and critical and are listed in "Table 1. In general, bridges that fall under "high" or "critical" risk grade often have fire resistance of much less than 60min [2,3].
Thus, suitable strategies are to be developed to enhance the fire risk
of these bridges to "low" or "medium" risk grade. This can be done
through developing relevant strategies to enhance fire resistance (FR)
of main structural members in the bridge.
Developing strategies through finite element simulation
Kodur et al. have shown that one practical strategy
is the application of fire protection to main structural members through
conducting finite element simulation [5].
This applied fire protection is provided to enhance fire resistance to
main structural members (to 60-90min) since previous bridge fire
incidents have shown that bridge steel girders can fail and collapse in
20-30min of fire exposure [4-7].
Thus, enhancing the fire resistance of structural members through fire
insulation can significantly lower the risk of collapse/damage to
bridge. The viability of fire protection is then verified through finite
element simulation. If fire resistance of insulated structural members
is higher than required fire resistance (of 60 to 90 minutes), then no
additional measures may be needed to enhance fire safety of such a
bridge. However, if fire resistance of structural members is less than
the required fire resistance, then the use of suitable fire mitigation
strategies is needed.
Case Study
The applicability of the above discussed approach to a
practical situation is illustrated by selecting a bridge that
experienced fire damage due to fire hazard.
Selection of a bridge and description of bridge fire incident
I-65 interchange in Birmingham, Alabama, USA is
selected for this case study. This bridge is comprised of steel girders
spanning 36.6 m on top of reinforced concrete piers. This bridge caught
fire on January 5, 2002 when a fuel tanker carrying 37,000 liters of
gasoline overturned near the bridge. The fire resulted in an intense
heat producing temperatures in the range of 1100 °C. This rapid rise in
steel temperature degraded strength and stiffness properties of steel
girders causing the girders to sag about 3m [7]. After this fire incident, the bridge had to be shut down and commuters were detoured to alternative highway routes.
Evaluating fire risk importance factor and inherent fire resistance of steel members
The above approach is applied to evaluate
vulnerability of this bridge to fire. Following the outlined procedure
proposed by Kodur & Naser [4],
the fire-based importance factor for this bridge works out to be 1.2,
which places the bridge under "high" risk grade. In order to evaluate
the fire resistance of I-65 steel bridge girder, a nonlinear
thermo-mechanical finite element model was developed to evaluate the
fire resistance of steel bridge girder under hydrocarbon-based fire
exposure (Figure 2(a))[5]. It can be seen from (Figures 2b & c)
that the uninsulated girder experiences rapid rise in sectional
temperature which leads to significant deflection at the early stage of
fire and premature failure within 11min of fire.
Developing strategies to enhancing fire resistance
Since the uninsulated steel girder fails early into
fire exposure, the fire performance of this girder is deemed poor and
can lead to substantial damage of this bridge in the event of fire (as
reported in actual fire incident [7]). The strategy to overcome such damage scenario in the event of fires is to enhance fire resistance of steel girders [5].
This can be achieved through provision of appropriate fire protection
to girders. In order to arrive at optimum insulation scheme (thickness),
a series of finite element simulations are carried out. Outcome of this
analysis indicate that using16mm thick insulation can enhance fire
resistance of steel girder to 75min (Figure 2(b)).
As discussed above, 60-90 minutes of fire resistance can significantly
lower the risk of collapse to bridge since steel girders can fail within
20-30 of fire.
Re-assessing fire risk based importance factor
To take the application of fire protection into
account, the fire-based importance factor is re-evaluated and was found
to be 1.0. Thus, this bridge falls under "medium" fire risk category to
be less susceptible to fire damage/collapse and no additional measures
may be needed to enhance fire safety of this bridge.
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