undamentals of fire engineering for ridges...nek ub institute of bridge engineering 1 october 25,...

NEK UB Institute of Bridge Engineering

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October 25, 2017

FUNDAMENTALS OF FIRE ENGINEERING FOR BRIDGES

NEGAR ELHAMI-KHORASANI

ASSISTANT PROFESSOR

DEPARTMENT OF CIVIL, STRUCTURAL AND ENVIRONMENTAL ENGINEERING

UNIVERSITY AT BUFFALO

NEK UB Institute of Bridge Engineering

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Session Objectives

• Define the risk of bridge failure due to fire

• Formulate structure behavior during fire

• Quantify mechanical properties of steel and concrete at elevated temperature

• Assess extent of damage (post-fire)

• Discuss probabilistic performance-based design

NEK UB Institute of Bridge Engineering

Bridges and fire

2013: I-81/US 22/US 322 interchange in Harrisburg, PA 2009: I-75 near Hazel Park, Mich.

2007: I-80/880 Highway bridge in Oakland, CA (MacArthur Maze)

2002: I-65 overpass at the I-20/I-59/I-65 interchange in Birmingham, AL

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• Fire poses a significant and frequent hazard to our bridge infrastructure

Bridges and fire

Failure means collapse or severe damage leading to demolition

NYDOT database summary

5219

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• Majority of bridges in the country are steel or concrete beams

with a concrete deck. Primary risk of fire is vehicle crashes.

Bridges and fire

• Not much structural fire safety provision for bridges exists.

• “NFPA 502: Standard for road tunnels, bridges, and other

limited access highways” states that:

“Protection of structure – critical structural members shall be protected from collision and high-temperature exposure that

can result in dangerous weakening or complete collapse of the bridge or elevated highway.”

• No guidance is given on how to protect bridges from fire.

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Designing for fire…

• Designing for fire implies:

fire resistance > fire severity

• Fire resistance: is a measure of the ability of the structure to

resist collapse or other failure during exposure to a fire.

• Fire severity is a measure of the destructive impact of a fire,

or a measure of the forces/temperatures which could result as

a consequence of fire.

• There are three methods to compare “fire severity” with “fire

resistance.”

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Influencing factors on a bridge fire model

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Structure behavior during fire

Define fire temp-time

curve

Thermal Analysis

Structural Analysis

Fire geometry Fuel

Section geometryThermal properties

Member geometry Applied load

Mechanical properties

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Modeling the fire

• Typically petrol fire (i.e. hydrocarbon fire or liquid pool fires)

• Fast heating rates, high temperatures within the first few

minutes.

• Bridge fires are open-air (no compartment)

• Need to know:

Fuel type and quantity

Shape and size of fuel spill

Wind speed and direction

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Modeling the fire

Germany

ISO

NetherlandsFrance

EC

• Hydrocarbon for tunnels Eurocode HC Modified (France) RABT ZTV (Germany) RWS (Netherlands)Adopted by NFPA 502

• Standard fire curve (cellulosic) for buildings ISO834/ASTM E119 Eurocode parametric curve

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Modeling the fire

Germany

ISO

NetherlandsFrance

EC

• Hydrocarbon for tunnels Eurocode HC Modified (France) RABT ZTV (Germany) RWS (Netherlands)Adopted by NFPA 502

• Standard fire curve (cellulosic) for buildings ISO834 Eurocode parametric curve

For bridge fires:• Use simplified assumptions (constant maximum temperature)• Advanced computational fluid dynamics (CFD)

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Structure behavior during fire

Define fire temp-time

curve

Thermal Analysis

Structural Analysis

Fire geometry Fuel load

Section geometryThermal properties

Member geometry Applied load

Mechanical properties

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Thermal analysis

• Need to calcualte section temperatures

Input data: gas temperatures near the structure (nominal fire curve) and/or heat flux to the structure

• Heat transfer:

Boundary conditions:

o Convection (movement of fluid in gas or liquid)

o Radiation (transfer of energy)

In the solid

o Conduction

=> Thermal properties of the materials are needed (thermal conductivity, specific heat, density)

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Thermal analysis

• Temperature distribution calculated by Finite Element after 30 min of ISO fire:

Steel (1/4 of the section)

ΔT = 22 °C

ΔT = 794 °C

• Thermal diffusivity of carbon steel is “high”, but not as high as common sense may indicate. The temperature in steel section is nearly uniform because the plates are thin, not because thermal diffusivity is high.

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Structure behavior during fire

Define fire temp-time

curve

Thermal Analysis

Structural Analysis

Fire geometry Fuel load

Section geometryThermal properties

Member geometry Applied load

Mechanical properties

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Effects of fire on structures

• In the fire situation, the structure is subjected to:

Mechanical loads

Temperature elevation

• The temperature elevation has several effects:

Decrease of strength and stiffness

Thermal elongation

Spalling (concrete)

=> Mechanical properties of the materials are needed

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Structure behavior during fire

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Effects of fire on structures

• Leads to thermal expansion of the heated elements

Case of a simply supported beam without axial restraint and uniform heating

In a real structure: no freedom to elongate

L = 0

Uniformtemperature rise ΔT

P PP = E εmA= - E εTA = - E A α ΔT

L = L α ΔT

Uniformtemperature rise ΔT

L ΔL

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Steel vs. concrete?

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Strength of steel as temperature

• Note that at elevated temperature, the slope of stress-strain curve is modified compared to the shape at room temperature.

• Instead of a linear-perfectly-plastic behavior as for normal temperature, the model at elevated temperature is an elastic-elliptic-perfectly plastic model.

• Yields corresponds to 2% total strain rather than conventional 0.2%

Effects of fire on materials: steel

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Strength of steel as temperature

Effective yield strength Young’s modulus Limit of proportionality

Effects of fire on materials: steel

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Strength of steel as temperature

Effective yield strength Young’s modulus Limit of proportionality

Effects of fire on materials: steel

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Effects of fire on materials: concrete

• Explosive spalling of concrete due to the combined effect of:

Stresses due to external applied loads Stresses due to differential thermal dilatation in the section

(temperature-induced) Increase in pore pressure (linked to moisture content and

permeability)

• High strength concrete is more prone due to lower permeability

• Occurrence can be largely decreased by adding polypropylene fibers (0.05-0.1% by weight) in a concrete mix

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600°C

20°C

Strain (%)

Normalised compressive strength

cu

200°C

400°C

800°C

1 2 3 4

1.0

0.8

0.6

0.4

0.2

0

Effects of fire on materials: concrete

• Behaviour slightly different between calcareous and siliceous concrete

• Strength reduced to 50% arround 600°C

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Post-fire assessment techniques

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Post-fire assessment

• Post fire assessment and repair strategy is needed to:

Evaluate the fire induced bridge damage

Determine the serviceability of bridge following fire

Develop repair techniques

Liberty bridge fire: September 2016$80 million reconstruction of the 88 year old bridge Closure due to fire: $213,000 per day

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Post-fire assessment

• Post fire assessment and repair strategy is needed for:

Evaluate the fire induced bridge damage

Determine the serviceability of bridge following fire

Develop repair techniques.

• NCHRP 12-85 - highway bridge fire hazard assessment:

published in 2013, provides guidelines for damage assessment.

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Post-fire assessment: steel bridges

• Most significant parameter for post-fire strength evaluation:

Maximum temperature of bridge members

• Overall damage mapping:

Regions with no direct fire exposure

Regions with high fire exposure and material damage

Regions with moderate fire exposure

• Approximate temperature reached in steel:

Based on geometry (deflection, buckling, etc.)

Steel material appearance

o Extreme overheating: pitting and flaking on steel (can scratch off

the mill scale)

Integrity of connections

o Temperatures below 650 °C do not affect bolt properties.

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• Visual evaluation:

Concrete color

Color Probable maximum temperature °C

No discoloration <315

Pink 315-593

Whitish-grey >593

Buff (light tan) >928

Post-fire assessment: concrete bridges

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• Visual evaluation:

Concrete color

Spalling

o Ranges from local spalling to moderate, and explosive spalling

Loss of concrete cover to reinforcement

o Exposure of steel reinforcement to fire, significant loss of

strength

Excessive cracking

o Indicator of reduced stiffness and strength that increases

deflection

• Concrete core samples: compression and hardness tests

• Remove and replace external layers of concrete (temp > 300 °C)

Post-fire assessment: concrete bridges

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Design bridges for fire?

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Performance-based approach

Define fire temp-time

curve

Thermal Analysis

Structural Analysis

Fire geometry Fuel load

Section geometryThermal properties

Member geometry Applied load

Mechanical properties

Sources of uncertainty

• Performance-based/probabilistic approach• Evaluate susceptibility of a bridge to fire hazard • little guidance is provided in the U.S. or European standards • Guidance to reduce the fire risk• Need to quantify uncertainties

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Example of quantifying uncertainties

Yield strength of steel:

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+/- one standard deviation

Yield strength of steel:

Example of quantifying uncertainties

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Summary and Conclusions

• Fire is a low probability but high-consequence event.

• Further data has to be collected to develop probabilistic

models for load and capacity parameters.

• Need to develop guidelines for bridge fire assessment”

Pre-fire assessment: specially fire models.

Post-fire assessment: evaluation and repair strategies

• Visual evaluation: for now, most effective and reliable

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• What are the influencing factors on a bridge fire model?• Fuel type and quantity, geometry of spill, and wind

• Which one is stronger when exposed to fire, steel or concrete?• Both have some form of weakness in fire

• Should we design bridges for fire?• It depends, risk assessment strategies should be developed to

answer the above question.

Summary and Conclusions

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Thank you!

NEGAR ELHAMI-KHORASANI

ASSISTANT PROFESSOR

DEPARTMENT OF CIVIL, STRUCTURAL AND ENVIRONMENTAL ENGINEERING

UNIVERSITY AT BUFFALO

NEGARKHO@BUFFALO.EDU