Post-fire Flexural Testing and Rating of Prestressed Concrete … · 2020-04-02 · Post-fire...
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Post-fire Flexural Testing and Rating of
Prestressed Concrete Bridge Girders
Eyosias Beneberu, Ph.D., P.E. Nur Yazdani, Ph.D., P.E.
Structural Engineer, Bridgefarmer & Associates Professor of Civil Engineering, UT Arlington
ACI CONVENTION, 2019CINCINNATI, OHIO
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Background➢Bridge fires are caused by crashing of fuel tanker trucks, wildfire
and arson.
➢Survey of DOTs showed that fire caused more bridge collapse
than earthquake.
➢Fire hazards on bridges is given less attention, even though it
can cause significant economic and public impact.
➢ I-85 Bridge collapse in Atlanta caused 20% increase in the unit
cost/mile for shipping of items, with rebuilding cost of $16.6 M.
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Objectives
➢Evaluate the in-fire and post-fire performance of a
typical full-scale one-span I-girder bridge under open
fuel fire.
➢Determine the post-fire residual strength and rating of
precast prestressed I-girders.
➢Performance of CFRP laminate strengthening and
fireproofing in fire.
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Test Bridge: Description➢ 33 ft. (10.1 m) long and 18 ft. (5.5 m) wide.
Girder -1 Girder -2 Girder -3
40 mm of
fireproofing
S e c tio n B -B
U -w ra p
L o n g itu d in a l F R P
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Test Bridge: Instrumentation
▪36 Type-K Inconel sheathed thermocouples installed.
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Test Bridge: Construction
➢ 32 days to construct, test and demolish.
➢ Zipper barriers used to simulate vehicular live load.
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➢ Two firefighter trucks and nine firefighters were at the
scene.
➢A minimal windy day selected.
➢ Test was conducted for an hour, burning 1140 gallons of
fuel.
Fire Test
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Fire Test
➢ Fire temperature reached as high as 1131oC.
➢ Temperature difference of up to 640°C was observed
within a one-minute interval.
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Girder-1 (with CFRP, no fireproofing)➢ Glass transition temperature,Tg, of the epoxy (60oC) was
exceeded 41 and 168 secs after the test began.
➢ CFRP fully debonded 6 minutes into the test.
➢ Temperature in the strands reached as high as 473oC; strands
lose 70% of their strength at this temperature.
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Exposed strand
Broken strands
Girder-1 (with CFRP, no fireproofing)
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➢ Fireproofing kept the temperature at the CFRP-concrete interface below glass transition temperature.
➢ Maximum temperature in the prestressing strands was 48°C; strands retain 100% of their strength at this temperature.
Girder-2 (with CFRP, fireproofing)
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➢ Had least fire exposure because of the prevailing
wind...it sustained minimal damage.
Girder-3 (no CFRP or fireproofing)
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Residual Strength Test
LVDT
Load cell
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➢ Failed at a load of 90 kip (399 kN), corresponding to a bending
moment of 707 kip-ft (958 KN-m).
➢ The undamaged nominal flexural capacity was 1780 kip-ft
(2348 kN-m).
➢ Fire caused a 59% reduction in flexural strength.
Girder-1 (with CFRP, no fireproofing)
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Girder-2 (with CFRP or fireproofing)
➢ Failed at a load of 224 kip (995 kN), corresponding to a
bending moment of 1761 kip-ft (2388 kN-m).
➢ Calculated flexural capacity was 1780 kip-ft (2348 kN-m), only
1.7% more than the failure moment.
➢ Fireproofing was successful in protecting the CFRP, concrete,
strands and mild reinforcement.
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Girder-3 (no CFRP or fireproofing)
➢ Failed at a load of 181 kip (805 kN), corresponding to
a bending moment of 1424 kip-ft (1930 kN-m).
➢ Calculated flexural capacity was 1384 kip-ft (1876 kN-
m), 2.8% less than the failure moment.
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Load Rating of Girders, Strength I
Girder φC φs φ Mn
(kip.ft)ϒDC MDC
(kip.ft)ϒDW MDW
(kip.ft)ϒL MLL+IM
(kip.ftRF
1 0.85 0.85 1.0 707 1.25 105 1.50 0 1.75 275 0.79
2 0.95 0.85 1.0 1761 1.25 124 1.50 0 1.75 275 2.63
3 0.95 0.85 1.0 1424 1.25 124 1.50 0 1.75 275 2.07
Inventory Level
Girder φC φs φ Mn
(kip.ft)ϒDC MDC
(kip.ft)ϒDW MDW
(kip.ft)ϒL MLL+IM
(kip.ftRF
1 0.85 0.85 1.0 707 1.25 105 1.50 0 1.35 275 1.02
2 0.95 0.85 1.0 1761 1.25 124 1.50 0 1.35 275 3.41
3 0.95 0.85 1.0 1424 1.25 124 1.50 0 1.35 275 2.68
Operating Level
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➢ The pioneering study helped in understanding the response of
concrete bridges to open hydrocarbon fires. May eventually
lead to the development of bridge fire safety provisions.
➢ The rating factor for Girder 1 and test results clearly show the
adverse effect of fire on the design load carrying capacity.
➢ From the performance of the girder without fireproofing, it is
apparent that the nation’s concrete bridges could be at a high
risk of failure due to extreme hydrocarbon fire events.
➢ Precast bridge girders can retain their flexural capacities and
integrity, including CFRP, concrete and the prestressing
strands, with proper fireproofing application.
Conclusions
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Acknowledgment
The study was performed at UT Arlington under
a contract from the Texas Department of
Transportation (TxDOT).
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Questions?
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Thank You!