Load Rating of the Bonners Ferry Bridge:A Case Study of Evaluating Post-Tensioned Steel Girders

Presented by:Daniel Baxter, P.E., S.E.

M ichael Baker International

Outline

Project location and bridge type

Post-tensioning configuration

Applicability to BrR

Structural analysis

Strength evaluation

Results

Load rating tool

Conclusions

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Bridge Location

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Idaho

W ashington

Oregon

M ontana

W yom ing

Bridge Location

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Bridge Location

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Bridge History

Opened in 1984

Longitudinally post-tensioned steel plate girders

Transversely post-tensioned deck

20% reduction in steel

Deck and superstructure condition rating of 7

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Plan and Elevation

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119’-6” 155’-0” 119’-6” 100’-0”100’-0”155’-0” 155’-0” 155’-0”155’-0” 155’-0”

119’-6” 155’-0” 119’-6”

100’-0”100’-0”

155’-0” 155’-0” 155’-0”155’-0” 155’-0”

S. Abut.

Pier 1 Pier 2 Pier 3 Pier 4 Pier 5

Pier 6 Pier 7

Pier 8 Pier 9 N. Abut.

S. Abut.

Pier 1

Pier 2

Pier 3 Pier 4 Pier 5 Pier 6 Pier 7

Pier 8

Pier 9 N. Abut.

Span 1 Span 2 Span 3 Span 4 Span 5 Span 6 Span 7 Span 8 Span 10Span 9

Span 1Span 2 Span 3 Span 4 Span 5 Span 6 Span 7 Span 8

Span 10Span 9

1,369’-0”

Spans 8 -10

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122’-3 3/8”

78’-1 1/4”A

B

C

D

Transverse Section

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69’-11”

9’-11 1/2” 18’-0” 18’-0” 18’-0” 9’-11 1/2”

A B C D

23’-2 5/8” 23’-2 5/8” 23’-3 1/2” 8’-0 1/4”7’-9”

6’-9 1/2” web

4’-10” web

Longitudinal Girder and Deck Tendons

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TendonA and B force after long-term losses = 197 kips(5 –0.6” diam eter strands –girder tendons)

Tendon C and D force after long-term losses = 159 kips(4 –0.6” diam eter strands –deck tendons)

Longitudinal Tendon Details

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Longitudinal Tendon Details

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Transverse Tendons

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• 4 –0.6” diam eter strands in 1” x 2” plastic ducts

• 30” typical tendon spacing

Applicability to BrR

Longitudinally post-tensioned steel plate girders and post-tensioned deck slabs are not supported

Girder spacing would be okay for 3D analysis

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General Load Rating Procedure

M odel bridge using general purpose FEA software

Identify points of interest using spreadsheet post-processing

Provide required design, legal, and perm it ratings

Extract influence lines for points of interest

Develop an autom ated load rating tool that can load rate user-defined perm it vehicles

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Larsa M odel

3D grillage m odel with construction staging

Girders m odeled with single lines of beam elem ents

Flange lateral bending stresses estim ated (C6.10.1)

Live loads applied to shell elem ents

Loads m ove transversely across deck using influence surfaces

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Larsa M odel

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Strength Evaluation

Designed with load factor design

Stress from post-tensioning after losses added to loads and com pared with resistance

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LRFR Load Rating

Initially followed a sim ilar approach for Strength lim it states

Low ratings in Girder D over Pier 9

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Girder D over Pier 9

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122’-3 3/8”

78’-1 1/4”A

B

C

D

Girder D over Pier 9 –HL93 Inventory(initial assum ption of uncrackedsection) nc_top = 45.5 ksi n_com posite_top = 1.6 ksi 3n_com posite_top = 1.4 ksi pt_top = -2.3 ksi top_total= 46.1 ksi fFnt= 50 ksi D/C = 0.92

nc_bottom = -24.6 ksi n_com posite_bottom = -21.6 ksi 3n_com posite_bottom = -4.0 ksi pt_bottom = -1.9 ksi bottom _total= -52.3 ksi fFnc -1/3fl= -42.5 ksi D/C = 1.23

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However, top of slab stress is 60% above fr. Therefore, com pute stresses using cracked section properties instead.

Girder D over Pier 9 –HL93 Inventory

nc_top = 45.5 ksi cracked_top = 49.6 ksi top_total= 95.1 ksi fFnt= 50 ksi D/C = 1.90

nc_bottom = -24.6 ksi cracked_bottom = -29.6 ksi bottom _total= -54.2 ksi fFnc -1/3fl= -42.5 ksi D/C = 1.28

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Ultim ate M om ent Strength

For Girder D over Pier 9, results are very sensitive to cracked section assum ption

Using strain com patibility provides a m ore consistent approach

Strain com patibility in conjunction with m axim um factored stress in bottom com pression flange is consistent with AASHTO LRFD steel and prestressedconcrete

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Strain Com patibility Procedure

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bottom _nc

top_nc

1. Find strain in extrem e fibers due to factored noncom posite dead load applied to noncom posite girder

2. Find additional strain that when added to com pression flange will cause buckling: bottom _m ax= bottom _nc + additional

bottom _m ax= fFnc /E-1/3fl

Strain Com patibility Procedure

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bottom _m ax= fFnc /E-1/3fl

ps

cdp

3. Choose a value for fps= fpe+ fs, set internal tension equal to internal com pression and solve for neutral axis location c

4.Use cand additional= bottom _m ax–ncto find s, additional strain in prestressing steel due to com posite loads

Strain Com patibility Procedure

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bottom _m ax= fFnc /E-1/3fl

ps

cdp

5. Using s, calculate fs, the additional stress in the prestressing steel due to com posite loads

Strain Com patibility Procedure

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6. Iterate the value chosen for fpsuntil fpe+ fs= fps. Bilinear stress-strain relationship used for prestressing steel

7. Check internal force equilibrium and sum internal m om ents to find m om ent capacity.

Girder D over Pier 9 –HL93 Inventory

M u = 12,904 k-ft

M n_strain_com patibility= 11,134 k-ft

D/C = 1.16

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122’-3 3/8”

78’-1 1/4”A

B

C

D

Service II Check

LRFD Service Lim it State provisions

Stresses from post-tensioning after losses added as loads

tf< 0.95RhFyt (6.10.4.2.2-1)

1/3l + bf< 0.95RhFyc (6.10.4.2.2-1)

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Transverse Analysis

AASHTO Equivalent Strip M ethod and influence lines

Typical deck, and widened deck cross-sections

Rated for Strength I, II and Service III

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Results

Longitudinal rating, Girder D over Pier 9, governs

HL-93 Strength I Inventory Rating = 0.74

HL-93 Service II Inventory Rating = 1.21

HL-93 Strength I Inventory Rating, Transverse Analysis = 1.37

HL-93 Service III RatingTransverse Analysis = 1.47

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Autom ated Load Rating Tool

User-defined perm it vehicles can be input

Option to load only one interior lane with perm it

Deterioration from field inspections can be input

Im pact factor can be specified

Strain-com patibility iterations are autom ated

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Autom ated Load Rating Tool

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Autom ated Load Rating Tool

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Conclusions

3D analysis revealed unexpected perform ance issues at skewed support

Strain com patibility useful for obtaining additional capacity with post-tensioned steel

Longitudinal rating, Strength I over Pier 9 controls

Autom ated load rating tool will facilitate future perm it evaluations

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Thank You!

Idaho Transportation Departm ent

M ichael Baker International: M itch Cook, P.E. Joe Kauzlarich, P.E. Parker Thom son, P.E., S.E. Frank Russo, Ph.D., P.E. Krista Stippelm ans, E.I.T. Yingchao Sun, E.I.T. Teresa W asheleski, E.I.T. Em ily W inn, P.E. Alan Yates, P.E. Jenny Zhou, P.E. Kent Zinn, P.E., S.E.

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Questions?

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