load rating of the bonners ferry bridge: a case study of...
TRANSCRIPT
Outline
Project location and bridge type
Post-tensioning configuration
Applicability to BrR
Structural analysis
Strength evaluation
Results
Load rating tool
Conclusions
8/9/20163
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
8/9/20167
Plan and Elevation
8/9/20168
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”
Transverse Section
8/9/201610
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
8/9/201611
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)
Transverse Tendons
8/9/201614
• 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
8/9/201615
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
8/9/201616
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
8/9/201617
Strength Evaluation
Designed with load factor design
Stress from post-tensioning after losses added to loads and com pared with resistance
8/9/201619
LRFR Load Rating
Initially followed a sim ilar approach for Strength lim it states
Low ratings in Girder D over Pier 9
8/9/201620
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
8/9/201622
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
8/9/201623
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
8/9/201624
Strain Com patibility Procedure
8/9/201625
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
8/9/201626
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
8/9/201627
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
8/9/201628
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
8/9/201629
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)
8/9/201630
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
8/9/201631
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
8/9/201632
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
8/9/201633
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
8/9/201636
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.
8/9/201637