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moving displacement based seismic design
- 1. MOVING TO DISPLACEMENT BASED SEISMIC DESIGN Johann Aakre, PE, SE Michael Baker International Chicago Bridge Department Manager ArDOT TRC May 25th, 2022
- 2. INTRODUCTION & LEARING OBJECTIVES FORCE BASED VS. DISPLACEMENT BASED DESIGN IDOT DBD POLICY TRANSITION IDOT DBD EVALUATION FINDINGS APPLICATION OF DISPLACEMENT BASED DESIGN PRINCIPLES Learning Objectives
- 3. FORCE BASED VS. DISPLACEMENT BASED DESIGN Force-Based Design • Period of structure, seismic acceleration based on elastic behavior • Design moments modified by Response Modification Factors (R-Factors) to indirectly account for damage • R-Factors based on broad parameters such as “multiple column bent” or “Wall-type pier” that do not account for sizes, reinforcement ratios, etc. • End result: Final design is very rough estimate (Picture Credit (BergerABAM/Lee Marsh) – 2017 COBS presentation)
- 4. FORCE BASED VS. DISPLACEMENT BASED DESIGN Displacement Based Design • Period of structure and seismic acceleration based on inelastic behavior • No R-Factors required • More exact estimation of seismic loads and more accurate design • Accurate approximation of damage allows for performance-based design (Picture Credit (BergerABAM/Lee Marsh) – 2017 COBS presentation)
- 5. FORCE BASED VS. DISPLACEMENT BASED DESIGN Performanced-Based Design • Bridge Performance Level based on Operational Classification • Start with desired performance and work design to deliver that. • Ability to achieve additional or enhanced criteria and better control of outcomes.
- 6. WHY????
- 7. ARKANSAS SEISMIC HAZARD M7.5 in 1811 (New Madrid) M6.0 in 1843 in Marked Tree M5.0 in 1976 – Poinsett County 34 of 75 Counties Included with NMSZ Catastrophic Planning Area
- 8. FORCE BASED VS. DISPLACEMENT BASED DESIGN Current Design Paradigm • All western states use DBD • All states in New Madrid Seismic Zone except IL, KY, and AR use DBD • Arkansas and Illinois currently switching to DBD.
- 9. FORCE BASED VS. DISPLACEMENT BASED DESIGN More Accurate and Economical • In FBD, wall-type piers have R-factors of 1.5 or 2.0 • Actual reduction in stiffness could be more like factor of 4 • This results in much lower stiffness, higher periods, lower loads • End result: smaller members, less piles, smaller footings
- 10. FORCE BASED VS. DISPLACEMENT BASED DESIGN Allows for use of Future Guide Specifications • Guidelines for Performance-Based Seismic Design • Specifications for Seismic Isolation Design
- 11. IDOT’S SHIFT TO DISPLACEMENT BASED DESIGN STEP 1: EVALUATE EXISTING BRIDGE DESIGN / DETAILS STEP 2: DEVELOP UPDATED POLICY AND GUIDELINES
- 12. IDOT DBD – STEP 1 EVALUATION BRIDGES Br 1 SN 016-1510 Br 2 SN 051-0075 Br 3 SN 014-0080 Br 4 SN 083-0067 Br 5 SN 080-0025 Total Length 256’ 858’ 617’-unit 1 449’- unit 2 198’ 712’ # Spans 3 10 8 3 5 Abut. Type Semi-integral on Steel H piles Stub on Steel H piles Stub on drilled shafts Integral on Steel H piles Stub on Steel H piles Pier Type 4 column with crashwall on pile cap foundation Pier Wall on pile cap foundation & Pile Bent Wall Piers Drilled shaft Bents with web walls Pile Bent Wall Piers Pier Walls on pile cap foundation Skew 9°35’33” 0 30° 0 0 SDC A ->C C B B B
- 13. EVALUATION REFERENCES BRIDGES DESIGNED TO: • 2012 IDOT Bridge Manual – Section 3.15 • 2011 – AASHTO Guide Specifications for LRFD Seismic Bridge Design • AASHTO LRFD Bridge Design Specifications • IDOT Design Guide 3.15 (May 2008) OTHER REFERENCES • DRAFT – IDOT Seismic Design Guides (March 2017) • Caltrans SDC – 2.0 • NHI 130093/130093A – LRFD Seismic Analysis & Design of Bridges Reference Manual
- 14. EVALUATION METHODOLOGY • Use the same Seismic Hazard (Design Response Spectrum) from Original Bridges Design • Life safety for the design earthquake event, 7% probability of exceedance in 75 years • Low probability of collapse but may suffer significant damage and that significant disruption to service is possible. Partial or complete replacement may be required. • Use IDOT 3-Tier Seismic ERS • Level 1 – Fusing Connections between Superstructure & Substructure • Level 2 – Adequate Support Lengths • Level 3 – Ductile or Elastic Substructure Response assuming anchor rods DON’T fuse • AASHTO Guide Specifications for LRFD Seismic Bridge Design, 2nd Edition with current interims (AASHTO Seismic) • Equivalent static analysis (ESA)- single mode spectral or uniform load analysis • Liquefaction/Lateral Spreading not considered • Simplified Methods for Soil-Structure Interaction
- 16. DISPLACEMENT BASED DESIGN FLOWCHART Special Steps for SDC C Unique to displacement based Common Steps in FBD and DBD 1. Seismic Proportioning 2. Effective Section Properties 3. Abutment Modeling 4. Foundation Modeling 5. Displacement Capacity
- 17. 1 - SEISMIC DESIGN PROPORTIONING
- 18. 2 - EFFECTIVE SECTION PROPERTIES Moment Curvature Analysis Caltrans eq. (SDC 2.0 Section C3.4.2) AASHTO Seismic Guide Figure 5.6.2.1 Ast (in2) Ag (in2) Ast / Ag per AASHTO figure, Ieff/Ig M-Phi, Ieff/Ig Pier 1 49.3 11259.6 0.004 N/A 0.13 Pier 2 142.2 11259.6 0.013 0.36 0.24 Pier 3 251.5 11259.6 0.022 0.48 0.33 Pier 4 49.3 11259.6 0.004 N/A 0.13 Wall Pier Example (Bridge 5)
- 19. 3 - ABUTMENT STIFFNESS LONGITUDINAL STIFFNESS TRANSVERSE STIFFNESS (Caltrans SDC)
- 20. 4 - FOUNDATION MODELING S. Abut Pier 1 Pier 2 N. Abut Case 1 T (sec) 0.578 Csm 0.570 No Pile Flexibility @ Piers Single Mode Spectral Disp (in) 3.570 0.034 0.008 1.743 Case 2 T (sec) 0.659 Csm 0.500 Pile Flexibility @ Piers Single Mode Spectral Disp (in) 3.38 1.95 1.45 1.62
- 21. 5 - DISPLACEMENT DEMAND VS. CAPACITY Concrete Cover Spalling Column Ductility = 3 DISPLACEMENT CAPACITY -2 0 2 4 6 8 10 12 0 10 20 30 40 Δc (in) Ho (ft) Δc (per Eq 4.8.1-2) Formula 0.12Ho
- 22. BRIDGE 1: SN 016-1510 LOCATION & FEATURES
- 23. BRIDGE 1: SN 016-1510 LOCATION & FEATURES DESIGN CRITERIA • SDC = C • Multi-Column Pier • Fixed or Elastomeric Bearings • Crashwall • Pile Cap Foundation • Battered Piles • Semi-Integral Abutment / No Expansion Joint • Elastomeric Bearings • Straight & Battered Piles
- 24. BRIDGE 1 EVALUATION RESULTS AASHTO Seismic 4.8.1 PL PT Capacity AASHTO Δc (in) 7.74 0.71 Pushover Δc (in) 7.5 2.98 Demand DL (in) 0.85 0.17 Longitudinal Transverse
- 25. BRIDGE 1 EVALUATION RESULTS
- 26. BRIDGE 4 LOCATION & FEATURES DESIGN CRITERIA • SDC = B
- 27. BRIDGE 4 ANALYSIS & RESULTS ABUTMENT STIFFNESS PIER STIFFNESS Abutments Pier 1 Pier 2 Abutment soil Longitudinal (kip/in) 317.1 20.0 17.1 399.4 Transverse (kip/in) 475.5 251.0 236.6 0 Abutment 1 Pier 1 Pier 2 Abutment 2 Longitudinal (in) 1.25 1.25 1.25 1.25 Transverse (in) 0.83 1.28 1.30 0.84 Longitudinal Direction Transverse Direction Ts = SD1/SDS 1.25Ts 0.43 s 0.41 s 0.363 s 0.454 s
- 28. BRIDGE 4 EVALUATION RESULTS FIX-FIX ABUTMENT PILE DISPLACEMENT CAPACITY FIX-FREE LONGITUDINAL PIER PILE DISPLACEMENT CAPACITY ∆𝑐 = ∆𝑦 = 𝑉 𝑦 𝐾𝑝𝑖𝑙𝑒 𝑉 𝑦 = 𝑀𝑦/(𝐿𝑚/2) 𝑀𝑦 = 𝐹𝑦𝑆 𝐾𝑝𝑖𝑙𝑒 = 12𝐸𝐼/ 𝐿𝑠 3 𝐿𝑚= 0.314 𝐿𝑠 ∆𝑐 = ∆𝑦 = 𝑉 𝑦 𝐾𝑝𝑖𝑒𝑟_𝐿 𝑉 𝑦 = 𝑛𝑀𝑦/(ℎ1 + 𝐿𝑚) 𝑀𝑦 = 𝐹𝑦𝑆 𝐿𝑚= 0.314 𝐿𝑠 • Fix-fix: - Abutment Transverse - Abutment Longitudinal - Pier Transverse • Fix-free: - Pier Longitudinal 𝐾𝑝𝑖𝑒𝑟_𝐿 𝑚𝑜𝑚𝑒𝑛𝑡 𝑎𝑟𝑒𝑎 𝑚𝑒𝑡ℎ𝑜𝑑
- 29. BRIDGE 4 EVALUATION RESULTS PILE SLENDERNESS & DUCTILITY DEMAND Limit for Essentially Elastic Member Limit for Ductile Member Steel HP 10x42 12.02<12.86 12.02>10.33 Steel HP 12x53 13.79>12.86 13.79>10.33 Steel HP 12x63 11.75<12.86 11.75>10.33 Steel HP 14X73 14.46>12.86 14.46>10.33 Steel HP 14x89 11.95>12.86 11.95>10.33
- 30. BRIDGE 2 LOCATION & FEATURES DESIGN CRITERIA • SDC = C Stub Abutment Pile Bent Wall PierWall Pier on Pile Cap
- 31. BRIDGE 2 ANALYSIS RESULTS AASHTO Seismic IDOT BM
- 32. IDOT SEISMIC DESIGN POLICY NEXT STEPS IDOT’s proposed 4 Step Policy Update Process 1. Seismic Details 2. Design Guidelines 3. Seismic Hazard 4. Performance Based Design Guidance Research Refinement Policy Test Designs Future Research... Expansion
- 33. SEISMIC DESIGN POLICY NEXT STEPS National AASHTO Policy Updates 1. Seismic Hazard NEW MAPS Balloted this year. 2. Displacement Based Design Still Discussion, but trend will be adoption 3. Performance Based Design Guidelines not Specifications
- 34. SEISMIC DESIGN POLICY NEXT STEPS – HAZARD UPDATES National AASHTO Policy Updates This software is preliminary or provisional and is subject to revision
- 35. SEISMIC DESIGN POLICY NEXT STEPS – HAZARD UPDATES (HOT SPRINGS, AR) https://earthquake.usgs.gov/ws/designmaps/aashto- 2009.json?latitude=34.5095&longitude=- 93.0518&siteClass=D&title=Example https://staging- earthquake.usgs.gov/ws/nshmp/designmaps/aash to-2023-conus/ 2009 HAZARD 2023 HAZARD Parameters: Longitude: 34.5095 | Latitude: -93.0518 | Site Class D (Vs30 = 900 ft/s ~ 275 m/s)
- 36. CONCLUSIONS DBD Conclusions • Prior Force base designs should not “Fail” under displacement based design. • Displacement Based Design can be more Accurate and Economical • DBD provisions have additional detailing requirements that should be followed
- 37. DISPLACEMENT BASED SEISMIC DESIGN QUESTIONS? ACKNOWLEDGEMENT
Editor's Notes
- Thank you Intro Name = Johann Aakre, Bridge Department Manager Before presentation – Thank you to ArDOT. You’re staff are so welcoming to out of town presenters. I definitely felt that southern hospitality.
- Read Slide Titles FBD vs. DBD DBD principles Get into the work that MBI has been working on with IDOT on using DBD
- FBD Stiffness and Mass of Structure Find our period Find Csm Apply Seismic Loading Solve for forces (moments and shears) Design elements with EE load combo with “Reduced” moments based on an “Assumed” level of ductility R-factors. Works well, but we don’t know how much the structure is displacing and how much damage.
- DBD -Period is more in tune with the inelastic stiffness of the system. R factors aren’t used. Compare elastically analyzed displacement with displacement capacity based on a tolerable level of non-liner displacement or post yield behavior. More accurate account of both Demand and design. Allows for damage
- Touch Briefly on PBD This is the next step in refinement of seismic design. Apply DBD principles, but the Displacement Capacity is set to an expected level of damage associated with the displacement and the design is worked to that. Substantial research in column behavior. We understand: Repair level associated with displacments Fix it Don’t just have to design for life safety, but an operational level based on owner performance needs.
- Why? Force Based may be easier and faster Displacement based more accurate but more complicated
- 1 – Not standing here unless Arkansas was considering. 2 – Hazard is very similar to Illinois & highly influenced by the NMSZ 3 – Large events in last 200 years SDZ 2, 3, & 4 4 – Arkansas = high seismic state.
- DBD is also the current Design Paradigm Read slide
- Now we don’t want to switch “Just because that’s what others are doing” DBD can be more economical. As example, consider Wall Type Pier R-factor = 1.5 or 2 depending on aspect ratios If properly detailed….Actual stiffness reduction could be more like 4. This lower stiffness would result in: Higher periods Lower Csm / Loads End Results = Smaller Members, less piles, smaller footings.
- Read Slide
- Now that I’m complete with DBD Commercial Get into work that MBI has done with IDOT on DBD. I’m not IDOT and don’t want any of this to be construed as me representing them, but we will share our experience working with them on this. IDOT’s 1st Step = Evaluate Existing Bridges with DBD. See how they perform and how they were detailed. Michael Baker performed these evaluations. IDOT’s 2nd Step, and where they are today, is develop updates to their policies based on DBD principles. We are helping as a peer reviewer.
- For Evaluation, IDOT assigned MBI 5 “representative bridges”. Which are listed with Abutment/Pier/SDC NOTE: In DBD we use SDC A, B, C, & D whereas FBD uses SZ 1, 2, 3, & 4. They are essentially the same. READ TABLE BR 1, 2, 3, 4, 5
- DESIGN CRITERIA developed 1 – Follow the ERS Strategy in the IDOT Bridge Manual, 2 – But use AASHTO Seismic Other References
- Evaluation Methodology Same seismic hazard - 7% probability in 75 years. Same performance – life safety – low probability of collapse, but damage may be expected. These are currently common between DBD and FBD. All bridges were considered “Regular” – Equivalent Seismic Analysis…. Liq/Lat Simple Methods for soil structure interaction. No geotech involvement
- As you get into the AASHTO Seismic Guide Specs, notice and emphasis on ERS. This is important – highlights where you expect to have ductility in structure and what code requirements there are for that. IDOT follows both Type 1 & Type 3 – Read Discuss ERS’s In ground hinging – Limited Ductility Response. Local Displacement ductility < 4.
- AASHTO Seismic has flow charts to guide designers through the code. *** What I hope people can get out of this is that several of the step/procedures are similar between DBD and FBD. What I have highlighted in RED are more unique to AASHTO Seismic these are: 1. 2 3 4. 5
- 1 – Seismic Proportioning For SDC D, but good practice for all bridges – want the relative stiffness of adjacent piers to be closer than 0.5. If not, potential for Increased damage in the stiffer elements, An unbalanced distribution of inelastic response throughout the structure, and Activation of other modes including twisting which can induce column torsion.
- Effective Stiffness. More guidance in AASHTO Seismic Two methods. Charts Caltrans SDC – Equation for charts. Moment Curvature Analysis. As an example – on evaluation bridge 5 – this table show the difference. LRFD – doesn’t give much guidance and says use Ieff/Ig = 0.5
- Abutment Stiffness also better covered. In longitudinal direction most of stiffness can come from abutment passive pressure. Two things: Know the height considered, seat vs. diaphragm If seat, know if displacements are expected to engage the backwall. Transverse Stiffness can be a bit more arbitrary. Caltrans SDC recommends rather than a fixed pin, use about 0.5 of the adjacent Bent.
- 4 - Foundation Modeling Depending on Kfound vs. Kpier, Foundation can have a larger effect. Consider this multi-column pier with a crashwall on pile foundation Columns are short, so pier is pretty stiff. If foundation is modeled, overall: Stiffness down, Period up Csm down Displacements up, but also Displacement capacity includes pile displacement capacity which goes up.
- 5 and last point – Displacement Demand vs. Capacity. You may think you need to do pushover analysis, and that’s complicated, BUT SDC’s B & C– Empirical equations related to Column Height Ho and Column Aspect Ratio X. These equations relate: Concrete cover spalling Column Ductility of 3 Columns < 15 ft, lower bound of 0.12Ho is used. Important that if your pier does look like a single or multi-column bent, you will end up doing pushover analysis.
- Now get into some findings or lessons learned from the evaluation bridges.
- Read parameters
- The behavior and displacement capacity of these piers were assessed both with the Empirical Equations and using pushover analysis. The pushover analysis was completed by knowing: Plastic hinge length Elastic displacement of the crashwall Yield displacement of the columns knowing the column yield curvature Ultimate curvature of column and associated of the column plastic displacement through rotation of the plastic hinge. Superposition of the yield and plastic displacements. Longitudinal – very close to the empirical equation. Transverse – Pushover provides more displacement capacity.
- Detailing of the columns was also evaluated. There is a provision in the AASHTO seismic that relates the maximum bar diameter to the column aspect length and diameter. If the bars are too large, bond was found to not be sufficient for plastic hinge development. Meeting this requirement can be challenging for short column as in this case it required ½” Dia. bars. As a note: this requirement has been since been omitted from the Caltrans SDC.
- Bridge 4 is shown here. Integral Abutments Wall Pile bent piers with Steel H Piles. SDC B
- For the ERS, because of the stiff abutment cap or stiff wall, in the transverse direction most of the ductility is provided by the piles BELOW ground. According to AASHTO seismic, when in-ground hinging occurs, you are supposed to check for limited ductility response, in other words we don’t want to push the elements we can’t see too far.
- Here we evaluated the displacement capacity just by looking at the yield displacement capacity of the pile. If we know the pile yield moment My = FyS And the stiffness for fixed fixed or fixed-free behavior. And the expected length to the point of maximum moment relative to the pile depth of fixity. We solve for the yield displacement delta Y based on P = K-Delta. In the end the yield displacement capacity was larger than the displacement demand and the local ductility is less than both 4 and 1.
- Steel H-piles there are also some detailing items. If there is some expected ductility (displacement beyond yield), you have to check slenderness ratios allow this behavior so you don’t have local buckling. We found that for 5 H-pile shapes, these ductility limits weren’t met, so need to be cognizant of this when selecting foundation type.
- For Bridge 2 10 span bridge Stub Abutments Pile Bent Wall Piers Wall Piers on Pile Cap SDC C
- For this bridge, all of the displacement checks were ok, but the main finding was in the detailing. For walls/columns extending into footings, the column confinement is expected to extend into the footings. This wasn’t part of the IDOT standard details.
- After we finished the evaluations, IDOT spent some time to develop their draft policy documents. MBI was then engaged and we are currently working on this as peer reviewers. We assembled a team of 6 reviewers from high seismic states including Cali, Nevada, Kentucky, Illinois and South Carolina. IDOT Steps for rolling out the policy are: Update Seismic Details Update Design Guidelines Update Seismic Hazard (in line with 2023 AASHTO Maps) Add performance based criteria.
- A couple national updates on seismic design. READ SLIDE
- From what I heard, It’s expected that the 2023 proposed AASHTO Seismic hazard will be adopted this year at by COBS. 2023 AASHTO is based on 2018 USGS Seismic Hazard. The items that are different include: No longer 3 point hazard curve, but a 22 point hazard. This minimizes the plateau. Site Factors are all built into the hazard. There are also 8 site classes (A, B, BC, C, CB, D, E & F) rather than 6. They are also only based on shear wave velocity rather than blow count data which I know a lot of states use for site class determination.
- For Hot Springs Arkansas, assuming Site Class D Here is a comparison of the hazard. Short period – less hazard Long Period, about the same.
- Read Slide
- I would like to say thank you and also extend a thank you to IDOT. It has been a real pleasure to work with them on these projects. With that I’ll open things up to any questions.