Seismic Design of a Railway Viaduct in a High Seismic Zone

Transcription

1 Seismic Design of a Railway Viaduct in a High Seismic Zone 9 th Small Bridges Conference, Australia 2019

2 Seismic Design of a Railway Viaduct in a High Seismic Zone 1. Project Overview 2. Typical Project Features 3. Design Preparation 4. Earthquake in the Philippines 5. Design Standards 6. Design Earthquake 7. Key Considerations for Railway Viaducts 8. Displacement Limit 9. Geotechnical Consideration 10. Structural Modeling 11. Determination of Natural Period 12. Key Learning Outline

3 1. Project Overview Item Guideway Number of Stations Structure Depot Trains Design Speed Expected Socio- Economic Impact Description Total Length:37.6 km Standard Superstructure Width:10.3m Track: Double Track Standard Gauge(1,435mm) 10 Stations by 2021,(5 Future Stations by 2030) Viaduct: 34.8km Embankment: 2.2km Bridges: center span & center span Balanced Cantilever Bridges; 18 cast-in-situ tapering superstructures at station approaches 1 Depot on, 14Ha Land, 15 Stabling tracks, Workshop, Light repair shop and OCC 8Trains (10 in Future) Max Design Speed: 120km/h Headway: 6 Minutes Cutting travel time into less than 1/5 th

4 2400 Walkway Walkway 2. Typical Project Features - 2 Typical Viaduct Substructure reinforced concrete pier supported on bored piles. Elastomeric bearings with steel seismic restraint pins to transfer seismic forces from superstructure to substructure. Drainage pipes hidden inside the box girder and the pier to discharge to ground level longitudinal drainage. Hole for Seismic Restrainer (Each Side) 300mm Drainage Hole 4 Elastomeric Bearings Drainage Outlet Cable Trough 4500 Typical Viaduct Superstructure Typical 40m spans of simply supported precast segmental box girder erected span by span with over head gantries 10.3m Width of Double Tracks Slab Track Bottom Slab of 4.5m wide Cable Trough below the emergency walkway PC Parapet wall Drain pipe at the center of the Box Four (4) 1500mm Bored Piles

5 3. Design Preparation Key Parameters Pier Height Range: H = 3.1m ~17.1m Design Height Cases: (m) Type of Super Structures Double Track, Single Track Classification on boreholes for Substructure design Span Length Typically 40m, Ranges from 25m to 50m Geotechnical Conditions: 485 Boreholes Design geotechnical model Cases: 6 Models

6 4. Earthquake Prone Areas in Philippines The Luzon area has great risk of major seismic activity and large seismic magnitude, with over a 100 recorded major earthquakes. Project Area

7 4. Earthquake Prone Areas in Luzon Sub Region Project Site Project Site West Valley Fault Susceptibility to Liquefaction Project Site The main source of seismic activity can be attributed to a nearby active fault, only 6 km South East of the project alignment West Valley Fault Imperative to conduct a thorough seismic design to ensure safe operations and resiliency Susceptibility to Ground Shaking

8 5. Design Standards Department of Public Works and Highways, Philippines - Bridge Seismic Design Specifications 1 st Edition 2013 (DPWH-BSDS) ; Department of Public Works and Highways, Philippines - Design Guidelines, Criteria and Standards; Volumes 1 to 6, 2015 (DPWH-DGCS); American Association of State Highway and Transportation Officials Guide Specifications for Load Resistance Factor Design Bridge Design 7 th Edition (2012) including amendments up to 2016 (AASHTO-LRFD); American Association of State Highway and Transportation Officials Guide Specifications for Load Resistance Factor Design Seismic Bridge Design 2nd Edition (2011) including amendments up to 2016 (AASHTO-LRFD-S); Japanese Design Standards for Railway Structures and Commentary (Seismic Design) (2012) (JDSRS); and Japanese Road Association Standard (JRA) (2012).

9 6. Design Earthquake Design Earthquake Level Probability of Occurrence Seismic Performance Requirement Design Seismic Acceleration Level 1 1:100 year Return Period Has the probability of occurring multiple times during the design life of the structure. Design is essentially to limit stresses in reinforcement to yield values and ensure an elastic design Structural damage due to earthquake is limit to the minimal. Train Derailment is prevented. 0.12g Level 2 1:1000 year Return Period The largest Earthquake that can be conceived for area of construction of the structure, and that may happen once or never during the design life of the structure Collapse of structures is prevented Damage in structures is limit to a level that repair and recovery can be achieve in a short period of time. 0.6g

10 7. Key Considerations for a Railway Viaduct While the Philippine Bridge Codes and AASHTO design codes address earthquake effects on a bridge, they do not specifically address the particular issues arising on railway bridges. The Japanese Design Standard for Railway Structures (Seismic Design) provides specific guidance to address earthquake design for railway structures. The key considerations include: 1. The increased stiffness of substructure required to limit the transverse displacement of the structure to prevent derailment of the train during the Level 1 Earthquake event. 2. A limit on the natural period of the structure to ensure robustness this is not directly required in the code which recommends more detailed investigation or analysis if the natural period of the structure exceeds 2 seconds in either the longitudinal or transverse direction of the bridge. Hence, the natural period was limited to 2 seconds. 3. Time History Analysis is recommended but due to the lack of time-history data in the Philippines, detailed Non Linear Static Analysis (pushover analysis) is specified.

11 8. Displacement Limit Lateral Oscillation of Train Wheel Uplift Lateral Vibration Displacement of Structure The principle to the displacement limit is concern that the Level 1 earthquake event is a 1:100 year event and expected to occur a few times during the design life of the structure. The likelihood of this event occurring during train operations is considered to be likely. During normal train operations, the train has a rocking motion on its suspension as it travels along the viaduct. At the same time, there is the corresponding vertical vibration of the train and the bridge girder. When the earthquake strikes, there is the risk that if the natural period is not controlled by limiting the natural period of the structure, the resulting overall movement could result in the over-turning or derailment of the train.

12 8. Displacement Limit Cont d In Japan, a specialist dynamic interaction analysis software is used to model: a) The rolling stock; b) Track structure interaction; c) Bridge substructure and soil interaction d) Earthquake excitation

13 8. Displacement Limit Cont d Displacement Limit Defined in the JDSRS codes

14 9. Geotechnical Considerations Approx 485 boreholes over the project = 1 borehole every 80m approx. 6 Different soil models based on shear wave velocity derived from average, idealised SPT N values method described in AASHTO LRFD; Liquefaction analysis to determine extent of liquefaction possible generally sandy layers with SPT N<10 considered to be liquefiable; In the liquefiable zones, soil springs ignored negative skin friction AFTER the seismic event is considered as additional load to the hinging loads. Considered very conservative as negative skin friction takes place after the seismic event and there is a case made to ignore this effect; Non Linear horizontal, vertical and pile toe springs behaviour allowed in accordance with the Japanese codes

15 10. Structural Modelling Member lengths defined by hinging lengths pier and piles; Important to model all loadings accurately in order to excite the correct mass; Generally, eigenvalue analysis carried out to determine natural period/frequency of structure. The cracked section properties are estimated and input. This is not accurate as required in the Japanese codes for major earthquakes. Non Linear material properties along with non linear soil springs are required to be modelled to get an accurate consideration of the natural period of the structure; Non linear material properties need to be specified not difficult, AASHTO prescribes what is required. A program capable of carrying out moment curvature relationships based on section defined and non linear material properties is required

16 10. Structural Modelling Consideration of Lateral Soil Forces on Piles Upper non-liquefied layer Liquefied layer Layer in which The effect of Lateral flow of Soil must be considered b Layer in which The effect of Non-liquefied layerlateral flow of Soil need not be considered Ground Displacement caused by lateral flow Of soil Analysis Model Vertical Load Force due to lateral flow of soil in non-liquefied layer Force due to lateral flow of soil in liquefied layer Subgrade reaction taking into Consideration effect of liquefaction Ground displacement caused by lateral Flow of soil induced to the piles Through subgrade reaction Consideration of Ground Displacement Caused by Lateral Flow of Soil Where liquefaction is a concern, lateral forces are induced in the piles due to the soil movement. This is automatically modelled in Japanese software, however, we had to adopt a different method. A pseudo static PLAXIS ground lateral displacement profiles; were obtained for the two (L1 and L2) earthquakes. Both linear elastic and Hardening Soil material models were defined and the deflection profiles are compared to the results from the Japanese software. The Hardening Soil (HS) model could not handle the earthquake L2, due to its large PGA, leading to large displacements but the linear elastic model produced results very similar to the Japanese.

17 11. Determination of Natural Period Japanese Code The figure on the left shows a typical Force Displacement curve obtained from the Pushover Analysis. In AASHTO an idealised bi linear moment curvature relationship is input and therefore, the corresponding Force Displacement curve is similar but different. Idealised Moment Curvature Diagram from AASHTO

18 11. Determination of Natural Period Japanese Method

19 11.Determination of Natural Period Japanese Method Cont d. According to the DSRS, the point D is deemed to the yield point of the whole structure and the effective natural period of the structure is calculated using this value from Teq = 2 SQRT(d eq /k heq ) Where K = the seismic intensity coefficient (related to stiffness) where k hm can be set to 1.0 so k heq is a fraction of 1. AASHTO however, derives the Natural Period of the Sructure from a dynamic eigenvalue analysis which does not permit non-linear soil springs and only models the cracked stiffness of the structure as manually input. In a simplified manner, T is calculated as follows

20 11. Determination of Natural Period Japanese Method Cont d. Generally, we found that the difference in Natural Period calculated in the two methods is very small typically 0.4 seconds difference in periods but as can be see the previous slide, a difference of 0.4 seconds is significant in softer soils (G4 to G6 soils) In these softer soils, or where the structure did not meet the transverse displacement limits, the following iterations were carried out: a) Increase rebar in piles upto max. 4% ; b) Increase pier rebar upto max. 4%; c) Increase column size and re-start.

21 12 Key Learnings Confirm the Design Criteria early and DOCUMENT the reasons why the values were determined; Confirm Durability Criteria as early as possible; Communication is a MUST! International codes are not all encompassing! Lot to learn from other Codes and Engineers! Check and verify the software before using the results; Pushover is a relatively easy, detailed method of analysis leading to a clearly economical and rational solution

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23 2.1 Soil Class- Ground Acceleration (Japanese Standard) Railway Standard Soil Class Soil Classes Ground Natural Frequency (TG(sec) Soil Profile Name G0 - Hard Rock G1 - Rock G2 ~0.25 Very Dense soil to Soft Rock G3 0.25~0.5 Dense soil G4 0.5~0.75 Dense to Soft soil G5 0.75~1.0 Soft soil G6 1.0~1.5 Very Soft soil G7 1.5~ Extremely soft soil Copyright Pacific Consultants Co., LTD.

24 2.5.Operational Safety and Deflection Limit Copyright Pacific Consultants Co., LTD.