Integral Bridges and the. Modeling of Soil-Structure Interaction

Transcription

1 Integral Bridges and the Click to edit Master title style Modeling of Soil-Structure Interaction Julian Moses, Principal Engineer, LUSAS

2 Introduction 2

3 Introduction 3

4 Introduction 4

5 About soil-structure interaction 5

6 About soil-structure interaction K p K H H Passive Movement 0.5 Active Movement K K a Wall movement / Height H 6

7 Seasonal changes 7

8 Seasonal changes Soil Pressure 3 rd summer 2 nd summer Pressures increase every year for cycles 1 st summer K o For design, consider maximum and minimum pressures K a Wall displacement 8

9 Analysis methods 1. Limiting equilibrium approach 2. Soil-structure interaction (continuum) 3. Soil-structure interaction (springs) 9

10 1 Limiting equilibrium Full height integral abutment on pad footing Full height integral abutment on piles Bank pad 10

11 1 Limiting equilibrium 11

12 1 Limiting equilibrium Full height integral abutment on pad footing Full height integral abutment on piles Bank pad (E s in MPa) 12

13 1 Limiting equilibrium 14

14 Structural idealisation Shell elements Shear, bending, twisting and in-plane forces Twisting M XY Bending M X Bending M X Twisting M XY 15

15 Structural idealisation Straight girders, Precast etc 3D beam elements (girders) Shell elements (slab) X Z Y 16

16 Structural idealisation Shell elements (slab) Skew or curved etc 3D beam elements (bracing) Shell elements (web) 3D beam elements (flanges & stiffeners) 17

17 Structural idealisation Y Z X 18

18 Grid models The conventional 2D-grid models used in current practice Substantially underestimate the girder torsional stiffnesses in I-girder bridges Substantially misrepresent the cross-frame responses Do not address the calculation of girder flange lateral bending in skewed I-girder bridges Section

19 Grid models...difficulties arise when in-plane effects are considered... the real problem is the occurrence of local in-plane distortions of the grillage members... which are clearly inconsistent with the behaviour of the bridge deck. Section

20 1 Limiting equilibrium Shell elements (slabs) 21

21 K* approach not adequate Wall Discrete piles Embedded wall integral abutment Full height integral abutment on single row of piles Bank pad on single row of piles 22

22 2 SSI with soil continuum 2D Beam elements Material Key Weald clay Tilgate Sands Limestone Concrete Check extents Plane strain elements 23

23 2 SSI with soil continuum Mesh checks 1. Width & depth of soil block 2. Element shapes and aspect ratios 1:1 ideal Avoid angles <30 1:3 max (areas of interest) 1:10 max (remote) 3. Mesh refinement 4. Element formulation quadratic better Need automatic re-meshing 24

24 2 SSI with soil continuum 2D Beam elements Material Key Weald clay Tilgate Sands Limestone Concrete Check shape & refinement Check extents Plane strain elements 25

25 10m 2 SSI with soil continuum Mohr Coulomb yield surface 10m 26

26 2 SSI with soil continuum 27

27 2 SSI with soil continuum 2D Beam elements Material Key Weald clay Tilgate Sands Limestone Concrete Check shape & refinement Mohr-Coulomb material Check extents Plane strain elements 30

28 2 SSI with soil continuum Cohesive soils Strain ratcheting may be ignored Granular soils Use modified elastic modulus Based on K* equations Determine effective height, H, iteratively 31

29 2 SSI with soil continuum 2D Beam elements Material Key Weald clay Tilgate Sands Limestone Concrete Check shape & refinement Modified elastic modulus* Mohr-Coulomb material Check extents Plane strain elements 32

30 2 SSI with soil continuum Interface between soil and structure Joint elements Contact slidelines Elasto-plastic interface materials Back of wall friction angle, δ Maximum δ=φ Smooth δ=0 Suggested δ=φ'/2 33

31 2 SSI with soil continuum Frictional interface δ=φ'/2 2D Beam elements Material Key Weald clay Tilgate Sands Limestone Concrete Check shape & refinement Modified elastic modulus* Mohr-Coulomb material Check extents Plane strain elements 34

32 K* approach not adequate Wall Discrete piles Embedded wall integral abutment Full height integral abutment on single row of piles Bank pad on single row of piles Continuum Soil modeled with nonlinear springs 35

33 3 SSI with soil springs Deck (shells/ 3D beams) Ka and K* pressures Discrete piles Soil springs / nonlinear joints End screens (shells/ 3D beams) Piles (3D beams) 36

34 3 SSI with soil springs 37

35 3 SSI with soil springs End screens shells & 3D beams Deck shells & 3D beams Soil springs Piles 3D beams 38

36 3 SSI with soil springs κ κ κ κ = 0 for cohesive soil under moderate loads κ = 0.5 for medium cohesive soil and non-cohesive soil above water table κ = 1.0 for non-cohesive soil below the groundwater level or under greater loads κ κ = for loose non-cohesive soil under very high loads 39

37 3 SSI with soil springs Stiffer, stronger Discrete piles Softer, weaker 40

38 3 SSI with soil springs Lateral pressure Passive Movement ' p k h ' a ' 0 Active Movement Horizontal deflection 41

39 3 SSI with soil springs 42

40 Software requirements Structural elements (beams, shells) Geotechnical elements (joints, springs, continuum) Automatic remeshing Nonlinear materials (e.g. Mohr Coulomb) Easy way to vary properties with depth 43

41 Summary Soil continuum Soil Springs UK Guidance: PD