An Improved Subgrade Model for Crash Analysis of Guardrail Posts - University of Windsor
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This document presents an improved subgrade model for analyzing guardrail posts during crash testing. The model combines continuum and subgrade methods to account for inertia effects. It models the soil-post interaction using spring stiffness calculated from bearing capacity, lumped soil masses, and viscous dampers. Simulation results matched well with four dynamic tests, improving accuracy over traditional subgrade models while maintaining computational efficiency compared to full continuum modeling. The proposed method can better simulate guardrail crash tests in cohesionless soils.
An Improved Subgrade Model for Crash Analysis of Guardrail Posts - University of Windsor
- 1. An Improved Subgrade Model for the Crash Analysis of Guardrail Posts Abdelmonaam SASSI, Ph.D. May 17, 2012 Dept. of Civil and Environmental Engineering, University of Windsor
- 2. Introduction: Regulations NCHRP 350 (1993) MASH (2009) Recommended Procedures Manual for Assessing for the Safety Performance Evaluation of Highway Safety Hardware Features TL3-11 TL3-10 TL3-11 TL3-10 2000 kg light 820 kg Sedan 2270 kg light 1100 kg Sedan truck truck V = 100kph V = 100kph V = 100 kph V = 100 kph Angle : 250 Angle : 200 Angle : 250 Angle : 250 Pickup truck impacting the guardrail, with 100 km/h speed at 25 deg impact angle, should not penetrate, under-ride or override the installation.
- 3. I- Full scale guardrail model L= 53.3 m D = 1950 mm N = 30 posts V = 100 km/h Angle = 25 deg Depth = 1100 mm
- 4. TYPICAL FULL SCALE GUARDRAIL TEST
- 5. II- Component testing of the guardrail post Post Impactor 550 mm 1830 mm 1100 mm Dynamic testing Set-up Dynamic testing Set-up used by Coon et al (1999) Soil Density Moisture Impact Speed Max Deflection Soil Density 2011 Slide #5 Sassi Moisture Test Kg/m3 Content m/s3 mm Kg/m3 Content Test #1 1980 Dry 4.6 234 42.8 42.8 Test #2 2110 Dry 5.4 314 43.9 43.9 Test #3 2240 Dry 5.9 348 47.3 47.3 Test #4 --- Dry 8.9 Override NA NA
- 6. III-1 Soil Modeling Subgrade Method Continuum Method -Fast -Accurate -Widely used -Does account for -Accurate after the peak the inertial effect -Does not account for -Computationally very costly the inertial effect -Soil parameters not available
- 7. III-2 Soil simulation with combining the twIo methods Kennedy et al. (2004) Continuum Method Subgrade Method Combined of two methods: -Subgrade method in all the guardrail post -Add continuum method in -Does account for the inertial the impact zone with no little effect or no stiffness and right -Computationally relatively density. costly
- 8. III-3 Typical Results of the FE Study of the dynamic testing of the guardrail post Plaxico (2002) Traditional subgrade modeling only with springs missed the inertia effect.
- 9. IV Proposed model Impactor Post Lumped soil mass Soil modeled as: Spring stiffness ( k ) Damper (c ) Lumped mass (m) C, k & m are not constant along the pile embedment
- 10. III-1 Stiffness Calculation (k) Method of Habibaghi and Langer (1984). (Based on the bearing capacity approach) ' kh Nq Nq is the bearing capacity factor y z Nq A B 0.1245y A 15.276 14.09 e Z is the depth B is the width of the post y is post deflection σ’ overburden pressure
- 11. III-2 Lumped Mass calculation Iso-displacement contour from Continuum model Iso-displacement defined cone centered around the rotation centre of the guardrail post . Lumped soil mass function of z M1 M2 M3 Parametric Study to determine the damping factor ξ
- 12. III-3 Damper calculation Parametric Study Results mx cx kx f Z Mass K Cc 5% Cc 20% Cc 12% Mm kg kN/mm N/s cc 2 mk 100 34.08 1.49 18.05 1.81 2.71 2.26 c 200 25.61 2.39 15.65 1.56 2.35 1.96 300 18.35 4.95 19.06 1.91 2.86 2.38 cc 400 12.30 7.62 19.36 1.94 2.90 2.42 500 7.46 10.37 17.59 1.76 2.64 2.20 600 3.83 13.13 14.19 1.42 2.13 1.77 700 1.41 16.09 9.53 0.95 1.43 1.19 800 0.20 19.04 3.92 0.39 0.59 0.49 Parametric Study to determine the damping factor ξ 900 0.65 22.12 9.38 0.76 1.13 0.95
- 13. IV Results of the simulation Maximum Deflection Average Force Peak Force (mm) (kN) (kN) Test Model Test Model Test Model Test #1 234 233 42.8 43.0 64.0 53.1 Test #2 314 296 43.9 45.9 66.9 57.8 Test #3 348 338 47.3 47.9 67.0 64.3 Test #4* Override Override NA 56.3 104.7 97.2 Good correlation between the 4 dynamic tests and the model results
- 14. V- Results of the simulation Modeled improved by defining space between the post and the lumped mass Model Continuum Method Spring model Spring/Damper Simulation time (S) 0.180 0.180 0.180 Run time 8.49T T 1.06T T = 40 minutes
- 15. CAE Results
- 16. IV-3-1 Full Scale guardrail test simulation
- 17. IV-3-2 Vehicle response Roll Angle Vehicle Speed
- 18. IV-3-2 Sequential of impact event of dynamic test Frontal View Overhead View
- 19. V- Conclusions Method developed for cohesionless and could be extended to cohesive soil Method accounts for the inertia effect Method accounts for the damping effect Method accurate and tunable Method computer time consumption efficient