Embed Size (px)
DESCRIPTION
In this study, a computer simulation model of a rigid impactor loading laterally a roadside W152x13.4 post has been developed. The interaction of cohesionless soil with a post was studied and compared to an existing dynamic test results from a published literature. Two approaches to simulate the soil have been studied: the continuum method where the soil is modeled as a solid element with Drucker and Prager material law, and the subgrade method where the soil reaction is simulated by a series of nonlinear springs. An improved method of the subgrade approach has been developed where the soil is modeled as a system of parallel springs and dampers with a lumped mass attached to the post. A simple procedure to calculate the lumped soil mass and the damping coefficient is presented.
Citation preview
Dept. of Civil and Environmental Engineering, University of Windsor
Abdelmonaam SASSI, Ph.D.
May 17, 2012
An Improved Subgrade
Model for the Crash
Analysis of Guardrail Posts
NCHRP 350 (1993)
Recommended Procedures
for the Safety Performance
Evaluation of Highway Features
TL3-11
2000 kg light
truck
V = 100 kph
Angle : 250
TL3-10
820 kg Sedan
V = 100kph
Angle : 200
MASH (2009)
Manual for Assessing
Safety Hardware
TL3-11
2270 kg light
truck
V = 100 kph
Angle : 250
TL3-10
1100 kg Sedan
V = 100kph
Angle : 250
Introduction: Regulations
Pickup truck impacting the guardrail, with 100 km/h
speed at 25 deg impact angle, should not penetrate,
under-ride or override the installation.
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
TYPICAL FULL SCALE
GUARDRAIL TEST
Dynamic testing Set-up
Test #4 --- Dry 8.9 Override NA NA
TestSoil Density
Kg/m3
Moisture
Content
Impact Speed
m/s3
Max Deflection
mm
Soil Density
Kg/m3
Moisture
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
TestSoil Density
Kg/m3
Moisture
Content
Impact Speed
m/s3
Max Deflection
mm
Soil Density
Kg/m3
Moisture
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
TestSoil Density
Kg/m3
Moisture
Content
Impact Speed
m/s3
Max Deflection
mm
Soil Density
Kg/m3
Moisture
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
55
0 m
m
Impactor
18
30
mm
11
00
mm
Post
II- Component testing of the guardrail
post
Dynamic testing Set-up used by Coon et al (1999)
Sassi 2011 Slide #5
Subgrade Method Continuum Method
-Fast
-Widely used
-Accurate after the peak
-Does not account for
the inertial effect
-Accurate
-Does account for
the inertial effect
-Computationally very costly
-Soil parameters not available
III-1 Soil Modeling
-Does account for the inertial
effect
-Computationally relatively
costly
Kennedy et al. (2004)
Combined of two methods:
-Subgrade method in all the
guardrail post
-Add continuum method in
the impact zone with no little
or no stiffness and right
density.
Continuum Method
Subgrade Method
III-2 Soil simulation with combining the twIo
methods
III-3 Typical Results of the FE Study of the
dynamic testing of the guardrail post
Traditional subgrade
modeling only with
springs missed the
inertia effect.
Plaxico (2002)
Impactor
Post Lumped
soil mass
IV Proposed model
Soil modeled as:
Spring stiffness ( k )
Damper (c )
Lumped mass (m)
C, k & m are not
constant
along the pile
embedment
III-1 Stiffness Calculation (k)
Method of Habibaghi and Langer (1984).
h q
'k N
y
q
zN A
B
0.1245yA 15.276 14.09 e
(Based on the bearing capacity approach)
Z is the depth
B is the width of the post
y is post deflection
σ’ overburden pressure
Nq is the bearing capacity factor
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
III-3 Damper calculation
Parametric Study Results
cc 2 mk
c
c
c
Z Mass K Cc 5% Cc 20% Cc 12%
Mm kg kN/mm N/s
100 34.08 1.49 18.05 1.81 2.71 2.26
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
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
900 0.65 22.12 9.38 0.76 1.13 0.95
m x c x k x f
Parametric Study to determine
the damping factor ξ
IV Results of the simulation
Good correlation between the 4 dynamic tests
and the model results
Maximum Deflection
(mm)
Average Force
(kN)
Peak Force
(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
V- Results of the simulation
Model Continuum Method Spring model Spring/Damper
Simulation time (S) 0.180 0.180 0.180
Run time 8.49T T 1.06T
Modeled improved by defining space between the
post and the lumped mass
T = 40 minutes
CAE Results
IV-3-1 Full Scale guardrail test simulation
IV-3-2 Vehicle response
Roll Angle
Vehicle Speed
Overhead View Frontal View
IV-3-2 Sequential of impact event of dynamic test
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
