Pole and Narrow Object Impacts - jhscientific.com centroids do not reach a common velocity for a CRASH lll or similar analysis ... “Analysis of Pole ... zCraig reports this method,

Pole and Narrow Object Impacts

Presented By:

John DailyJackson Hole Scientific Investigations, Inc

Nate ShigemuraThe Traffic Safety Group, LLC

Copyright © 2005, 2007 by J. Daily, N. Shigemura, R. Strickland

The Problem

Vehicles often leave the road and impact with narrow objects.Narrow objects do not move in the same sense as an unconstrained object.However, narrow objects, particularly poles, can break away.The vehicle coming into the collision is the only energy source to do damage.For our purposes, a narrow object is less than ¼ the width of the vehicle.


The Problem, cont’d.

We must account for all of the energy dissipated by the vehicle.A Combined Speed equation enables us to add KE.In Section 1, we will examine vehicle impact damage and EBS.In Section 2, we will look at pole properties, including fracture energy.In Section 3, we will determine the EBS for pole movement.In Section 4, we will examine test results from a staged crash.


What Do We Need?

Identify the overall path of the vehicle and list what it did.Pre-Impact skidding? CSY?Pole or other narrow object collision?How much vehicle damage?How much damage to the pole?How much did the pole move?Post-Impact motion?


Section 1

Vehicle Impact Damage and EBS


Vehicle Impact Damage


Vehicle Impact Damage

Vehicles may impact with poles in any number of configurations:

FrontalSideRearOther Surface Sideswipe


Vehicle Impact Damage, cont’d.

Frontal ImpactsMost information availableSeveral different methods useful for analysis

Maximum crush techniques (ex. Craig Method)Damage profile techniques (ex. CRASH lll)



Side ImpactsNot as much information availableSide structures contain some hard points

A-pillar (hard) v doors (soft)Analysis limited to damage profile techniquesGood stiffness data lacking



Rear ImpactsInformation lackingDamage profile technique

Not much stiffness data available



Other surface impacts



Other surface impactsProblematic

No stiffness or strength data availableSignificant bending of whole structure may be presentAs of now, no techniques availablePerhaps FEA in the future



SideswipesCannot use maximum crush depth or damage profile techniquesNot enough information for maximum depthDamage centroids do not reach a common velocity for a CRASH lll or similar analysisMIGHT(?) be able to approximate with slide to stop and a drag factor sliding past the pole.Be very careful with this…



Sometimes we have a collision on the frontal surface of the vehicle….

And it is a REALLY good WHACK!!



This pickup drove off the road and hit squarely an immovable object.The speed change was obviously quite high.There is too much damage to apply a CRASH III type of analysis.From examining the damage to the vehicle, we see the collision force was virtually straight back.



We will look inside the vehicle for evidence of speed.Specifically, we will be looking for both speedometer and tachometer slap.In certain limited cases of this type, such evidence may be viable.It is best if the two instruments will corroborate each other.The speedometer shows about 57 mph.



The tachometer shows paint transfer from the needle slap at about 2400 rpm.This is consistent with the speedometer reading of 57 mph.This is a TRUE impact speed for this case.Speedometer needles with molded in color may not transfer to face.


Determining Vehicle EBS

The term EBS stands for “Equivalent Barrier Speed”.It may also be called BEV, for “Barrier Equivalent Velocity”A “Barrier” is a non-movable object that absorbs none of the collision energy.Hence, the EBS is the impact speed of the vehicle into a barrier with no motion of the vehicle after impact. Thus, EBS represents the kinetic energy required to crush the vehicle to its noted damage profile.



With pole impacts, there are two methods commonly used to determine vehicle EBS:

Maximum Crush TechniquesDamage Profile Techniques



Maximum Crush TechniquesSeveral different equations have been developed by researchers which use the maximum crush of the vehicle to determine EBS.Of these, we will examine the Craig models.Victor Craig published the following article in the Sept/Oct 1993 AR Journal: “Analysis of Pole Barrier Test Data and Impact Equations”.In this article, he suggests a “rule of thumb” of one mph per inch of deformation in a frontal crash with a small, front wheel drive car. A “small” car is less than 15 feet long with a weight under 3000 lbs.



Craig further reports this “rule of thumb” may be modified for crush depth in the following manner:

For crush depths up to 20 inches, use the crush depth with no modification.For depths between 21 & 25 inches, add one inch.For depths between 26 & 30 inches, add 2 inches.For depths between 31 & 35 inches, add 3 inches.For depths between 36 & 40 inches, add 4 inches.For depths between 41 & 45 inches, add 5 inches.Thus, a small front wheel drive car with a measured depth of crush of 45 inches would have an EBS of 50 mph.



Craig did additional work on the subject, published in the ARJ, Sept/Oct 1995, “Speed Estimation in Head-on Vehicle Pole Impacts –Update”.The following four equations are presented, which account for different vehicle sizes and crush depth regions.



Small FWD with maximum crush less than or equal to one foot:

EBS = 0.47Cmax + 4.0Small FWD with maximum crush greater than one foot:

EBS = 1.30Cmax – 6.0Mid/Full size with max crush less than or equal to 1.5 feet:

EBS = 0.54Cmax + 4.0Mid/Full size with max crush greater than 1.5 feet:

EBS = 1.18Cmax – 7.0



Craig reports this method, after the analysis of 47 crash tests, will predict the EBS within 5 mph 100% of the time.Expert AutoStats Crush Factor:

For narrow object impacts


MIDS )6.12(30=

Where:S = Speed in MPHMID = Maximum Indentation Depth (in feet)


Damage Profile TechniqueThe CRASH III model, which was developed from the initial work of Emori and Campbell uses the actual damage profile for EBS calculations.Essentially, the CRASH III model assumes certain uniform stiffness characteristics over the damage face of the vehicle.The model further assumes the vehicle deforms according to Hooke’s Law, which describes the deflection of a linear spring.



Damage Profile TechniqueThe CRASH III Impact Model:

The impact model is derived from a simultaneous solution of the COLM and Conservation of Energy equationsAs such, the limitations on the model are the same as those for COLMThe model does NOT handle significant external, impulsive forces, such as a pole planted in the ground.If the pole does not break, then the impact model may give reasonable results for ΔV, which is a vector quantity.If the pole breaks and the vehicle moves through it, then the method may over estimate the impact speed.If the pole breaks and the vehicle moves through it, then it maybe better to calculate a damage EBS for use in a combined speed equation.



Damage Profile TechniqueThe CRASH III Damage Energy Model:

The energy model, which uses the damage profile to calculate the energy required to deform the vehicle.This model requires we know the area of the damage, the depth of the damage centroid, and the A, B, & G stiffness coefficients.We must also specify the angle the collision force makes with respect to the damage face so the proper magnification factor, (1 + tan2α) may be calculated.We MAY use the energy model to put into an equation for EBS.The CRASH III model is NOT valid for sideswipe collisions.



Damage Profile TechniqueThe CRASH III method provides us with good information.However, further discussion of it is beyond the scope of this presentation.In order to use CRASH III, it is recommended the investigator take a complete course in the subject.


Section 2

Pole Properties and Fracture Energy


Properties of Wooden Poles

National standards and specifications for wooden utility poles are found in the American National Standards Institute (ANSI) database.These standards specify types of wood from which utility poles may be made and the preservative treatments which may be used.



Poles will come in different sizes, depending on the job the pole must do.Poles come with classifications indicating circumference at the top and bottom, length, wood species, and preservative treatment.



There are 15 classes of poles. Class refers to the relative load bearing capability of the pole.Classes run from H-1 through H-6 and then 1 through 10.Class H-6 poles have the largest top circumference, while a #10 pole has the smallest.



Poles are designated by a series of numbers indicating the class of the pole as well as its length.A common pole often cited in the literature is known as a “4-40”This means the pole has a strength classification of “4” and a length of 40 feet.



A “4-40” pole would be set into the ground about 6 feet and would have a circumference at ground level of 33.5 inches minimum. It would weigh about 1200 pounds.As the pole within a given class gets longer, its ground level circumference will increase.A “4-70” pole will have a ground level circumference of 41.5 inches.


Pole Identification

Wooden poles are often branded to identify the following:ManufacturerThe place where the pole was made The year the pole was treated,The species code The preservative code The class and the height


Pole Identification

Example of Species code:SP = Southern pineRW = RedwoodWF = Western firOthers…


Pole Identification

Example of Preservative code:SK = CCAC = CreosoteP = PentaOthers…


Pole Identification Example

This light standard was treated in 1991It is made of Southern Pine and is treated with creosote.It is a “3-35” class/length. Its ground circumference is a minimum of 34 inches.ET&T and 7.5A are manufacturer codes.


Pole Life

Wooden utility poles have a finite life.Unless damaged beyond repair and depending on the part of the country in which it is located, this useful life may be between 20 and 40 years.Morgan & Ivey, in SAE 870607, report wooden utility poles may lose up to half their strength in 20 years, depending on environmental conditions.Areas of the pole most affected are at and just below ground level.


Pole Break Energies (BFE)

Some types of metallic poles are designed to break away if there is a sharp enough impact.These poles are rated in break away energy, usually in foot pounds.A simple speed equation will show the energy equivalent speed loss:

WKES 30

=



Wooden Poles are not so predictableAs we may see, there are many factors effecting pole strength.These include, but are not limited to:

Pole classPole ageEnvironmental conditionsPrevious damageGuy wires and electrical wires

Tree fractures MAY NOT be analyzed with the same empirical equations as wooden poles.



Because of these factors, it is difficult to accurately access the energy required to fracture the utility pole.In SAE 870607, Morgan and Ivey report the following:

A class “4-40” pole will begin to fracture at impact speeds of 50 to 60 mph by a 2000 pound vehicle.If the vehicle weight is increased to 4000 pounds, the impact speed to begin fracture reduces to 35 to 40 mph range.They also report most cars will be stopped by a “4-40”pole without fracture below 30 mph.



There are some published methods to determine pole fracture energies:

Mak, et al (early 1980’s)Kent & Strother (SAE 980214)Either of these should be used with caution!



Mak, et alWorked in conjunction with the CRASH lll vehicle deformation modelEmpirical equations for both partial and complete fractures



( )[ ]38.42104.1200005.0 CBFE −×−=


Pole Circumference less than or 26 inches

Full breakBFE = 20,000 ft-lbThis value seems to be arbitrary

Partial break


Pole Circumference greater than 26 inches:

Full Break

Partial Break

( ) 38.42104.1 CBFE −×=


[ ]20000)104.1(5.0 38.42 −×= − CBFE


Mak results for 42 inch circumference pole: (This may be a “2-50” pole)

Partial Break: 80,142 ft-lb

Full Break: 180, 285 ft-lb



Kent and StrotherEmpirical equations for actual pole breaks

Uses properties of pine poles (conservative)Treats pole as cantilever beamPartial and complete fracture equations available



⎟⎟⎠

⎞⎜⎜⎝

⎛+⎟⎟

⎠

⎞⎜⎜⎝

⎛−=

4782.39

40051.0

424 rrFEtotππ

Kent and Strother Equations:Full Break:

Partial Break:


R

R

o Er

ar

FE 2

42

64 ⎟⎟

⎠

⎞⎜⎜⎝

⎛

=

πσ


Kent and StrotherFor the Partial Break

σR = Modulus of Rupture (from data base)ER = Radial Modulus of Elasticity – calculated from bending modulus of elasticity EL



Kent and Strother – Pine pole with 42 inch circumference:

Partial Break:8898 ft-lb

Full Break:49,736 ft-lb



As we may see, the two methods result in seriously different values for fracture energy.


Effect of Pole Break Energy

Consider a 4000 lb vehicle impacting a 42 inch circumference pole.The EBS from vehicle damage is 50 mph.

The pole fracture EBS using the Mak, et al analysis is 36.77 mph.Thus, the impact speed would be 62 mph if the vehicle stopped at impact.The Kent & Strother EBS is 19.31 mphThe impact speed would be 53 mph.


Effect of Pole Break Energy

We may see the difference between the two cases is 9 mph.If we ignore the pole break entirely, the EBS for the vehicle crush will potentially underestimate the impact speed between 3 and 12 mph.If there is a post-impact spin/skid, then this difference will become smaller.We must also look at the pole movement.


Section 3

Pole Movement


Pole Movement

The pole may be dislodged and move in its hole, compressing the dirt.The force to move the pole comes from the impact of the car. Newton’s Third Law says the forces are equal and opposite.The pole pushes on the car just as hard as the car pushes on the pole.


Pole Movement

Recall:

Work = Change in KE

Work = ΔKE

Work = F(d)


Pole Movement

F = Collision Force This may be calculated from CRASH IIIIt may also be calculated from the estimated collision impulse: FΔt = MΔv

d = Pole displacementIf pole deflects 4 inches and the average collision force is 19,121 lb:

Work = 19,121(4/12)Work = 6373 ft-lb


Pole Movement

WKES 30

=


For the example: if W = 3000

S = 7.98 mph

Section 4

Crash Test Analysis


An Analysis of Three Pole Impacts

Institute of Police Technology and Management’s 25th Annual Special Problems in Traffic Crash Reconstruction

Presented By

John DailyJackson Hole Scientific Investigations, Inc

IPTM Adjunct Faculty

Russell StricklandFairfield, OH Police Department

IPTM Adjunct Faculty


Hold my Beer and Watch This!!!


Purpose of the Research

These three pole strikes are the beginning of a series of experiments designed to further examine the mechanics of pole fractures.Vehicle impacts provide a dynamic loading to the pole.This dynamic loading is different than slow (quasistatic) force application.The research goal is to hit and fracture a total of 35 poles.


Purpose of the Research

We wish to examine the published fracture models and then develop models based upon this research.We will also examine energy methods to analyze pole impacts.We will need to account for all of the energy possessed by the vehicle at impact in order to compute a reasonable impact speed.


Where does the energy go?

The vehicle is crushed to some damage profile upon impact with the pole.There are several impact models we may use to determine vehicle EBS for narrow object impacts.

CRASH III Energy ModelSimple Craig ModelExpert AutoStats Crush FactorOthers



The pole will fracture. There are several published models for pole fractures.For the purpose of this presentation, we will use the Mak equation for a full break:

( ) 38.42104.1 CBFE −×=Where C = Pole Circumference in inches



Often times, the pole will also move in the ground.From Newton’s Third Law, we know the collision force acts upon the pole, which causes the movement.This force will act for a distance at the area of impact on the pole. ΔKe = Work = F(d)If the pole tips significantly, then the effective force will change.We may calculate an average collision force using CRASH III methods as outlined in FTCR, Chapter 18.We may calculate the effectiveness of this force using the Mean Value Theorem, (MVT). (Pg 96, FTCR)We integrate over the angle change of the pole.



We may also have post-impact movement. This post-impact movement may be calculated with standard skid or spin analysis.If there is significant post-impact rotation, we may account for the spin energy by using rotational mechanics techniques. (Chapter 9,

FTCR)If we account for all of the energy, we may then compute a reasonable impact speed.


Test Design and Implementation

We will need to deliver test vehicles into the pole impact with a reasonable degree of consistency of speed and impact location.The speeds of the vehicles should be fast enough to fracture the pole and then move through its location. The desired target speed was 45 mph.Vehicles will be similar in structure.Instrumentation will be installed in the vehicles for proof of concept purposes.


Test Design and Implementation

Once the crash has occurred, we will use standard reconstruction techniques to gather, preserve, and interpret the evidence.We will then use this evidence to compute the impact speed for each vehicle, and will then compare this computed speed to the known impact speed.We will examine the tests and results next.


Test Implementation

The pole impacts were conducted at Reynolds Industrial Park in Green Cove Springs, FL.The first pole impact was on April 20, 2007.The second pole impact was on April 23, 2007.The third pole impact was on April 24, 2007.


1996 Mercury Sable


1996 Mercury Sable


1996 Mercury Sable Specifications

Rear Axle to Front Bumper: 149 inchesWeight: 3400 lbUndamaged Width, L: 61.5 inchesRear Axle to Measuring Jig: 14 feetUndamaged Profile to Understructure, accounting for free space:

C1=14.75; C2=5.75; C3=4.5; C4=4.5; C5=5.75; C6=14.75



Crush Profile:C1=0; C2=5.75; C3=11.25; C4=13.5; C5=1.25; C6=0Maximum Crush Depth: 20.0 inchesStiffness Coefficients:A = 418.4 lb/in; B = 161.9 lb/in2; G = 540.5 lbPost Impact Movement: 15 feet


1999 Mercury Sable


1999 Mercury Sable




C1=14.75; C2=5.75; C3=4.5; C4=4.5; C5=5.75; C6=14.75



Crush Profile:C1=0; C2=0.75; C3=6.75; C4=14; C5=2.0; C6=0Maximum Crush Depth: 14.75 inchesStiffness Coefficients:A = 418.4 lb/in; B = 161.9 lb/in2; G = 540.5 lbPost Impact Movement: 42 feet


1998 Ford Taurus


1998 Ford Taurus


1998 Ford Taurus Specifications


C1=14.75; C2=5.75; C3=4.5; C4=4.5; C5=5.75; C6=14.75


1998 Ford Taurus Specifications

Crush Profile:C1=0; C2=5.0; C3=15.75; C4=9.0; C5=1.5; C6=0Maximum Crush Depth: 15.5 inchesStiffness Coefficients:A = 418.4 lb/in; B = 161.9 lb/in2; G = 540.5 lbPost-Impact Movement: 127 feet

Thanks to DV3 at Expert AutoStats for all the vehicle specifications and the

following slow motion video!


Damage on Understructure, Typical

Understructure Damage


1996 Sable Impact Video


1999 Sable Impact Video


1998 Taurus Impact Video






1996 Sable Test Results

CRASH III EBS: 19.15mphPole Break EBS: 19.79mph from Mak, et alPole Movement EBS: 34.30mph

Effective Force Coefficient: 0.76 from MVT

Post-Impact Movement: 13.41mph to 15.0mphPost Impact Drag: 0.40 to 0.50Pole Circumference at Break: 30.5 inComputed Impact Speeds: 45.98mph to 46.48mphActual Impact Speed:

46mph to 47mph (Brakes applied just before impact)


1999 Sable Test Results

CRASH III EBS: 18.03mphPole Break EBS: 17.18mphPole Movement EBS: 28.24mph

Effective Force Coefficient: 0.70 from MVT

Post-Impact Movement: 22.45mph to 25.09mphPost Impact Drag: 0.40 to 0.50Pole Circumference at Break: 28.5 inComputed Impact Speeds: 43.83mph to 45.24mphActual Impact Speed:

43mph to 44mph (Brakes applied just before impact)


1998 Taurus Test Results

CRASH III EBS: 19.92mphPole Break EBS: 17.58mphPole Movement EBS: 16.00mphPost-Impact Movement: 33.80mph to 39.03mphPost Impact Drag: 0.30 to 0.40 (0.33 from VC3000)Pole Circumference at Break: 28.5 inComputed Impact Speeds: 45.87mph to 49.85mphImpact Speed using f = 0.33: 47.27mphActual Impact Speed:

48mph to 49mph (Brakes applied just before impact; released at impact)


Summary

Vehicle pole and narrow object impacts may be solved by means of energy methods.All energy must be accounted for.All energy comes from the vehicle.If we can account for all the energy, we can get a reasonable vehicle speed.We may combine EBS speeds together to obtain a speed at a point in the trajectory, including impact.


Summary

Remember to gather as much evidence as possible. If track is kept of vehicle motions and damage, and all energy is accounted for, we may obtain reasonable speeds for vehicles into poles or narrow objects.An Excellent Reference:

“The Investigation of Automobile Collisions with Wooden Utility Poles and Trees”, Joseph Cofone. IPTM, Jacksonville, FL. 1996


Documents

Pole and Narrow Object Impacts - jhscientific.com centroids do not reach a common velocity for a CRASH lll or similar analysis ... “Analysis of Pole ... zCraig reports this method,