7/24/2019 design vehicle barrier

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STRUCTURE magazine September 201022

A Rational Method to Design Vehicular BarriersBy Mohammad Iqbal, D. Sc., P.E., S.E., Esq.

In an earlier STRUCTURE magazinearticle (October 2008), the author pre-sented an algorithm to determine designforce on a barrier during a vehicular im-pact. The algorithm, based on energyprinciple and empirical car crash data,showed that the impact force depends on

four factors: mass, speed and crush char-acteristics of the vehicle, and the barrierstiffness. The article concluded that theimpact force on a barrier during a head-oncollision can be significantly larger thanthe code-specified force of 6,000 lbs.A vehicular barrier is defined as an el-ement or a system that, when placed inthe path of a moving vehicle, would stopthe vehicle after it collides with the barrier(Figure 1). The barrier may be active orpassive, located at grade or at an elevatedlevel. In general, vehicular barriers are

used to protect life, limbs and propertyfrom intruding vehicles. In a parkingstructure, barriers are used at floor edgesto prevent the vehicles from plunging tostreet below. In other structures, barriersare installed outside a building to keepvehicles from slamming into the building.Generally, the barriers are passive type,such as concrete walls, upturn beams,spandrel beams, steel guardrails, bollardsand prestressed cables. If a vehicle canplow through or go over an obstacle, theobstacle is not considered an effective

barrier. A barrier that either fails duringan impact with a colliding vehicle (Fig-ure 2) or flexes so much that the vehiclebreaches it without stopping, is not aneffective barrier. Recent fatal incidentsinvolving failure of the barriers in parkingstructures during vehicular impacts andthe use of vehicles to slam into buildingshave put the barriers under focus andtheir design adequacy has become moreimportant than ever.A vehicle crashing into a barrier presentsa complex analytical problem. In order to

calculate the impact force Fon a barrier,one needs to know the weight, speed andcrashing characteristics of the vehicle, as

well the stiffness properties of the barrier.In this respect, the International BuildingCouncils (IBC) approach to design barriersto use a single force of 6,000 lbs. appearsarbitrary and unreasonable.

Historical BackgroundDesigning a vehicular impact barrier is not

a straight-forward task. Modern passenger

vehicles are made of various materials, in-cluding metals, alloys and fiber-reinforcedpolymer composites. The impact lastsfraction of a second and then the vehicleretreats or rebounds away from the barrier.During a collision with a barrier, metals inthe vehicle body fold and collapse like anaccordion causing polymer fibers to breakaway or de-bond from polymer matrix.

An ana lytical model that incorporatesthese effects and accurately predicts how

a car crushes in a crash is useful in thebarrier design. The finite-element crashand crush analyses procedures have beenused abundantly in automotive industry;however, the analyses require specializedsoftware and large models consisting ofseveral hundred thousands, if not millions,finite elements. Though the method isappealing, it may be cost-prohibitive foruse in building projects.In contrast to the above approach that

relies on sophisticated analysis, the U.S.military has used field testing to design

barriers used to protect its bases againstenemy vehicles. The testing methodpresumably requires building a test bar-rier, subjecting it to a moving vehicle ata specified speed and then standardizing

it on a scale of 0 to 10. For exampaccording to the Military Field Mansteel pipes embedded in 4 foot deep foings (Figure 3) have been approved 4,500-pound vehicles travelling at mph. The protection rating of this sysis poor 1.0 on a scale of 0 to 10. Anotstandardized system is a reinforced concretaining wall shown in Figure 4 (p

24). The wall is 21 inches thick and been approved for 15,000-pound vehi

travelling at 30 mph, with a protectrating of 3.6. Recently, ASTM F-2607, Standard Test Method for Vehicle CrTesting of Perimeter Barriers, has bdeveloped to standardize testing of barrThe method requires building a test brier and subjecting it to a moving vehat the design speed. The barriers tesare then classified. The experimenprocedure is definitive, but expensive.

Proposed Design MethodIn contrast, the authors approach

been to seek synergy by integrating well-known energy principle with available vehicular crash data to determimpact force for all types of barriers, not limit the design to a few standardibarrier types. As explained in the previarticle, the impact force on a vehicle barcan be determined by the equation:

F = Equation

Where m= the vehicle mass [= ]

v= the vehicle speed at the impac c= vehicle crush b= barrier deflection under impaEquation 1 does not capture the p

force a barrier experiences. Ratherprovides an average force during the crand rebound duration that lasts a fractof a second. The four parameters noin Equation 1are discussed below, witfocus on the effects of barrier stiffnessthe impact force F.

Figure 1: Barrier impact force and its arm above floor.

Figure 2: A concrete barrier failed prematurelyat impact.

mv2

2(c+b)

Wg

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Mass

It was concluded in the previous article thata vehicular weight of 6,000 pounds should beused in barrier design in parking structures.In case the clear floor height exceeds 7 feet, ataller and heavier vehicle should be consideredin designing the barrier.

Vehicular Speed

The most significant parameter affecting the

impact force is the vehicle collision speed;the impact force increases with the square ofthe vehicular speed. The anticipated speeddepends on the distance and slope available fora vehicle to accelerate before slamming intothe barrier. Further, in a parking structure,a vehicle may roll down the ramp withoutany aid from its engine and gain considerablespeed, as its potential energy is converted intokinetic energy. A formula to compute thespeed at bottom of the ramp was presented inthe 2008 article.

Vehicle CrushWhen a vehicle hits a barrier, parts of thevehicle deforms, bends or crushes, and thevehicle length decreases. The decrease in vehiclelength after an impact is termed car crushand is denoted as c in Equation 1. Basedon the National Highway Traffic Safety

Adminis tration (NHTSA) vehicle crash-worthiness tests, the car crush distance ccanbe approximated by the following equation:

c= (ft) Equation 2

where v is the car speed in miles per hour(mph).Since vehicles are manufactured by many au-

tomakers in many models with changes madeevery year, the vehicle crush characteristics maychange as the technology progresses. As such, thecar crush data need to be updated accordingly.

Barrier Deflection

During an impact, a part of vehicles kinenergy is transferred to the barrier. One barsystem may absorb energy as elastic str

while another system may rely on local ymechanisms. The amount of energy absorand accompanying deformation depends the barrier type. For barriers exhibiting linbehavior, the deflection can be represented

b= Equation

where kbis the barrier stiffness. Substitutthe value of band cin Equation 1, and asome algebra, the impact force, F, cancomputed using the following equation:

F= 0.5kb[ + + ]Equation

where m, kband vare in ft-lb. units.Equation 4 can be used to plot the imp

force-speed relation for a given vehicuweight. For example, Figure 5 (page 24)sho

the relationship for a 6,000 pound car imping against a barrier of stiffness kb. Figure 5 (p

24) shows that the impact force decreasesthe barrier stiffness is reduced (i.e. its flexibiincreases). However, barrier rigidity canbe reduced ad infinitum because, after ctain reduction in stiffness, a barrier cea

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1/2" THICK

STEEL PIPE

2'-0

"

4

'-

0"

4'-

0"

3'-

0"

CONCRETE

FILLED

Figure 3: Steel Bollards used as Barriers for 4,500lbs. vehicle travelling at 30 mph.

v

3

Fkb

2mv2

kb

v13.2

v

3.63

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3/3STRUCTURE magazine September 201024

to be effective. Concrete barriers,such as cantilever walls, upturnedbeams and precast spandrel beamsare nearly rigid. Such barriers ex-hibit negligible b and, unlessthey fail prematurely (e.g. Figure2, page 22), they experience theseverest impact force. The steelguardrails show some flexibilityand the multi-strand steel cablesundergo large deformations under

impact loading. However, thedetermination of impact forceand associated deflection is nota straightforward task for a non-rigid barrier, as it may requireconsideration of the P- effects.For example, the prestressed cablebarrier system is a non-linearsystem that requires an iterativeprocess to determine Fand b.

Comparison between

Test Results andProposed DesignMethod

Two barriers tested by the U.S.Military are analyzed to determineif the proposed method can rea-sonably predict the barrier designforce. Since all necessary test data isnot available, some assumptionsare made in the comparison.

Pipe Barrier

As shown in Figure 3 (page 22), a series ofpipes cantilevering from a nearly-rigid baseform a barrier to a moving vehicle. The pipesare spaced 4 feet apart. Each pipe is 8 inches indiameter and is filled with concrete. The pipesare extra strong ASTM A501, Fy= 36 ksi. Fromthe American Institute of Steel Constuctions(AISC) handbook, the pipe section I=106in4, S=24.5 in3 and plastic modulus Z= 33in3. The colliding car weighing 4,500 pounds(m=139.75 ft-sec.2/lb.) impacts a single pipebarrier at 30 mph (44 ft./sec.). Its bumperheight is assumed to be 18 inches. The pipes

stiffness kb= = 106lbs./in. Using Equation5, F= 68 kips. The pipe starts yielding at Fy= 49 kips when its deflection is 0.042 inch.The pipe becomes fully plastic when Freaches

56.7 kips. The contribution of concrete inresisting impact is minimal, if at all, and so itis neglected. As the pipe continues to deflectin plastic mode under load, it keeps absorbingenergy. When the pipes deflection (at the pointof impact) b= 6.8 inches, the vehicle wouldstop and rebound, leaving behind a bent pipeleaning 13.6 inches at the top. Accordingly,the load factor for the pipe is 0.83 less thanunity, which is unsatisfactory. Similarly, theMilitarys protection rating for the pipe barrieris very poor 1 on the scale of 0 to 10.

Wall BarrierThe vehicle used in testing the bar-

rier (Figure 4) weighed 15,000 pounds(m = 466 ft-sec.2/lb.). It is assumedto be a commercial truck, such asFord F-450with a bumper height of21 inches. For a rigid barrier, the impactforce F= 248 kips. The test wall wasreinforced each way, each face and wasthus capable of developing yield-lineson both faces and in both directions.

Mohammad Iqbal is a l icensed P.E., S.E.and a licensed attorney. He serves onseveral ACI and ASCE committees and isa member of the American Bar AssociationConstruction Forum. Mohammad can bereached at mi@iqbalgroup.us.0

0

25

50

75

100

125

150

10 20 30 40

Rigid Barrier

K = 100 Kips/ft.

K = 10 Kips/ft.

IBC-2009

IMPACT SPEED (mph)

IMPACTFORCE(kips)

Figure 5: Vehicular Speed Impact Force plot for a 6,000 lb.vehicle and barrier stiffness, k.

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Assuming a rigid wall, fixed band impact load spread overinches long line, the wall capaappears to be adequate at F254 kips. In addition, since

wall footing is narrow, it is pected to rotate under impto absorb energy. The analshows that the wall will be ato stop the truck speeding atmph; however, it will experie

severe cracking and its base wundergo noticeable rotation. TMilitarys protection rating the wall barrier is a low 3.6the scale of 0 to 10. The coparison of tests results with proposed design method shothat the proposed method is able in predicting the impact fo

Summary andGuidelines

A frontal vehicular impact volves an enormous amountenergy. The magnitude of impenergy imparted to the bardepends upon vehicular mspeed and crush characteristic

well as on barrier characteristBoth IBC and ASCE-7 prescra minimum design force that p

tains to the vehicular speed of approximatemph (Figure 5); they do not provide a ratiobasis to design a vehicular barrier. It is suggesthat the building codes should require the

of energy principles in vehicular barrier desto protect public safety, and that design pfessionals use the following guidelines:

1) Select vehicular speed based on distaand slope available for acceleration, bnot less than 10 mph in parking stall

2) Select vehicular weight based onthe ceiling height available, but notless than 6,000 pounds in facilities

with ceiling heights of less than 8feet. When complying with ADArequirements, the vehicular weightshould be increased accordingly.

3) Incorporate barrier deformationcharacteristics and load flow to avoida progressive collapse.

1'-9" 9"

1'-6"

3'-3"

2'-9"

1/2"

3'-0"

1'-4

1/2

"

IMPACT FORCE

1 1/2" CONCRETE COVER

(3 SIDES)

#5 BAR STIRRUPS

@ 6" O.C.

(11) #7 BARS

@ 5" O.C.

10 #4 BARS

(TIES) @ 6" O.C.

#5 BARS @ 6" O.C.

#5 BAR

STIRRUPS

@ 6" O.C.

GROUND SURFACE

3" CONCRETECOVER

(3 SIDES)

BACKFILL

2" x 10 1/2" KEY

(8) #7 BARS

Figure 4: Concrete retaining wall used as barrier for 15,000 lb. vehicle travelingat 30 mph.