8/12/2019 C StructuralPractices Iqbal Oct08

1/3STRUCTURE magazine October 2008

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Designing Edge Barriers in Parking StructuresBy Mohammad Iqbal, F. ASCE, D. Sc., P.E., S.E., Esq.

Numerous motorists have died in re-

cent years as their vehicles hit edgebarriers in parking structures, breachedthem and plunged below (Figure 1). Thebuilding codes have historically requiredthe vehicle barriers to resist a horizontalstatic force at a certain height above thefloor level, as shown in Figure 2. Theheight pertains to the bumper heightand the force roughly equals the weightof a fully loaded vehicle. For example,IBC 2006 prescribes that the vehicle bar-riers should resist a static force of 6,000pounds applied at a height of 18 inches

above floor. In light of the fact that tallerand heavier SUVs and pick-up truckshave become popular in recent years,the IBC 2009 is expected to modify andincrease bumper height requirements.Though IBC has moved in the rightdirection, using a single force to coverall locations may be arbitrary and inad-equate to provide safe barriers.Several types of highway and military

barriers are designed to stop vehicles fromveering from the roadway. To ascertainthat the barriers perform properly whenhit by a vehicle, they are pre-tested andcertified according to their capacity. How-ever, the barriers in parking structures areneither pre-tested nor certified. They areperceived to be low-risk because parkingstructures deal mostly with lighter vehi-cles, such as passenger cars, that move ata relatively slow speed therein. However,recent fatal incidents involving failure ofthe barriers caused by vehicular impactshave put the design of edge barriers underfocus and raised about their inadequacy.The probability exists that the vehicles

will hit the barriers head-on,endangering the occupants

within and pedestrians be-low unless the barriers areproperly designed.This article introduces the

use of energy principles to

design vehicular barriers. Itdiscusses factors affectingthe magnitude of impactload under various condi-tions and outlines frame-

work to formulate a rationaldesign approach for design-ing vehicle edge barrier inparking structures.

Energy-BasedDesign Method

The kinetic energy of a moving ob-ject can be determined using the well-known equation:

The expressions m and v represent themass and velocity of a moving body, re-spectively. However, a vehicle crashing intoa barrier presents a complex analyticalproblem. As a vehicle approaches a bar-rier, it impacts the barrier, as shown inFigure 3. The impact lasts a fraction ofa second, and then the vehicle retreats

or rebounds away from the barrier. Thphenomenons are non-linear and complex. As a result of the impact, the vehiclekinetic energy is consumed by (a) vehcle crush and (b) barrier deformationThe impact force on a vehicle barriecan be approximately determined b

the equation:

Where m= the vehicle massv= the vehicle speed at the impactc= vehicle crushb= barrier deflection under impactEquation 2 does not capture the pea

force a barrier experiences for a few milliseconds. Rather, it provides an averagforce during the crush and rebounduration. The following sections discu

factors affecting the impact force.

Impact-causing Vehicle

There are several makes and modelof numerous vehicles in the US, anevery vehicular type has its uniqucharacteristics. Three attributes of vehicle that affect the frequency anseverity of a potential impact on thbarrier are: its curb weight, bumpeheight and market share. An attributanalysis of 2006 model SUV and pickup trucks shows that Chevrolet Silverad

Figure 1: A car hanging precariously off Chicago Marina CityTowers as part of Allstate Insurance ad campaign. Courtesy ofchicagobusiness.com.

Figure 2: Barrier impact force and its arm above floor.

Figure 3: A car crash test on a 2009 model vehicle speeding at 35 mph against a rigid barrier.Car crush = 1 ft. 11 inches (576 mm.). Courtesy of http://nhtsa.gov.

K.E.=mv2

2Equation 1

F= mv2

2(c+ b)Equation 2

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2/3STRUCTURE magazine October 200825

1500 is the most likely vehicle to cause the

severest impact. It weighs 5,360 pounds atcurb and 6,930 pounds when fully loaded. Ithas a bumper height of 25 inches and over10% share of the market. On the other hand,Hummer is the heaviest SUV, weighing 8,800pounds at curb and 10,300 pounds whenfully loaded. Its bumper height is 30 inches,but has low market share of about 1% amongSUVs and pick-up trucks. It is suggestedthat the most popular vehicle be used in thebarrier design.

Mass

Once the design vehicle is selected, thedetermination of vehicular mass for im-pact purposes needs estimation of probable

weight of passengers and luggage it is likelyto carry at the time of impact. A conserva-tive approach is to assume that the vehicleis fully loaded. However, the approach maynot be realistic because the vehicles thatplunged thru the barriers were not fullyloaded, but had just one occupant thedriver in them. A 500 pound allowancefor the weight of one occupant, gasoline andluggage seems reasonable. Therefore, the de-

sign weight for the Silverado would be 5,860pounds It is suggested the design weight of6,000 pounds be used in barrier design.

Speed

The most significant parameter affecting theimpact force is the vehicle speed at the timeof impact. The speed a vehicle can gain in aparking structure depends on the approachdistance the vehicle has to accelerate. Thisarticle focuses on a drivers loss of his vehiclecontrol when rolling down a ramp, as shownin Figure 4. Other situations where a drivermay intentionally and recklessly accelerate hiscar are outside the scope of this analysis. Asa vehicle rolls down the ramp, its potentialenergy is converted into kinetic energy. Thespeed gain depends on the slope and lengthof the ramp. Assuming the vehicle is in astationary condition at the top of the ramp(point A) and that it moves down on its own,its speed at the bottom of ramp (point B) canbe determined by the following equation:

After some algebra, speed at the bottom of

ramp is given by:

Where = co-efficient of rolling frictbetween the driving surface and vehicle tirs = length of the sloping ramph = ramp heightIt is noteworthy that the car speed v

independent of its mass. The friction fac may vary depending on the drivewand the tires, but can be taken as 0.0assuming a concrete surface and radial tiSee the design example later in this arti

to compute the vehicle speed v. (MoVehicle Accident Reconstruction and Ca

Analysis, Limpert, 1999)

Figure 4: A passenger car rolling down a ramp.

m.g.h = + .m.g.sm.v2

2Equation 3

v= 2.g.(h- .s) Equation 4

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3/3STRUCTURE magazine October 200826

Barrier DeflectionDuring the impact a part of vehicles kinetic

energy is transferred to, or is consumed in,deflecting the barrier. Some barrier systemsabsorb energy as elastic strain and distributingit to the parking structure, while others mayrely on local yield mechanisms. The impactforce depends on the type of barrier. The rigidbarriers experience the severest impact force.The most popular types of barriers used in

parking structures are: Cast-in-place concrete cantilever walls Post-tensioned concrete upturn beams Precast concrete spandrel beams acting

at barrier walls Multi-strand steel cables Steel members and rails

Every barrier has its unique characteristics.In general, steel barriers are ductile. Thesteel guards employing various steel shapesare considered very desirable, as they canbe readily designed to deflect and yieldunder load. Similarly, barrier cables exhibit

flexibility and offer considerable deflection,b. On the other hand, concrete walls andprecast spandrels generally are neitherdetailed to have ductility nor expected tohave ductility. The cast-in-place and post-tensioned concrete upturn beams are quiterigid when connected to columns and bracedby a diaphragm. They exhibit negligible b.For non-rigid barriers, b can be readilydetermined using an iterative process.

Vehicle Crush and ReboundWhen a vehicle hits a barrier, parts of the

vehicle deform, bend or crush and the vehiclelength decreases, as shown in Figure 3 (page24). The decrease in vehicle length after animpact is termed car crush and is denotedas cin Equation 2 (page 24). After impactinginto a barrier, the vehicle rebounds and movesaway from the barrier and stops. The NationalHighway Traffic Safety Administration(NHTSA) has tested thousands of vehiclesto determine vehicle crash-worthiness. Thetarget vehicle speed in the tests has been35 mph. While most of the test data is not

c= (ft) Equation 5v

3

relevant to the barrier design, some is useful.A limited survey of the test results showsthat, for rigid barriers, the car crush distance

cranges from 1.1 feet (0.32 meters) to 2.2feet (0.66 meters) at the impact speed of 35mph. Assuming a second-degree relationshipbetween car crush and impact speed, c canbe approximated by the following equation:

Where vis car speed in miles per hour. Bysubstituting the values of c into Equation

2 (page 24), an impact force-velocity graphcan be obtained as a design aid Figure 5. TheIBC-prescribed force of 6,000 lbs. for edge

barrier design is considerably smaller thanthat predicted by the energy principles asshown in Figure 5. Therefore, it is suggestedthat the code requirements should be revisedto reflect the anticipated force levels usingenergy principles.

Design ExampleConsider a 6,000 pound vehicle rolling

down a ramp and crashing into a rigid barrier,as shown in Figure 4 (page 25).Ramp length, X = 200 feetStory height, H = 10 feetCoefficient of friction, = 0.017

Assuming the car starts rolling down frompoint A and using Equation 4 (page 25),Velocity vat B = 14.3 mph.For rigid barriers, b= 0. There are two ways

to determine the impact force:Using Equation 5,c= 1.26 feet. Substituting

the c value in Equation 2 (page 24), theforce F= 31,500 pounds. Alternately, use theforce-velocity graph in Figure 5to computethe force.

Mohammad Iqbal is Senior Vice Presidentand General Counsel at Walker ParkingConsultants. Dr. Iqbal is a member of the

bar in Illinois, holds a D.Sc. degree in civilengineering and is a licensed P.E. and S.E.in several states. Mr. Iqbal may be reachedat mo.iqbal@walkerparking.com.

Figure 5: Impact force on a rigid barrier. Impacting vehicle weight = 6,000 lbs.

0 .0

2 0 .0

4 0 .0

6 0 .0

8 0 .0

0 5 1 0 1 5 2 0 2 5

ImpactForce(Kips)

Vehicle Speed (mph)

Energy Eq.

IBC Code

SummaryA vehicular impact at edge barriers in

parking structure involves an enormouamount of energy that needs to be absorbeby the barrier and the vehicle. The magnitudof impact energy depends upon vehicular masand speed as well as on barrier characteristicsas illustrated by the force-velocity relationshishown in Figure 5. Therefore, the presenIBC approach to use one force level to desigbarriers sited along various locations in oalong the perimeter of a parking structuris improper and inadequate. In order teliminate or curtail fatalities caused by thbarrier failures, the building codes shoul

incorporate the energy principles in edgbarriers design requirements. A minimumspeed of 10 mph and vehicular weight o6,000 pounds are recommended in design. Alocations in a parking structure where vehiclecan gain greater speed, such as on dowramps, anticipated vehicular speed should bcalculated and the barrier force requirementshould be increased accordingly.

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Acknowledgement

The author is thankful to WalkerParking Consultants/Engineers, Inc. fortheir support. The opinions expressedin this article are the authors own andnot necessarily of any other individual,

association or entity.