Burns & McDonnell 7 TECHBriefs 2003 No. 2

By Philip Terry, P.E., and Mike Tholen, P.E.Vehicle barriers are used to restrict vehicles fromentering areas where they might endanger peopleor damage property. Light (usually movable orremovable) barriers inform drivers that vehiclesare not permitted beyond certain points. Theeffectiveness of these barriers is predicated onthe public’s desire not to cause even minor dam-age to their vehicles. Typically these barriers aresmall-diameter, movable, closely spaced postsand are not designed, but they may be designedfor a 6,000-pound lateral load in accordancewith ASCE 7-02, Minimum Design Loads forBuildings and Other Structures. Although effec-tive at hindering normal traffic, they are easilydeformed and will not stop drivers intent ondriving over them.

Heavier barriers are used for more positive pro-tection against heavy and/or rapidly movingvehicles. Examples of these barriers are posts attruck access doors or at the corners of buildings,and Jersey or proprietary barriers in constructionzones. Typically, protective bollards or posts arenot designed for project-specific loads. Rather,standard architectural or civil engineer details,developed many years ago, are used.

The U.S. Department of Transportation usesstandard crash tests to evaluate highway barriersystems. These barriers effectively limit the trav-el of errant vehicles but, even when linkedtogether, can move many feet.

Another class of vehicle barrier is becomingmore common, the security barrier. There aretwo types: Flexible/movable barriers that gradu-ally slow a vehicle, and rigid barriers that takethe impact with little or no movement. Flexiblebarriers include cables, posts, guard rails, buriedtires, planters, fences and gates that encapsulateor disable vehicles. Both the flexible and rigidbarriers dissipate energy through elastic andinelastic deformation of the vehicle and barriermaterials. Flexible/movable barriers can move20 feet or more.

The fixed/rigid barrier is of particular interestbecause it must withstand very large forces.Rigid barriers must be stiff enough to provide an

impediment that a vehicle can not run over orpush out of the way.

The force that a vehicle barrier must resist islargely dependent on the mass and velocity ofthe vehicle. The force required to stop a largevehicle at a relatively low speed may be thesame as that required to stop a much smallervehicle traveling at a higher speed. This conceptis expressed in terms of the kinetic energy (KE)of the moving vehicle. The key to an effectivevehicle security barrier is to determine a way todissipate/absorb the kinetic energy of the vehicle.

Kinetic EnergyA moving vehicle has kinetic energy (KE) =1/2mv2, where "m" = mass of the vehicle and "v"= velocity of the vehicle. A medium-weightautomobile or light truck, traveling at cityspeeds, could easily have a KE of 135,500 foot-pounds.

If a barrier is permitted to move when hit, muchof the energy from the impact is expended infriction from the barrier being dragged along theground. Typically, flexible barriers are linked ortied together to form a barrier system. When oneelement is hit other parts of the barrier systemare engaged, thus adding to the mass that isdragged along the ground and deformed. For aflexible barrier, energy transfer usually occursover a period of several seconds.

For rigid barriers deceleration rates are veryhigh. Numerous instrumented tests show thatmost energy transfer in a head-on vehicle impactwith a rigid barrier occurs within 0.2 secondsand can be as short as 0.07 to 0.12 seconds.

Equivalent Static Design ForceStructural engineers may not be accustomed tocalculating and considering kinetic energy.Design criteria for a vehicle impact wouldinclude such items as: vehicle weight (which canbe converted to mass), approach speed, directionand approach angle, vehicle width, vehicle track(tire centerlines), and height of impact.

Two methods can be used to determine thedesign force for the barrier: 1) determination of

Vehicle Barrier DesignDetermining Degrees of Effectiveness

For more information on this subjectplease send an email to the followingaddresses:Philip Terry <techbriefs@burnsmcd.com> Mike Tholen <techbriefs@burnsmcd.com>

TECHBriefs 2003 No. 2 8 Burns & McDonnell

average deceleration rate and 2) direct calcula-tion of force value based on vehicle crush data.The average deceleration rate is determined bycomparing the KE of the design vehicle with KEof test data culled from published literature.Thus, the basic equation for the design force onthe barrier is: F = ma, where "F" = force on bar-rier and "a" = deceleration rate of the vehicle.In the direct calculation method, crushing char-acteristics of tested vehicles are used. The forcerequired to stop a design vehicle can be deter-mined by equating the KE of the design vehicleto the work energy required to crush a similarvehicle.

Design Vehicle Collision Information The following items summarize some of thepublished literature available on flexible andrigid barriers and vehicle crushing:a. There is considerable variation in acceleration,force test data and computed results, even forsimilar vehicles and similar tests. Following arethe reasons for these variations: • Approach angle and size of barrier affect

deceleration values because different portionsof the vehicle frame are involved in the crash

• Differences in vehicle construction (automo-biles versus trucks and vans)

• Readings from accelerometers at various loca-tions (engine tops, engine bottoms, dummyoccupants, within crushing zone) vary consid-erably due to restraint, damping and springaction of the support.

b. Despite the large variation in data and testingmethods, it is possible to deduce that an averageautomobile or light truck impacting a rigid barri-er at city speeds would have a lower-bounddeceleration in the range of 16 to 22g. The max-imum/peak deceleration value is in the range of62 to 100g. This peak deceleration occurs for avery short period of time (0.01 second). Theaverage deceleration value is in the range of 25to 31g. For barrier design, the average decelera-tion rate (rather than the maximum rate) shouldbe used to account for crushing of the designvehicle.c. Figure 1 shows typical variations in accelera-tion with respect to time in a head-on impact.This graph is from Society of AutomotiveEngineers (SAE) Technical Paper 930899.

d. Other sources of ener-gy dissipation are notconsidered directly (suchas rotations of the vehi-cle or barrier, plastic andelastic deformation ofthe barrier, elongation ofthe anchor rods, and damage to the barrier).

Design Force on Rigid BarrierThe design force could occur at any point on thebarrier, so it must be assumed that it will occurat the points that cause the most overturning,shear, axial compression or tension, bending orrotation of the barrier, and to the supportingstructure. For barrier design, several simplifyingassumptions are used:a. The barriers are considered rigid and deflec-tions are intended to be negligible. Assumingrigid barriers means that the designer will haveto make it rigid compared to the vehicle.b. Some kinetic energy will be converted to elas-tic deformation of the vehicle (rebound), elasticand inelastic deformation of the barrier, damageto the barrier, sound, light, and heat, but mostkinetic energy will be converted to permanentdeformation (crushing) of the vehicle. Based onan average deceleration of 28g, an average vehi-cle at city speeds would produce a design forceof approximately 130,000 pounds.

Additional Design Assumptions For security barrier design, it may not be neces-sary or practical to treat the impact force as alive load or to apply live load factors. The typi-cally large forces will make it necessary toreplace the barrier and maybe parts of the sup-porting structure if it is hit. It may be more prac-tical to treat the impact force as already factored,thereby assuming that the full ultimate/inelasticcapacity of the barrier will be used.

Material strength reduction factors should beapplied for the materials because they relate toconstruction tolerances and variations in materialproperties. If the design vehicle impact occurs,local damage to the existing supporting structureand to the barrier is expected. These vehicle bar-riers are not crash-worthy and vehicle occupantscould be severely injured.

Time (seconds)

Acce

lera

tion

(g)

0 0.1 0.2 0.3-100

0

80

Figure 1 Vehicle Acceleration vs. Time

(from SAE Technical Paper930899)

Mike Tholen, Ph.D., P.E., is astructural engineer with theAviation & Architecture divi-sion of Burns & McDonnelland has six years of experi-ence in the design of govern-ment and industrial facilities.

Philip Terry, P.E., S.E., is anassociate structural engineerwith the Aviation &Architecture division of Burns& McDonnell, and has 25years of experience specializ-ing in the design of industrial,government and aircraft facil-ities.