Impact Reg Has Some 'Give'

Jan. 1, 1998
If the new trailer guards work as predicted, will side-impact protection for trailers and rear-impact guards for straight trucks be next?New trailers and semitrailers delivered after January 26 will not be sporting the familiar ICC bumpers. After nearly 50 years of service, the long-time guardians of rear ends and some-time equalizers of the disparity between trailer floors and automobile hoods have

If the new trailer guards work as predicted, will side-impact protection for trailers and rear-impact guards for straight trucks be next?

New trailers and semitrailers delivered after January 26 will not be sporting the familiar ICC bumpers. After nearly 50 years of service, the long-time guardians of rear ends and some-time equalizers of the disparity between trailer floors and automobile hoods have yielded to a new generation of protection.

The new rear-impact guards are 22 in. high, which is 8 in. closer to ground level than their predecessors. But according to the National Highway Transportation Safety Administration's (NHTSA) new rear-impact rule, 49 CFR 571, the latest generation of guards must also conform to the rigorous performance requirements set forth in Federal Motor Vehicle Safety Standard 223.

FMVSS 223 requires that the new guards be able to absorb "by plastic deformation" some of the energy generated in a rear-end accident. Thus, they can help reduce the severity of rear-end car/trailer collisions, giving air bags, seat belts, and sheetmetal-crush strategies designed into today's autos more opportunity to control crash forces.

A companion standard, FMVSS 224, states that all new trailers rated at or over 10,000 lb. GVW must have the new guards. The only exceptions are dump trailers with rear spreader pans, low chassis with wheels permanently positioned at the rear, specialty vehicles, temporary living quarters, and pole trailers.

Accident statistics compiled by Failure Analysis Associates for the Truck Trailer Manufacturers Assn. (TTMA) show that the average annual fatality rate between 1978 and 1993 for rear-impact accidents is relatively low, 32 per year. But NHTSA wants to save as many lives as possible. When all applicable trailers are equipped with the new rear-impact guards, the Agency believes 4 to 15 PCI (passenger compartment intrusion) episodes will be prevented annually. If so, NHTSA might want to extend the rear-guard mandate to new straight trucks.

Since trailers can be hit from behind by a wide range of cars and light trucks traveling at varying speeds, it wasn't easy to determine just how compliant the new impact guards should be. In the end, NHTSA determined that impact should be capable of absorbing 5,650 joules -- or 4,170 lb.-ft of impact -- through "plastic deformation" within the first 5 in. of deflection. With this degree of compliancy, the guards are said to be strong enough to resist PCI at speeds of 30-35 mph.

When the energy absorbency requirement, published as part of the Final Rule, appeared for the first time in the Federal Register on January 24, 1996, it took the industry by surprise. Trailer and guard makers began a wild flurry of finite-element analysis and physical testing.

Guard makers say that it was not that difficult to come up with designs that met the dimensional requirements for the many different types of trailers. In a few cases, the oft-maligned ICC bumpers came reasonably close to fulfilling the strength requirements set forth in FMVSS 223. The real challenge was to produce whole families of guards that could economically meet the energy-absorbency requirement.

For many guard makers, it seemed as though energy-absorbency demand might be satisfied using a fixture that held prototype units in place while a hydraulic ram applied the requisite amount of pressure to three test points mandated by the standard.

The new impact guards must also be able to withstand a minimum of 11,240 lb. of static force on the curbside, roadside, and center points of the horizontal member, and 22,480 lb. where the horizontal member attaches to the vertical member.

Test stands were a helpful start. As it turned out, however, assuring proper levels of energy absorbency in the most efficient manner was not something that could be handled by the guard alone. In fact, in almost every case, trailer structure had to be incorporated into the equation.

As a result, most trailer makers elected to weld the guards directly to structural attachment points beneath the trailers. Some manufacturers, Great Dane Trailers, for example, back-braced the impact guard into the upper bogie rails. With a unitizing approach, contact points at the rear of the trailer and the guard are united to form a single compliant system.

However, some characteristics of the new guards could present problems. For instance, it's not difficult to imagine a scenario in which a tractor driver has to pull a heavily loaded 53- or 57-ft. trailer (with slider moved forward) up a steep slope: A guard that's less than 22 in. off the ground could easily scrape the surface and sustain severe damage to itself, as well as the attaching trailer structure. A slightly damaged guard could -- but probably won't -- be grounds for slapping an out-of-service sticker on a trailer at a roadside safety inspection.

But what will ultimately give fleets heartburn is maintaining the energy absorption factor that manufacturers certify is inherent in the guards. Shop technicians are not physicists. They have no means of verifying whether a replacement guard also restores original energy absorbency.

Repairs are also complicated by the practice of welding, rather than bolting, guards to new trailers. If a guard that is welded on is severely damaged, it must be replaced. The original weldments will have to be ground away so a new guard can be installed. Strive as they may to faithfully replicate an original installation, shops can never be certain whether what they do results in a weakened or even an overly strengthened system.

The FHWA is the first to admit that there's no way for fleets to know how close or far they are from restoring original energy absorbency. If a guard receives major damage, some of the underlying metal structure will undoubtedly be fatigued. The consequence of indirect damage to the trailer is additional loss of energy absorbency. How much of this lost absorbency can be recovered is an unanswerable question.

Unfortunately, this is precisely where common law and applicable safety standards could collide: Whether it's a piece of wire or a rear-impact guard, metal inevitably weakens when bent. This results in a loss of some of the original strength, as well as some of the structure's ability to absorb energy.

Let's assume that a trailer with one of the new impact guards is rear-ended by a car, resulting in a PCI. The lawyer for the driver of the car investigates and finds that the trailer's rear impact guard had sustained minor damage before the accident. He then claims that his client was severely injured because the energy absorbency of the guard had not been properly restored.

This scenario opens the door to a charge of negligence. The carrier would have to summon expert witnesses and argue that the guard did contain sufficient energy absorbency, which could prove to be a costly, difficult defense.

The only rear-impact guards that anyone can say with certainty have the proper amount of energy absorbency are those bearing certification labels on totally undamaged trailers. Any others -- to one degree or another --have been compromised.

Oddly enough, at presstime there was no regulation stating that fleets are responsible for maintaining the new rear-impact guards. However, one is expected to be issued soon -- perhaps in a matter of weeks.

According to Larry Minor, an FHWA mechanical engineer, the maintenance requirement will take the form of an amendment to the existing Federal Motor Carrier Safety Regulation on rear-end protection, 49 CFR Part 393.86. "The revised reg will restate the need to retain clearance and location distances of the devices, and inform fleets that they must keep the guards on trailers -- and maintain them in good condition," he says.

At present, there is no requirement to retrofit rear impact guards or to fit side impact guards onto trailers, although the FHWA is said to be considering such moves.

This much is certain -- side impact protection, if it is ever mandated, will produce significant cost and weight penalties.

Rear-guard action in the shop Fleets with a mix of new and old trailers will soon have two styles of rear guards to maintain: the old ICC guards, which are bolted on; and the new energy-absorbent types, which are welded on.

Heavy-vehicle rear-impact guard maintenance is specified in Motor Carrier Safety Regulation 49 CFR 393.86. Currently, heavy trailers and semi-trailers must be equipped with firmly attached rear-end devices designed to prevent rear impact.

The regulation's technical requirements for the ICC guard are familiar to maintenance departments: it cannot be more than 30 in. off the ground when the vehicle is empty; it must be located not more than 24 in. forward of the rear of the vehicle; and it must extend laterally to within 18 in. of each side. The scope of this regulation will soon be expanded to include maintenance of the new guards.

Since straightening a bent ICC bumper would be a time-consuming task, most fleets have instructed their trailer makers to bolt in the ICC bumpers at the time of trailer manufacture. This way, when an ICC bumper gets mangled it can easily be replaced.

Replacement will be much harder, however, with the new welded-in guards. According to Randy Cornell, vp-maintenance for Joplin, Mo.-based Contract Freighters Inc., rear-impact guards are already a fairly high-maintenance item. "They rank as our sixth highest trailer-maintenance item in terms of frequency, and eighth in terms of dollars expended," he says.

The new energy-absorbent impact guards pose a maintenance dilemma for the industry, Cornell explains. "We are expecting to take delivery of some new trailers later in the year. But at present there is no test that will enable us to determine whether we have successfully restored the energy absorbency to one of the new guards after it has been damaged," he says. "We are going to pay special attention to the condition of the guards, however. At the very least, we will do whatever it takes to make sure any damaged guards are repaired as quickly as possible."

About the Author

Stewart Siegel

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