ABSTRACT

Impacts with narrow fixed objects such as poles, trees, retaining wall ends, and other obstacles require special consideration from the reconstructionist. For example, real-world poles may be fixed (or not, if they are mounted on a slip base), but they are not infinitely rigid. Nor are they infinitely massive. They can be separated from their base, pushed over in the ground, or totally or partially fractured (splintered). While they are not infinitely massive, their mass can be significant relative to that of the vehicle. Trees usually are not broken unless they are fairly small, but they can bend and sometimes recover. Trees can dissipate energy aerodynamically if their foliage whips about in the air, and by vibrating. Trees are living things and can grow and heal; some can even straighten out somewhat after being hit by a vehicle. On the other hand, some narrow objects are, for all intents and purposes, infinitely massive and infinitely rigid, such as the end of a concrete abutment.

Then again, the very narrowness of objects has effects on the vehicle. By definition, narrow objects are narrower than the vehicle, and do not engage the entire structure. The part that is engaged is exercised differently than in a barrier crash (in shear or bending, for example—not just in compression). In a barrier crash, the behavior of its more compliant parts gets averaged with the action of the stiffer parts, but such “averaging” may not occur when something narrow is hit. In any case, frontal narrow-object crash tests are comparatively few in number, effects are not well understood, and ignorance abounds. Lord Kelvin would surely note the plethora of theories and the paucity of numbers.

Chapter 23 addresses the topics of wooden utility poles, poles that move, vehicle crush profiles, and impact speeds. Finally, information is presented regarding recent rigid side pole impacts conducted by the National Highway Traffic Safety Administration (NHTSA).