An analytical solution of the impact behavior of sign posts

Present highway design concepts for multilane facilities have resulted in the placement of large signs near the edge of the traffic lane. Collisions with the supports of these signs have resulted in a number of fatalities. The Texas Transportation Institute, at Texas A&M University, has explored methods of sign support construction that will limit impact forces to tolerable limits. The concept chosen as the most feasible is the "break-away" post. In this concept the post disengages from its foundation at impact and is knocked out of the vehicle path. Two mechanical features are employed to achieve this action: a base plate that is capable of restrained movement and a "plastic hinge," or weakened moment section, in the upper portion of the post. In this research a mathematical model was developed which simulates the behavior of the "break-away" post under the impact of a vehicle. In the development of the model a procedure is presented for the response of a finite inelastic beam subject to time-dependent forces. Provision is made for various support conditions. The mathematical analysis is based on a lumped-mass idealization. The differential equations of motion for this system are solved by the use of a constant acceleration forward step in time integration technique. The method developed for the finite inelastic beam is applied to the "break-away" post problem. The mechanical features employed in the "break-away" concept are simulated in the model. While the exact characteristics of these devices are not known, the model is formulated in such a fashion that the "break-away" parameters can be introduced as input information when test data becomes available. The mathematical model of the "break-away" post was verified by correlation with test results from a full scale field test. This correlation shows that the model can be used to predict the behavior of a full scale sign with satisfactory engineering accuracy. Further studies are presented that show the effects of the various parameters of the system on the response of the "break-away" post. Studies of this type are valuable for proving specific designs and developing design criteria. It is anticipated that the model will be used to select promising designs for full scale testing. It will also be valuable in designing and selecting test instrumentation