INTRODUCTION
It is estimated that out of half a million existing bridges in the 50 states, nearly 105,000 are rated critically deficient.(1,2) The number of structurally deficient bridges reported in the last six years in the federal-aid system has risen from 28,070 to 37,300 - an increase of 33%.(3) In Florida thousands of existing highway bridges are older than 20 years. Throughout the state some bridges are posted for lower than original design loads.
In many cases the proper rating of a bridge cannot be achieved by the present methods. of analysis. In most cases the bridge is small and on an off-system road; such bridges do not seriously impact commercial users. However, other bridges are on major systems and the resulting detours do impact the public and the commercial users.
Bridges of questionable strength that are posted for lower loads or are scheduled to be replaced can be examined through a load test. The information collected from such a test can be analyzed to evaluate the true strength of the structure. This information can be used in making decisions on the future of such bridges. The Federal Highway Administration (FHWA) requires states to consider all possible alternatives, including rehabilitation; before approving bridge replacement. In spite of all attempts to police loads, overweight vehicles do use our roads and bridges everyday. Furthermore, higher loads are expected in the future. Therefore, one needs to know the actual safe loads that these bridges can carry.
Bridge load testing will allow a satisfactory overall strength evaluation of any bridge under question. The information provided will greatly increase the possibility of selective rehabilitation, rather than the current practice of replacing the entire structure.
In Florida a large number of new and old bridges have been field tested during the past five years. The experience gained from these tests indicates that load limitations imposed by theoretical analyses are not representative of the structures real capacities. Proof loading has consistently indicated that structures have greater residual strength than indicated by analysis or design.