| The objective of this thesis is to investigate the feasibility of using a Falling Weight Deflectometer (FWD) as an impact source to rapidly assess the condition of an in service bridge through the evaluation of its modal properties. Tests were conducted on two bridges, a Parker Pony Truss and a modern concrete deck on rolled steel beam bridge. Two full tests were performed on these structures with a hand held impact hammer and a falling weight deflectometer as sources for excitation. The advantage of using a FWD is that many transportation agencies already use these machines for pavement testing and this would provide a means of quantitatively evaluating bridges. This thesis will discuss the advantages and disadvantages of using a FWD and how it compares to the use of a modal impact hammer. The experimental characterization results indicated that the FWD can be an effective device for the dynamic testing of highway bridges. The FWD enabled the identification of natural frequencies and mode shapes almost identical to those by typical hammer testing. Also, the extracted modal flexibility was in good agreement with modal flexibility extracted from the hammer test and static flexibility from the FE model. The FWD has some positive qualities that make it attractive for the impact dynamic testing of bridges. First, many departments of transportation already have these devices. Second, the FWD produces an excitation force significantly larger than what can be produced by a standard hand-held impact hammer; the larger force leads to better signal to noise ratios in the measurements and more closely approximates service loads on the bridge. Finally, the FWD provides a fairly consistent impact force from hit to hit and from day to day, which is difficult and labor intensive to accomplish with a hand-held impact hammer. |
