Influence of intermediate diaphragms on load distribution in box girder bridges | |
| Author | Pichai Nimityongskul |
| Call Number | AIT Diss. no.D7 |
| Subject(s) | Girders Bridge construction Bridges |
| Note | A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Engineering of the Asian Institute of Technology, Bangkok, Thailand |
| Publisher | Asian Institute of Technology |
| Abstract | The static behavior of simply supported box girder bridges with or without intermediate diaphragms subjected to concentrated joint loads is investigated. The elements in the box section are treated as rectangular plates subjected to lateral and in-plane boundary forces. The end diaphragms are assumed to be infinitely rigid in and flexible normal to their planes. The intermediate diaphragm is assumed to act in such a way that it exerts only concentrated vertical and horizontal reactions on the joints of the box section without introducing resisting moments against joint rotations. The values of these reactive forces are determined from the conditions that the rigid body displacements of the intermediate diaphragm are compatible with the joint displacements under the simultaneous effect of the applied loads and the reactive forces, and that the reactive forces on the diaphragm are in self equilibrium. Continuous spans over intermediate supports are also treated in a similar manner. Numerical results for a simply supported double cell box girder bridge subjected to unit loads applied separately at the middle and side joints are initially obtained for the case without intermediate diaphragms. These results are then used as influence coefficients in developing the solutions for cases with one intermediate diaphragm at the midspan and three intermediate diaphragms at the quarter points. Numerical solutions for a two-span continuous box girder bridge with and without intermediate diaphragms are also obtained by setting the rigid body displacements at the joints of the diaphragm at the intermediate support equal to zero. The displacements, transverse bending moments, normal forces and membrane shear forces for cases without intermediate diaphragms, with one intermediate diaphragm and three intermediate diaphragms due to unit loads applied at different positions along the span are plotted together and compared in order to visualize the influence of the intermediate diaphragms. The results show that the distortion of the cross section of a box girder without intermediate diaphragm is more prominent when loaded along the side joints. With one intermediate diaphragm, the distortion at the loaded section remains practically the same when the diaphragm is sufficiently far from the applied loads, but is considerably reduced when the diaphragm is located near the loads. The use of intermediate diaphragms decreases effectively this cross sectional distortion throughout the span and the associated transverse bending moments, increases the overall stiffness of the bridge and redistributes the longitudinal normal forces. For the set of parameters investigated, the use of three intermediate diaphragms at quarter spans seems sufficient to practically eliminate the cross sectional distortion even when the loads are applied midway between two diaphragms. The proposed method of analysis with unit antisymmetrical joint loads is an accurate and efficient means for analysing the behavior of multi-cell box girder bridges with or without intermediate diaphragms and supports. With the availability of computers nowadays, the proposed solution can be readily used to obtain the design of box girder bridges with optimum numbers of intermediate diaphragms. |
| Year | 1973 |
| Type | Dissertation |
| School | AIT Publication (Year <=1978) |
| Department | Other Field of Studies (No Department) |
| Academic Program/FoS | Dissertation (D) (Year <=1978) |
| Chairperson(s) | Lee, S. L.;Pama, R. P. |
| Degree | Thesis (Ph.D.) - Asian Institute of Technology, 1973 |