Carden, Itani, and Buckle 2006
The authors evaluate the performance of I-girders with ductile end cross frames that use single angle X braces acting as structural fuses. Shake table experiments were conducted in this study. Component testing was also conducted on single angles to determine their yield strengths for comparison with the stresses induced in the system testing.
System Concept
Employing ductile members to brace the transverse direction of bridges is intended to reduce the base shear on the substructure, where damage is more difficult to identify than the superstructure. The authors evaluate if single angles are ductile enough to reduce base shear through energy dissipation from their yielding. After the ductile members yield, they are intended to harden, but also to experience post-yield stiffness degradation. This allows them to deform greater amounts without breaking, acting as energy-dissipating fuse elements.
Experimental Study, Results and Discussion
A bridge model was tested under reverse static loading and a shake table with single angle X bracing welded or bolted to a plate connecting the bridge girders. A double angle served as a bottom chord, with a top chord in some tests. The top and bottom chord served as rocking mechanisms to allow for large girder drifts. Component testing also occurred on seventeen single angles.
Component testing on the angles showed initial failure in tension, followed by hardening. In compression, the angles buckled and immediately lost most of their strength.
The full bridge model was found to have greater shear forces when only elastic cross members were used. The base shear with ductile single angle cross members was 78% of that with elastic members for seismic loading of 1.5 times El Centro. The base shear was even further reduced with larger scaling of the forces of El Centro. Large drifts occurred with the single angle X bracing, but the self-centering capabilities of the top and bottom chord limited residual drifts.
It was also found that the concrete deck would not be distressed if minimum connection between the deck and girders is detailed.
Reference
Carden, L. P., Itani, A. M., and Buckle, I. G. (2006). "Seismic Performance of Steel Girder Bridges with Ductile Cross Frames Using Single Angle X Braces." Journal of Structural Engineering, 132.3, pp. 329-337.