1. Cyclic Response and Low Cycle Fatigue Characteristics of Plate Steel
Research Assistant: Peter Dusicka
Experimental evaluation of the stress-strain behavior and the low cycle fatigue life was conducted on five grades of plate steel to determine their suitability for use in earthquake engineering applications. The figure below shows the test set-up that was for this investigation. The steel grades ranged from high performance steel HPS 70 ksi to low yield point steel LYP 14.5 ksi. The coupons were tested using reverse cyclic axial strains of primarily constant strain amplitude between 1% and 7% strain at a constant rate 0f 0.1%/sec. The cyclic stress increased with strain amplitude for all of the steel, but the experimental results indicated a strong dependency on the steel grade. The cyclic stress-strain response was idealized by a power law relationship and cyclic hardening was evident in specific cases when compared to the monotonic response. The cyclic stress was nearly two times the yield stress for A709 Gr 50 and nearly five times the yield stress for low yield point steel LYP Gr 14.5 ksi. These experiments showed that all of the steel would be suitable for earthquake engineering applications, although the effects of welding or multi-axial stresses were not considered.
View of Test Set-up Used for Low Cycle Fatigue of Plate Steel
2. Displacement Ductility of Steel Members under Axial Tension
Research Assistant: Jeremy Woodgate
Experimental investigation was conducted on seven ASTM A36 steel members subjected to increasing axial tension until fracture. The members were chosen with various ratios of net cross- sectional area An to gross sectional area Ag. The figure below shows the test-sup for this investigation. The results of this study showed that the displacement ductility is highly dependent on the ratio of An/Ag and the geometrical shape of the members. It was also found if the ratio of An/Ag is less than the specified yield to ultimate strength Fy/Fu, the displacement ductility will be close to 1, which means the member will fail in a brittle manner.
View of Test Set-up Used to Determine Axial Displacement Ductility
3. Cyclic Behavior of Single Angles for Ductile End Cross Frames
Research Assistant: Francisco Garcia-Alvarez
The cyclic inelastic behavior of single angles specimens with dimensions typical of those used for the diagonals of X-braces in bridge cross frames was investigated. The figure below shows the test set-up that was used for this investigation. It was shown that angles can be designed to achieve a ductile response with large inelastic strains prior to failure. Failure of single angles is dictated by the properties of the connections, such as bolted connections with An/Ag ratio of 0.81 failed with limited axial ductility due to the fracture around bolt holes. Using a thickening plate to increase the An/Ag ratio and move the fracture of the specimen to outside the connection region resulted in improved behavior. Connections with balanced welds between the angles and the gusset plates exhibited the most favorable cyclic behavior with the largest maximum strain and cumulative plastic strain. For members where failure was prevented in the connections, the resulting maximum stains are greater than 6% and effective cumulative plastic strains greater than 113%
View of the Test Set-up Used for Axial Member Experiments
4. Cyclic Behavior of Buckling Restrained Braces for Ductile End Cross Frames
Research Assistant: Lyle P. Carden
The cyclic behavior of buckling restrained braces was investigated for potential implementation in ductile end cross frames at the supports of steel plate girder bridges. The figure below shows the test set-up that was used for this investigation. The BRB exhibited excellent hysteretic behavior with similar properties in tension and compression. Slippage in the connections was observed in some of the braces, although this did not have a significant effect on the energy dissipation characteristics of the brace. The braces were subjected to different loading histories with displacements up to an equivalent axial strain in the core of the brace of 3.7%.
View of Test Set-up Used for BRB Cyclic Axial Experiments
5. Flange and Web Limit States in Beams Subjected to Patch Loading
Research Post Doctorate: Lyle P. Carden
Recent field observations during bridge construction revealed excessive localized deformations in sill and cap beams that support falsework posts. Experimental and analytical investigations were carried out to determine the limit states of beams that are used under falswork. The figure below shows the test-setup that was used for this investigation. Critical limit states were found to be related to flange bending, post compression, and the interaction associated with patch loading between beam flange and timber or steel post.
View of Steel Post and Sill Beam Experiment