I completed my Masters in May 2011 with my advisor Dr. Richard DeVries at Milwaukee School of Engineering (MSOE). We were testing the equations regarding bond length at end conditions. I specifically tested hooked rebar and straight bars as a control. This research was part of a greater series of test. A co-researcher was testing headed rebar. The abstract follows, and you can view the full PDF document here. All of the electronically accessible Structural Engineering research can be found here.
The purpose of this capstone design project report is to discuss the behavior of a hooked bar in concrete carrying a tension force. The cover and bonded length are varied to observe the effect on the load distribution between the hook portion and the bonded length portion of a hooked bar. Each specimen had 1 or 2 inches of cover. Each specimen had a bonded length of 8, 12, or 16 inches. Straight rebar was also tested to provide a control and comparison to the hooked bars with similar cover and bond length variables.
All hooked specimens experienced a steel failure. The straight bar specimens with 8 inch bond length and the straight bar specimen with 12 inch bond length and 1 inch cover experienced a concrete failure in splitting. The straight bar specimen with 12 inch bond length and 2 inch cover and the straight bar specimens with 16 inch bond length experienced a steel failure.
The addition of the hook adds enough strength to prevent concrete failure when compared to the straight rebar with similar bond length and cover. With bond lengths less than 12 inches and minimal cover, the addition of the hook adds enough strength for the steel to reach yield. The hooked bar stiffness was larger than the straight bar stiffness in every specimen except for the straight bar specimen with 16 inches of bonded length and 2 inches of cover.
An interesting trend is the effect of bonded length and cover on load distribution between hook and bond. As the bond length increased, the bond took a greater portion of the load. At shorter bond lengths, the hook takes the greatest portion of the load. An increase in cover increased the load carried by the bond in all cases. This effect was greater at shorter bond lengths since the hook carries the greater portion of the load.
It is suggested in further work to continue to include a load cell, slip measurement, and strain gauges to allow for comparison of maximum loading, load distribution, and stiffness. The method for measuring lead and end slip might be improved. The lead slip in this capstone report was erratic or failed to record due to the LVDT slipping off the angle. The bulk of research conducted on hooks concentrates on a pullout failure. Further research and data will be required to make any statements regarding splitting-controlled hook configurations. The data obtained in this capstone report are not sufficient to create a model that can be applied to the code.
Structural Engineer with a knack for creative solutions using code and ingenuity.
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