New Methodology for Design and Construction of Concrete Members with Complex Stress Fields Using Steel Fiber–Reinforced Concrete

Reinforced concrete members with significant geometric discontinuities or D-regions experience complex stress fields under loading, which require considerable analytical effort and usually complicated reinforcement detailing. Large openings in RC members can interrupt the direct load transfer provided by concrete struts, thereby leading to overstressed localized regions and unexpected failure modes. Empirically based strut-and-tie models (STMs) are generally used to design the reinforcement detailing of these RC members. However, many prior studies indicate that the resulting details can be very complicated while the actual stress fields deviate significantly from that assumed by STMs, thereby leading to unpredictable failure modes. This study investigates the feasibility of using steel fibers to replace the majority of conventional reinforcing bars in RC deep beams with significant D-regions. The test beams have two large openings, which are located between the loading point and the supports, thus disrupting the direct flow of forces. A simplified procedure is proposed for designing and detailing the reinforcement of steel fiber–reinforced concrete (SFRC) specimens based on the stress fields from elastic finite-element analyses. Experimental results show that when the critical regions of a test specimen were reinforced appropriately by conventional reinforcing bars and the remaining portion of that specimen was reinforced by SFRC with 1.0% volume fraction of fibers, the reinforced SFRC specimen exhibited a ductile failure mechanism with very large plastic deformation. The reinforced SFRC specimens also showed much higher strength than the nominal design load and experienced slow postpeak strength loss. In comparison, although the RC specimen reached very high strength, it also showed an unexpected brittleness and localized failure behavior. This study also shows that finite-element simulation based on the modified compression field theory (MCFT) is able to identify possible failure mechanisms of reinforced SFRC specimens.