Torsional Behavior of Trapezoidal and Rectangular Concrete Box-Girders Reinforced with BFRP Bars and Steel Stirrups – An Experimental Study

Mariwan Mirhaj Mohamed-Salih, Ali Ramadhan Yousif


Experimental research on hollow members under combined loading is limited due to the complex nature of the problem and the huge experimental costs involved. This study aims at examining the differences in torsional strength and stiffness, cracking pattern, and failure modes between trapezoidal and rectangular concrete box girders reinforced with longitudinal basalt fiber reinforcement polymer (BFRP) bars and steel stirrups. This unique study utilizes exclusively rectangular and trapezoidal high-strength concrete (HSC) box-girders reinforced with BFRP bars as internal longitudinal reinforcement under the combined effect of torsion, bending, and shear. Six equal size and length specimens consisting of two rectangular and four trapezoidal specimens were prepared and tested. Every specimen contains about 2.5 percent total reinforcement, equally distributed between the longitudinal BFRP bars and transverse steel stirrups. The 5 m long specimens were restrained against torsion at the supports and subjected to an eccentric load at midspan, which produces a combination of torsion, shear, and bending but with torsion as the dominant action. The study variables were the effect of shape (rectangular vs. trapezoidal), the strength of concrete (high-strength vs. normal-strength concrete), stirrup configuration, and effect of overhangs on the ultimate torsional capacity to find which variable can enhance the ultimate capacity the most (without any increase in the amount of reinforcement). The most significant finding was that the transversely stiffened specimen, B-16, was able to offer the highest torsional capacity; this is achieved without any increase in the total amount of reinforcement but with a new confinement technique in which additional one-leg stirrups act as transverse stiffeners that hold all the cage together by interconnecting internal and external legs of the stirrups to resist the twisting action as one compact unit.


Keywords: box-girder, high-strength concrete, fiber reinforcement polymer bar, torsion, combined loading.

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