Behaviour and strut-and-tie predictions of high-strength concrete deep beams with trapezoidal web openings

Abstract

The present paper investigates the effects of the size of trapezoidal web openings and shear span-to-overall-height a/h ratio on the behaviour and shear strength of high-strength concrete deep beams. A total of six deep beams with compressive strengths in the range 84 MPa ≤ f′c ≤ 103 MPa (12 200 psi ≤ f′c ≤ 15 000 psi) were tested to failure under two-point symmetric top loading to determine their failure modes, diagonal cracking, mid-span deflection and ultimate shear strengths. All six beams were reinforced with 2·61% longitudinal main steel to prevent flexural failure at mid-span. They had an overall depth of 1000 mm and a web thickness of 150 mm with an a/h ratio of 0·5, 1·0 and 1·5. Trapezoidal web openings with different sizes were located in the middle interior of the beams. Test results indicate that the ultimate strength of a deep beam with a web opening is comparable with that of a solid beam, if the web opening does not interpose the force path—that is, does not reduce the strut width between the load point and the support. Otherwise, the ultimate strength will decrease significantly. A strut-and-tie approach is also proposed to analyse the shear strength of a deep beam with this kind of web opening.