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

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.