تاثیر دهانه های وب بر رفتار ساختاری تیرهای عمیق بتن مسلح با مقاومت بالا

The objective of this study is to experimentally and analytically estimate the influence of web openings in reinforced concrete deep beams. Thirty-two reinforced high-strength concrete deep beams with or without openings were tested under two-point top loading. Test variables included concrete strength, shear span-to-depth ratio, and the width and depth of the opening. Test results indicated that the strengths at diagonal crack and at peak were closely related to the angle of the inclined plane joining the support and the corner of the web opening. Also, the influence of concrete strength on the ultimate shear strength remarkably decreased in deep beams with openings rather than solid deep beams. From comparisons of predictions and test results, the equations proposed by Kong and Sharp, and Tan, Tong and Tang would be suitable for reinforced high-strength concrete deep beams with openings and θ3≥30∘θ3≥30∘.

Reinforced concrete deep beams are members in which a significant amount of the load is carried to the support by a compression thrust joining the loading and reaction point. ACI 318-02 Section 11.8 [1] specifies that deep beams should be loaded on loading points and supported on reaction points so that compression struts can develop between the loads and supports. For many years, the deep beam has been designed based on empirical methods for slender beams. However, some experimental results and analytical reports have indicated that shear strength, redistribution of internal forces before failure, and internal force mechanisms in deep beams are quite different from those in slender beams. Thus, in recent years, the methods for reasonable deep beam design, such as the strut-and-tie model, have been created. Openings are inevitably installed in deep beams to facilitate conduits, air conditioning, electricity, and computer network cables. If the openings interrupt the load path joining the loading and reaction point, it is obvious that the simple load path changes to a more complex one, and the shear capacity will be reduced. In spite of significant effects of the openings on the structural behavior of deep beams, there is no clear design guidance for deep beams with openings. In addition, most of its tests have focused on solid deep beams except for only very few research papers on deep beams to have compressive strengths in the range of 18–30 MPa including the effects of openings [2], [7], [8] and [10]. Based on the test results of high-strength concrete deep beams, Tan et al. [11] and Yang et al. [13] concluded that the effect of concrete strength on the nominal shear strength appears more significant in deep beams than in slender beams because most load is transferred by concrete struts. Thus, the application of high-strength concrete to deep beams is gradually increasing with the enhancement in concrete strength. Nielsen [9] proposed that the effective load-carrying capacity of a strut decreases with the increase in concrete strength. Also, in the Canadian Code [3] it is assumed that the effective strength of the strut is expressed as a function of the transverse tensile strain in the strut. It is potentially desirable to alleviate the effect of concrete strength for enhancing the load-carrying capacity of deep beams with openings, because of higher transverse tensile strains and a great deal of cracking around the corners of openings. This test program is to understand the structural behavior of, and to evaluate the effect of, concrete strength on shear strength in high-strength concrete deep beams with openings. The test results were compared with predictions proposed by Kong and Sharp [7], and Tan et al. [12].

This study illustrated the effects of web openings through the experiment to utilize concrete strength, shear span-to-depth ratio, and width and depth of opening as test variables, and analytical comparisons with the methods proposed by Kong and Sharp, and Tan, Tong and Tang. The results of this study are summarized as follows. 1. The width and depth of opening did not affect the mid-span deflection at initial loading stages, but it significantly affected the deflection after the occurrence of diagonal cracks. The concrete strength did not largely affect the rigidity of beams with web opening due to the existence of the web opening. 2. The slope of inclined plane to exhibit maximum crack width decreased with the increase in concrete strength, opening size and shear span-to-depth ratio. 3. The shear strengths at initial diagonal crack and at peak were closely related to θ3θ3. 4. The influence of concrete strength on the ultimate shear strength significantly decreased in deep beams with openings rather than solid deep beams. In deep beams with openings, it was evaluated that the strength of a compressive concrete strut increased at a lower rate with the increase in concrete strength. 5. The equations proposed by Kong and Sharp, and Tan, Tong and Tang would be suitable for reinforced high-strength concrete deep beams with openings and θ3≥30∘θ3≥30∘.