Behaviour of RC beams with a fibre-reinforced polymer (FRP)-strengthened web opening
Introduction
For reinforced concrete (RC) frames subjected to seismic effects, there are mainly two possible failure modes: beam-sway mechanism (i.e., plastic hinges first form at the ends of beams) and story-sway mechanism (i.e., plastic hinges first form at the ends of columns). Beam failure usually affects only a limited part of the structure, while column failure may lead to progressive collapse of the whole structure. Therefore, beam-sway mechanism is preferred to story-sway mechanism if the failure of RC frames cannot be avoided. The strong column-weak beam design philosophy which can realize the beam-sway mechanism has been widely adopted in the seismic design of RC frames, according to a comprehensive literature review conducted by the authors [1]. To achieve the strong column-weak beam hierarchy in RC frames, a flexural strength ratio (i.e., ratio of the sum of flexural capacities of the columns at a joint to that of the beams framing into the joint) greater than 1 has been stipulated in the relevant design codes from different countries [1]. For instance, the current ACI code [2] and Eurocode [3] specify a flexural strength ratio of 1.2 and 1.3 respectively, and the current Chinese code [4] specifies a range of flexural strength ratio from 1.1 to 1.7.
However, studies have shown that the beam-sway mechanism rarely formed in failed RC frames after large earthquakes [5], [6], [7], although these frames were designed to achieve the strong column-weak beam hierarchy. For example, after the magnitude (Ms) 8.0 Wenchuan earthquake in 2008, only RC frames without floor slabs or with precast floor slabs exhibited the beam-sway mechanism, while the cast-in-place RC frames commonly failed at the ends of columns [6]. This is mainly because that the contribution of cast-in-place slab to the flexural capacity of the beam was not properly considered in design codes of previous versions, leading to an underestimation of the flexural capacity of beam and thus the violation of the strong column-weak beam hierarchy. For instance, the Chinese seismic design code of previous versions [8] ignored the contribution of cast-in-place floor slab to both the negative flexural capacity (i.e., the flange of the beam is in tension) and the positive flexural capacity (i.e., the flange of the beam is in compression) of the supporting beam [9]. Existing experimental studies on RC exterior/interior joints with a cast-in-place floor slab have confirmed that the floor slab can significantly enhance the flexural capacity of the beam, especially when the beam is in negative bending [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. Therefore, it is not unreasonable to believe that many existing RC frames in China and other countries/regions are likely to violate the strong column-weak beam requirement. The current design codes [2], [3], [4] require the consideration of the contribution of the cast-in-place slabs to the flexural capacity of the beams. A larger number of existing RC frames, however, cannot meet this requirement and need to be retrofitted.
In order to achieve the strong column-weak beam hierarchy of an existing cast-in-place RC frame which violates such hierarchy, the strengthening of column is a straightforward option [21], [22], [23], [24], [25], [26], [27]. There are mainly three common column strengthening techniques: (1) concrete jacketing [21], [22]; (2) steel jacketing [23], [24]; and (3) fibre-reinforced polymer (FRP) jacketing [25], [26], [27]. However, concrete jacketing or steel jacketing results in the increase in seismic forces by increasing the mass and/or stiffness of the columns, while the FRP jacketing is not effective enough for non-circular columns, compared with that for circular columns [28]. Furthermore, the successful strengthening of column may simply shift the failure location from the column ends to the beam-column joint, which is more difficult to retrofit. Against the above background, a novel seismic retrofit method was proposed to achieve good seismic performance of cast-in-place RC frames which violate the strong column-weak beam requirement, based on the concept of Beam-end Weakening in combination with FRP Strengthening (referred to as BWFS method hereinafter for simplicity) [29]. Three specific techniques were proposed to implement the BWFS method [29]: (1) the beam opening (BO) technique; (2) the slab slit (SS) technique; and (3) the beam section reduction (SR) technique. More details of the above three techniques can be found in Ref. [29], and the present study is focused on the BO technique. The BO technique involves the creation of a web opening in the T-section beam adjacent to the beam-column joint and the installation of local shear strengthening system (e.g., using FRP wraps and FRP U-jackets) around the opening (as shown in Fig. 1). It should be noted that the strong column-weak beam design philosophy requires that the Sum of Flexural Capacities (referred to as SFC hereafter for simplicity) of the beam at a joint (i.e., the sum of positive and negative flexural capacities of the beam) are smaller than that of the column at the same joint. If the size of the web opening is suitable, the SFC of the T-section beam can be ideally reduced to a desired value (i.e., the SFC of the corresponding rectangular beam). Compared with other shapes of web openings such as circular and elliptical web openings, rectangular web opening can be more flexibly designed to achieve a required reduction of the flexural capacity of the T-section beam, and at the same time, rectangular web opening leads to regular shapes of the chords which can facilitate the development of design methods. Therefore, rectangular web opening is adopted in the BO technique. Meanwhile, to prevent the brittle shear failure of the beam with a web opening, the shear strengthening using FRP is applied around the opening. Actually, creating web openings in existing RC beams is not a new thing. It has been widely adopted in practice for the passages of utility pipes [30], [31], [32].
A preliminary study has been conducted by the authors [33] to experimentally assess the concept of the BO technique in reducing the flexural capacity of the T-section beam. In the preliminary study [33], a total of eight RC beams, including one rectangular beam and seven T-section beams were tested. The rectangular beam (CB-Rec) and one T-section beam (CB-T) did not contain web openings and served as control specimens, while the remaining six T-section beams had a web opening. Two different web opening sizes (length × height being 700 mm × 300 mm and 800 mm × 280 mm respectively, referred to as web openings of large size) were examined. For each large web opening size, three beams were tested: one with an un-strengthened web opening (tested under negative bending) and two with an FRP-strengthened web opening (one tested under negative bending and the other one tested under positive bending).
In the preliminary study [33], the adopted web opening sizes were found to be too large so that the flexural capacity of the T-section beam was largely over-weakened. In addition, the web openings in the preliminary study [33] were pre-formed through manipulating the formwork for casting concrete, which is a simplification for test and not feasible for retrofitting of existing structures. The post-cut web opening, which is more practical for existing structures, needs to be investigated. To more comprehensively verify the effectiveness of the BO technique and to gain an improved and in-depth understanding on the behaviour of RC beams with a web opening, large-scale tests on RC beams with a pre-formed/post-cut web opening of more reasonable sizes were conducted and the test results are presented and analysed in the present study. In this paper, unless otherwise specified: (1) when presenting the test results, it is assumed that the web opening is located in the right shear span of the beam (i.e., the left opening edge refers to the edge closer to the loading point while the right opening edge refers to the edge closer to the right support); and (2) for the sake of brevity, the concrete chords in the beam flange and beam web are respectively referred to as flange chord and web chord.
Section snippets
Specimen details
A total of six large-scale RC T-section beams were tested under three-point bending in the present study. The studied parameters included the web opening size (i.e., length × height) and the effect of FRP strengthening. Four different web opening sizes (length × height being 600 mm × 220 mm, 700 mm × 200 mm, 600 mm × 280 mm and 700 mm × 260 mm respectively) were investigated. For web openings of 600 mm × 220 mm and 700 mm × 200 mm (referred to as web openings of small size for simplicity), each
Failure mode
The failure modes of all test specimens are shown in Fig. 11. For the two specimens with an un-strengthened web opening of small size (i.e., O-600 × 220 and O-700 × 200), the first crack appeared at the top-left corner of the web opening (i.e., the opening corner nearest to the loading point) and extended diagonally towards the loading point (around 45 degrees with respect to the horizontal direction). Then a horizontal crack between the flange and the web appeared at the bottom-right corner of
Effect of web opening size
From the above comparisons, it can be seen that a larger web opening unsurprisingly gives a lower load-carrying capacity and stiffness of the RC T-section beam. It can be found from Fig. 12 and Table 2 that the reduction in the load-carrying capacity of the RC T-section beam caused by an increase of 100 mm in the web opening length is comparable to that caused by an increase of 20 mm in the web opening height (e.g., O-600 × 220 and O-700 × 200; F-600 × 280 and F-700 × 260), which indicates that
Effect of FRP strengthening
As can be seen from Fig. 11a–d, the existence of the FRP strengthening system which included CFRP U-jackets on the beam web with their ends anchored with the beam flange using spike anchors and CFRP wrap on the web chord contributed to changing the failure mode of the web opening-weakened RC T-section beam from shear failure in the web chord to flexural failure at the two ends of the web and flange chords. Moreover, it can be found from Fig. 12 that the adopted FRP strengthening system not only
Assessment of the effectiveness of BO technique
As has been mentioned earlier, the proposed BO technique aims to reduce the SFC of the T-section beam to the desired value (i.e., the SFC of the corresponding rectangular beam). A strength model for RC beams with an FRP-strengthened web opening has been proposed by the authors [37] based on the test results of the preliminary study [33]. In this section, the comparison of the strength of RC beams with an FRP-strengthened web opening between the prediction of the proposed model and the test
Concluding remarks
As the contribution of cast-in-place floor slabs to the flexural capacities of RC beams was not properly considered in the design based on previous codes, a large number of existing RC frames may violate the strong column-weak beam hierarchy. Based on the concept of Beam-end Weakening in combination with FRP Strengthening (BWFS) method, a novel seismic retrofit technique (BO technique) which involves the creation of a web opening in the T-section beam and the installation of FRP strengthening
Data availability
The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
CRediT authorship contribution statement
X.F. Nie: Investigation, Methodology, Writing - original draft. S.S. Zhang: Conceptualization, Methodology, Supervision, Writing - review & editing, Funding acquisition. T. Yu: Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors are grateful for the financial support received from the Research Grants Council of the Hong Kong Special Administrative Region (Project No.: PolyU 5273/11E) and the National Natural Science Foundation of China (Project No. 51878310). The work presented in this paper was undertaken under the supervision of Prof. Jin-Guang Teng from The Hong Kong Polytechnic University. The authors are grateful to Prof. Teng for his contributions to this work.
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2021, Composite StructuresCitation Excerpt :To resolve this problem, a more advanced 3-D FE model needs to be developed in the future. The FE predicted ultimate loads of all beams tested by the authors [19,20] are compared with test results in Fig. 9 and Table 3. It should be noted that in Fig. 9 and Table 3, for the RC T-section beams with an FRP-strengthened web opening tested in negative bending, the term “DP model” means the DP model with the confinement effect from FRP strengthening being considered.