Behaviour of RC beams with a fibre-reinforced polymer (FRP)-strengthened web opening

Abstract

The beam opening (BO) technique, which involves (1) the creation of a web opening in the beam to weaken its flexural capacity and (2) the fibre-reinforced polymer (FRP) shear strengthening around the opening to avoid shear failure of the beam, was proposed to realize the strong column-weak beam hierarchy in existing reinforced concrete (RC) frames, which were designed using previous codes and thus violated such hierarchy. In the present study, six large-scale T-section beams with a web opening of different sizes and made by different methods (pre-formed and post-cut) were tested to assess the effectiveness of BO technique and the test results are presented. The experimental study shows that the post-cutting method is feasible for making web openings in existing RC beams, the BO technique can effectively reduce the flexural capacity of the beam to the desired value if the size of web opening is appropriate, and the FRP strengthening can well compensate the loss of shear capacity and ductility of the beam caused by the creation of web opening. The present study provides a basis for the safe use of BO technique to achieve the strong column-weak beam hierarchy in existing RC frames.

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.