An Analytical Study of Improving Beam-Column Joints Behavior Under Earthquakes
Many existing worldwide Reinforced Concrete (RC) structures, such as non-ductile RC frames, were designed for gravity loads only during the 1950s through 1970s or earlier. Due to variations in the identification of seismic active zones by national codes, such structures may not satisfy the current design requirements, especially when lying in a recently identified seismic active zone. This is because such structures, as a result of poorer reinforcement detailing, may generally do not possess the adequate ductility and strength needed to withstand an expected earthquake. Consequently, older RC frames may undergo substantial damage during earthquakes. One of the main damage aspects in such case is clear cracks around and within the beam-column connections. This is the case where the failure of beam-column joints is governed by bond and shear failure mechanisms which are usually brittle. This may be attributed to inadequate shear reinforcement in the beam-column joints region. Accordingly, several techniques of repairing and strengthening beam-column joints in older RC frames have been reported especially in earthquake prone countries. In this paper, a finite element model for an exterior beam-column joint is presented to simulate the behaviour of such joints in older gravity load designed RC frame structures. Several specimens are studied, one for the unstrengthened case, and others represent strengthened cases with different techniques. Studied strengthening techniques include using banded joints with CFRP sheets as a proposed technique, or joints reinforced with steel jackets as observed from older research in literature. Each case is modelled then analysed when loaded incrementally till failure. The stress and deformation results are evaluated then compared for each case. Numerical results show that the beam-column joint strengthened with CFRP can increase their structural stiffness, strength and energy dissipation capacity in contrast to other techniques. The proposed strengthening technique is even advantageous for practical requirements.
KeywordsRetrofitting RC frames Beam-column joint CFRP Earthquake
Reinforced Concrete (RC) structures including residential, administrative as well as historic and transportation structures widely exist worldwide. Each of these structures has its own role and importance. These structures thus need to be conserved against sudden failures that would cause consequent human life and economic losses. In the meantime, earthquakes may abruptly occur anywhere around the globe. The effect of earthquakes on concrete structures varies, and ranges from mild responses for well- designed structures to severe responses with extensive damage in case of poorly designed structures.
In addition, one of the popular RC structural systems is the moment resisting frame system. This is the case such that many moment resisting RC frames were designed in the last century or earlier and still exist worldwide. Meanwhile, such older RC frames may not satisfy the current seismic design requirements, and do not possess adequate ductility. Previous research studies attributed that to inadequate shear reinforcement in the beam-column joint region, as stated in Alemdar and Sezen (2010) and Ghobarah and Said (2002). The behavior of the beam-column joint in RC frames is thus a crucial point that requires good design and detailing when strengthening RC moment resisting frames (Kam et al. (2011), Alemdar and Sezen (2010), and Sasmal et al. (2011a)). Several techniques of repairing and strengthening of RC joints, to better withstand earthquakes, have been reported in earlier research in the literature. Some techniques used steel jackets as Jalil et al. (2014) and Sasmal et al. (2011b), while others used Concrete jacketing as Karayannis et al. (2008), and sometimes FRP wrapping as Naveeena and Ranjitham (2016), and Sasmal et al. (2011b).
1.1 Problem Definition and Research Objectives
Prior to modern codes that include detailed information for designing structures under seismic loads, older RC frame structures were designed only for gravity loads. This is even the case when considering that the seismic active zones varied in the national codes by time, such that some older structures are presently lying in active seismic zones, though this was not the case in the past. Such existing RC frame structures, though performing well under conventional gravity load case, could lead to questionable structural performance under earthquakes. In most cases, those structures are vulnerable to any moderate or major earthquake, and thus need immediate assessment and retrofitting to avoid a sudden full or partial collapse scenario bringing considerable losses in human lives and economic assets.
In this paper, objectives include developing an effective rehabilitation to strengthen beam-column joints in older structures to improve their seismic performance in terms of lateral strength and serviceability. Accordingly, an analytical model of a beam- column joint is proposed, and simulations are performed to study the strength and serviceability of retrofitted beam-column joints using several techniques. The advantages and disadvantages of the different studied strengthening techniques are identified, and results are compared to a proposed strengthening technique using CFRP sheets.
2 Literature Review
2.1 Beam-Column Joints
Joints Behavior Under Seismic Loading
2.2 Fibre Reinforced Polymer (FRP)
Fibre Reinforced Polymer (FRP) composites comprise fibres of high tensile strength within a polymer matrix such as epoxy. FRP composites are used in a lot of applications such as aircraft, helicopters, space-craft, satellites, ships, submarines, automobiles, chemical processing equipment, sporting goods and civil infrastructure (Liyoung et al. (2002)), Abhishek (2012), and Sreelatha (2013).
In general, one of the advantages of using FRP products would be strengthening of the existing or new RC structures with the possibility of application without disturbing the existing functionality of the structure (Liyoung et al. (2002) and Sreelatha (2013). In addition, FRP composites had proved to be extremely useful for strengthening of RC structures against both normal and seismic loads as stated in previous research as shown above. Moreover, most of the elements of a structure can be applicably strengthened with FRP composite materials. Currently, this method has been applied to strengthen structural elements as columns, beams, walls, slabs (Nikita et al. (2015), Sasmal et al. (2011b) and Obaidat et al. (2010)). This means in fact that FRP composites can take up the majority of the forces developed in a structure as long as they are transmitted by the strengthened element to the composite one as tensile stresses. Furthermore, strengthening with externally bonded FRP fabric has shown to be applicable to many kinds of structures. The use of external FRP reinforcement may be classified in: (i) Flexural strengthening, (ii) Improving the ductility of compression members, and (iii) Shear strengthening.
Furthermore, Carbon Fibre-Reinforced Polymer, Carbon Fibre-Reinforced Plastic, or Carbon Fibre-Reinforced Thermoplastic (CFRP, CRP, CFRTP) are extremely strong and light FRP which contain carbon fibres. This is the case where carbon fibres give CFRP its strength and rigidity in terms of increasing the ultimate stress and elastic modulus respectively. Unlike isotropic materials like steel and aluminium, CFRP has directional strength properties. The properties of CFRP depend on the layouts of the carbon fibre and the proposition of the carbon fibres relative to the polymer. Advantages for CFRP include high tensile strength, high strength to weight ratio, low weight to volume ratio, excellent fatigue behaviour, and quicker application (Naveeena and Ranjitham (2016)). Thus, CFRP composites are able to strengthen beam column joints in terms of the shear capacity and ductility.
3 Method Statement
Within the context of this paper, an exterior beam column connection is studied in both the strengthened and the unstrengthened cases. It is proposed to increase the shear capacity of the beam column joints using CFRP sheets as shown below in this paper in contrast to using steel or FRP jackets as stated earlier in the literature. The studied beam-column connection is modelled and loaded incrementally till failure to simulate the joint strength against overturning moments, and to compare the joint behaviour on using different strengthening techniques versus the unstrengthened case. Results are studied and compared. Moreover, the load deflection response is compared for each of the retrofitted beam-column joints and the unstrengthened joint case.
A finite element model using ANSYS® software is generated, and used for testing the beam column connection problem. ANSYS® is a general purpose Finite Element (FE) modelling and analysis software. The properties of the different elements used in the studied model are explained below.
Discrete modelling is used in modelling the steel reinforcement with assuming that steel and concrete are perfectly bonded. All the reinforcements are modelled separately using LINK8 element, a 3-D spar element which is a uniaxial tension- compression element that is defined by two nodes with three translational degrees of freedom at each node. This LINK8 element is also capable of undergoing plastic deformation.
3.2 Numerical Case Study
The fixation plates and steel plates are modelled using Solid45 elements. Figure 7(A) shows the control specimen-1 model. It is important to note that FE modelling of beam-column joints in ANSYS® typically consists of three stages: selection of element type; assigning material properties; and modelling the geometrical mesh. In addition, the meshing process of the structure must be optimized to not only help in reducing considerable amount of time, but also in reducing the memory requirements of the system and to link structural elements together. Figure 7(B) shows the mesh model for the control specimen-1 model.
Retrofitting with Prestressed Steel Angles
Retrofitting with Surface CFRP Sheets
Retrofitting with Steel Jacket
Retrofitting with Reinforced CFRP Diagonal Wrapping
4 Analysis of Results
From the above mentioned figures, it is clear the cracks were concentrated for the unretrofitted case that showed the lowest strength. In spite of the fact that cracks propagation are much less in the case of specimen-4, yet, the ductility was less than the case of the other specimens which undergo higher deflections. The cracks in the case of specimen-5 are more than the case of specimen-4, however, there is not clear difference in terms of the strength of the specimen.
In summary and as clear from Fig. 20, it can be concluded that the retrofitting technique used in specimen-5 may be considered the best among the different specimens. This is the case especially that applying the proposed retrofitting technique in specimen-4 for GLD structures may not be practical due to clear difficulties in applying without harming or varying the structures facades. In contrast, retrofitting using the technique of diagonal wrapped CFRP sheets can be easily applied without varying the old facades.
A survey study was undergone in this paper to identify a suitable retrofitting technique for beam-column joints. Different retrofitting techniques were tested based on previous studies from the literature together with a proposed retrofitting technique using reinforced CFRP diagonal wrapping. Results indicate that beam-column connections strength can clearly increase by using steel jacketing or the proposed technique of wrapping CFRP sheets in a diagonal setup to be perpendicular to the expected shear cracks in the beam-column joint zone. Furthermore, using the proposed technique proved to show better ductile response than other techniques suggested in some previous research related to the same study point. From the overall study, it can be concluded that the strengthening with CFRP sheets as proposed will increase the serviceability of the structure without damaging the facades.
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