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Experimental investigation of using mechanical splices on the cyclic performance of RC columns
Structures 24 (2020) 717–727
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Experimental investigation of using mechanical splices on the cyclic performance of RC columns
T
⁎
Ali Kheyroddina, , Amirhossein Mohammadkhaha, Hamed Dabirib, Ahmad Kaviania a b
Faculty of Civil Engineering, Semnan University, Semnan, Iran School of Science and Technology, University of Camerino, Camerino, Italy
A R T I C LE I N FO
A B S T R A C T
Keywords: RC column Mechanical splice Force–displacement hysteresis Ductility Absorbed energy Stiffness
In this study, the influence of using mechanical splices on the cyclic performance of RC columns under axial and cyclic loads was investigated experimentally. Totally six specimens were fabricated and examined; the first one was fabricated by non-spliced reinforcement bars which was considered as reference specimen. Overlap splice in the middle of the column was used in the second specimen. Threaded mechanical splices were used for the all of the longitudinal bars at 50 mm above the column-foundation interface in the third column. The fourth column was fabricated the same as the third one but half of the couplers were staggered at 600 mm above the bottom couplers. The fifth and sixth specimens were fabricated similar to the third and fourth column but shear screwthreaded couplers were incorporated instead of threaded couplers. Force-displacement hysteresis loops were presented as the output and parameters including lateral load bearing capacity, ductility, absorbed energy and stiffness are considered and discussed. Based on the obtained results, it was concluded that mechanical splices can considerably affect cyclic behavior of RC columns.
1. Introduction Cutting and splicing reinforcement bars is one of the most important topics raised in the reinforced concrete (RC) structures. In terms of construction cost, steel bars form a considerably large proportion of the total cost of a RC building. Therefore, applying economically optimized methods to patch bars has attracted researchers’ attention to reduce the construction cost. Reinforcement bars can be spliced by several methods including: bar overlapping, mechanical patching, couplers, and welding bars head to head. When overlap method is used the length of the overlapped bars must be equal or greater than the anchorage length of the bar. The significant problem with this method is that it is not economically beneficial especially when it is applied to the bars with diameter greater than 30 mm. As a result, using couplers has become common because not only it decreases the bar weight, but also it is easier and less timeconsuming to apply. Due to importance of bar splice methods in RC structures in seismically behavior and total construction cost and time, they have become one of the major researchers’ concern. Some studies have been conducted on the effective parameters such as length and pattern in overlap methods on the behavior of RC elements. El-Hacha et al. [8] evaluated the bonding strength and the ⁎
necessary anchorage length of the bars spliced by overlapping method. Isa and Nasr [10] studied side-by-side welding patches in order to get the proper welding length. They suggested that the welding length should be greater and equal to 1.5 times of the bar diameter. Moreover, their study concluded that welded splicing can be considered as a reliable, efficient, and consistent approach for bar splices. The influence of different overlap splice lengths on the cyclic performance of RC columns was evaluated by Goksu et al. [9]. In the study it was concluded that incorporating 180-degree hooks at the end of the overlap splice can improve the poor behavior of column due to inadequate overlap length even in case of using low-strength concrete. Moreover, all the specimens with different overlap lengths reached their flexural strength up to large drift ratios. In order to minimizing the trim loss through automating the process of lab splicing pattern generation, cutting pattern generation and trim loss optimization was proposed by Nadoushani et al. [12]. Flexural behavior of RC beam with lap spliced bars under reversed quasi-static cyclic loading was assessed experimentally by Najafgholipour et al. [13]. Their study showed that the beam which was designed based on ACI318-14 could not stand the reverse cyclic loading up to flexural failure, whilst the modified specimens (in terms of splice length and transverse reinforcement) survived cyclic loading with acceptable flexural ductility. The considerable benefits of mechanical splices, in terms of
Corresponding author at: Semnan University, Semnan, Iran. E-mail addresses: [email protected] (A. Kheyroddin), [email protected] (H. Dabiri).
https://doi.org/10.1016/j.istruc.2020.01.043 Received 17 September 2019; Received in revised form 21 January 2020; Accepted 28 January 2020 2352-0124/ © 2020 Institution of Structural Engineers. Published by Elsevier Ltd. All rights reserved.
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Nomenclature Ag Ccr D db Dc Fu Fmax
f’c Lc M Fy Sy Δy Δu μ
gross area of column section concrete crack diameter or width of column bar diameter external diameter of coupler ultimate force maximum force
compressive strength of concrete length of coupler diameter of threaded area in coupler yield force yield of reinforcement yield displacement in Eq. (1) ultimate displacement in Eq. (1) ductility
and ductility of RC elements. In addition, ductility and energy dissipation can be affected considerably by using threaded mechanical couplers [5]. An experimental study on the effect of elevated temperature on the mechanical properties of spliced steel reinforcement showed that using couplers affect ductility characteristics of threaded splices of elevated temperature [3]. Tazarv and Saiidi [17] proposed methods for designing seismic columns on different bridges by using mechanical splices. The research evaluated the influence of couplers on the higher plasticity of reinforced concrete columns and using mechanical splices in the precast segments of the seismic districts. Bompa and Elghazouli [4] evaluated different coupling systems by comparing their key performance parameters experimentally and numerically. The results indicated that the bar with threaded couplers had higher strains in comparison to the non-spliced reference bar within the elastic cyclic test, without notable slip at the threads. Furthermore, the couplers showed ultimate strains around or above 7.5%. It means that they comply with requirements for high ductility reinforcement according to European codes. Shear screw couplers are another type of mechanical splice which are evaluated by a few researchers. Cruz and Saiidi [6] examined the influence of utilizing 14db length shear screw couplers at two locations of a two-circular column bent experimentally. According to their results, using couplers led to more than 5% increase in drift ratio capacity without any failure in bars or couplers. As it was mentioned, many studies on the performance of RC elements with mechanical spliced bars have been conducted. However, the
structure behavior, construction cost and time, has motivated researchers to generate and assess various types of couplers. Navaratnarajah [14] introduced a new method for splicing bars by putting two heads of the bars in a steel case and then injecting epoxy adhesive (instead of grout) through it. A mechanical model was presented by Zanuy and Diaz [18] to evaluate the fatigue behavior of lap splices by presenting a mechanical model. Among various coupler types, threaded couplers are the most known type of mechanical splices which are evaluated in several studies experimentally and numerically. Lehman et al. [11] compared the performance of RC columns with threaded couplers. They considered couplers in two locations of the column: bottom couplers were installed in the footing while the other couplers were installed 1.5 times of column diameter above the bottom splices. Based on the results, the column with couplers exhibited 30% higher lateral strength and 40% higher displacement capacity in comparison with the column without couplers. Moreover, Saiidi et al. [16] studied the seismic performance of RC columns with 4db length threaded couplers at two different locations (below the footing surface and above it). The conclusion of their study was increase in drift capacity up to 14% drift ratio when compared to the column without couplers. In the similar study conducted by Saiidi and Wang [15], the column with 4db length threaded couplers at two locations (102 mm below the footing surface and 1.4D above bottom couplers) showed large displacement capacity with no bar fracture. Based on another experimental study, the shape and location of threaded couplers have a big impact on the inelastic cyclic response
Fig. 1. Dimensions and reinforcing details of the specimens. 718
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studies on the effect of using shear screw-threaded splices are rare. Therefore, in this experimental study cyclic performance of RC columns with overlap and mechanical spliced bars, including threaded and shear screw-threaded splices, is compared with the reference specimen with no splice in longitudinal bars. The specimens were examined under axial and cyclic loads and force–displacement hysteresis loops are presented. Moreover, lateral load bearing capacity (LLBC), ductility, cumulative (CAEabsorbed energy) and stiffness of each model are presented and discussed. 2. Experimental program Fig. 2. Mechanical splices: a) two-way threaded coupler, b) shear threaded coupler (http://irancoupler.ir/services-v1/).
The main objective of this study is to evaluate the influence of incorporating mechanically spliced bars in RC columns under axial and cyclic loading. Therefore, six square section RC columns were fabricated and examined. As could be seen in Fig. 1, the experimental specimens were made of three main parts: foundation, column and cap. Width, height and length of the columns were considered 300 mm, 350 mm and 1100 mm, respectively. Dimensions of foundation, on the other hand, were considered 300 × 350 × 1100 mm. A reinforced concrete cap sized 300 × 350 × 800 mm was located above the column for applying loads. Eight 16 mm diameter bars provided column longitudinal reinforcement whilst 8 mm diameter stirrup bars at every 50 mm considered for transverse reinforcement. Table 1 presents the details of reinforcement for different parts of the specimens. The properties of concrete and steel bars for all parts of the specimens were the same. For determining concrete mechanical properties, totally eight samples were examined under compressive test: four 150 × 150 × 150 mm cubic samples and four Ø150 × 300 mm cylindrical samples which were cast into steel forms. After removing forms, the samples were submersed in water for 28 days to be prepared for testing. Based on the test results, the average 28 days concrete compressive strength of the cylinder and cubic samples were 25.6 and 28.4 MPa, respectively. The steel reinforcing bars used in the experimental specimens were type AIII and made in Steel Complex Factory located in Kashan, Iran. The mechanical splices assessed in this study can be classified in two main categories: 1) threaded couplers and 2) shear screw-threaded couplers, also known as shear bolt splice, which are shown in Fig. 2. The measured dimensions of each type of the couplers are provided in Table 2. As illustrated in Fig. 3, three steel bars were tested under tensile load according to ASTM E8 code [2]. The measured characteristics of mechanically spliced bars are given in Table 3. It is worth to mention that based on the mechanical splice requirements stated in ACI 318, section 12.14.3.4, “a full mechanical connection shall develop in tension or compression, as required, at least 125 percent of specified yield strength of the bar” [1]. This requirement means that in case of failure, yielding occur in the reinforcing bar before failure in the mechanical connection. As shown in Fig. 3c and d, failure of bars occurred out of the couplers. Therefore, the splices used in this study meet this requirement. Fig. 4 depicts the load-deformation response of test on non-spliced and mechanically spliced bars. As observed from these curves, the bar
Table 2 Characteristics of mechanical splice.
Bar diameter External diameter of coupler Coupler length Diameter of threaded
Threaded coupler
Shear screw-threaded coupler
mm mm
db Dc
16 23
16 44
mm mm
Lc M
40 –
95 14
with threaded splice (dashed curve) had the highest deformation and load in comparison to other tested bars. Moreover, the ultimate deformation of all of the bars is in the range of 60 mm while their ultimate load is in the range of 120 KN. The other important note which should be taken into account is that the bars with mechanical splices showed almost similar performance, which can be considered appropriate comparing to non-spliced bar, under monotonic load. As it was mentioned before, totally six experimentally RC columns were investigated in this research. Four of which contained mechanical splices in different locations of the longitudinal bars. In order to prevent the stress concentration due to axial and lateral loads, a steel cap for loading is considered on top of the columns which transfers axial and lateral loads to the RC columns. The loading cap was a 200 mm height steel beam with six 10 mm thickness stiffener plate welded to its web and flange. A steel sheet was positioned under the steel beam in order to transfer lateral load to the specimens. For applying axial load a threaded shaft were used to connect loading cap to the load cell which was connected to a hydraulic jack. This shaft was made of a high resistance steel to avoid any deformation during the test. The load cell to loading cap connection was considered pin. Otherwise stated, just vertical and horizontal degrees of freedom were restrained. It is also worth to mention that the load cell to frame connection was modeled roller to be able to move under lateral loads. All above mentioned details are schematically shown in Fig. 5. As it is depicted in Fig. 5, four Linear Variable Differential Transformer (LDVT) sensors were installed: two at the end of beams and two at the plastic hinge of the column. A wired LDVT was also considered at the beam side surface. The applied axial load was equal to 0.1f’cAg while the lateral load was considered based on the protocol and the
Table 1 Details of dimensions and reinforcement of the specimens. Specimen name
Column
Foundation
Loading cap
RSP OS-M TC-A5 TC-H5-H60 STC-A5 STC-H5-H60
Dimensions: 300 × 350 × 1100 mm Longitudinal bars: 8Ø16 mm Stirrups: Ø8@50 mm
dimensions: 300 × 350 × 1100 mm Longitudinal bars: 8Ø16 mm Stirrups: Ø8@50 mm
dimensions: 300 × 350 × 800 mm Longitudinal bars: 8Ø16 mm Stirrups: Ø8@50 mm
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4. Splice details of the specimens In this section, details of the used splices in all the specimens are presented. As provided in Table 4, three types of splices were used in two different locations of longitudinal reinforcement bars. Specimen RSP is considered as the reference specimen with nonspliced bars. As demonstrated in Fig. 7b, overlap splices at the middle of column were considered in the specimen OS-M. Threaded couplers were used for the specimen TC-A5 in order to patch longitudinal bars. As illustrated in Fig. 7c, all of the threaded splices were located at 50 mm above the footing surface. In the specimen TC-H5-H60 half of the threaded couplers installed at 50 mm above the footing surface whilst other couplers incorporated 600 mm (2D) above the bottom couplers. Shear screw-threaded couplers were used for the specimen STC-A5 at 50 mm above the footing surface. Shear screw-threaded couplers were also installed in the specimen STC-H5-H60 but in this specimen half of the splices were moved 600 mm upper. Type, position and number of splices considered for the specimens are depicted in Fig. 7. 5. Results and observations 5.1. Hysteresis curves To evaluate the influence of overlap and mechanical splices on the cyclic performance of RC columns, totally six experimental specimens were fabricated and examined under axial and cyclic loads. The variable parameters were the type and location of considered splices in longitudinal reinforcement bars. Force-displacement hysteresis curves of all the specimens are shown in Fig. 8. It should be noted that blue rectangles and red circles on the diagrams are the start point of flexural and shear cracks, respectively. Fig. 3. The tension examination of bars with mechanical splices: a) threaded splice, b) shear screw-threaded splice, c) bar fracture in threaded coupler, d) bar fracture in shear screw-threaded coupler.
5.2. Crack analysis Observed damage and crack patterns of all the six specimens are demonstrated in Fig. 9. The general behavior and crack propagation of each specimen during test is described as follow:
Table 3 Results of test on the reinforcement bars. Splice type
Yielding stress (MPa)
Ultimate stress (MPa)
Ultimate strain (%)
Non-spliced bar Threaded splice Shear screw-threaded
400.36 510.158 501.020
602.52 643.254 605.598
25.10 26.03 25.22
5.2.1. Rsp In the reference specimen with no bar splice, the first flexural crack commenced under the load of 32.5 KN at the column-footing interface. As expected, most of the flexural cracks took place at the bottom of the column by increasing lateral load. Cracks in the middle of column were also initiated under 43 KN lateral load and they propagated up to load 75 KN. Flexural-shear cracks occurred during the loads of 43, 71, 77.5 and 81 KN. Shear cracks, on the other hand, started under the load of 45.5 KN at the column-footing interface and they were also observed at the middle and top of the column under 81 and 77.5 KN loads, respectively. In the meanwhile, vertical cracks occurred under the load of 81 KN. Concrete spalling started at the corner of column-footing interface (at the crack due to 32 KN lateral load) by crushing concrete cover and orthogonal cracks in foundation which led to total failure during the load of 49.47 KN.
control displacement shown in Fig. 6.
3. Time history of cyclic loading Displacement control condition which is shown in Fig. 6 was applied at top of the column as a semi-static cyclic load based on ACI 374.1-05 code. According to the definition of loading history, two effective parameters should be taken into consideration: the first parameter is the increasing rate of displacement and the second one is the number of repetition at each level of displacement. The displacement should be applied in a way that not only investigates the response of the specimens but also tests the performance level of specimens. The first threshold of the applied displacement should be chosen in a way that the sample shows elastic response. Therefore, based on ACI 374.1-05 recommendation, displacement was chosen 0.2 percent as the threshold of the first cycle. According to this instruction, the ratio of the increased consecutive cycles from the threshold of the former cycles should not be less than 1.25 or more than 1.5 times of the next one.
5.2.2. OS-M Considering the column with overlap splices, under the load of 35.5 KN the first flexural crack was observed at the corner of columnfooting interface and its length increased up to load 38.5 KN. As lateral load increased, more cracks took place at the bottom of the specimen. The significant point is that a few cracks were observed at the middle of column (under the load of 67 and 70.5 KN) while almost no crack was observed at the top of column. During applying the load of 25 KN, shear cracks initiated at the column-footing corner whilst they were found under loading of 51.5 KN at the middle of column. Concrete damage started at the first flexural crack location (column-footing interface). 720
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Fig. 4. Load-deformation curves for non-spliced and mechanically spliced bars.
Fig. 5. Schematic demonstration of test setup.
Fig. 6. Loading Protocol of ACI 374.01-05.
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Table 4 Types and locations of splices in the specimens. Specimen name
Splice method
Description
RSP OS-M TC-A5 TC-H5-H60 STC-A5 STC-H5-H60
*** Overlap Splice Threaded Coupler Threaded Coupler Shear screw-Threaded Coupler Shear screw-Threaded Coupler
Non-spliced bars All of the overlap at the Middle of the column All the couplers at location one Half of the couplers at location one and other Half at location two All the splices at location one Half of the couplers at location one and other half at location two
*Location one: 50 mm above the footing surface. *Location two: 600 mm (2D) above the bottom couplers.
Fig. 7. Bar splicing details of the six specimens: a) RSP, b) OS-M, c) TC-A5, d) TC-H5-H60, e) STC-A5, f) STC-H5-H60.
resulted to concrete cover damage. Subsequently, orthogonal cracks took place under the column in the foundation which resulted to failure.
Then, by increasing lateral load, crack width increased up to the load of 66.22 KN and finally after concrete cover crush, the specimen failed.
5.2.3. TC-A5 It worth to recall that in this specimen all of the threaded couplers were positioned at 50 mm upper the foundation surface. The first flexural crack occurred at the vicinity of couplers under the load of 25 KN. Next, the number of cracks increased at the bottom of column by increasing lateral load. A few flexural cracks were observed at the middle and top of column under the loads of 50.5 and 62.5 KN, respectively. Shear cracks initiated at the column-footing corner under the load of 60 KN and they occurred at the bottom of column by increasing load. In addition, some shear cracks were also observed at the middle of column during the load of 72.5 KN. By increasing load up to 67.5 KN, a few vertical cracks occurred in the column. Under the load of 81.61 KN, crack width at the column-footing interface increased which
5.2.4. TC-H5-H60 In the specimen with threaded couplers at two considered locations, no crack was observed up to the load of 33 KN. After that, flexural cracks took place at the corner of concrete column-footing interface. During the load of 48 KN lateral load, flexural cracks were observed at the location where the bottom couplers were installed. In the meanwhile, by increasing load, flexural cracks commenced at the bottom of specimen. It should be noted that under the load of 56 KN and 80 KN, flexural cracks were found at the vicinity of upper couplers. Most of the shear cracks also occurred at the bottom of the column between the loads of 48 to 65 KN. A limited number of vertical cracks were also observed under the load of 87 KN. Specimen failure started at the 722
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Fig. 8. The force–displacement hysteresis curves of the six specimens: a) RSP, b) OS-M, c) TC-A5, d) TC-H5-H60, e) STC-A5, f) STC-H5-H60 (Blue rectangle: start point of flexural cracks, red circle: start point of shear cracks). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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hand, commenced under the load of 50.5 at the column-footing interface and propagated at the bottom of column by increasing load. It is noteworthy that shear cracks were also observed at the location of upper couplers under the load of 80 KN. At this load, vertical cracks also occurred at the middle of column. By reaching to the load of 61.39, after increasing crack width at the column-footing corner, concrete cover crushed and the specimen failed. Considering the behavior of specimens, it could be said that in the specimen STC-A5 flexural cracks started at the middle of column while in other specimens they initiated at the bottom of column. Moreover, specimen OS-M (column with overlap splice) was the only column without any vertical crack. Furthermore, it can be stated that almost in all the specimens, shear cracks commenced at the bottom of column. To be able to make an easier comparison between performances of the columns, load and displacement corresponding to the concrete cracks and bar yield are provided in Table. 5. The positive, negative and average values of ultimate and maximum loads and their corresponding displacements are also reported in Table. 5. It should be noted that data related to concrete crack are obtained by experiment observations while other data are calculated using load–displacement hysteresis curves (e.g. Fmax and Fu are the maximum and ultimate load obtained from envelope of hysteresis curve of each specimen). 6. Discussion 6.1. Elasticity The ultimate lateral load capacity for each specimen is provided in Table 6. The comparison of specimens 2–6 with reference specimen is also made in the last column. According to the data provided in Table 6, using overlap splice at the middle of column increases ULLC insignificantly by 7.47%. Moreover, in case of using threaded couplers, the location of couplers play an important role on the value of ULLC. To explain it in much more details, when all the couplers are installed at the bottom of column, ULLC increment is negligible. On the other hand, by incorporating them in two mentioned locations, ULLC increases notably by 26.99%. When the results of specimens with shear screw-threaded couplers are considered, two notable points can be figured out: firstly, location of the couplers does not affect the results considerably. Secondly, ULLC values of the specimens with shear screw-threaded couplers are higher than the reference column. In general, the column with threaded couplers at two locations has the highest ULLC in comparison to the other specimens.
Fig. 9. Damage propagation and crack patterns for all the specimens after the last cycle.
flexural cracks due to the load of 48 KN and after increase in crack width, damage occurred under the load of 68.2 KN by footing cover crush. 5.2.5. STC-A5 No crack occurred up to the load 28 KN on the column with shear screw-threaded couplers all in the bottom of column. During the load of 33 KN, unlike other specimens, flexural cracks started at the middle of column. By increasing load, however, flexural cracks took place mostly in the bottom of the column. It should be taken into consideration that flexural cracks took place at the vicinity of couplers under the loads of 47.5 and 64 KN. At the bottom of column, flexural-shear cracks were also observed under the loads of 35–72 KN. In the meanwhile, a few cracks were occurred at the middle of column. At the last cycles (loads of 79, 74.5 and 82 KN) vertical cracks were observed at the bottom of the column. Damage in the specimen started from the column-footing interface (the flexural-shear cracks due to the load of 47.5 KN) and then its width increased by increasing load up to 72.5 KN. Orthogonal cracks in the foundation were also observed at this cycle. Finally, under the load of 66.91 KN, concrete cover crushed and the specimen failed.
6.2. Ductility Ductility can be calculated using Eq. (1):
μ=
Δu Δy
(1)
Δu is the displacement corresponding to 15% drop of Fmax (which is obtained from the envelope of hysteresis curve) while Δy can be obtained by different proposed methods. The Method which is based on the balance of energy is used to find yield displacement in this study. As shown in Fig. 10 [8,7], a secant line which starts from the origin point O and cut the curve at point I, intersects the peak strength at point A. The secant line should be adjusted until two equal areas obtained (A1 = A2). Consequently, the displacement corresponding to the secant line (point A) is considered as the yield displacement. Table 7 provides the data related to calculated ductility and comparison between all the specimens and the reference model. It should be noted that the average of positive and negative displacement is used to calculate ductility. Base on the data given in Table 7, it can be said that in the specimen OS-M with overlap splice at the middle of column, ductility decreases slightly compared to the reference column. Considering specimens with threaded couplers, specimen TC-H5-H60 possesses the lowest ductility
5.2.6. STC-H5-H60 Shear screw-threaded couplers were incorporated in this RC column at two mentioned locations. Firstly, it should be noted that no crack was observed up to the load of 30 KN. Flexural cracks, then, started under the load of 35.5 KN at the bottom of the column. As load increased, in addition of occurring flexural cracks at the bottom of the column, some cracks were observed at the location of upper couplers under the loads of 52 and 69.5 KN. Moreover, at the middle of column, flexural cracks occurred under the load of 43 to 69.5 KN. Shear cracks, on the other 724
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Table 5 Results of the experimental tests. Specimen name
Parameter
RSP
Ccr Sy Fu
Load (KN) 32.5 65.72 Positive direction Negative direction Average Positive direction Negative direction Average 35.5 65.287 Positive direction Negative direction Average Positive direction Negative direction Average 25 66.83 Positive direction Negative direction Average Positive direction Negative direction Average 33 81.61 Positive direction Negative direction Average Positive direction Negative direction Average 28 81.87 Positive direction Negative direction Average Positive direction Negative direction Average 30 67.15 Positive direction Negative direction Average Positive direction Negative direction Average
Fmax
OS-M
Ccr Sy Fu
Fmax
TC-A5
Ccr Sy Fu
Fmax
TC-H5-H60
Ccr Sy Fu
Fmax
STC-A5
Ccr Sy Fu
Fmax
STC-H5-H60
Ccr Sy Fu
Fmax
Ultimate lateral load capacity (KN)
Difference percentage
RSP OS-M TC-A5 TC-H5-H60 STC-A5 STC-H5-H60
77.76 83.57 81.735 98.75 84.01 86.01
*** 7.47% 5.11% 26.99% 8.04% 10.61%
94.46 94.37 94.415 47.18 47.18 47.18
94.49 94.52 94.505 71.26 71.26 71.26
141.73 141.36 141.545 60.77 36.9 48.835
94.11 93.72 93.915 71.243 71.35 71.2965
141.08 141.67 141.375 94.44 94.58 94.51
138.54 141.02 139.78 92.466 95 93.733
As provided in the last column of Table 7 and shown in Fig. 8, specimen OS-M has the lowest Δu. Moreover, ultimate displacement increases in all the specimens with mechanical splices. It is noteworthy that incorporating all of the couplers at the bottom of column results in higher Δu comparing to other patterns. To be able to compare displacement of the specimens easier, the obtained force-displacement push curves are displayed in Fig. 11.
Table 6 The ultimate lateral load capacity of the specimens. Specimen name
38.3 23.34 49.47 55.92 52.695 80.5 75.01 77.755 12.31 28.863 66.22 63.99 65.105 86.18 80.96 83.57 11.08 19.21 68.2 61.36 64.78 86 77.47 81.735 29.08 94.11 81.61 75.34 78.475 102.727 94.79 98.7585 11.38 36.025 61.39 68.59 64.99 86.17 81.85 84.01 7.68 31.845 66.91 61.24 64.075 89.6 82.43 86.015
Displacement (mm)
6.3. Absorbed energy whilst specimen TC-A5 has the highest. It shows that location of threaded couplers affect column ductility significantly. Otherwise stated, installing all of the threaded couplers at the bottom of column increases ductility by 112.24% in comparison to the reference column. Incorporating half of the couplers at 600 mm above the bottom couplers, however, decreases ductility by 4.12%. On the other hand, columns with shear screw-threaded couplers exhibited higher ductility in comparison to the reference one. Unlike the columns with threaded couplers, splice location does not have a considerable impact on the column ductility in the specimens STC-A5 and STC-H5-H60.
By integrating the area enclosed by the hysteresis loop the absorbed energy (AE) can be calculated for each load cycle. Cumulative absorbed energy at each cycle can be also obtained by summating AE in previous cycles. Fig. 12a, depicts the absorbed energy by each specimen at each cycle. CAE for all the columns is also compared together in Fig. 12b. Considering Fig. 12b, it can be realized that at the first cycles (up to 15th cycle), all the columns have approximately similar performance with low values of CAE. It simply means that different types of bar splices cannot affect cyclic behavior of a RC column under low lateral loads. From the 15th cycle to 30th cycle, however, CAE increases 725
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considerably in all the specimens and then it increases sharply at the rest cycles. The most important point which can be found from Fig. 12b is that the column with shear screw-threaded couplers all at the bottom of column (STC-A5) possesses the highest CAE up to cycle 35 whereas From cycle 35 on, RSP and TC-H5-H60 have the highest CAE. On the other hand, the specimen with threaded couplers all in the bottom of column (TC-A5) has the lowest CAE. According to the demonstrated curves, it can be figured out that the reference column (with nonspliced bars) and TC-H5-H60 (with threaded couplers at the both considered locations) exhibit almost the same behavior during the test. Therefore, in terms of CAE, incorporating threaded couplers can be considered as the most appropriate bar splice method. 6.4. Stiffness Secant stiffness, which is known as the ability to resist deformation, is defined as the slope of a line passing through peak loads at both directions in each hysteresis loop [8]. Fig. 13 demonstrates the stiffness for all the specimens. As depicted in Fig. 13, stiffness decreases rapidly at the first cycles (up to 10 mm). Next, it decreases slightly up to the displacement of 100 mm and finally, it reaches to an almost constant value for all the specimens. By comparing stiffness curves, it can be understood that in the displacements between 10 mm and 25 mm specimen STC-A5 has the lowest stiffness while specimen TC-H5-H60 possesses the highest stiffness in the other displacements. In general, similar to the results obtained for CAE, specimen with threaded couplers all at the bottom of column has the lowest stiffness and the specimen with threaded couplers exhibited the best performance. As a result, it can be said that using threaded couplers at two considered locations lead to the most acceptable result in comparison to other splicing methods.
Fig. 10. The method used to find Δy and Δu. Table 7 Calculated ductility for each specimen and comparison between the RC columns with reference one. Specimen name
RSP OS-M TC-A5 TC-H5-H60 STC-A5 STC-H5H60
Δy mm
23.342 22.863 21.189 32.485 36.025 31.845
Δu mm
70.11 67.512 134.436 93.546 138.92 128.865
μ
3.0035 2.95 6.3446 2.8796 3.8562 4.0466
The percentage of ductility difference for each specimen
The percentage of Δu difference for each specimen
– −1.78% 112.24% −4.12% 28.39% 34.72%
– −3.70% 91.70% 33.42% 98.14% 83.80%
7. Conclusion The main objective of this study was to find out the effect of incorporating mechanical splices on the cyclic performance of RC columns. To achieve the aim, totally six experimental RC columns were fabricated and examined under axial and cyclic loads. A column with non-spliced bars was considered as the reference model to be compared with other specimens. Overlap splice in the middle of column was used in the second model. Threaded and shear screw-threaded mechanical splices in two different locations were considered for all and half of the
Fig. 11. Force-displacement envelope curves of the specimens. 726
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Fig. 12. a) Absorbed energy by the specimens, b) cumulative absorbed energy by the specimens.
column for half of bars and 2D upper for the other bars may lead to the results similar to a column without splice. Using overlap splice or threaded couplers all at the bottom of a column reduces stiffness considerably in comparison to the column without splice. Acknowledgments We would like to thank the people who helped us to fabricate and test the specimens. We also thank the anonymous reviewers for their constrictive comments on an earlier version of this article. References [1] ACI Committee 318. Building Code Requirements for Structural Concrete: (ACI 318-14); and Commentary (ACI 318R-14). Farmington Hills, MI: American Concrete Institute, 2014. Print. [2] ASTM, ASTM E8/E8M-16a Standard Test Methods for Tension Testing of Metallic Materials, West Conshohocken: ASTM International, 2018. [3] Bompa DV, Elghazouli AY. Ductility considerations for mechanical reinforcement couplers. Structures 2017;12:115–9. https://doi.org/10.1016/j.istruc.2017.08.007. [4] Bompa DV, Elghazouli AY. Monotonic and cyclic performance of threaded reinforcement splices. Structures 2018;16:358–72. https://doi.org/10.1016/j.istruc.2018.11.009. [5] Bompa DV, Elghazouli AY. Inelastic cyclic behaviour of RC members incorporating threaded reinforcement couplers. Eng Struct 2019;180:468–83. https://doi.org/10.1016/ j.engstruct.2018.11.053. [6] Cruz Noguez CA, Saiidi MS. Shake-table studies of a four-span bridge model with advanced materials. J Struct Eng 2012;138(2):183–92. https://doi.org/10.1061/(asce)st. 1943-541x.0000457. [7] Dabiri H, Kheyroddin A, Kaviani A. A numerical study on the seismic response of RC wide column–beam joints. Int J Civil Eng 2018;17(3):377–95. https://doi.org/10.1007/ s40999-018-0364-2. [8] EL-Hacha R, Elagroudy H, Rizkalla S. Proposed modification to the ACI 318-02 Code equation on bond strength for steel bars. 5th Structure Specialty Conference of the Canadian Society for Engineering, Saskatoon, Saskatchewan, Canada. 2004. [9] Goksu C, et al. The effect of lap splice length on the cyclic lateral load behavior of RC members with low-strength concrete and plain bars. Adv Struct Eng 2014;17(5):639–58. https://doi.org/10.1260/1369-4332.17.5.639. [10] Issa CA, Nasr A. An experimental study of welded splices of reinforcing bars. Build Environ 2006;41(10):394–1405. https://doi.org/10.1016/j.buildenv.2005.05.025. [11] Lehman DE, Gookin SE, Nacamuli AM, Moehle JP. Repair of earthquake-damaged bridge columns. ACI Struct J 2001;98(2). https://doi.org/10.14359/10192. [12] Moussavi Nadoushani ZS, et al. Minimizing cutting wastes of reinforcing steel bars through optimizing lap splicing within reinforced concrete elements. Construct Build Mater 2018;185:600–8. https://doi.org/10.1016/j.conbuildmat.2018.07.023. [13] Najafgholipour MA, et al. The performance of lap splices in RC beams under inelastic reversed cyclic loading. Structures 2018;15:279–91. https://doi.org/10.1016/j.istruc. 2018.07.011. [14] Navaratnarajah V. Splicing of reinforcement bars with epoxy joints. Int J Adhes Adhes 1983;3(2):93–9. [15] Saiidi MS, Wang H. Exploratory Study of Seismic Response of Concrete Columns with Shape Memory Alloys Reinforcement. ACI Struct J 2006;103(3). https://doi.org/10. 14359/15322. [16] Saiidi MS, O'Brien M, Sadrossadat-Zadeh M. Cyclic Response of Concrete Bridge Columns Using Superelastic Nitinol and Bendable Concrete. ACI Struct J 2009;106(1). https://doi. org/10.14359/56285. [17] Tazarv M, Saiidi MS. Seismic design of bridge columns incorporating mechanical bar splices in plastic hinge regions. Eng Struct 2016;124:507–20. [18] Zanuy C, Díaz IM. Stress distribution and resistance of lap splices under fatigue loading. Eng Struct 2018;175:700–10. https://doi.org/10.1016/j.engstruct.2018.08.067.
Fig. 13. Stiffness curves of the all specimens.
longitudinal bars in other four specimens. In other words, the variable parameters were the location and type of splices in longitudinal reinforcement bars. Considering the results obtained from the experiment, it could be concluded that using mechanical splices can affect cyclic performance of reinforced concrete columns notably. Following conclusions can be drawn:
• Overlap and mechanical splices can increase ULLC of a column • • •
•
under cyclic loads. When different types and locations of splices are taken into account, using threaded couplers at the bottom of a column for half of the bars and 2D upper for the other bars, may lead to appropriate results. Although using overlap splice at the middle of a column decreases ultimate displacement, incorporating mechanical splice can increase it. Based on the obtained results, installing all of the couplers at the bottom of a column result in higher Δu in comparison to other arrangements. Incorporating mechanical splices all at the bottom of a column may lead to high values of ductility. Using either overlaps splices or couplers at two locations (the bottom of column and 2D upper), however, decreases the ductility. Using overlap splice or couplers at the bottom of a column for all the longitudinal bars may reduce AE. On the other hand, considering threaded couplers at the bottom of column for half of the bars and 2D upper for the other bars result in the same value of the column without splice. Moreover, incorporating shear screw-threaded couplers can also exhibit the same results as the column without splice. In terms of stiffness, using threaded couplers at the bottom of a 727
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Structures journal homepage: www.elsevier.com/locate/structures
Experimental investigation of using mechanical splices on the cyclic performance of RC columns
T
⁎
Ali Kheyroddina, , Amirhossein Mohammadkhaha, Hamed Dabirib, Ahmad Kaviania a b
Faculty of Civil Engineering, Semnan University, Semnan, Iran School of Science and Technology, University of Camerino, Camerino, Italy
A R T I C LE I N FO
A B S T R A C T
Keywords: RC column Mechanical splice Force–displacement hysteresis Ductility Absorbed energy Stiffness
In this study, the influence of using mechanical splices on the cyclic performance of RC columns under axial and cyclic loads was investigated experimentally. Totally six specimens were fabricated and examined; the first one was fabricated by non-spliced reinforcement bars which was considered as reference specimen. Overlap splice in the middle of the column was used in the second specimen. Threaded mechanical splices were used for the all of the longitudinal bars at 50 mm above the column-foundation interface in the third column. The fourth column was fabricated the same as the third one but half of the couplers were staggered at 600 mm above the bottom couplers. The fifth and sixth specimens were fabricated similar to the third and fourth column but shear screwthreaded couplers were incorporated instead of threaded couplers. Force-displacement hysteresis loops were presented as the output and parameters including lateral load bearing capacity, ductility, absorbed energy and stiffness are considered and discussed. Based on the obtained results, it was concluded that mechanical splices can considerably affect cyclic behavior of RC columns.
1. Introduction Cutting and splicing reinforcement bars is one of the most important topics raised in the reinforced concrete (RC) structures. In terms of construction cost, steel bars form a considerably large proportion of the total cost of a RC building. Therefore, applying economically optimized methods to patch bars has attracted researchers’ attention to reduce the construction cost. Reinforcement bars can be spliced by several methods including: bar overlapping, mechanical patching, couplers, and welding bars head to head. When overlap method is used the length of the overlapped bars must be equal or greater than the anchorage length of the bar. The significant problem with this method is that it is not economically beneficial especially when it is applied to the bars with diameter greater than 30 mm. As a result, using couplers has become common because not only it decreases the bar weight, but also it is easier and less timeconsuming to apply. Due to importance of bar splice methods in RC structures in seismically behavior and total construction cost and time, they have become one of the major researchers’ concern. Some studies have been conducted on the effective parameters such as length and pattern in overlap methods on the behavior of RC elements. El-Hacha et al. [8] evaluated the bonding strength and the ⁎
necessary anchorage length of the bars spliced by overlapping method. Isa and Nasr [10] studied side-by-side welding patches in order to get the proper welding length. They suggested that the welding length should be greater and equal to 1.5 times of the bar diameter. Moreover, their study concluded that welded splicing can be considered as a reliable, efficient, and consistent approach for bar splices. The influence of different overlap splice lengths on the cyclic performance of RC columns was evaluated by Goksu et al. [9]. In the study it was concluded that incorporating 180-degree hooks at the end of the overlap splice can improve the poor behavior of column due to inadequate overlap length even in case of using low-strength concrete. Moreover, all the specimens with different overlap lengths reached their flexural strength up to large drift ratios. In order to minimizing the trim loss through automating the process of lab splicing pattern generation, cutting pattern generation and trim loss optimization was proposed by Nadoushani et al. [12]. Flexural behavior of RC beam with lap spliced bars under reversed quasi-static cyclic loading was assessed experimentally by Najafgholipour et al. [13]. Their study showed that the beam which was designed based on ACI318-14 could not stand the reverse cyclic loading up to flexural failure, whilst the modified specimens (in terms of splice length and transverse reinforcement) survived cyclic loading with acceptable flexural ductility. The considerable benefits of mechanical splices, in terms of
Corresponding author at: Semnan University, Semnan, Iran. E-mail addresses: [email protected] (A. Kheyroddin), [email protected] (H. Dabiri).
https://doi.org/10.1016/j.istruc.2020.01.043 Received 17 September 2019; Received in revised form 21 January 2020; Accepted 28 January 2020 2352-0124/ © 2020 Institution of Structural Engineers. Published by Elsevier Ltd. All rights reserved.
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Nomenclature Ag Ccr D db Dc Fu Fmax
f’c Lc M Fy Sy Δy Δu μ
gross area of column section concrete crack diameter or width of column bar diameter external diameter of coupler ultimate force maximum force
compressive strength of concrete length of coupler diameter of threaded area in coupler yield force yield of reinforcement yield displacement in Eq. (1) ultimate displacement in Eq. (1) ductility
and ductility of RC elements. In addition, ductility and energy dissipation can be affected considerably by using threaded mechanical couplers [5]. An experimental study on the effect of elevated temperature on the mechanical properties of spliced steel reinforcement showed that using couplers affect ductility characteristics of threaded splices of elevated temperature [3]. Tazarv and Saiidi [17] proposed methods for designing seismic columns on different bridges by using mechanical splices. The research evaluated the influence of couplers on the higher plasticity of reinforced concrete columns and using mechanical splices in the precast segments of the seismic districts. Bompa and Elghazouli [4] evaluated different coupling systems by comparing their key performance parameters experimentally and numerically. The results indicated that the bar with threaded couplers had higher strains in comparison to the non-spliced reference bar within the elastic cyclic test, without notable slip at the threads. Furthermore, the couplers showed ultimate strains around or above 7.5%. It means that they comply with requirements for high ductility reinforcement according to European codes. Shear screw couplers are another type of mechanical splice which are evaluated by a few researchers. Cruz and Saiidi [6] examined the influence of utilizing 14db length shear screw couplers at two locations of a two-circular column bent experimentally. According to their results, using couplers led to more than 5% increase in drift ratio capacity without any failure in bars or couplers. As it was mentioned, many studies on the performance of RC elements with mechanical spliced bars have been conducted. However, the
structure behavior, construction cost and time, has motivated researchers to generate and assess various types of couplers. Navaratnarajah [14] introduced a new method for splicing bars by putting two heads of the bars in a steel case and then injecting epoxy adhesive (instead of grout) through it. A mechanical model was presented by Zanuy and Diaz [18] to evaluate the fatigue behavior of lap splices by presenting a mechanical model. Among various coupler types, threaded couplers are the most known type of mechanical splices which are evaluated in several studies experimentally and numerically. Lehman et al. [11] compared the performance of RC columns with threaded couplers. They considered couplers in two locations of the column: bottom couplers were installed in the footing while the other couplers were installed 1.5 times of column diameter above the bottom splices. Based on the results, the column with couplers exhibited 30% higher lateral strength and 40% higher displacement capacity in comparison with the column without couplers. Moreover, Saiidi et al. [16] studied the seismic performance of RC columns with 4db length threaded couplers at two different locations (below the footing surface and above it). The conclusion of their study was increase in drift capacity up to 14% drift ratio when compared to the column without couplers. In the similar study conducted by Saiidi and Wang [15], the column with 4db length threaded couplers at two locations (102 mm below the footing surface and 1.4D above bottom couplers) showed large displacement capacity with no bar fracture. Based on another experimental study, the shape and location of threaded couplers have a big impact on the inelastic cyclic response
Fig. 1. Dimensions and reinforcing details of the specimens. 718
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studies on the effect of using shear screw-threaded splices are rare. Therefore, in this experimental study cyclic performance of RC columns with overlap and mechanical spliced bars, including threaded and shear screw-threaded splices, is compared with the reference specimen with no splice in longitudinal bars. The specimens were examined under axial and cyclic loads and force–displacement hysteresis loops are presented. Moreover, lateral load bearing capacity (LLBC), ductility, cumulative (CAEabsorbed energy) and stiffness of each model are presented and discussed. 2. Experimental program Fig. 2. Mechanical splices: a) two-way threaded coupler, b) shear threaded coupler (http://irancoupler.ir/services-v1/).
The main objective of this study is to evaluate the influence of incorporating mechanically spliced bars in RC columns under axial and cyclic loading. Therefore, six square section RC columns were fabricated and examined. As could be seen in Fig. 1, the experimental specimens were made of three main parts: foundation, column and cap. Width, height and length of the columns were considered 300 mm, 350 mm and 1100 mm, respectively. Dimensions of foundation, on the other hand, were considered 300 × 350 × 1100 mm. A reinforced concrete cap sized 300 × 350 × 800 mm was located above the column for applying loads. Eight 16 mm diameter bars provided column longitudinal reinforcement whilst 8 mm diameter stirrup bars at every 50 mm considered for transverse reinforcement. Table 1 presents the details of reinforcement for different parts of the specimens. The properties of concrete and steel bars for all parts of the specimens were the same. For determining concrete mechanical properties, totally eight samples were examined under compressive test: four 150 × 150 × 150 mm cubic samples and four Ø150 × 300 mm cylindrical samples which were cast into steel forms. After removing forms, the samples were submersed in water for 28 days to be prepared for testing. Based on the test results, the average 28 days concrete compressive strength of the cylinder and cubic samples were 25.6 and 28.4 MPa, respectively. The steel reinforcing bars used in the experimental specimens were type AIII and made in Steel Complex Factory located in Kashan, Iran. The mechanical splices assessed in this study can be classified in two main categories: 1) threaded couplers and 2) shear screw-threaded couplers, also known as shear bolt splice, which are shown in Fig. 2. The measured dimensions of each type of the couplers are provided in Table 2. As illustrated in Fig. 3, three steel bars were tested under tensile load according to ASTM E8 code [2]. The measured characteristics of mechanically spliced bars are given in Table 3. It is worth to mention that based on the mechanical splice requirements stated in ACI 318, section 12.14.3.4, “a full mechanical connection shall develop in tension or compression, as required, at least 125 percent of specified yield strength of the bar” [1]. This requirement means that in case of failure, yielding occur in the reinforcing bar before failure in the mechanical connection. As shown in Fig. 3c and d, failure of bars occurred out of the couplers. Therefore, the splices used in this study meet this requirement. Fig. 4 depicts the load-deformation response of test on non-spliced and mechanically spliced bars. As observed from these curves, the bar
Table 2 Characteristics of mechanical splice.
Bar diameter External diameter of coupler Coupler length Diameter of threaded
Threaded coupler
Shear screw-threaded coupler
mm mm
db Dc
16 23
16 44
mm mm
Lc M
40 –
95 14
with threaded splice (dashed curve) had the highest deformation and load in comparison to other tested bars. Moreover, the ultimate deformation of all of the bars is in the range of 60 mm while their ultimate load is in the range of 120 KN. The other important note which should be taken into account is that the bars with mechanical splices showed almost similar performance, which can be considered appropriate comparing to non-spliced bar, under monotonic load. As it was mentioned before, totally six experimentally RC columns were investigated in this research. Four of which contained mechanical splices in different locations of the longitudinal bars. In order to prevent the stress concentration due to axial and lateral loads, a steel cap for loading is considered on top of the columns which transfers axial and lateral loads to the RC columns. The loading cap was a 200 mm height steel beam with six 10 mm thickness stiffener plate welded to its web and flange. A steel sheet was positioned under the steel beam in order to transfer lateral load to the specimens. For applying axial load a threaded shaft were used to connect loading cap to the load cell which was connected to a hydraulic jack. This shaft was made of a high resistance steel to avoid any deformation during the test. The load cell to loading cap connection was considered pin. Otherwise stated, just vertical and horizontal degrees of freedom were restrained. It is also worth to mention that the load cell to frame connection was modeled roller to be able to move under lateral loads. All above mentioned details are schematically shown in Fig. 5. As it is depicted in Fig. 5, four Linear Variable Differential Transformer (LDVT) sensors were installed: two at the end of beams and two at the plastic hinge of the column. A wired LDVT was also considered at the beam side surface. The applied axial load was equal to 0.1f’cAg while the lateral load was considered based on the protocol and the
Table 1 Details of dimensions and reinforcement of the specimens. Specimen name
Column
Foundation
Loading cap
RSP OS-M TC-A5 TC-H5-H60 STC-A5 STC-H5-H60
Dimensions: 300 × 350 × 1100 mm Longitudinal bars: 8Ø16 mm Stirrups: Ø8@50 mm
dimensions: 300 × 350 × 1100 mm Longitudinal bars: 8Ø16 mm Stirrups: Ø8@50 mm
dimensions: 300 × 350 × 800 mm Longitudinal bars: 8Ø16 mm Stirrups: Ø8@50 mm
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4. Splice details of the specimens In this section, details of the used splices in all the specimens are presented. As provided in Table 4, three types of splices were used in two different locations of longitudinal reinforcement bars. Specimen RSP is considered as the reference specimen with nonspliced bars. As demonstrated in Fig. 7b, overlap splices at the middle of column were considered in the specimen OS-M. Threaded couplers were used for the specimen TC-A5 in order to patch longitudinal bars. As illustrated in Fig. 7c, all of the threaded splices were located at 50 mm above the footing surface. In the specimen TC-H5-H60 half of the threaded couplers installed at 50 mm above the footing surface whilst other couplers incorporated 600 mm (2D) above the bottom couplers. Shear screw-threaded couplers were used for the specimen STC-A5 at 50 mm above the footing surface. Shear screw-threaded couplers were also installed in the specimen STC-H5-H60 but in this specimen half of the splices were moved 600 mm upper. Type, position and number of splices considered for the specimens are depicted in Fig. 7. 5. Results and observations 5.1. Hysteresis curves To evaluate the influence of overlap and mechanical splices on the cyclic performance of RC columns, totally six experimental specimens were fabricated and examined under axial and cyclic loads. The variable parameters were the type and location of considered splices in longitudinal reinforcement bars. Force-displacement hysteresis curves of all the specimens are shown in Fig. 8. It should be noted that blue rectangles and red circles on the diagrams are the start point of flexural and shear cracks, respectively. Fig. 3. The tension examination of bars with mechanical splices: a) threaded splice, b) shear screw-threaded splice, c) bar fracture in threaded coupler, d) bar fracture in shear screw-threaded coupler.
5.2. Crack analysis Observed damage and crack patterns of all the six specimens are demonstrated in Fig. 9. The general behavior and crack propagation of each specimen during test is described as follow:
Table 3 Results of test on the reinforcement bars. Splice type
Yielding stress (MPa)
Ultimate stress (MPa)
Ultimate strain (%)
Non-spliced bar Threaded splice Shear screw-threaded
400.36 510.158 501.020
602.52 643.254 605.598
25.10 26.03 25.22
5.2.1. Rsp In the reference specimen with no bar splice, the first flexural crack commenced under the load of 32.5 KN at the column-footing interface. As expected, most of the flexural cracks took place at the bottom of the column by increasing lateral load. Cracks in the middle of column were also initiated under 43 KN lateral load and they propagated up to load 75 KN. Flexural-shear cracks occurred during the loads of 43, 71, 77.5 and 81 KN. Shear cracks, on the other hand, started under the load of 45.5 KN at the column-footing interface and they were also observed at the middle and top of the column under 81 and 77.5 KN loads, respectively. In the meanwhile, vertical cracks occurred under the load of 81 KN. Concrete spalling started at the corner of column-footing interface (at the crack due to 32 KN lateral load) by crushing concrete cover and orthogonal cracks in foundation which led to total failure during the load of 49.47 KN.
control displacement shown in Fig. 6.
3. Time history of cyclic loading Displacement control condition which is shown in Fig. 6 was applied at top of the column as a semi-static cyclic load based on ACI 374.1-05 code. According to the definition of loading history, two effective parameters should be taken into consideration: the first parameter is the increasing rate of displacement and the second one is the number of repetition at each level of displacement. The displacement should be applied in a way that not only investigates the response of the specimens but also tests the performance level of specimens. The first threshold of the applied displacement should be chosen in a way that the sample shows elastic response. Therefore, based on ACI 374.1-05 recommendation, displacement was chosen 0.2 percent as the threshold of the first cycle. According to this instruction, the ratio of the increased consecutive cycles from the threshold of the former cycles should not be less than 1.25 or more than 1.5 times of the next one.
5.2.2. OS-M Considering the column with overlap splices, under the load of 35.5 KN the first flexural crack was observed at the corner of columnfooting interface and its length increased up to load 38.5 KN. As lateral load increased, more cracks took place at the bottom of the specimen. The significant point is that a few cracks were observed at the middle of column (under the load of 67 and 70.5 KN) while almost no crack was observed at the top of column. During applying the load of 25 KN, shear cracks initiated at the column-footing corner whilst they were found under loading of 51.5 KN at the middle of column. Concrete damage started at the first flexural crack location (column-footing interface). 720
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Fig. 4. Load-deformation curves for non-spliced and mechanically spliced bars.
Fig. 5. Schematic demonstration of test setup.
Fig. 6. Loading Protocol of ACI 374.01-05.
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Table 4 Types and locations of splices in the specimens. Specimen name
Splice method
Description
RSP OS-M TC-A5 TC-H5-H60 STC-A5 STC-H5-H60
*** Overlap Splice Threaded Coupler Threaded Coupler Shear screw-Threaded Coupler Shear screw-Threaded Coupler
Non-spliced bars All of the overlap at the Middle of the column All the couplers at location one Half of the couplers at location one and other Half at location two All the splices at location one Half of the couplers at location one and other half at location two
*Location one: 50 mm above the footing surface. *Location two: 600 mm (2D) above the bottom couplers.
Fig. 7. Bar splicing details of the six specimens: a) RSP, b) OS-M, c) TC-A5, d) TC-H5-H60, e) STC-A5, f) STC-H5-H60.
resulted to concrete cover damage. Subsequently, orthogonal cracks took place under the column in the foundation which resulted to failure.
Then, by increasing lateral load, crack width increased up to the load of 66.22 KN and finally after concrete cover crush, the specimen failed.
5.2.3. TC-A5 It worth to recall that in this specimen all of the threaded couplers were positioned at 50 mm upper the foundation surface. The first flexural crack occurred at the vicinity of couplers under the load of 25 KN. Next, the number of cracks increased at the bottom of column by increasing lateral load. A few flexural cracks were observed at the middle and top of column under the loads of 50.5 and 62.5 KN, respectively. Shear cracks initiated at the column-footing corner under the load of 60 KN and they occurred at the bottom of column by increasing load. In addition, some shear cracks were also observed at the middle of column during the load of 72.5 KN. By increasing load up to 67.5 KN, a few vertical cracks occurred in the column. Under the load of 81.61 KN, crack width at the column-footing interface increased which
5.2.4. TC-H5-H60 In the specimen with threaded couplers at two considered locations, no crack was observed up to the load of 33 KN. After that, flexural cracks took place at the corner of concrete column-footing interface. During the load of 48 KN lateral load, flexural cracks were observed at the location where the bottom couplers were installed. In the meanwhile, by increasing load, flexural cracks commenced at the bottom of specimen. It should be noted that under the load of 56 KN and 80 KN, flexural cracks were found at the vicinity of upper couplers. Most of the shear cracks also occurred at the bottom of the column between the loads of 48 to 65 KN. A limited number of vertical cracks were also observed under the load of 87 KN. Specimen failure started at the 722
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Fig. 8. The force–displacement hysteresis curves of the six specimens: a) RSP, b) OS-M, c) TC-A5, d) TC-H5-H60, e) STC-A5, f) STC-H5-H60 (Blue rectangle: start point of flexural cracks, red circle: start point of shear cracks). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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hand, commenced under the load of 50.5 at the column-footing interface and propagated at the bottom of column by increasing load. It is noteworthy that shear cracks were also observed at the location of upper couplers under the load of 80 KN. At this load, vertical cracks also occurred at the middle of column. By reaching to the load of 61.39, after increasing crack width at the column-footing corner, concrete cover crushed and the specimen failed. Considering the behavior of specimens, it could be said that in the specimen STC-A5 flexural cracks started at the middle of column while in other specimens they initiated at the bottom of column. Moreover, specimen OS-M (column with overlap splice) was the only column without any vertical crack. Furthermore, it can be stated that almost in all the specimens, shear cracks commenced at the bottom of column. To be able to make an easier comparison between performances of the columns, load and displacement corresponding to the concrete cracks and bar yield are provided in Table. 5. The positive, negative and average values of ultimate and maximum loads and their corresponding displacements are also reported in Table. 5. It should be noted that data related to concrete crack are obtained by experiment observations while other data are calculated using load–displacement hysteresis curves (e.g. Fmax and Fu are the maximum and ultimate load obtained from envelope of hysteresis curve of each specimen). 6. Discussion 6.1. Elasticity The ultimate lateral load capacity for each specimen is provided in Table 6. The comparison of specimens 2–6 with reference specimen is also made in the last column. According to the data provided in Table 6, using overlap splice at the middle of column increases ULLC insignificantly by 7.47%. Moreover, in case of using threaded couplers, the location of couplers play an important role on the value of ULLC. To explain it in much more details, when all the couplers are installed at the bottom of column, ULLC increment is negligible. On the other hand, by incorporating them in two mentioned locations, ULLC increases notably by 26.99%. When the results of specimens with shear screw-threaded couplers are considered, two notable points can be figured out: firstly, location of the couplers does not affect the results considerably. Secondly, ULLC values of the specimens with shear screw-threaded couplers are higher than the reference column. In general, the column with threaded couplers at two locations has the highest ULLC in comparison to the other specimens.
Fig. 9. Damage propagation and crack patterns for all the specimens after the last cycle.
flexural cracks due to the load of 48 KN and after increase in crack width, damage occurred under the load of 68.2 KN by footing cover crush. 5.2.5. STC-A5 No crack occurred up to the load 28 KN on the column with shear screw-threaded couplers all in the bottom of column. During the load of 33 KN, unlike other specimens, flexural cracks started at the middle of column. By increasing load, however, flexural cracks took place mostly in the bottom of the column. It should be taken into consideration that flexural cracks took place at the vicinity of couplers under the loads of 47.5 and 64 KN. At the bottom of column, flexural-shear cracks were also observed under the loads of 35–72 KN. In the meanwhile, a few cracks were occurred at the middle of column. At the last cycles (loads of 79, 74.5 and 82 KN) vertical cracks were observed at the bottom of the column. Damage in the specimen started from the column-footing interface (the flexural-shear cracks due to the load of 47.5 KN) and then its width increased by increasing load up to 72.5 KN. Orthogonal cracks in the foundation were also observed at this cycle. Finally, under the load of 66.91 KN, concrete cover crushed and the specimen failed.
6.2. Ductility Ductility can be calculated using Eq. (1):
μ=
Δu Δy
(1)
Δu is the displacement corresponding to 15% drop of Fmax (which is obtained from the envelope of hysteresis curve) while Δy can be obtained by different proposed methods. The Method which is based on the balance of energy is used to find yield displacement in this study. As shown in Fig. 10 [8,7], a secant line which starts from the origin point O and cut the curve at point I, intersects the peak strength at point A. The secant line should be adjusted until two equal areas obtained (A1 = A2). Consequently, the displacement corresponding to the secant line (point A) is considered as the yield displacement. Table 7 provides the data related to calculated ductility and comparison between all the specimens and the reference model. It should be noted that the average of positive and negative displacement is used to calculate ductility. Base on the data given in Table 7, it can be said that in the specimen OS-M with overlap splice at the middle of column, ductility decreases slightly compared to the reference column. Considering specimens with threaded couplers, specimen TC-H5-H60 possesses the lowest ductility
5.2.6. STC-H5-H60 Shear screw-threaded couplers were incorporated in this RC column at two mentioned locations. Firstly, it should be noted that no crack was observed up to the load of 30 KN. Flexural cracks, then, started under the load of 35.5 KN at the bottom of the column. As load increased, in addition of occurring flexural cracks at the bottom of the column, some cracks were observed at the location of upper couplers under the loads of 52 and 69.5 KN. Moreover, at the middle of column, flexural cracks occurred under the load of 43 to 69.5 KN. Shear cracks, on the other 724
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Table 5 Results of the experimental tests. Specimen name
Parameter
RSP
Ccr Sy Fu
Load (KN) 32.5 65.72 Positive direction Negative direction Average Positive direction Negative direction Average 35.5 65.287 Positive direction Negative direction Average Positive direction Negative direction Average 25 66.83 Positive direction Negative direction Average Positive direction Negative direction Average 33 81.61 Positive direction Negative direction Average Positive direction Negative direction Average 28 81.87 Positive direction Negative direction Average Positive direction Negative direction Average 30 67.15 Positive direction Negative direction Average Positive direction Negative direction Average
Fmax
OS-M
Ccr Sy Fu
Fmax
TC-A5
Ccr Sy Fu
Fmax
TC-H5-H60
Ccr Sy Fu
Fmax
STC-A5
Ccr Sy Fu
Fmax
STC-H5-H60
Ccr Sy Fu
Fmax
Ultimate lateral load capacity (KN)
Difference percentage
RSP OS-M TC-A5 TC-H5-H60 STC-A5 STC-H5-H60
77.76 83.57 81.735 98.75 84.01 86.01
*** 7.47% 5.11% 26.99% 8.04% 10.61%
94.46 94.37 94.415 47.18 47.18 47.18
94.49 94.52 94.505 71.26 71.26 71.26
141.73 141.36 141.545 60.77 36.9 48.835
94.11 93.72 93.915 71.243 71.35 71.2965
141.08 141.67 141.375 94.44 94.58 94.51
138.54 141.02 139.78 92.466 95 93.733
As provided in the last column of Table 7 and shown in Fig. 8, specimen OS-M has the lowest Δu. Moreover, ultimate displacement increases in all the specimens with mechanical splices. It is noteworthy that incorporating all of the couplers at the bottom of column results in higher Δu comparing to other patterns. To be able to compare displacement of the specimens easier, the obtained force-displacement push curves are displayed in Fig. 11.
Table 6 The ultimate lateral load capacity of the specimens. Specimen name
38.3 23.34 49.47 55.92 52.695 80.5 75.01 77.755 12.31 28.863 66.22 63.99 65.105 86.18 80.96 83.57 11.08 19.21 68.2 61.36 64.78 86 77.47 81.735 29.08 94.11 81.61 75.34 78.475 102.727 94.79 98.7585 11.38 36.025 61.39 68.59 64.99 86.17 81.85 84.01 7.68 31.845 66.91 61.24 64.075 89.6 82.43 86.015
Displacement (mm)
6.3. Absorbed energy whilst specimen TC-A5 has the highest. It shows that location of threaded couplers affect column ductility significantly. Otherwise stated, installing all of the threaded couplers at the bottom of column increases ductility by 112.24% in comparison to the reference column. Incorporating half of the couplers at 600 mm above the bottom couplers, however, decreases ductility by 4.12%. On the other hand, columns with shear screw-threaded couplers exhibited higher ductility in comparison to the reference one. Unlike the columns with threaded couplers, splice location does not have a considerable impact on the column ductility in the specimens STC-A5 and STC-H5-H60.
By integrating the area enclosed by the hysteresis loop the absorbed energy (AE) can be calculated for each load cycle. Cumulative absorbed energy at each cycle can be also obtained by summating AE in previous cycles. Fig. 12a, depicts the absorbed energy by each specimen at each cycle. CAE for all the columns is also compared together in Fig. 12b. Considering Fig. 12b, it can be realized that at the first cycles (up to 15th cycle), all the columns have approximately similar performance with low values of CAE. It simply means that different types of bar splices cannot affect cyclic behavior of a RC column under low lateral loads. From the 15th cycle to 30th cycle, however, CAE increases 725
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considerably in all the specimens and then it increases sharply at the rest cycles. The most important point which can be found from Fig. 12b is that the column with shear screw-threaded couplers all at the bottom of column (STC-A5) possesses the highest CAE up to cycle 35 whereas From cycle 35 on, RSP and TC-H5-H60 have the highest CAE. On the other hand, the specimen with threaded couplers all in the bottom of column (TC-A5) has the lowest CAE. According to the demonstrated curves, it can be figured out that the reference column (with nonspliced bars) and TC-H5-H60 (with threaded couplers at the both considered locations) exhibit almost the same behavior during the test. Therefore, in terms of CAE, incorporating threaded couplers can be considered as the most appropriate bar splice method. 6.4. Stiffness Secant stiffness, which is known as the ability to resist deformation, is defined as the slope of a line passing through peak loads at both directions in each hysteresis loop [8]. Fig. 13 demonstrates the stiffness for all the specimens. As depicted in Fig. 13, stiffness decreases rapidly at the first cycles (up to 10 mm). Next, it decreases slightly up to the displacement of 100 mm and finally, it reaches to an almost constant value for all the specimens. By comparing stiffness curves, it can be understood that in the displacements between 10 mm and 25 mm specimen STC-A5 has the lowest stiffness while specimen TC-H5-H60 possesses the highest stiffness in the other displacements. In general, similar to the results obtained for CAE, specimen with threaded couplers all at the bottom of column has the lowest stiffness and the specimen with threaded couplers exhibited the best performance. As a result, it can be said that using threaded couplers at two considered locations lead to the most acceptable result in comparison to other splicing methods.
Fig. 10. The method used to find Δy and Δu. Table 7 Calculated ductility for each specimen and comparison between the RC columns with reference one. Specimen name
RSP OS-M TC-A5 TC-H5-H60 STC-A5 STC-H5H60
Δy mm
23.342 22.863 21.189 32.485 36.025 31.845
Δu mm
70.11 67.512 134.436 93.546 138.92 128.865
μ
3.0035 2.95 6.3446 2.8796 3.8562 4.0466
The percentage of ductility difference for each specimen
The percentage of Δu difference for each specimen
– −1.78% 112.24% −4.12% 28.39% 34.72%
– −3.70% 91.70% 33.42% 98.14% 83.80%
7. Conclusion The main objective of this study was to find out the effect of incorporating mechanical splices on the cyclic performance of RC columns. To achieve the aim, totally six experimental RC columns were fabricated and examined under axial and cyclic loads. A column with non-spliced bars was considered as the reference model to be compared with other specimens. Overlap splice in the middle of column was used in the second model. Threaded and shear screw-threaded mechanical splices in two different locations were considered for all and half of the
Fig. 11. Force-displacement envelope curves of the specimens. 726
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Fig. 12. a) Absorbed energy by the specimens, b) cumulative absorbed energy by the specimens.
column for half of bars and 2D upper for the other bars may lead to the results similar to a column without splice. Using overlap splice or threaded couplers all at the bottom of a column reduces stiffness considerably in comparison to the column without splice. Acknowledgments We would like to thank the people who helped us to fabricate and test the specimens. We also thank the anonymous reviewers for their constrictive comments on an earlier version of this article. References [1] ACI Committee 318. Building Code Requirements for Structural Concrete: (ACI 318-14); and Commentary (ACI 318R-14). Farmington Hills, MI: American Concrete Institute, 2014. Print. [2] ASTM, ASTM E8/E8M-16a Standard Test Methods for Tension Testing of Metallic Materials, West Conshohocken: ASTM International, 2018. [3] Bompa DV, Elghazouli AY. Ductility considerations for mechanical reinforcement couplers. Structures 2017;12:115–9. https://doi.org/10.1016/j.istruc.2017.08.007. [4] Bompa DV, Elghazouli AY. Monotonic and cyclic performance of threaded reinforcement splices. Structures 2018;16:358–72. https://doi.org/10.1016/j.istruc.2018.11.009. [5] Bompa DV, Elghazouli AY. Inelastic cyclic behaviour of RC members incorporating threaded reinforcement couplers. Eng Struct 2019;180:468–83. https://doi.org/10.1016/ j.engstruct.2018.11.053. [6] Cruz Noguez CA, Saiidi MS. Shake-table studies of a four-span bridge model with advanced materials. J Struct Eng 2012;138(2):183–92. https://doi.org/10.1061/(asce)st. 1943-541x.0000457. [7] Dabiri H, Kheyroddin A, Kaviani A. A numerical study on the seismic response of RC wide column–beam joints. Int J Civil Eng 2018;17(3):377–95. https://doi.org/10.1007/ s40999-018-0364-2. [8] EL-Hacha R, Elagroudy H, Rizkalla S. Proposed modification to the ACI 318-02 Code equation on bond strength for steel bars. 5th Structure Specialty Conference of the Canadian Society for Engineering, Saskatoon, Saskatchewan, Canada. 2004. [9] Goksu C, et al. The effect of lap splice length on the cyclic lateral load behavior of RC members with low-strength concrete and plain bars. Adv Struct Eng 2014;17(5):639–58. https://doi.org/10.1260/1369-4332.17.5.639. [10] Issa CA, Nasr A. An experimental study of welded splices of reinforcing bars. Build Environ 2006;41(10):394–1405. https://doi.org/10.1016/j.buildenv.2005.05.025. [11] Lehman DE, Gookin SE, Nacamuli AM, Moehle JP. Repair of earthquake-damaged bridge columns. ACI Struct J 2001;98(2). https://doi.org/10.14359/10192. [12] Moussavi Nadoushani ZS, et al. Minimizing cutting wastes of reinforcing steel bars through optimizing lap splicing within reinforced concrete elements. Construct Build Mater 2018;185:600–8. https://doi.org/10.1016/j.conbuildmat.2018.07.023. [13] Najafgholipour MA, et al. The performance of lap splices in RC beams under inelastic reversed cyclic loading. Structures 2018;15:279–91. https://doi.org/10.1016/j.istruc. 2018.07.011. [14] Navaratnarajah V. Splicing of reinforcement bars with epoxy joints. Int J Adhes Adhes 1983;3(2):93–9. [15] Saiidi MS, Wang H. Exploratory Study of Seismic Response of Concrete Columns with Shape Memory Alloys Reinforcement. ACI Struct J 2006;103(3). https://doi.org/10. 14359/15322. [16] Saiidi MS, O'Brien M, Sadrossadat-Zadeh M. Cyclic Response of Concrete Bridge Columns Using Superelastic Nitinol and Bendable Concrete. ACI Struct J 2009;106(1). https://doi. org/10.14359/56285. [17] Tazarv M, Saiidi MS. Seismic design of bridge columns incorporating mechanical bar splices in plastic hinge regions. Eng Struct 2016;124:507–20. [18] Zanuy C, Díaz IM. Stress distribution and resistance of lap splices under fatigue loading. Eng Struct 2018;175:700–10. https://doi.org/10.1016/j.engstruct.2018.08.067.
Fig. 13. Stiffness curves of the all specimens.
longitudinal bars in other four specimens. In other words, the variable parameters were the location and type of splices in longitudinal reinforcement bars. Considering the results obtained from the experiment, it could be concluded that using mechanical splices can affect cyclic performance of reinforced concrete columns notably. Following conclusions can be drawn:
• Overlap and mechanical splices can increase ULLC of a column • • •
•
under cyclic loads. When different types and locations of splices are taken into account, using threaded couplers at the bottom of a column for half of the bars and 2D upper for the other bars, may lead to appropriate results. Although using overlap splice at the middle of a column decreases ultimate displacement, incorporating mechanical splice can increase it. Based on the obtained results, installing all of the couplers at the bottom of a column result in higher Δu in comparison to other arrangements. Incorporating mechanical splices all at the bottom of a column may lead to high values of ductility. Using either overlaps splices or couplers at two locations (the bottom of column and 2D upper), however, decreases the ductility. Using overlap splice or couplers at the bottom of a column for all the longitudinal bars may reduce AE. On the other hand, considering threaded couplers at the bottom of column for half of the bars and 2D upper for the other bars result in the same value of the column without splice. Moreover, incorporating shear screw-threaded couplers can also exhibit the same results as the column without splice. In terms of stiffness, using threaded couplers at the bottom of a 727