An experimental study on the structural performance of reinforced concrete low-rise building columns subjected to axial loading

This paper reports an experimental programme to study the effectiveness of stirrup detailing on the structural performance of columns having small sectional dimensions that are common in low-rise building structures. Nine column specimens with the same geometrical dimensions of 220 mm × 220 mm × 880 mm in three batches were detailed with different stirrup categories, have been gradually axially loaded to failure. The test data have revealed that although the presence of stirrups can generally enhance the axial load capacity of the column specimens, the enhancing levels are much dependent to the shapes of the stirrups. Selected interesting aspects of the test results have also been discussed, which set a concrete base for recommendations for design and detailing of such vertical structural elements.

Journal of Science and Technology in Civil Engineering NUCE 2020 14 (1): 103–111 AN EXPERIMENTAL STUDY ON THE STRUCTURAL PERFORMANCE OF REINFORCED CONCRETE LOW-RISE BUILDING COLUMNS SUBJECTED TO AXIAL LOADING Pham Xuan Data,∗, Nguyen Anh Vua a Faculty of Building and Industrial Construction, National University of Civil Engineering, 55 Giai Phong road, Hai Ba Trung district, Hanoi, Vietnam Article history: Received 04/12/2019, Revised 11/01/2020, Accepted 14/01/2020 Abstract It has been commonly recognized by the international research and practice community that the presence of both outer and inner stirrups may significantly enhance the axial load capacity of reinforced concrete (RC) columns However, there is limited testing evidence to support this conclusion that has been published nationally This paper reports an experimental programme to study the effectiveness of stirrup detailing on the structural performance of columns having small sectional dimensions that are common in low-rise building structures Nine column specimens with the same geometrical dimensions of 220 mm × 220 mm × 880 mm in three batches were detailed with different stirrup categories, have been gradually axially loaded to failure The test data have revealed that although the presence of stirrups can generally enhance the axial load capacity of the column specimens, the enhancing levels are much dependent to the shapes of the stirrups Selected interesting aspects of the test results have also been discussed, which set a concrete base for recommendations for design and detailing of such vertical structural elements Keywords: experimental investigation; low-rise building columns; axial load capacity; stirrups https://doi.org/10.31814/stce.nuce2020-14(1)-09 c 2020 National University of Civil Engineering Introduction Reinforced concrete columns are often detailed with outer stirrups that tie together all longitudinal rebars and inner ones tying some of the rebars along the column sectional dimensions The common shapes of inner stirrups are either cross-link or diamond shapes Stirrup detailing serves for two purposes The first is to keep column longitudinal rebars align with the formwork and stable during the concreting process Along the column height, closed outer stirrups should be placed at a spacing less than a codified value [1] Meanwhile, providing inner stirrups is optional unless the column cross-section is long-narrow rectangular The second purpose is to improve the axial load capacity by confining concrete material and preventing the rebars from buckling Previous researches have shown that if a column is properly detailed, confinement effects could increase the concrete strength as high as 40% [2] Also, it can be expected that closer-spacing stirrups could reduce the buckling length of ∗ Corresponding author E-mail address: phamxdatcdc@gmail.com (Dat, P X.) 103 Dat, P X., Vu, N A / Journal of Science and Technology in Civil Engineering rebars so that they can share more compressive stress with the concrete core at the pre-failure stage of a column Although the enhancement effects by column stirrups on the axial load capacity are well supported by previous theoretical prediction [2], the applicability of such enhancement in design practice is very limited, particularly for columns with small sectional dimensions in low-rise building structures The limited applicability can partly be explained by the lack of the experimental data for such small column sizes Furthermore, the guidelines for evaluating the effects in the current Vietnamese code of practice are not so informative For the demand for column axial load capacity is getting higher and higher in modern buildings nowadays due to more stringent architect requirements for column sectional sizes, such enhancement, if significant, should be taken into account This paper reports a series of tests to examine the effectiveness of stirrups on enhancing the axial load capacity of RC columns Nine column specimens whose cross-sectional dimensions were extracted from typical low-rise building structures were detailed and constructed with different stirrup detailings that consist of inner and outer stirrups were axially loaded statically to failure The test results including the load-displacement curves and failure modes will be discussed to clarify the contribution of stirrups to the overall structural performance of the test structures Based on the discussions, some recommendations for design, analysis and construction of low-rise building columns are also addressed Experimental programme 2.1 Design and detail of test specimens The cross-section of test specimens is selected to be 220 mm × 220 mm, that is the typical size of columns in low-rise buildings in North Vietnam The specimen height is 880 mm equal to four times the width of its section to satisfy the basic requirement for this type of testing units All test specimens were detailed constructed with concrete material and stirrup detailing which are the same as actual building structures Meanwhile, due to the capacity of the compressing machine used for this investigation, the diameter of longitudinal reinforcing bars in all specimens is selected to be mm, much smaller than those in the actual building columns, which are rarely smaller than 14 mm The use of small diameter for rebars here can be acceptable since the main objectives of the experiments focus on the contribution of stirrup detailing, not that of the rebars, to the structural performance of the columns specimens Nine specimens were divided in three groups; each group has its own testing objective Figs 1(a), 1(b), and 1(c) show design of the specimens The first group (Fig 1(a)) was aimed to examine the effectiveness of outer stirrups in confining concrete The test specimens, namely V-01, 02 and 03, were reinforced with outer stirrups mm in diameter with spacings of 280 mm (V-01), 50 mm (V-02) and 80 mm (V-03) In Group (Fig 1(b)), three specimens were reinforced with three different stirrup configurations at the same spacing of 50 mm The stirrup configurations are: (i) only outer stirrups in Specimen M50-V; (ii) Outer stirrups together with cross ties in Specimen M50-Đ, (iii) Outer stirrups together with inner diamond stirrups in Specimen M50-TR Similarly, specimens in Groups were reinforced with the same configurations at a spacing of 80 mm Detail of specimens in this group are shown in Fig 1(c) To prevent any local damage when being subjected to the compressive forces, both ends of the specimens are strengthened with a double value of the common reinforcement ratios as shown in Section 1-1 in Figs 1(a), 1(b), and 1(c) Fig presents a photo of the reinforcement cage of each type of specimens before the concreting process 104 stirrups together with cross ties in M50-Đ, (iii) Outer stirrups together with inner stirrups together with cross in Specimen Specimen M50-Đ, Outer stirrups together stirrups together with cross tiesties in Specimen M50-Đ, (iii)(iii) Outer stirrups together withwith innerinner diamond stirrups M50-TR Similarly, specimens in Groups Groups were reinforced with diamond stirrups inSpecimen Specimen M50-TR Similarly, specimens in 33 were reinforced diamond stirrups inin Specimen M50-TR Similarly, specimens in Groups were reinforced with with the same configurations aaspacing Detail of in this this group group are shown shown in the same configurations spacing of 80 80 mm mm Detail of specimens specimens in are the same configurations at at aatspacing of of 80 mm Detail of specimens in this group are shown in in Figure 1(c) Dat, P X., Vu, N A / Journal of Science and Technology in Civil Engineering Figure 1(c) Figure 1(c) (a) Group 1: Outer stirrups spacingsofof50/80/280 50/80/280 (b) Group (b)2:Group 2:stirrups Mixed stirrups at a spacing of 50 mm (a)(a) Group 1: Outer stirrups at at spacings Mixed at a spacing of 50mm Group Outer stirrups spacings of of 50/80/280 50/80/280 (b) Group 2: Mixed stirrups (a) Group 1:1:Outer stirrups atatspacings stirrups at at aa spacing spacing of of50mm 50mm (c) Group 3: Mixed stirrups at a spacing of 80mm Figure 1: Reinforcement detail of test specimens To prevent any local damage when being subjected to the compressive forces, both ends of the specimens are strengthened with a double value of the common reinforcement ratios as shown in (c) Group 3: Mixed stirrups at a spacing of 80 mm Section 1-1 in Figures 1(a), 1(b) and 1(c) Figure presents a photo of the reinforcement cage of each type of specimens before theReinforcement concreting process.detail Figure of test specimens Longitudinal bars Cross ties Outer stirrups Inner diamond stirrups Figure 2: A photo of reinforcement cages of test specimens Figure A photo of reinforcement cages of test specimens Both longitudinal reinforcing bars with a diameter of mm and stirrups with a diameter of mm used in this experimental programme was the same steel grade CB240-T, whose yield strength ofreinforcing 240 N/mm2 It is emphasized three specimens cast withwith the a diameter of Both longitudinal bars with that a diameter of 8in each mmgroup andwere stirrups concrete batch programme The equivalent cylinder compressive strength for specimen groups 1, 2, mm used in thissame experimental was the same steel grade CB240-T, whose yield strength and were 20.3 MPa, 25.2 MPa, 26.9 MPa, respectively 2.2 Test setup and instrumentations 105 Figure (a) shows a side view of the test setup The specimens were axially loaded by a compression table with 500-Ton capacity The testing force was measured by a load cell placed on top of specimens To extract the compressive strain, three Linear Variable Differential Transformers (LVDT) with 50-mm stroke were attached on three out of four specimen faces, Dat, P X., Vu, N A / Journal of Science and Technology in Civil Engineering of 240 N/mm2 It is emphasized that three specimens in each group were cast with the same concrete batch The equivalent cylinder compressive strength for specimen groups 1, 2, and were 20.3 MPa, 25.2 MPa, 26.9 MPa, respectively 2.2 Test setup and instrumentations Fig 3(a) shows a side view of the test setup The specimens were axially loaded by a compression table with 500-Ton capacity The testing force was measured by a load cell placed on top of specimens To each extract the compressive Linear Variable Differential Transformers with LVDT was used to measure thethree relative displacement at two separated 150 mm each LVDT was used tostrain, measure the relative displacement atsections two sections separated 150(LVDT) mm 50 mm stroke on three outcalculated of four specimen faces, each LVDT was used to measure as shown Theattached compressive strainstrain was as follows: aswere shown The compressive was calculated as follows: the relative displacement at two sections (separated 150 mm as shown The compressive strain was +, ( +,+0 +0+1 +1 𝜀"#$%.𝜀"#$% = ) ((- ) (Eq = )+ ((- ++ (- + )1)(Eq 1) (- (- (- calculated as follows: f1of theofftest specimen; Where 𝜀"#$%.𝜀"#$% is theisaverage compressive and f1and , f2, fand aref3the Where the average compressive testf3specimen; are the 1, f2,f3and εcomp = strainstrain + the+ (1) 150 150 150 relative displacements measured at three faces of the specimen relative displacements measured at three faces of the specimen where εcomp is compressive of the test and 3a) f1 , f2 , and f3 are the relative All test data were recorded by a data-logger with 30 channels (Figure 3a) Allthe testaverage data were recorded by a strain data-logger with 30specimen; channels (Figure displacements measured at three faces of the specimen Figure (b) 3provides a closer look on theontest Both Both ends ends of each specimen was capped Figure (b) provides a closer look thesetup test setup of each specimen was capped All test data were recorded by a data-logger with 30 channels (Fig 3(a)) Fig 3(b) provides a by a couple of steel cagescages 5-mm5-mm thickness to make sure there is no islocal damage during the the a couple of steel to make sure capped there no damage closer look onbythe test setup Both ends of thickness each specimen was by local a couple of during steel cages mm test run test run thickness to make sure there is no local damage during the test run a) Aa)photo ofphoto a side view of aof side viewview (a)AAphoto a side b) Ab) photo ofAa closer look A photo of a closer (b) photo of a look closer look Figure 3: Detail of theoftest Figure 3: Detail thesetup test setup Figure Detail of the test setup 106 The test specimens were gradually loaded to the failure point which was signed abysudden a sudden specimens were gradually loaded to the failure point which signed TheThe testtest specimens were gradually loaded to the failure point which waswas signed by aby sudden decrease of the acting force due to the force-controlled procedure After the applied load was decrease acting force to the force-controlled procedure After applied decrease of of thethe acting force duedue to the force-controlled procedure After the the applied loadload was was Dat, P X., Vu, N A / Journal of Science and Technology in Civil Engineering gradually decreased to the zero, the same procedure was repeated to confirm peak axial load gradually decreased to zero, the same procedure repeated to confirm the the peak axial load gradually decreased to zero, same procedure waswas repeated to confirm the peak axial load The2.3 test specimens were gradually loaded to the failure point which was signed by a sudden Failure modes of test specimens: 2.3 Failure modes of test specimens: 2.3 Failure modes of test specimens: decrease of the acting force due to the force-controlled procedure After the applied load was gradually The failure mode test specimens the crushing of concrete combined typical failure mode test specimens the crushing of axial concrete combined decreased totypical zero, the same procedure was repeated towas confirm theofpeak load TheThe typical failure mode of of testof specimens waswas the crushing concrete combined withwithwith buckling of longitudinal reinforcing bars which mainly occurred at the middle-section of every buckling of longitudinal reinforcing which mainly occurred at middle-section the middle-section of every buckling of longitudinal reinforcing barsbars which mainly occurred at the of every 2.3 Failure modes of test specimens specimen The failure was initiated with diagonal/horizontal cracks at one orfaces more specimen failure initiated diagonal/horizontal cracks at one or more faces of of specimen TheThe failure waswas initiated withwith diagonal/horizontal cracks at one or more offaces The typical failure mode of test specimens was the crushing of concrete combined with buckling specimens, that were gradually and progressively spread to the other faces (Figure 4(a)) With specimens, that were gradually progressively spread tomiddle-section the other faces 4(a)) With that were gradually andand progressively spread the other faces (Figure 4(a)) With ofspecimens, longitudinal reinforcing bars which mainly occurred attothe of(Figure every specimen The a small increase of applied load, concrete cover started spalling (Figure 4(b)), which was failure was initiated with diagonal/horizontal cracks at one or more faces of specimens, that were a small increase of applied load, concrete cover started spalling (Figure 4(b)), which was a small increase of applied load, concrete cover started spalling (Figure 4(b)), which was gradually and progressively spread to theofother faces (Fig 4(a)) With aconcrete small increase ofasapplied immediately followed buckling longitudinal bars and heavy crushing shown immediately followed by by buckling of longitudinal and heavy concrete crushing as shown immediately followed by buckling of longitudinal barsbars and heavy concrete crushing as shown load, concrete cover started spalling (Fig 4(b)), which was immediately followed by buckling of Figure 4This (c) This failure mode, concrete crushing combined with rebar buckling, is well in in Figure (c) This failure mode, concrete crushing combined with rebar buckling, is concrete well in Figure bars (c) failure mode, concrete crushing combined with rebar buckling, is well longitudinal and heavy concrete crushing as shown in Fig 4(c) This failure mode, crushing combined with rebar buckling, is well consistent with previous seismic tests on consistent with previous seismic tests on V-shape columns [3,4] and other of RC consistent with previous seismic on V-shape columns [3,4] and other types ofV-shape RC consistent with previous seismic teststests on V-shape columns [3,4] and other types oftypes RC columns [3, 4] and other types of RC structures [5–10] It worth-noting that at the final failure stage, structures [5,6,7,8,9,10] It worth-noting at the final failure stage, both of most structures [5,6,7,8,9,10] It worth-noting at the final failure stage, both ends of most test test structures [5,6,7,8,9,10] It worth-noting thatthat at that the final failure stage, both ends ofends most test both ends of most test specimens were intact specimens were intact specimens were intact specimens were intact a) Onset of failure a) Onset of failure a) of Onset of failure (a) Onset failure b) Concrete spalling c) Buckling of longitudinal bars bars b) Concrete spalling c) Buckling of longitudinal b) Concrete spalling c) Buckling of longitudinal longitudinal bars (b) Concrete spalling (c) Buckling of bars and concrete crushing andand concrete crushing andconcrete concretecrushing crushing Figure 4: Failure mode of test specimens Figure 4: Failure mode of test specimens Figure 4: Failure mode of test specimens Figure Failure mode of test specimens Discussions The stress-strain curves of test specimens presented in this section were constructed with the horizontal axis describing the relative compressive strain εcomp calculated by Eq (1) The vertical 107 Discussions The stress-strain curves of test specimens presented in this section were constructed with the horizontal axis describing the relative compressive strain 𝜀"#$% calculated by Equation The X., Vu, N / Journal ofstress Science and Technology in Civil Engineering verticalDat, axisP.describes theA compressive given by: s"#$% axis describes the compressive stress given by:= 𝑃/𝐴67#88 (Eq 2) Where 𝑃is the compressive force acting on the specimens; and 𝐴67#88 =220 mm x 220 mm is σcomp = P/Agross the area of the specimen gross section (2) Since the specimensforce were repeatedly loaded confirm the peak load=value, where P is the compressive acting on the to specimens; and axial Agross 220 the mmoriginal × 220 mm is the area of the specimen stress-straingross curvessection had several repeated ascending and descending branches as shown in Since theFigure specimens were repeatedly to confirm the peak load value, In the following discussions,loaded these repeated segments have beenaxial omitted to make the the original stress-strain curves curvesclearer had several repeated ascending and descending branches as shown in Fig In the following discussions, these repeated segments have been omitted to make the curves clearer 40.0 Compressive stress (MPa) 35.0 30.0 25.0 20.0 15.0 10.0 5.0 0.0 0.0000 0.0005 0.0010 0.0015 0.0020 Compressive strain Figure5.5:The Theoriginal original stress-strain of Specimens M80-TR Figure stress-strainrelationships relationships of Specimens M80-TR 3.1 Enhancement on the concrete stress strain curve by the outer stirrups 3.1 Enhancement on the concrete stress strain curve by the outer stirrups Fig compares the stress-strain curves of three specimens in Group whose outer stirrup spacings are respectively 280 mm for Specimen V-1, 50 mm for Specimen V-2, and 80 mm for Specimen V-3 As can be seen, the ascending and descending parts before and after reaching the peak point of these stress-strain relationships are pretty similar regarding to the curve tendency In particular, the descending parts of Curve V-1 and V-3 are almost identical In terms of compressive stress, the peak value in Test V-2 with the closest spacing of 50 mm expectedly reached the highest peak, that is 33.6 MPa, followed by the peak value of 26.5 MPa in Test V-3 with a spacing of 80 mm, and then 22.8 MPa in Test V-1 where the specimen was reinforced with stirrup spacing of 280 However, the increasing in the peak compressive stress is generally lower and not proportional to the increasing in the stirrup amount used in the specimens As can be seen in Table 1, the ratios of the peak stress values between specimens V-3 and V-1 (V-2 and V-3) are 1.16 (1.26), while the ratios of the stirrup amount are 3.50 (1.60), respectively 3.2 Enhancement of the internal diamond-shaped stirrups Effectiveness of diamond-shaped stirrups can be evaluated by comparing specimens with and without this type of internal stirrups which should be cast with the same concrete batches and detailed with the same stirrup spacings Figs 7(a) and 7(b) compares the stress-strain curves of such specimens reinforced with stirrups at a spacing of 80 (mm) in Group and at a spacing of 50 (mm) in Groups As can be seen in Fig 7(a), the peak axial stress of Specimen M80-V (without diamond 108 Dat, P X., Vu, N A / Journal of Science and Technology in Civil Engineering 40.0 33.6 Compressive stress (MPa) 35.0 30.0 26.5 25.0 22.8 20.0 15.0 V-2 (9D6@50) V-3 (5D6@80) V-1 (1D6@280) 10.0 5.0 0.0 0.0000 0.0005 0.0010 Compressive strain 0.0015 Figure 6: Stress-strain relationships of the specimens in Group Figure Stress-strain relationships of the specimens in Group Figure compares the stress-strain curves of three specimens in Group whose outer stirrup spacings respectively mm for Specimen V-1, mm for Specimen V-2, and 80 mm for Table 1.areThe ratios of280 stirrup amounts and the50peak values of compressive stress Specimen V-3 As can be seen, the ascending and descending parts before and after reaching the the stirrup amountare pretty similar Ratio of theto peak compressive peak point Ratio of theseof stress-strain relationships regarding the curve tendency In stress particular, the descending parts of Curve V-1 and V-3 are almost identical In terms of compressive V-3/V-1 3.50 1.16 stress, the peak value in Test V-2 with the closest spacing of 50 mm expectedly reached V-2/V-3 1.60 1.26 the highest without this type of internal stirrups which should be cast with the same concrete batches and Effectiveness of diamond-shaped stirrups can be evaluated by comparing specimens with and peak, that is 33.6 MPa, followed by the peak value of 26.5 MPa in Test V-3 with a spacing of 80 ratio spacings of stirrup amounts be evaluated by thecurves ratio detailed Note: with the The same stirrup Figures 7(a) andcan 7(b) compares the stress-strain of the stirrup spacings mm, and then 22.8 MPa in Test V-1 where the specimen was reinforced with stirrup spacing of of such specimens reinforced with stirrups at a spacing of 80 (mm) in Group and at a spacing 280.be However, the7(a), increasing in stress the peak compressive of 50 (mm) in Groups As can seen in Figure the peak axial of Specimen M80- stress is generally lower and not stirrups) reached at a value of 29.2 MPa, while that of Specimen M80-TR (with diamond stirrups) was 33.6 MPa,33.6 15% greater than the values former value Theindifference terms of the peak axial stress the ratios of greater the peak between specimens V-3 and V-1in(V-2 and V-3) are 1.16 (with diamond stirrups) was1, MPa, 15% thanstress the former value The difference for a spacing of 50 (mm) is even more impressive, that is as high as 30%, as can be observed in terms of the peak axial stress(1.26), for a spacing (mm) of is even more impressive, that3.50 is as(1.60), high respectively whileof the50ratios the stirrup amount are It isinworth-noting that, although all specimens were loaded with load-controlled procedure, as 30%, Fig as can 7(b) be observed Figure 7(b) is worth-noting that, although specimens were Table 1: ItThe ratios of stirrup amountsalland the peak values of compressive stress the load-controlled specimens procedure, having the diamond was more ductile as their strain values corresponding to the loaded with specimensstirrups having diamond stirrups was more Ratio of the stirrup amount Ratio of the peak compressive stress stresses were significantly greater than those of the specimens having not such closed stirrups ductile aspeak their strain values corresponding to the peak stresses were significantly greater than V-3/V-1 3.50 1.16 those of This the specimens having notV-2/V-3 suchthe closed stirrups This alsoofaddressed the effectiveness also addressed effectiveness the stirrups in 1.60 diamond 1.26enhancing the structural ductility of of the diamond stirrups in enhancing ductility of RCcan columns RC columns Note:the Thestructural ratio of stirrup amounts be evaluated by the ratio of the stirrup spacings to theofincreasing in thethat stirrup amountM80-TR used in the specimens As can be seen in Table V (without diamond stirrups)proportional reached at a value 29.2 MPa, while of Specimen 3.2 Enhancement of the internal diamond-shaped stirrups 40.0 35.0 33.6 30.0 29.2 25.0 20.0 15.0 M80-TR 10.0 M80-V 5.0 0.0 0.0000 0.0005 0.0010 Compressive strain a) Specimens with a stirrup spacing of 80 mm (Group 3) Compressive stress (MPa) Compressive stress (MPa) 40.0 35.2 30.0 26.9 25.0 20.0 15.0 10.0 M50-TR 5.0 M50-V 0.0 0.0000 0.0015 (a) Specimens with a stirrup spacing of 80 mm (Group 3) 35.0 0.0005 0.0010 0.0015 0.0020 Compressive strain b) Specimens with a stirrup spacing of 50 mm (Group 2) (b) Specimens with a stirrup spacing of 50 mm (Group 2) Figure 7: Stress-strain curves of Specimens w/o diamond-shaped stirrups Figure Stress-strain curves of Specimens w/o diamond-shaped stirrups 3.3 Effectiveness of diamond stirrups and cross ties It has been traditionally believed that diamond stirrups can be a better choice over the cross 109 links in terms of enhancing structural performance of vertical elements such as RC columns and walls since the former type is closed, while the latter is not In some current design guidelines for high-rise building structures, it is compulsory to reinforced all primary columns Dat, P X., Vu, N A / Journal of Science and Technology in Civil Engineering 3.3 Effectiveness of diamond stirrups and cross ties 40.0 Compressive stress (MPa) It has been traditionally believed that diamond stirrups can be a better choice over the cross links 35.0 33.6 in terms of enhancing structural performance of vertical elements such as RC columns and walls since the former type is closed, while the latter is not In30.0 some current design guidelines for high-rise 34.0 building structures, it is compulsory to reinforced all 25.0 primary columns with these closed stirrups, especially for those at the ground and basement floors 20.0 15.0 However, for low-rise building columns, it is not convenient and very time-consuming to fabriM80-TR cate their reinforcement cages with the diamond stirrups.10.0 Meanwhile, the detailing processM80-Đ with cross links is preferable for it can be done faster and independently from other processes Given the con5.0 tradictions in terms of traditional application and easiness 0.0 of detailing procedure at site, Figs 8(a) 0.0005M50-TR 0.0010that were 0.0015reinforced 0.0020 and 8(b) compares the stress-strain curves of Specimens 0.0000 M80-TR and Compressive strain with diamond stirrups, and two Specimens M80-Đ and M50-Đ that were reinforced with cross links a) Specimens with a stirrup spacing of 80 mm (Group 3) 35.0 33.6 30.0 Compressive stress (MPa) Compressive stress (MPa) 40.0 34.0 25.0 20.0 15.0 M80-TR M80-Đ 10.0 5.0 0.0 0.0000 0.0005 0.0010 0.0015 Compressive strain 40.0 35.0 29.7 25.0 20.0 15.0 10.0 M50-TR M50-Đ 5.0 0.0 0.0000 0.0020 35.2 30.0 0.0005 0.0010 0.0015 0.0020 0.0025 Compressive strain a) Specimens Specimens with spacing of 80ofmm 3) Specimens with with aastirrup spacing of 50ofmm 2) (a) witha astirrup stirrup spacing 80(Group mm (Group 3) (b)b)Specimens stirrup spacing 50(Group mm (Group 2) Figure 8: Stress-strain curves of Specimens with diamond-shaped stirrups and cross-links Compressive stress (MPa) 40.0 35.0 Figure Stress-strain curves 35.2 of Specimens with diamond-shaped stirrups and cross-links 30.0 29.7 As can be seen in these Figures, in terms of the peak axial stress, the specimens reinforce 25.0As can be seen in these Figures, in terms of the the specimens reinforced with M80-TR at with cross linkspeak can beaxial eitherstress, comparable (Specimen M80-Đ versus Specimen 20.0 links can be either comparable (Specimen cross M80-Đ versus Specimen M80-TR at a spacing of M50-TR at spacing of 80 mm), or only slightly lower (Specimen M50-Đ versus Specimen 8015.0 mm), or only slightly lower (Specimen M50-Đ versus Specimen M50-TR at a spacing of 50 mm) spacing of 50 mm) than those reinforced with diamond stirrups Furthermore, the use of cross lin 10.0 those reinforced with diamondM50-TR than stirrups Furthermore, the use of cross links seems to be more M50-Đseems to be more effective in enhancing structural ductility as the strain values corresponding 5.0 effective in enhancing structural ductility as the strain values corresponding to the peak axial stress the peak axial stress of two specimens M80-Đ and M50-Đ are considerably greater than those 0.0 of two specimens M80-Đ and M50-Đ are considerably greater than those of specimens M80-TR and 0.0000 0.0005 0.0010 0.0015 0.0020 0.0025 M50-TR It seems that cross links are more suitable to reinforce the low-rise building columns than Compressive strain diamond-shaped stirrups b) Specimens with a stirrup spacing of 50 mm (Group 2) Figure 8: Stress-strain curves of Specimens with diamond-shaped stirrups and cross-links Conclusions As can be seen in these Figures, in terms of the peak axial stress, the specimens reinforced ith cross links can This be either comparable (Specimen M80-Đ versus Specimen M80-TR a paper presents an experimental programme on theateffectiveness of stirrups on improving the batches consisting of column specimens thatreinforced were detailed withstirrups different stirrupthe types been statically tested to failure under conpacing of 50 mm) than those with diamond Furthermore, use ofhave cross links axial load procedure The including the specimen’s failure modes and eems to be morecentrically effective in enhancing structural ductility as the test strainperformances values corresponding to the axial stress-strain relationships were consistent throughout all specimens, showing the reliability e peak axial stress of two specimens M80-Đ and M50-Đ are considerably greater than those of of the test setup and the testing method axial RC low-rise columns Three pacing of 80 mm), or load-resistance only slightly lower of (Specimen M50-Đ building versus Specimen M50-TR at a 110 Dat, P X., Vu, N A / Journal of Science and Technology in Civil Engineering The current test data have shown that providing an increasing number of stirrups can generally help to improve the structural performance of columns in low-rise building structures The stirrups can be either the outer wrapping around all longitudinal bars or that combined with cross ties and the diamond shaped A close spacing less than 100 mm can be recommended for columns subjected to high axial compression load, especially for the column end sections where the internal stresses are most complicated Whenever the use of inner stirrups is necessary, providing cross ties, instead of diamond links, is recommended for the columns due to the easiness of the fabricating process for such inner stirrups while the enhancing effects on the load capacity of columns can be kept the same Due to limited funding, there have been only column specimens tested in this experimental programme Further investigation, both experimental and theoretical, are therefore needed in order to quantify the enhancement of stirrup detailing Acknowledgements The study presented in this paper was financially supported by National Foundation For Science and Technology Development (NAFOSTED), Vietnam through Grant 107.01-2016.07 The financial supports are greatly appreciated References [1] TCVN 5574:2018 Vietnamese design of concrete and reinforced concrete structures Vietnam National Standard [2] Mander, J B., Priestley, M J N., Park, R (1988) Theoretical stress-strain model for confined concrete Journal of Structural Engineering, 114(8):1804–1826 [3] Nguyen, X.-H., Pham, X.-D., Luong, X.-C (2015) Shaking table test on seismic performance of L-and V-sectioned reinforced concrete columns Journal of Earthquake and Tsunami, 9(04):1550010 [4] Hung, N V., Huy, N X., Thuy, P T T., Linh, N N., Dat, P X (2019) Shaking table tests on V-shaped RC columns at the weak ground storey of a building Magazine of Concrete Research, 1–14 [5] Dat, P X., Tan, K H 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Double-curvature test of reinforced concrete columns using shaking table: A new test setup Civil Engineering Journal, 5(9):1863–1876 111 ... of stirrups to the overall structural performance of the test structures Based on the discussions, some recommendations for design, analysis and construction of low-rise building columns are... Buckling of longitudinal longitudinal bars (b) Concrete spalling (c) Buckling of bars and concrete crushing andand concrete crushing andconcrete concretecrushing crushing Figure 4: Failure mode of. .. effectiveness the stirrups in 1.60 diamond 1.26enhancing the structural ductility of of the diamond stirrups in enhancing ductility of RCcan columns RC columns Note :the Thestructural ratio of stirrup