BEHAVIOR OF COLUMNS LATERALLY REINFORCED WITH WELDED WIRE MESH

Transcription

1 Proceedings of the First Makassar International Conference on Civil Engineering (MICCE2), March 9-, 2, ISBN BEHAVIOR OF COLUMNS LATERALLY REINFORCED WITH WELDED WIRE MESH B. Kusuma 1, Tavio 2, and P. Suprobo 3 ABSTRACT: An experimental study was carried out to investigate the behavior of short columns laterally reinforced with welded wire mesh under monotonically increasing concentric compression. The test variables included volumetric ratio, spacing and lateral steel configuration, and amount of longitudinal reinforcement. The effects of these variables on the uniaxial behavior of reinforced concrete columns are presented and discussed. The results indicate that welded wire mesh can be effective in confining the core concrete, resulting in significant improvements in strength and ductility of columns if compared with that of columns laterally reinforced with conventional ties. Keywords: Confined concrete, ductility, reinforced concrete, strength, tie, welded wire mesh. INTRODUCTION Inelastic deformability of reinforced concrete columns is essential for overall strength and stability of a structure during earthquakes and large wind loads. Deformability of columns can be achieved through proper confinement of core concrete by closely spaced transverse and longitudinal reinforcement. Confinement of concrete improves strength and deformability of columns. It is now well documented that the desired ductility can be attained in case of normal strength concrete columns by providing well-detailed lateral conventional confinement reinforcement (Sheikh and Uzumeri 198; Scott et al. 1982; Mander et al. 1988; Sheikh and Toklucu 1993). However, the gradual development of material technology has promoted the use of welded wire mesh (WWM) owing to its wide range of advantages over conventional confinement reinforcement. One of the advantages of WWM is that it is prefabricated and incurs less labor cost during construction. The first study in recent years on WWM for use in concrete columns was by Razvi and Saatcioglu (1989). They tested 34 small columns with mm section reinforced with conventional ties and with WWM wrapped around the columns, under concentric compression. The intention was to use WWM as an additional confining agent. Although the results were favorable, it was concluded that there existed practical difficulties in placing WWM in columns in that configuration. Furthermore, wrapping WWM around a column would require overlap of the WWM, which means more material and more construction labor. Until recently, a different approach with regard to the use of WWM was adopted (Mau, Holland, and Hong 1998). The WWM was utilized as transverse reinforcement, instead of longitudinal reinforcement. It was distributed throughout the core and completely replaced the conventional ties. The first study stage of the study concentrated on the effects of the spacing of twodimensional WWM on the behavior. No longitudinal bars were included in the column specimens (Holland 1995). The only parameters studied by Holland were the vertical WWM spacing, horizontal grid spacing, and wire size. Based on the results of his tests, Holland concluded that the primary benefit of the use of WWM as transverse reinforcement was strength enhancement. Hong (1997) also tested 36 small-scale and six fullscale columns in the second phase of the research program at the University of Houston. The specimens in this phase differed from those in the first phase in that they were all reinforced with the same amount of longitudinal bars. The results of Hong s study showed that the ductility enhancement for full size columns was highest with the Gage 4 WWM and lowest with Gage 12.5 WWM for the same volumetric ratio of transverse steel. The confinement of concrete with longitudinal bars improved over that of columns without longitudinal bars tested by Holland (1995), and negligible ductility enhancement was observed in columns with volumetric ratio of transverse steel less than 3.5 percent. In 1999, Saatcioglu and Grira tested under simulated lateral seismic loading columns confined with WWM and made from NSC with a 28-day cylinder strength equal 1 Ph.D. Candidate, Sepuluh Nopember Institute of Technology (ITS), Surabaya 6111, INDONESIA 2 Associate Professor, Sepuluh Nopember Institute of Technology (ITS), Surabaya 6111, INDONESIA 3 Professor, Sepuluh Nopember Institute of Technology (ITS), Surabaya 6111, INDONESIA 1

2 to 34 MPa. The variables that were investigated in the study were the volumetric transverse steel ratio, spacing, and arrangement of the welded grids. The results showed that WWM can be used effectively as confinement reinforcement, provided that the steel used has sufficient ductility and the welding process employed does not alter the strength and elongation characteristics of the steel. Recently, Lambert-Aikhionbare and Tabsh (21) investigated the effectiveness of WWM in confining HSC columns. In the study, the WWM sheets were placed in bundles at a uniform spacing in the columns. The results showed that strength and ductility of columns laterally reinforced with WWM can be achieved if the volumetric ratio of transverse steel was above 3.5 percent. Strength increases of at least 15 percent were observed, while ductility increases of 25 percent when compared with the corresponding specimens with conventional ties. The few studies carried out on confinement of reinforced concrete with WWM in the recent past, when more data on the confinement of reinforced concrete with WWM are needed. Therefore, as a part of ongoing experimental study to fully discover the strength and ductility of normal strength concrete columns, this paper reports the results of concentrically tested square columns confined with WWM. The performance of columns laterally reinforced with WWM was compared with that of columns laterally reinforced with conventional ties. EXPERIMENTAL PROGRAM the spacing of lateral reinforcement. The fourth digit ( W2 in this example) is a reference to the steel configuration. The different steel configurations tested were S, C, 1-W2, 2-W2, 1-W3, and 2-W3. The C and 2-W2 configurations contained eight longitudinal bars. The 2-W3 configuration had twelve longitudinal bars. The S, 1-W2, and 1-W3 configurations contained four corner bars only. The last digit is a reference to the volumetric ratio of the lateral steel. Number 4.8 refer to a volumetric ratio of approximately 4.8 percent. The volumetric ratio of transverse reinforcement is based on the ratio of volume of transverse reinforcement to volume of core (out-to-out of lateral steel), ACI 318-8, 28. Table 1 Properties of Column Specimens Specimen No. Specimen Label Longitudinal Reinforcement Number & Diameter, mm r l % f y MPa Dia. mm Transverse Reinforcement Arrangement s mm 1 N1-3W N1-45W D N1-72W Grid 2 x N2-3W D N2-45W N1-3W N1-45W D N1-72W Grid 3 x N2-3W D N2-45W N1-4S D Hoop 12 N1-6S N2-6C D Ties 14 N2-9C SPN r s % f yh MPa Test Program This paper presents an experimental study of the behavior of short NSC columns confined by WWM and conventional ties tested under concentric compression. A total of 15 NSC concrete columns, each with a 18 mm square cross-section of all columns, while the length was 72 mm. Ten columns were confined by transverse WWM and four columns confined by conventional ties. The remaining one column were unreinforced, or plain. The columns were designed to investigate the parameters of confinement, including the volumetric ratio, spacing and lateral steel configuration; and the amount of longitudinal reinforcement. All the details regarding the various column specimens are illustrated in Table 1 and Fig. 1. Test specimens are identified with specific names, using a five digit alphanumeric number such as N1-3W2-4.8 for example. The first digit represents column is a normal strength concrete column. The second digit ( 1 in this example) refers to the amount of longitudinal reinforcement as a percentage of the gross column area. Numbers 1 and 2 represent approximately 1.5 and 3. percent longitudinal steel, respectively. The third digit is a reference to the spacing of transverse reinforcement. Number 3 refers to Tie WWM S C 1-W2 2-W2 1-W3 2-W3 Strain Gauges : Longitudinal Bar Lateral Steel Fig. 1 Overall dimensions of test specimens and instrumentation Ratios of the amount of lateral reinforcement in the specimens to the amount of lateral reinforcement required by the ACI Building Code 28 for seismic design ranged from 125 percent for the lowest confined specimen to 3 percent for the highest. The tie spacing and diameter requirements for lateral support of the longitudinal reinforcement specified in the ACI Code for seismic design are respected for all specimens except for N2-9C-3.2 2

3 specimen. However, for specimen N2-9C-3.2, a tie spacing of 9 mm is adequate when seismic design is not a concern. Column specimens were cast vertically. After 24 hours, the form was removed and the specimens were submerged in a water tank in order to obtain the 28-day specified concrete strength. The water-curing period lasted two weeks, after which the specimens were left in the laboratory at ambient temperature until the time of testing. In all the specimens, the ratio of gross area of the section, A c, to the core area, A cc, measured to the outside of the lateral reinforcement was approximately 1.3. Longitudinal reinforcement was provided in all the confined column specimens. The lateral conventional ties of all specimens had 135 hooks and a development length as per the ACI 318 Code provisions. Fig. 1 and Table 1 show the various properties of longitudinal and transverse reinforcement. The notation l and f y are respectively longitudinal steel ratio and yield strength of longitudinal steel, s, s, and f yh are respectively spacing, volumetric ratio and yield strength of lateral steel. The spacing of WWM and conventional ties were varied from approximately one six to half of the total lateral dimension of column in order to give varying volumetric ratios of lateral confining steel. Failure of the specimens was forced in the test region, which has a length of 36 mm in the mid-height of the specimen by reducing the spacing of lateral reinforcement outside the test region which has the same spacing of 3 mm. Unless, in the case of traditionally-tied specimens, the spacing in the end zones was reduced into only one-fourth of the total lateral dimension of column of that in test region to facilitate proper placing of concrete in the cover of columns in these end regions. All the specimens were also externally confined by mm thick steel collar clamped in the end regions of 15 mm to further prevent premature failure. Material Properties The target compressive strength was 35 MPa with specified 28-day strength were produced in the Concrete and Building Material Laboratories Department of Civil Engineering at the Sepuluh Nopember Institute of Technology (ITS) Surabaya of Indonesia. The materials consisted of Ordinary Portland Cement produce by Gresik Cement Type I, natural river sand, crushed stone aggregate of maximum size mm, tap water for mixing and curing, and superplasticizer admixture used was Viscocrete- produce by PT. SIKA Indonesia to achieve workability of mix. The final mixes had 28 days average cylinder (15 3 mm) compressive strength of 43.4 MPa and the average peak strain was.258 percent. Table 2 shows the mix proportion. The measured tangent elastic moduli (E c ) TENSILE STRESS (MPa) was 3,122 MPa. A concrete slump of 2 mm was used to ensure that the concrete could be placed through the dense reinforcement cages. Table 2 Mix proportion Target Strength (MPa) Cement (kg/m 3 ) Sand (kg/m 3 ) Coarse Aggregate (kg/m 3 ) Water (kg/m 3 ) Superplasticizer (kg/m 3 ) 1.54 (,35%) Three standard concrete cylinders (15 3 mm) were cast along with each set of four columns. The properties of unconfined concrete were obtained by testing plain unreinforced column, SPN specimen. It was observed that concrete strength of column specimen were generally lower than the concentric strength measured on standard cylinders. The unreinforced specimen concrete strength was measured as 87 percent of the average concrete cylinder strength. The commonly used ratio of.85 was then used for evaluating the concrete section capacity of the specimens tested in the present study. Table 3 Tensile Test Results of WWM Wires and Steel Bars WWM & Steel Bar (d, mm) Yield Stress (MPa) Yield Strain Rupture Strain Fig. 2 Average stress-strain curves for WWM wires and steel bars Deformed steel bars were used for the longitudinal and lateral reinforcement. The longitudinal reinforcement consisted of mm and 13 mm. Two different sizes of reinforcing steel were used as WWM reinforcement, while the conventional lateral reinforcement consisted of mm diameter deformed bars. The properties of the WWM wires and conventional bars are listed in Table 3. Stress-strain w/c slump (mm),52 2 Modulus of Elasticity (MPa) 5.97 (WWM) (WWM) WR7 WR6 Steel bar DM13 DM WWM Wire TENSILE STRAIN (%) 3

4 relationships, established by performing at least three coupon tests for each reinforcement type, are illustrated in Fig. 2. Due to the absence of well-defined yield points, the yield stresses were determined at strain offsets of.2 percent for each type of WWM wire and 13 mm diameter deformed bar tested. respect to the transverse plane of the specimen. The mode of failure is shear. No long cracks were visible before the peak stress was reached. The strain at peak stress was obtained at a value.233 percent. Steel Collar Instrumentation and Testing Procedures Tr-1 Tr-4 Tr-3 Tr-6 Tr-2 Longitudinal and lateral reinforcement deformations were measured by electrical-resistance strain gauges glued to the steel bars. The strain gauges were pasted to the two or three opposite longitudinal steel bars at their middle lengths. Similarly, two or four gauges were glued at the two or four locations on the lateral steel at approximately middle length of the specimen as shown in the Fig. 1. The axial displacement of the specimens was recorded using four linear variable differential transformers (LVDTs) located at each faces of the specimens and attached to a top and bottom steel collar clamped to the specimens to give a gage length of 32 mm. The LVDTs were secured on threaded rods that had been cast into the column core concrete. Loads were recorded through a 4 kn load cell. The recorded strain data from the strain gauges and four LVDTs, along with the corresponding axial load data from load cell were fed to a data acquisition system and stored on a computer. Overall view of the test setup and a 5 kn capacity hydraulic universal testing machine with load controlled capabilities are shown in Fig. 3. The monotonic concentric compression was applied at very slow rate to capture the post-peak part of the measured load deformation curves by manually controlling the oil pressure. The load was applied from zero to failure, which was determined primarily by either rupture of the lateral reinforcement or excessive crushing of the core, together with buckling of the longitudinal bars. The time taken to complete each test ranged from 2 to 4 minutes depending upon the degree of confinement in the specimen. To ensure concentric loading, an initial load of approximately 2 percent of the total ultimate load was applied and the readings of the four LVDTs were monitored. If the readings of the LVDTs were not approximately equal, this signals that the load is not concentric. The column was then unloaded and the additional packing was given under the wooden plies unless the load becomes almost concentric. OBSERVED BEHAVIOR AND TEST RESULTS The testing result of the plain concrete specimen underwent a relatively brittle type of failure, with a large diagonal crack opening violently at a 72-degree angle with Tr-5 SGB-1 SGB-2 Steel Collar Tr-5 (a) Test setup (b) Test machine SGB-2 SGB-1 Tr-1 Fig. 3 Overall view of test setup and test machine All columns initially behaved in a similar manner until the spalling of cover concrete occurred. The test results showed that the columns reinforced with WWM began displaying longitudinal cracks at approximately 8 percent of the peak load, which corresponded to an average longitudinal strain on the surface equal to.23 percent. The cracks gradually increased and widened with increasing column load until the concrete cover began spalling off at a strain approximately equal to.3 percent. The general behavior of confined specimens was comparatively ductile and complex unlike plain unconfined column. These columns were characterized sequentially by the development of surface cracks, cover spalling, yielding of longitudinal steel, yielding of lateral steel, buckling of longitudinal bars and crushing of core concrete. The results of the experimental work are given in Table 4. Fig. 4 shows the appearance of all the specimens after testing and Fig. 5 illustrates the total column axial load normalized to unconfined column capacity versus average longitudinal strain curves obtained from the four LVDTs for all the specimens tested. Tr-4 Tr-3 4

5 (a) 2 2 Grids Relative Column Load (P/P o ) Col 1 Col 3 Specimen r s s (mm) f yh (MPa) r l Col 2 Col 4 Col 5 1; 2; 3 4; Column Axial Strain (mm/mm) (a) Specimens reinforced with WWM grids 2 2 (b) 3 3 Grids Relative Column Load (P/P o ) Col 6 Col 7 Col 8 Specimen r s s (mm) f yh (MPa) r l Col 9 Col 6; 7; 8 9; Column Axial Strain (mm/mm) (b) Specimens reinforced with WWM grids 3 3 (c) Hoops / Ties Fig. 4 Appearance of test specimens after testing Table 4 Experimental Results Relative Column Load (P/P o ) Col 11 Col 12 Col 13 Specimen r s s (mm) f yh (MPa) r l Col 14 11; 12 13; Column Axial Strain (mm/mm) (c) Specimens reinforced with conventional tie Fig. 5 (a), (b), and (c) Column axial load versus axial strain curves Specimen no. Specimen Label r s P max. (MPa) P max. P o f' cc /f' co (1) (2) (3) (4) (5) (6) (7) (8) (9) () 1 N1-3W N1-45W N1-72W N2-3W N2-45W N1-3W N1-45W N1-72W N2-3W N2-45W N1-4S N1-6S N2-6C N2-9C SPN e cc At.85f' cc, e cc85 m e, e cc /e co e cc85 /e co Generally, the load-strain curve for the specimen shows a strength gain and reaches a second peak. The second peak, may be lower or higher than the first peak, depending on the confinement efficiency of the specimen as can be seen from Fig. 5. Load resistance of columns again increased to a second peak normally for well-confined specimens only. This behavior indicates that the passive confinement becomes active only after the cover spalled and postspalling behavior depended solely upon the confinement 5

6 level of specimens. The second peak was observed for all columns confined with wire grids 2 2 and 3 3, and a few columns confined by conventional tie (specimens 11 and 13). In the case of 1, 4, 6, 7, 9, 11 and 13 specimens the load values at second peak were in fact more than those at respective first peak. This may be attributed due to the fact that 1, 6 and 11 specimens have a high volumetric ratio and closer spacing of lateral reinforcement. While columns 4, 13 and 9 had also high volumetric ratio and the lateral tie arrangement of eight and twelve bars respectively, which provided better arrangement than an arrangement of four bars. Overall, it was observed that for similar confinement columns reinforced with grids 3 3 had a better post peak behavior than columns reinforced with grids 2 2.At the stage of cover spalling, in all the specimens longitudinal steel yielded but, the level of stress in lateral confining steel was considerably less than their respective yield strengths. Similar observations were made in some earlier studies also for conventional tie (Cusson and Paultre 1994; Foster 1999). Lateral steel yielded either at the second peak or even after that in the descending part of load-deformation behavior. In the case of columns laterally reinforced with WWM, buckling of longitudinal steel was observed after of breaking at a welded joint of the WWM. The buckling phenomenon was usually found after a second peak load. The columns were analyzed to obtain the stress-strain curves of confined concrete, as suggested by Sheikh and Uzumeri (198), Cusson and Paultre (1994) and Sharma et al. (25). The concrete contribution at a certain deformation is determined by subtracting the contribution of longitudinal steel from the applied load (Fig. 6). The concrete contribution curves were non-dimensioned with respect to gross concrete area force P oc and core concrete area force P occ (Fig. 7), where P max P. 85 f A (1) Column Axial Load, P (kn) Column Axial Load, P (kn) P oc P conc = P max - P s occ Longitudinal Steel, P s c c c. 85 f A (2) Column Axial Strain (mm/mm) P conc = P max - P s P max Longitudinal Steel, P s Column Axial Strain (mm/mm) Fig. 6 Calculation of concrete contribution cc P o c Ag Ast f y Ast f (3). 85 Fig. 7 Nondimensionalized concrete contribution curve (Sharma et al. 25) while the gross concrete area A c represented the column behavior before the cover start to spall, only the core area A cc resists the applied load after concrete cover was completely spalled. Therefore, a smooth transition was assumed to take place from the lower curve to the upper curve. In Fig. 7, dark line shows the behavior of confined concrete. The load sustained by the confined core concrete P cc was calculated by subtracting the load carried by the cover concrete from P c. The maximum loads, P max was normalized by P o and this value was compared with each other as shown in Table 4, where Table 4 shows the experimental results obtained for each specimen. The maximum axial load, P max, applied on each specimen during testing, varied between 16 and 242 kn. These maximum loads are compared with their corresponding axial strength computed according to the ACI Code. The ratio P max /P o ranges from 1.13 to 1.59 for columns reinforced with WWM and 1.17 to 1.39 for columns reinforced conventional ties. The lower ratios are observed for specimens made with low levels confinement. The stress ratio f cc /f co ranges from 1.47 to 2. for columns reinforced with WWM and 1.43 to 1.55 for columns reinforced conventional ties. Strength enhancement of core concrete due to confinement is indicated in the table by the ratio f cc /f co. Ductility of confined concrete is indicated in the same table by the ratio cc85 / co. This ratio, ranging from 3.62 for low-confined specimens to 11.6 for well-confined specimens, clearly shows that excellent ductility can be achieved if sufficient lateral reinforcement is provided. The results shown in Table 4 and Fig. 5 indicate that column reinforced by conventional tie (N2-6C-4.8) has a slightly better ductility efficiency than columns reinforced by WWM when the volumetric ratios and amount of longitudinal reinforcement are equal. This is because the characteristic lateral 6

7 reinforcements have different size diameter of transverse reinforcement and modes of failure. Conventional ties can withstand lateral pressure from the concrete core to a larger extent than WWM due to hook stretching, while the welded joint of WWM brakes and therefore cannot restrain the lateral buckling that occurs in longitudinal steel. Evaluation of Column Strength The concrete strength used in capacity calculations is the in-place strength of concrete, which is known to be somewhat less than that determined by a standard cylinder test. The difference in strength is usually attributed to differences in size, shape, and concrete casting practice between actual column members and standard cylinders. The in-place strength of column concrete is represented in the SNI Building Code (SNI ) or ACI Code by.85f c, where f c is the strength of concrete determined by a standard cylinder test. The following expression is used in SNI or ACI to compute the concentric capacity P o of a reinforced concrete column made with normal-strength concrete is given by Eq. (3). The test data was further analyzed along with data reported by other researchers. Fig. 8 shows the relationship between the parameter s f yh /f c and the ratio of experimentally obtained axial load capacity for 111 square columns transversely reinforced with WWM and 93 square columns transversely reinforced with conventional ties to that predicted by Eq. (3). From this plot it could be observed that columns with a low volumetric ratio of transverse reinforcement may not achieve their strength as calculated by SNI or ACI 318; however, wellconfined columns can result in strength in excess of that calculated by SNI or ACI 318. Excess strength of columns with relatively higher amounts of transverse reinforcement is generally obtained after spalling of cover concrete. This strength enhancement comes as a result of an increase in strength of the confined core concrete. Those tested by Hong (1997), Lambert-Aikhionbare (1999) and some of the columns reinforced with conventional ties tested by Yong et al. (1988), Cusson and Paultre (1994), Saatcioglu and Razvi (1998) and Sharma et al. (25) contained widely spaced low volumetric ratio transverse reinforcement. These columns were able to develop unconfined column capacities (P o ) as specified in (3). Columns tested by Holland (1995) did not have any cover and consistently showed higher capacities than those computed based on gross cross-sectional area and unconfined concrete, since they did not suffer strength loss due to cover spalling. While, some of the columns tested by Li (1993), with higher amounts of transverse reinforcement, consistently show test strengths higher than the calculated values. P test /P o Fig. 8 Concentric capacities of square columns reinforced by conventional ties and WWM Effect of Test Variables This Investigation (Grid) This Investigation (Ties) Lambert Aikhionbare (1999) Hong (1997) Holland (1995) Sharma et al. (25) Razvi & Saatcioglu (1998) Cusson & Paultre (1994) Li Bing (1993) Yong et al. (1988) r s f yh /f' c (%) The test data was processed to establish stress-strain relationships of core concrete from recorded data. These relationships were used to assess the significance of test variables. Axial strain ductility ratio was also used as a measure of concrete deformability, defined as the ratio of axial strain of confined concrete at 15 percent strength decay ( cc85 ) to strain, corresponding to peak stress of unconfined concrete ( co ). These strain ratios are listed in Table 4. Volumetric ratios and spacing of lateral steel The importance of the amount of lateral confining steel as a factor that affects the behavior of confined concrete is well recognized. An increase in the volumetric ratio of confinement steel may be directly translated into a proportional increase in lateral confining pressure. Also, the spacing of transverse reinforcement is an important parameter that affects the distribution of confinement pressure and stability of longitudinal reinforcement. Behavior of columns with different tie or WWM spacings is shown in Fig. 9. These columns also have different volumetric ratios of tie or WWM reinforcement. As expected, the larger the volumetric ratio or closer the spacing of lateral steel, the more ductile is the behavior of columns. The columns with low volumetric ratio or increased spacing of lateral steel exhibit brittle behavior, showing faster rate of strength decay after the peak. Therefore, the relationship between the effect of tie or WWM spacing and volumetric ratio is not linear. The effectiveness of transverse reinforcement diminishes quickly with increasing tie or WWM spacing. An increase of 4 percent in strength and 145 percent in strain ductility was noticed for the columns confined by WWM as the volumetric ratio increased from 2. to 4.8 percent. Similarly, 3 percent gain in strength and 127 percent enhancement in 7

8 strain ductility were observed for columns reinforced with conventional ties due to the increase in volumetric ratio of lateral ties from 3.2 to 4.8 percent Col. 3 s = 72 mm r s = 2.% 1. Col. 1 s = 3 mm Col. 2 Strain (%) Col. 5 Col. 4 s = 3 mm column specimens transversely reinforced with WWM and ties, which have almost, equal volumetric ratio of lateral steel (Fig. ). The reinforcement arrangements only consisted of 4 bars. Figure shows stress-strain curves for the confined concrete in the column core. The results are presented for transverse steel volumetric ratio 3.2 and 4.8 percent. In general, Fig. shows the superior behavior of NSC column cores transversely reinforced with WWM over corresponding column cores reinforced with hoops. Also, columns with WWM consisting of 3 3 grids (with 5 5 mm cells) consistently resulted in greater strength than that obtained from the use of WWM with 2 2 grids (with mm cells) Col. 8 s = 72 mm r s = 2.% (a) 2 2 Grids Col. 6 s = 3 mm Col. 7 Col. Col. 9 s = 3 mm Grid Grid 7 6 Hoop 2 2 Grid 3 3 Grid Hoop Strain (a) 4.8 percent transverse steel 1. Strain (%) Grid (b) 3 3 Grids Grid Col. 12 s = 6 mm 1. Col. 11 s = 4 mm Strain (%) Col. 14 s = 9 mm Col. 13 s = 6 mm (c) Ties Fig. 9 Effect of volumetric ratio and spacing of lateral steel Configuration of lateral steel The transverse steel configuration and the resulting distribution of the longitudinal steel play a significant role in the confinement of concrete. If the lateral confining pressure applied by the transverse reinforcement on concrete is well distributed around the perimeter of the core concrete, the efficiency of confinement is improved. In this study it was possible to observe the effect of varying the lateral steel configuration by comparing the behavior of Hoop Hoop 2 2 Grid 3 3 Grid Strain (b) 3.2 percent transverse steel Fig. Effect of configuration of lateral steel The amount of increase in ductility due to the use of smaller cell size varied between 1 and 24 percent. Although there is a difference in strength between the 2 2 grids and 3 3 grids, there is no identifiable trend. For volumetric ratio of 4.8 percent, the tests showed that the strength enhancement ratio is larger with 3 3 grids than with 2 2 grids. Table 4 shows the results of the strength and ductility analyses, which indicate that the WWM in a NSC column provided between 17 to 4 percent more strength to the core than was achieved by using rectilinear ties. Further, columns laterally reinforced with WWM possessed between 1.3 and 1.6 times more ductility than corresponding tied columns with the same transverse steel 8

9 ratios and distribution of longitudinal reinforcement and resulting tie arrangement, although the tied columns exhibited larger residual ductility at higher strain levels. Amount of longitudinal reinforcement A larger amount of longitudinal bars, provided by a larger bar diameter, would prevent premature buckling of longitudinal bars. Fig. 11 shows two different pairs of specimens, and within each matched pair, two specimens varying only in their ratios of longitudinal reinforcement are compared Strain Col. 5 r l = 3.9% Col. 7 r l = 1.54% (a) 2 2 Grids (b) 3 3 Grids Fig. 11 Effect of amount of longitudinal reinforcement Pairs col. 2-col. 5 and col. 1-col. 4 show no increase in the strength gain and low enhancement in ductility gain when the longitudinal reinforcement ratio is increased from 1.54 to 3.9 percent for specimens reinforced with 2 2 grids. On the other hand, pairs col. 7-col. and col. 6-col. 9, with 3 3 grids, show good enhancements in the strength gain (4 percent) and in ductility gain (8-12 percent) when the longitudinal reinforcement ratio is increased from 1.54 to 2.78 percent. CONCLUSIONS s = 3 mm Col. 2 r l = 1.54% s = 3 mm Col. r l = 2.78% 4 D 13 8 D 13 Col. 4 r l = 3.9% 4 D 13 Col. 9 r l = 2.78% Col. 1 r l = 1.54% 12 D Col. 6 r l = 1.54% Strain This research was based on 15 square column specimens reinforced with WWM or ties and longitudinal bars. The columns were tested under concentric axial loading and can be summarized as follows: 1. The strength and ductility of confined concrete columns have increased with the increase in the amount of lateral steel. However, the strength enhancement is not as much as the ductility enhancement. 2. Among the test variables studied, volumetric ratio and spacing of lateral steel has a more pronounced effect on the behavior of confined columns than the other parameters like longitudinal steel ratio and configuration of lateral steel though, improvement in each of the variables considered, translated into enhancements in strength and ductility. 3. For the same volumetric ratio of transverse steel, the use of WWM provided higher strength in axial compression than conventional rectilinear hoops or ties. For the tested specimens, columns reinforced with WWM provided 17 to 4 percent more core strength enhancements than tied columns. Further, column cores reinforced with 3 3 grids were consistently stronger than corresponding cores reinforced with 2 2 grids. 4. The WWM in the columns provided between 1.3 and 1.6 times more ductility enhancements than corresponding tied columns with the same transverse steel volumetric ratios. The tied columns, however, exhibited larger residual ductility at higher strain levels than the columns with the WWM if the volumetric ratio of transverse steel was above 4 percent. 5. For the same volumetric ratio and spacing of the WWM, the tests showed that the ductility enhancement ratio is larger with 3 3 grids than with the 2 2 grids. 6. Within the range of values used in these tests, the amount of longitudinal reinforcement appears to have little effect on the behavior of the confined concrete. ACKNOWLEDGEMENTS This research project was financially supported by the Competitive Funding from the Directorate General of Higher Education, Ministry of National Education. The welded wire mesh was donated by PT. Union Metal, Indonesia. The superplasticizer used for concrete was granted by PT. SIKA Indonesia. The head of the Experimental Station for Building Structure and Construction of the Research Center for Human Settlements Bandung, Mr. Lutfi Faizal, is gratefully acknowledged for his support during the experimental program. The authors also would like to extend their sincere gratitude to all the technicians of the Laboratory of Concrete and Building Materials Department of Civil Engineering, Sepuluh Nopember Institute of Technology (ITS) Surabaya and the Experimental Station for Building Structure and 9

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