The Effect of Steel Jacketing on Strength Capacity and
Ductility ofReinforced Concrete Columns
M. Sheikhi H. Haji-Kazemi A. Attari
Department of Civil Engineering, FerdowsiUniversity of Mashhad,
P.O. Box 91775-1111, Mashhad, Iran
ABSTRACTThere are various methods for strengthening reinforced concrete columns.Among them, coveringcolumns with steel plates is an easy solutiondue to the versatility and availability of needed materials.Applying this method will cause an increase not only in axial and flexural strength,but also in the ductility of the member. In this research, the moment-curvature curves ofstrengthened concrete columns confined by stirrups and steel jackets were studied by investigating the effective factors in column’sbehavior.The results show that, there isnot any noticeable differencebetween ultimate curvatures of the initiallyreinforced concrete column and thestrengthened one, but the proportion of final curvature to yielding curvature, so called curvature ductility, will increase substantially.
Keywords: Reinforced Concrete Column, Strengthening, Confinement, Axial Force, Moment CurvatureInteraction, Ductility.
Introduction
During the last decades,strengthening concrete structures, due to earthquakes, revising the codes, the process of deterioration of concrete and other related issues, has become the center of attentions of the construction industry. Among the structural elements, columns are one of the most crucial members.So in this paper, the method ofcovering columns with steel jackets and their confining effects on the behaviorof the section,especially on its moment-curvature relationis chosen for study.
In this strengthening approach, steel plates are usually stuck to the reinforced concrete columns by the use of resins or screws. Also it is possible to place the steel box as a cover within a little distance of the concrete column, and to fill the intervening space with high quality concrete, self consolidating or self expansive concrete. The resulting element gains more flexural and compressive strength. Theincreasein flexural strength is due to the fact that the concrete core does not let the steel jacket buckle towardsinside the section. Also because of the interaction effects of orthogonal elements, the section will gain extra stiffness at its corners which lead to a reduction in both buckling length and boundary resistant of the member.
Experiments show that the buckling strength of such columns is about 2.5 timesthe strength of steel box sections without a concrete core [1, 2].On the other hand, upon further increase of axial load, the stress in concrete reachesbeyond the elastic limit of the concrete strength, and consequently causes the formation of small cracks in concreteand an increase in the concrete volume [3]. This increasein volume will be confined by steel jackets, causingthe concrete section to experience triaxial stresses which increases its axial strength. The remarkable point here is the increasing of ductility due to the improvement of concrete section. This kind of behavior is important in seismic areas.
Although experimental studies onthe effects of stirrups on concrete columns behavior have been conducted by Saatcioglu et al [4], and also extended researches have been done bySusantha et al [5], on the behavior of steel box columns filled with concrete, but there haven’t been done any research on the effects of stirrups and strengthening steel plates simultaneously on the behavior of reinforced concrete columns.
Effects of stirrups on reinforced concrete columns
Transverse rebars hold longitudinal reinforcements on their correct positions and increase shearing strength,besides causingcolumn’s ductility and axial strength to improve.Equation (1) states the effects of lateral pressure on the axial strength of concrete columns[6].
/ (1)/ (2)
Where fccis confined compressive strength of concrete; is unconfined compressive strength of concrete; is lateral pressure.is a coefficient of increase in strength, which depends on the poisson’s ratio and is defined by equation (2).
k1is dependent upon the kind of concrete used, and the value of pressure applied.Researchers also suggested some other equations for the value of. One of themost credible equationswhich has been suggested by Richart et al. [6], is:
/ (3)This equation is also verified by other researchers such as Balmer [7] and Sato [8].
The lateral pressure,resulting fromaxial force applied on the column is resisted by transverse reinforcements or covering steel jackets. This lateral pressure is notuniform in the sides of a rectangular section.The equations suggested by Saatcioglu et al.[4]for equivalent lateral pressure in square sections will be utilized here.
/ (4)/ (5)
/ (6)
/ (7)
/ (8)
WhereAsis area section of transverse reinforcements; αis inclination of stirrups towards the transverse axis of the section; sis stirrups spacing; bis section width; and sl is distance between two adjacent longitudinal rebars.
The stress-strain diagram of concrete (Fig. 1), can now be drawn after obtaining confined compressive strength of concrete (fcc) from above equations, and corresponding strains usingsimplified equations (9)to(12). The subscripts of strains denote the percent of the relative stresses.
/ (9)/ (10)
/ (11)
/ (12)
The first nonlinear part of diagram (A) of Fig.1 is obtained by equation (13) proposed by Saatcioglu et al.[4]. Diagram (B) represents the stress – strain curve of unconfined concrete section.Since there are not any experimental data onεB1andεB.85, for the usual concrete, these two items will be taken 0.002 and 0.0038 respectively.
/ (13)Effects of steel jackets on concrete filled steel boxes
Some researchers such as Susantha et al. [4] have studied the behavior of concrete filled steel box columns without longitudinal and transverse reinforcement. The following equations are experimentalresults suggested bySusantha et al. on concrete specimens withcompressive strength of 20 to 50 MPa.
(14)
(15)
(16)
Where frp is the lateral pressure applied to concrete core due to the presence of cover; fy and Es are yield strength and modulus of elasticity of steel of steel; fc is concrete compressive strength, andm is a coefficient taken from 4 to 6. m should be taken as 4 for concrete filled steel box sections.
Suggested equations for the behavior of reinforced concrete columns strengthened with steel plates
In this section, a concrete column with transverse and longitudinal reinforcement strengthened with a confining steel boxis studied. So far, no research has been reported on the behavior of concrete columns confined with both steel box and transverse reinforcement. In this research, it is intended to establish new equations which consider confining effects of both steel box and stirrupssimultaneously, by combining classic formulations and test results.
Lateral pressure applied to the concrete core due to the presence of steel box is obtained using equations (14) and (15); and lateral pressure resulting from transverse and longitudinal reinforcements is obtained using equations (5) to (8). Combining those, results the following equations:
/ (17)/ (18)
/ (19)
/ (20)
/ (21)
/ (22)
Which lead to the followinguseful statements for drawing the stress-strain diagram.
/ (23)
Section analysis and moment-curvature diagram
Regarding the subject of lateral loads and the importance of ductility in structural members, the moment-curvature relation is been an appropriatecriteria to study the behavior of such sections. Infact, curvature is the rotation per unit length of the element and as the capability of rotation in joints is one of the most important factors affecting the issue of ductility, the moment-curvature curve can be a suitable criterion for evaluating this characteristic.
With respect to the above mentioned methodology,some specimens of ordinary reinforced concrete columns, and reinforced concrete columns strengthened with steel platesare analyzed here,using the trial and errorstechnique; and their moment-curvature curves are presented.
Assuming plane sections remain plane before and after loading, andconsidering an assumed value for the extreme compressive fiber’s strain, it is possible to locate section’s neutral axis by usingthe equilibrium equations.
Finding internal forces of the section, including forces due to concrete coreresistance, forces due to concrete cover, and forces due to tensile and compressive rebars, one may utilize the equations of equilibrium, and determine thelocation of neutral axis by trial and error method. After verifying the location of neutral axis (c), resisting moment of the section is calculated.
/ (24)/ (25)
Although the ultimate strain of confined concrete may reaches to about 0.008,present codes have suggested it to be taken about 0.003to0.0035 for design purposes. In this paper,with respect to the fact that concrete is confined and its ultimate compressive strain is possibly higher than unconfined section, the ultimatestrain of concrete is assumed to be 0.007.Therefore the maximum resisting moment of the section isattained. A computer program is preparedfor thisprocess.
Sections are denoted as bx-py,where x is referred to the width of the squaresection and y is referred to the thickness of steel plate. For example, b30-p1.5 is indicatinga concrete section with the dimensions of30x30 cm, strengthened with a steel plate having thickness of 1.5 cm.Material specifications of the specimens are shown in Table 1.
Table 1 Materials specification of the specimens
E (MPa) / fy (MPa) / f’c (MPa) / Materials25000 / - / 25 / concrete
200000 / 300 / - / Longitudinal reinforcement
200000 / 300 / - / Transverse reinforcement
200000 / 240 / - / Strengthening steel plates
The famous Hognestad formula[10] is applied for the stress-strain relationship of concrete, and steel’s behavior has been modeled using an elastoplastic two-linear stress – strain diagram.
Results
In order to provide a better knowledge of the column’s behavior, especially within practical size limits, the moment-curvature curves of some column specimens are drawn, while changing different parameters.
Besides making a comparison between ordinary reinforced concrete and strengthened composite sections, it is possible to study the effects of parameters involved, separately. Some parameters considered here are the dimensions of the section, length to thickness ratio of the steel plates, and also the value of axial force applied to the column.
It should be mentioned that when each section reaches its ultimate flexural strength (balance region), its axial stress is within 0.2 to 0.4 times its ultimate values corresponding to zero eccentricity. But diagrams here are drawn assuming axial stress of the section to be 0.3 to 0.7 times its ultimate values to show the behavior of the column section in the practical region of material failure.
In figures (3 to 14), A is the point when concrete experiences tensile stresses for the first time.B is the point when steel yields in tension side; C is the point when compressive stress of the section’s rebars reaches to the yielding value; E and F are the points corresponding to yielding steel plates at tension and compression sides of the section respectively. Eventually, D indicates that concrete has reached to its ultimate strain of 0.007 in compression.
In Figure 3, it is shown that final curvature of both plated and unplated sections is almost the same. On the other hand, the ductility of composite column is more than the ordinary reinforced concrete column, because of a small difference which exists between the values of sections’ yielding curvature (point B). It should be mentioned that diagrams aredrawn while the value of axial load for the ordinary concrete section is within its balance region, causing it to show more flexural strength; but this value of axial load isnot enough for the composite section to arrive at its balance region.
In Fig. 4, it is observed that bending resistance of reinforced concrete section has decreased by increasing the axial load ( C to D is descending ), while bending resistance of the composite section has not changed evidently. Also diagrams show that final curvature of both sections has reduced by increasing the axial load.
In figures(5) to (14), the effect of some differentparameters on moment-curvature curve is studied. In figures(5) to (10), the effect of length to thickness ratio (b/t) of strengthening plates are demonstrated. In figures (11) to (14), the effect of axial load on moment-curvatures are shown.
Figures(5) to (10) are categorized in two groups.First group with low axial load is located under the balance state in moment-axial load interaction diagram. In this group, final curvature ductility is decreased by increasing b/t ratio due to the reduction of axial load, which cause the section to exit its balance region. But second group areapplied high axial load, and so are in close proximity to their balance region;in this group, final curvature will increase by increasing b/t ratio. This phenomenon mayhappen because confinement of concrete is more effective in higher axial loads.
Figures(11) and (12) will showonly the effect of axial load on moment-curvature curves while all other parameters are constant.
Figures 11 to 14 are expressing the fact that tensile longitudinal reinforcementsare not yielded when axial load applied to the section is too high or too low, which will cause a reduction in section's ductility.
Therefore, designers should always try to utilize sections within their balanceregion.
Conclusion
- Covering reinforced concrete columns with steel jackets is one of the easiest andmost efficient methods of strengthening them. This method has some advantages such as ease of performance and availability of materials. Furthermore,confinement of concrete core will increase the column's strength and will control its ductility.
- There isnot any noticeable difference between the ultimate curvatures corresponding to assumed ultimate compressive strains of an ordinary reinforced concrete column and the strengthened one. But, on the other hand, curvature ductilities, as explained and demonstrated in result section, will be increased if the section is strengthened.
- Upon further, increase of axial load causethe curvature ductility of plated and unplated sections to decrease, but the rate of decreasing the ductility of plated sections are noticeably lower than the unplated ones.
- The flexural strength capacity of the strengthened sections is quite higher than the unplated sections comparatively. Some records show up to 200 percent increase in moment capacity
References
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