Effect of Jacket sizing

Analyzing the thickness of the jacket is important in designing the jacket to efficiently meet the column’s structural requirements.

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Choi et al. (2008) evaluated the use of different steel jacket thicknesses or multiple jackets on axially loaded circular reinforced concrete columns, as shown in Figure 3.6. The main variables were the strengths, lateral confining pressure, thickness of the jacket, adhesive

presence, and welding quality. Jackets with two layers versus one layer of an equivalent overall thickness behave approximately the same. Additionally, jacket thickness and peak strength have a nearly linear relationship.

Li et al. (2005) tested reinforced concrete cylinders, like the one shown in Figure 3.8, with varying concrete strengths, jacket thicknesses, and type of lateral steel reinforcement under axial loading. Logically, thicker steel jackets provided more confinement, increasing the stress of the confined concrete.

Figure 3.8: Standard steel jacket on circular reinforced concrete columns

Priestley et al. (1994) examined how different loading, aspect ratio’s, reinforcing, jacket thickness, and jacket strength affect circular and rectangular reinforced concrete columns under lateral loading with a constant axial force. Cross-sections of these columns are shown in Figures 3.2 and 3.3a. The thinner jacket used on the circular columns could not confine the column sufficiently at large ductility factors, even though all the columns surpassed the shear requirements.

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Using different steel jacket and cage cross-sections and spacings, Belal et al. (2015) investigated how steel jackets made from cross sections contribute to the strength of retrofitted reinforced concrete columns under axial loading. Columns with angles, channels, and plate cross sections of the same area were used with different sizes and numbers of batten plates resulting in the same cross-section area as well, as shown in Figure 3.9. Steel plates were found to be less effective due to the thinness of the plate, in relation to using steel cages made from angles or channels. More information about the results can be found in section 3.2.4.

Figure 3.9: Original column; steel cage with 3 battens; steel cage with 6 battens; steel plating Original column Angle or channel Batten plates Steel plating

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Table 3.5: Summary of jacket sizing effect on retrofit performance Choi et al. (2008) Jackets with two layers versus one layer of an equivalent overall

thickness behave approximately the same. Additionally, jacket thickness and peak strength have a nearly linear relationship.

Li et al. (2005) Logically, thicker steel jackets provided more confinement, increasing the stress of the confined concrete.

Priestley et al. (1994)

The thinner jacket used on the circular columns could not confine the column sufficiently at large ductility factors, even though all the columns surpassed the shear requirements.

Belal et al. (2015) Steel plates were less effective due to the thinness of the plate, in relation to using steel cages made from angles or channels. More information about the results can be found in section 3.2.4.

Two jacket layers versus one of equivalent size perform essentially the same, due to the degree of confinement. Jacket thickness and peak strength have a nearly linear relationship. Thin steel plates may have problems due to ductility or buckling.

3.1.5 Effect of Cross-Section

For columns without significant space constraints, the column may have flexibility with the cross-section shape. As such, the optimum column cross-section should be chosen. Studies have compared a variety of shapes of steel jackets including square, rectangular, elliptical, circular, or octagonal cross-sections for square or rectangular columns; and circular or elliptical jackets for circular columns.

Lin et al. (2010) investigated using octagonal or elliptical steel jackets, shown in Figure 3.3, on rectangular lap-splice deficient reinforced concrete columns under lateral loading. While both cross-section options were successful at improving strength and ductility capacities and preventing non-ductile splice failures, the octagonal greatly prevented lap-splice failure while

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enhancing ductility. The octagonal steel jacket also had better energy dissipation and lateral capacity further preventing seismic failures, although it used a thicker jacket. Additionally, octagonal jackets are more preferable from a constructability aspect, since they only require 8 bends, while the elliptical jacket requires continuous bending of the steel plate. In addition to improving strength, energy dissipation, and other aforementioned factors, octagonal jackets may also be preferable due to being lower cost, and taking up less space than elliptical jackets.

Priestley et al. (1994) evaluated elliptical and circular steel jackets on rectangular and circular reinforced concrete columns, respectively, under lateral loading with a constant axial load. These cross-sections are shown in Figures 3.2 and 3.3a. Loading, aspect ratios,

reinforcing, jacket thickness, and jacket strength varied between the columns tested. Rectangular columns with elliptical steel jackets and circular columns with circular steel jackets increased the elastic stiffness by 64% and 30%, respectively. This demonstrates how the retrofitted columns will experience higher shear than non-retrofitted columns. Both were very effective at

improving shear strength and flexural ductility of under-designed columns for shear.

Belal et al. (2015) also evaluated steel jackets of different shapes, thicknesses, and spacings. The results of this experiment are described above in 3.1.4: Jacket sizing

Uy (2002) tested different cross-sections and lengths of rectangular and square RC columns with steel plates on two or four sides under axial loading with different glue and bolting options, shown in Figure 3.7. The tall slender columns experienced the greatest increase in axial capacity after jacketing by 100%. Nevertheless, the steel jackets were effective for all columns tested.

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Table 3.6: Summary of effect of retrofit cross-section performance

Lin et al. (2010) While both cross-section options were successful at improving strength and ductility capacities and preventing non-ductile splice failures, the octagonal section greatly prevented lap-splice failure while enhancing ductility. The octagonal steel jacket also had better energy dissipation and lateral capacity further preventing seismic failures, although it used a thicker jacket. Additionally, octagonal jackets are more preferable from a constructability aspect, since they only require 8 bends, while the elliptical jacket requires continuous bending of the steel plate. Octagonal jackets may be preferable due to having a lower cost and taking up less space than elliptical jackets.

Priestley et al. (1994)

Rectangular columns with elliptical steel jackets increased the elastic stiffness by more than double the circular steel jackets. Both were very effective at improving shear strength and flexural ductility of under- designed columns for shear.

Belal et al. (2015) The results of this experiment are described above in 3.1.4.

Uy (2002) Tall slender columns experienced the greatest increase in axial capacity after jacketing by 100%. Nevertheless, the steel jackets were effective for all columns tested.

The use of an octagonal instead of an elliptical jacket for rectangular columns prevents lap-splice failure, enhances ductility, improves energy dissipation and lateral capacity, is easier to construct, has a lower cost, and takes up less space. Elliptical steel jackets were twice as effective at increasing elastic stiffness as circular jackets on rectangular columns. Both improved shear strength and flexural ductility sufficiently. Steel jackets are more effective at improving axial capacity for tall slender columns.

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3.1.6 Effect of Loading

Evaluating the effect of different loadings on retrofitted columns is important in simulating realistic loading conditions. Therefore, preloading levels and loading columns in strong and weak direction were compared to understand the steel jacket’s success.

Aboutaha et al. (1999) tested laterally loading partial and solid steel jacketed rectangular RC columns, like those shown in Figure 3.1, in either the weak or strong direction. The jackets were successful and strengthening the columns previously inadequate in shear under loading from both the weak and strong directions.

Table 3.7: Summary of loading results on retrofit Aboutaha et

al. (1999)

The jackets were successful at strengthening the columns previously

inadequate in shear under loading from both the weak and strong directions.

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