# Time-Dependent Deformations of Eccentrically Loaded Reinforced Concrete Columns

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## Abstract

If reinforced concrete columns and walls in high-rise buildings are subjected to axial load for a sustained duration, long-term lateral deformation (or curvature) as well as axial shortening increase over time due to the creep and shrinkage of concrete. In the present study, sustained load tests were performed to evaluate the time-dependent axial shortening and lateral displacement occurring in eccentrically loaded columns. A test set-up using post-tensioning steel rods was introduced for the sustained loading with or without eccentricity. The test results showed that the lateral displacement as well as the axial shortening increased over the sustained duration but the increasing rate of the long-term deformations rapidly decreased with increasing time. The time-dependent axial shortening and lateral displacement increasing with time were predicted by using the age-adjusted effective modulus method based on the creep and shrinkage models of ACI 209. The predictions agreed well with the test results.

## Keywords

long-term deformation sustained load eccentric loading long-term curvature high-rise building columns reinforced concrete## 1 Introduction

For eccentrically loaded concrete columns, stress redistribution between concrete and reinforcement occurring over time due to sustained loading is more complicated than that in pure compression members. Previous studies were carried out to investigate the time-dependent behavior of such eccentrically loaded columns (Viest et al. 1955; Green and Breen 1969; Balaguru and Nawy 1982; Tatsa 1989; Mickleborough and Gilbert 1991; Bradford 1997). Viest et al. (1955) investigated the sustained load-carrying capacity of eccentrically loaded short columns under very high axial compressive load. From the sustained load tests of pin-ended columns without restraints, Green and Breen (1969) reported that the bending moments and curvatures increased with time. Balaguru and Nawy (1982) proposed an analysis method for eccentrically loaded columns based on the creep model of plain concrete. Tatsa (1989) proposed a numerical method for time-dependent analysis, based on the sustained tests on eccentrically loaded panels. Mickleborough and Gilbert (1991) investigated the creep instability of eccentrically loaded slender columns. Bradford (1997) theoretically investigated the flexural rigidity of eccentrically loaded columns under sustained loading.

In the present study, sustained load tests of eccentrically loaded reinforced concrete columns were performed to investigate deformations due to sustained eccentric moment. The test parameters were the magnitude of sustained loads and the presence of eccentricity. The lateral displacement (curvature) and axial shortening due to the sustained eccentric moment were measured through the tests, and the results were compared with the predictions. In addition, simplified methods for estimating the time-dependent deformations of eccentrically loaded columns were proposed, based on the creep and shrinkage models of ACI 209R-92 and the age-adjusted effective modulus method.

## 2 Test Program

*P*

_{sus}) and the presence of eccentricity (

*e*) were considered as the test parameters. Concentrically and eccentrically loaded column specimens were named as C-

*xx*and E-

*xx*-

*y*, respectively.

*xx*(= 15 and 30) denotes the magnitude of sustained load,

*P*

_{sus}, applied to each specimen and

*y*(= 1, 2, and 3) denotes the sequential number used for the eccentrically loaded column specimens. C-15 and E-15 were subjected to

*P*

_{sus}= 421 kN (= 0.15 \(A_{g} f^{\prime}_{c}\)) and C-30, E-30-1, E-30-2, and E-30-3 were subjected to

*P*

_{sus}= 842 kN (= 0.30 \(A_{g} f^{\prime}_{c}\)).

*A*

_{g}= cross section area of column (= 60,000 mm

^{2}), and \(f^{\prime}_{c}\)= concrete strength at 28 days (= 47.3 MPa). For the eccentrically loaded column specimens E-15, E-30-1, E-30-2, and E-30-3, the sustained loads

*P*

_{sus}were applied with an eccentricity of

*e*= 50 mm from the geometrical center of the cross section. Therefore, in addition to

*P*

_{sus}, E-15, E-30-1, E-30-2, and E-30-3 were subjected to the sustained bending moment

*M*

_{sus}=

*P*

_{sus}

*e*. As shown in Fig. 2b, the sustained moment

*M*

_{sus}was uniform along the column height. The magnitude of the eccentricity was carefully taken as

*e*= 50 mm from a section analysis for short-term loading of

*P*

_{sus}and

*M*

_{sus}so that the neutral axis lies at the edge of the cross-section (i.e., no tensile stress and no cracking). Note that E-30-1, E-30-2, and E-30-3 were tested under the same sustained load condition in order to improve the reliability of long-term measurement.

Test parameters of column specimens.

Specimen | Sustained axial load | Eccentricity | Duration (days) |
---|---|---|---|

C-15 | 421 | 0 | 64 |

C-30 | 842 | 0 | 64 |

E-15 | 421 | 50 | 64 |

E-30-1 | 842 | 50 | 64 |

E-30-2 | 842 | 50 | 64 |

E-30-3 | 842 | 50 | 64 |

Figure 2a shows the test set-up for sustained loading. The compression load *P*_{sus} was applied by using two external post-tensioning high-strength steel rods (diameter = 38 mm) and a steel block placed on top of the column. At the bottom, the post-tensioning steel rods were pin-jointed to the steel blocks embedded in the concrete pedestal. Twelve shear studs (diameter = 19 mm and extension = 125 mm) were used in each embedded steel block to transfer the post-tensioning loads of the steel rods to the concrete of the pedestal (see Fig. 2a). In order to exclude the contribution of the steel rods to the lateral stiffness of the column, the steel rods were pin-jointed at a height of 90 mm from the column base.

To prevent concrete spalling due to high compressive stress, a steel cap consisting of 60 mm-thick bearing plate and 10 mm-thick side plates was used at the top of the column specimens. (see Fig. 2). In order to let shrinkage deformation of the concrete occur without restraint during curing, the steel cap was not installed for 2 days before the sustained loading. After the steel cap was placed, the gap between the steel cap and the concrete surface of the columns was filled with high-strength epoxy mortar (see Fig. 2). The epoxy mortar was hardened for 2 days.

*M*

_{sus}and continuously increase with time due to the creep and shrinkage of concrete. Therefore, LVDTs were placed at the height of 900 mm from the column base to measure the long-term lateral displacement (see Fig. 3).

## 3 Sustained Load Test

### 3.1 Testing Method

*t*

_{0}= 37 days). The tests were carried out during winter season in Korea. As shown in Fig. 4, the column specimens were placed in the basement laboratory where the variation of temperature and relative humidity between day and night was relatively insignificant. The average temperature and relative humidity of the basement laboratory during the sustained load tests were approximately 8 degrees Celsius and 40%, respectively.

As shown in Fig. 2a, the sustained axial loads were applied by using external post-tensioning high-strength steel rods. The post-tensioning steel rods were stressed by using the oil jacks at the top. Once the target posttensioning force (= 0.5*P*_{sus}) was attained by the oil jacks, the steel nuts were tightened in order for the post-tensioning steel rods to be in place by without loss of the posttensioning force. For precise post-tensioning, the post-tensioning force acting on the steel rods was measured with the load cells. The loss of the post-tensioning force of the steel rods during the sustained load tests were monitored using the strain gages attached to the steel rods.

### 3.2 Results of Sustained Load Tests

*ɛ*

_{PT}; (b) is the lateral displacements measured at the top of the column specimens by LVDTs,

*δ*

_{LVDT}; (c) and (d) are the compressive strains of the right and left reinforcing steel bars,

*ɛ*

_{RS}and

*ɛ*

_{LS}, respectively; and (e) and (f) are the compressive strains at the right and left concrete faces,

*ɛ*

_{RC}and

*ɛ*

_{LC}, respectively. The measured strains of the post-tensioning steel rods, reinforcing steel bars, and concrete surface,

*ɛ*

_{PT},

*ɛ*

_{RS},

*ɛ*

_{LS},

*ɛ*

_{RC}, and

*ɛ*

_{LC}, are the mean of two measured strains. In Figs. 5c–f and 6c–f, the blue, red, and green lines indicate the strains of the reinforcing bars and concrete at 100, 400, and 700 mm heights from the column base, respectively. As shown in Figs. 5a and 6a, the strains of the posttensioning steel rods decreased over time due to loss of the post-tensioning force. Therefore, when 6, 13, and 41 days passed after the initial posttensioning (i.e.

*t*−

*t*

_{0}= 6, 13, and 41 days), the posttensioning force of the steel rods were restored to the target force by re-tightening the steel nut using a hand wrench. During the re-tightening process, the measurements of the LVDTs at the top of the columns were disturbed due to the impact of the wrench (see Figs. 5b and 6b). The measurements of the strain gages attached to reinforcing bars and concrete surface were also disturbed during the re-tightening process of the post-tensioning loads (see Figs. 5c–f and 6c–f).

Figures 5 and 6 clearly show the difference in the time-dependent deformations between the concentrically and eccentrically loaded column specimens. As shown in Figs. 5c–f, for concentrically loaded column specimen C-30, the compressive strains of the reinforcing bars and concrete measured at the left and right ends of the cross section were almost identical. Furthermore, the compressive strains increased with time due to creep and shrinkage of the concrete. However, as shown in Fig. 5b, the lateral displacement measured by the LVDT at the top did not increase during the sustained duration. Although sudden changes in the lateral displacement were detected by the LVDT during the re-tightening process, substantial increase in the lateral displacement with time was not observed.

For eccentrically loaded column specimen E-30-1, however, the lateral displacement occurred on applying the axial load (see Fig. 6b). Furthermore, as shown in Fig. 6c–f, the strains of concrete and reinforcing bars were different at the right and left ends of the cross section. The compressive strains of concrete and reinforcing bars continuously increased with time, but the increasing rates differed. The compressive strains of concrete and reinforcing bars measured at three different heights were nearly identical, which indicates that E-30-1 with an aspect ratio 3.0 (i.e. the ratio of the shear span-to-overall section depth) was subjected to uniform bending moment *M*_{sus} over the height (compare blue, red, and green lines in Fig. 6c–f).

### 3.3 Creep and Shrinkage of Concrete Cylinders

Figure 7a, b show the creep and shrinkage strains measured for 90 days. The creep and shrinkage strains of one concrete cylinder were significantly deviated from those of the other two concrete cylinders. Therefore, only the results measured from the latter two cylinders were used for the investigation. From a regression analysis, the ultimate creep coefficient and ultimate shrinkage strain of the plain concrete corresponding to the standard conditions of ACI 209R-92 (ACI Committee 209 1992) were determined as \(\phi^{\prime}_{u}\) = 2.25 and \(\varepsilon^{\prime}_{shu}\) = 0.000672, respectively. Note that \(\phi^{\prime}_{u}\) and \(\varepsilon^{\prime}_{shu}\) are the modified values corresponding to the loading age of 7 days and the relative humidity of 40%, though the actual loading age and relative humidity during the tests were 28 days and 45.3%, respectively.

## 4 Evaluation of Time-Dependent Deformations

Long-term deformations occurring in eccentrically loaded reinforced concrete columns can be evaluated in terms of the axial shortening due to sustained axial compression *P*_{sus} and the lateral displacement (or curvature) due to sustained eccentric moment *M*_{sus}. The eccentrically loaded column specimens of the present study were subjected to compressive stress and strain over the entire cross section without significantly tensile cracking (refer to Fig. 6c–f). Therefore, the long-term axial shortening and curvature were evaluated based on an un-cracked section analysis.

### 4.1 Axial Shortening

*P*

_{sus}and eccentric moment

*M*

_{sus}. The strain distributions due to the immediate loading (i.e.

*t*=

*t*

_{0}) and long-term loading are presented as the dotted and thick solid lines, respectively. If the immediate and long-term strain distributions are assumed as linear over the section, the axial shortening strain of eccentrically loaded column can be represented as the strain at the geometrical center of the section (i.e.

*ɛ*

_{a}and

*ɛ*

_{a0}of Fig. 8a). Furthermore, since the compressive stress–strain relationship of concrete can be approximated as a linear elastic behavior if the compressive stress is not greater than 0.4 \(f^{\prime}_{c}\) (European Committee for Standardization 2004), the axial shortening of eccentrically loaded column is the same as that of concentrically loaded column (see Fig. 8b).

*E*

_{caa}(

*t*,

*t*

_{0}) and an age-adjusted transformed area

*A*

_{traa}as follows.

*ɛ*

_{cr}(

*t*,

*t*

_{0}) = creep shortening of reinforced concrete at time

*t*;

*t*

_{0}= time at loading from concrete placing;

*E*

_{ct}(28) = elastic modulus of the concrete at the age of 28 days;

*χ*(

*t*

_{0}) = aging coefficient addressing the reduction of compression load in the concrete due to creep;

*ϕ*(

*t*,

*t*

_{0}) = creep coefficient at time

*t*;

*A*

_{c}and

*A*

_{s}= areas of the concrete and longitudinal reinforcement, respectively;

*n*

_{aa}= age-adjusted modular ratio between the reinforcement and concrete (=

*E*

_{s}/

*E*

_{caa}(

*t*,

*t*

_{0})); and

*E*

_{s}= elastic modulus of the reinforcement (= 200 GPa). The aging coefficient

*χ*(

*t*

_{0}) and creep coefficient

*ϕ*(

*t*,

*t*

_{0}) of concrete are defined as follows [McGregor (McGregor 1997) and ACI 209R-92 (ACI Committee 209 1992)].

*ɛ*

_{a0}at loading age

*t*

_{0}as follows.

*ɛ*

_{a0}= elastic shortening of reinforced concrete at time

*t*

_{0};

*A*

_{tr}= transformed area of the column section at time

*t*

_{0}(=

*A*

_{c}+ [

*E*

_{s}/

*E*

_{ct}(

*t*

_{0})]

*A*

_{s});

*n*= modular ratio between the reinforcement and concrete (=

*E*

_{s}/

*E*

_{ct}(

*t*

_{0})); and \(\bar{\rho }\) = reinforcement-to-concrete area ratio (=

*A*

_{s}/

*A*

_{c}). According to ACI 209R-92 (ACI Committee 209 1992) and Fintel et al. (1987), the elastic modulus

*E*

_{ct}(

*t*

_{0}) at time

*t*

_{0}of normal-weight concrete can be defined as follows.

*E*

_{caa}(

*t*,

*t*

_{0}) of Eq. (1) accounts only for the creep effects, the shrinkage shortening needs to be additionally addressed. As shown in Fig. 8b, the shrinkage shortening

*ɛ*

_{sh}(

*t*,

*t*

_{0}) of reinforced concrete column at time

*t*is smaller than the shrinkage shortening

*ɛ*

_{cs}(

*t*,

*t*

_{0}) of plain concrete column, because the longitudinal reinforcement restrains shrinkage in concrete. From the force equilibrium, the increase in the compression force of the longitudinal reinforcement due to the shrinkage shortening,

*ɛ*

_{sh}(

*t*,

*t*

_{0})

*E*

_{s}

*A*

_{s}, should be equal to the decrease in the compression force of the concrete, [

*ɛ*

_{cs}(

*t*,

*t*

_{0}) −

*ɛ*

_{sh}(

*t*,

*t*

_{0})]

*E*

_{caa}(

*t*,

*t*

_{0})

*A*

_{c}(refer to Fig. 8b). Therefore,

*ɛ*

_{sh}(

*t*,

*t*

_{0}) can be calculated as follows.

*ɛ*

_{cs}(

*t*,

*t*

_{0}) of the plain concrete at time

*t*from loading age

*t*

_{0}can be determined as follows [ACI 209R-92 (ACI Committee 209 1992)].

*t*

_{s}= age of the concrete in days when shrinkage begins.

*t*

_{s}, which is earlier than

*t*

_{0}, can be taken as the age of the end of moisture curing.

*ɛ*

_{a}(

*t*,

*t*

_{0}) of concentrically and eccentrically loaded column is then calculated by adding the creep and shrinkage shortenings in Eqs. (6) and (9).

*ɛ*

_{RC}and

*ɛ*

_{LC}measured at the three heights, 100, 400, and 700 mm (see Figs. 5 and 6). For the predictions,

*P*

_{sus}= 421 and 842 kN,

*A*

_{c}= 58,800 mm

^{2},

*A*

_{s}= 1200 mm

^{2}, and \(\bar{\rho }\) = 0.0204 (see Fig. 2b) were applied. For the creep and shrinkage of the concrete increasing with time,

*t*

_{0}= 37 days,

*t*

_{s}= 7 days,

*χ*(

*t*

_{0}) = 0.859, \(f^{\prime}_{c}\) = 47.3 MPa, and

*n*= 5.69 were used. The ultimate creep coefficient

*ϕ*

_{u}and shrinkage strain

*ɛ*

_{shu}of concrete in the column specimens were modified considering the actual loading age and member size as follows. According to ACI 209R-92 (ACI Committee 209 1992), the creep correlation factor for loading age

*t*

_{LA}was taken as

*γ*

_{LA}= 1.25(

*t*

_{LA})

^{−0.118}= 0.816 by using

*t*

_{LA}=

*t*

_{0}= 37 days. The creep and shrinkage correlation factors for member size (i.e. the volume-to-surface ratio

*VS*= 60 mm) were taken as \(\gamma_{VS} = {\raise0.5ex\hbox{$\scriptstyle 2$} \kern-0.1em/\kern-0.15em \lower0.25ex\hbox{$\scriptstyle 3$}}[1 + 1.13\exp ( - 0.0213\,VS)]\) = 0.877 and 1.2 exp (− 0.00472

*VS*)] = 0.904, respectively. Therefore, the ultimate creep coefficient and shrinkage strain for the column specimens were determined as \(\phi_{u}\) = \(\gamma_{LA} \gamma_{VS} \phi^{\prime}_{u}\) = 0.816·0.877·2.25 = 1.61 and \(\varepsilon_{shu}\) = \(\gamma_{VS} \varepsilon^{\prime}_{shu}\) = 0.904·0.000672 = 0.000607. Since the sustained load tests of the column specimens were started at time

*t*

_{0}= 37 days, the effects of the creep and shrinkage of concrete on the axial shortenings of the column specimens were predicted between

*t*= 37 days and 101 days (duration = 64 days). The time-dependent creep coefficient

*ϕ*(

*t*,

*t*

_{0}) and shrinkage strain

*ɛ*

_{cs}(

*t*,

*t*

_{0}) increasing with time were calculated by Eqs. (5) and (10), respectively. Note that the elastic shortening

*ɛ*

_{a0}of each column specimen was used as the values measured by the strain gages so that uncertainty in the prediction of elastic shortening could not influence the prediction of long-term axial shortening. The elastic shortenings

*ɛ*

_{a0}of the column specimens are presented in Fig. 9.

As shown in Fig. 9a, the predicted axial shortenings of concentrically loaded column specimens C-15 and C-30 were greater that the measured shortenings. The low temperature of laboratory might affect discrepancies between the measured and predicted results. If the unexpected increase in the measured axial shortenings that occurred during the re-tightening process of posttensioning rods were taken into account (particularly at *t* − *t*_{0} = 43 days), the correlations between the predictions and measurements were reasonable. Figure 9(b) compares the predicted and measured axial shortenings of eccentrically loaded column specimens E-15, E-30-1, E-30-2, and E-30-3. Despite non-uniform strain distributions, Eq. (11) based on the hypothesis of uniform strain distribution over the section predicted well the axial shortenings increasing with time.

### 4.2 Curvature (or Lateral Displacement)

*κ*

_{cr }(

*t*,

*t*

_{0}) of the uncracked section can be defined based on the age-adjusted effective modulus

*E*

_{caa}(

*t*,

*t*

_{0}) and an age-adjusted transformed moment of inertia

*I*

_{traa}as follows (see Fig. 10a).

*M*

_{sus}= sustained bending moment and

*I*

_{c}and

*I*

_{s}= moments of inertia of the concrete and reinforcement calculated about the geometrical center of the section. Figure 10 illustrates

*I*

_{c}and

*I*

_{s}of a rectangular section with asymmetric reinforcement arrangement. The creep curvature

*κ*

_{cr}(

*t*,

*t*

_{0}) in Eqs. (12) and (13) are formulated similar to the creep shortening

*ɛ*

_{cr}(

*t*,

*t*

_{0}) in Eqs. (1) to (3), based on the linear elastic behavior and reduced elastic modulus

*E*

_{caa}(

*t*,

*t*

_{0}) of the concrete. By replacing

*ɛ*

_{cr}(

*t*,

*t*

_{0}) and

*ɛ*

_{c0}in Eq. (6) with

*κ*

_{cr}(

*t*,

*t*

_{0}) and

*κ*

_{0}, respectively, the creep curvature

*κ*

_{cr}(

*t*,

*t*

_{0}) can be defined as the function of the elastic curvature

*κ*

_{0}at time

*t*

_{0}.

*I*

_{tr}= transformed moment of inertia (=

*I*

_{c}+ [

*E*

_{s}/

*E*

_{ct}(

*t*

_{0})]

*I*

_{s}) and \(\bar{\eta }\) = ratio between moments of inertia of reinforcing steel bars and concrete (=

*I*

_{s}/

*I*

_{c}). Equation (14) represents the curvature increasing with time by the creep of concrete.According to Park and Paulay (1975), shrinkage can also cause a long-term curvature increasing over time particularly in unsymmetrically reinforced columns. Figure 10b illustrates the long-term curvature

*κ*

_{sh}(

*t*,

*t*

_{0}) caused by the shrinkage of concrete. For formulation, it is assumed that the column has different reinforcement areas

*A*

_{st}and

*A*

_{sb}at both ends and

*A*

_{st}is greater than

*A*

_{sb}(see Fig. 10a). Due to the unequal restraints of

*A*

_{st}and

*A*

_{sb}against the shrinkage of concrete, a shrinkage curvature

*κ*

_{sh}(

*t*,

*t*

_{0}) as well as the shrinkage shortening

*ɛ*

_{sh}(

*t*,

*t*

_{0}) increasing with time occur (see Fig. 10b). Since there is no gain or loss of moment at the section after the concrete shrinkage, the moment created by the concrete should be cancelled out that by the unequal reinforcements

*A*

_{st}and

*A*

_{sb}. Thus,

*y*

_{b}and

*y*

_{t}= distances of the reinforcements

*A*

_{sb}and

*A*

_{st}from the geometrical center of the section. In Eq. (15), the shrinkage shortening

*ɛ*

_{sh}(

*t*,

*t*

_{0}) is calculated from Eq. (9). By using

*I*

_{s}≈

*A*

_{st}

*y*

_{ t}

^{2}+

*A*

_{sb}

*y*

_{ b}

^{2},

*n*

_{aa}=

*E*

_{s}/

*E*

_{caa}, and \(\bar{\eta } = I_{s} /I_{c}\), Eq. (15) can be rewritten as follows.

*κ*(

*t*,

*t*

_{0}) of eccentrically loaded column can be calculated by adding or subtracting the shrinkage curvature

*ɛ*

_{sh}(

*t*,

*t*

_{0}) to the creep curvature

*ɛ*

_{cr}(

*t*,

*t*

_{0}).

*A*

_{sb}>

*A*

_{st}). Note that the shrinkage curvature

*κ*

_{sh}(

*t*,

*t*

_{0}) of Eq. (17) is vanished in symmetrically reinforced sections because

*A*

_{sb}=

*A*

_{st}and

*y*

_{b}=

*y*

_{t}.The long-term curvatures of eccentrically loaded column specimens E-15, E-30-1, E-30-2, and E-30-3 were evaluated using Eq. (17). Since the column specimens were symmetrically reinforced, the effect of the shrinkage on the curvature was ignored:

*κ*

_{sh}(

*t*,

*t*

_{0}) = 0. Furthermore, since the column specimens were subjected to uniform moment

*M*

_{sus}, the immediate curvature

*κ*

_{0}and long-term curvature

*κ*(

*t*,

*t*

_{0}) of Eq. (17) can be replaced with the lateral displacements

*δ*

_{0}and

*δ*(

*t*,

*t*

_{0}) at the top, respectively.

*δ*

_{0}= elastic lateral displacement at time

*t*

_{0}and

*δ*(

*t*,

*t*

_{0}) = long-term lateral displacement at time

*t*.

*δ*(

*t*,

*t*

_{0}) increasing with time. The predicted and measured are presented with the circles and solid lines, respectively. For the prediction of

*δ*(

*t*,

*t*

_{0}),

*ϕ*

_{u}= 1.61,

*ɛ*

_{shu}= 0.000607,

*t*

_{0}= 37 days,

*t*= 37–101 days,

*χ*(

*t*

_{0}) = 0.859,

*n*= 5.69,

*I*

_{c}= 4.37·10

^{8}mm

^{4},

*I*

_{s}= 1.26·10

^{7}mm

^{4}, and \(\bar{\eta }\) = 0.0287 were used. The creep coefficient

*ϕ*(

*t*,

*t*

_{0}) was calculated by Eq. (5). The elastic lateral displacement

*δ*

_{0}of each column specimen was used as the values measured by the LVDTs, shown in Fig. 11. The lateral displacements measured by the LVDTs were disturbed at (

*t*−

*t*

_{0}) = 42, 50, and 78 days during the re-tightening process of post-tensioning rods. If such measurement errors were taken into account, the agreements between the predicted and measured lateral displacements might be better.

The lateral displacement at the top of the eccentrically loaded column specimens can also increase due to the second-order effect (i.e. *P*-*Δ* effect). However, the specimens were the short columns where the slenderness effect can be neglected [ACI 318 (ACI Committee 318 2014)].

## 5 Summary and Conclusions

- 1)
For the eccentrically loaded column specimens, the lateral displacement due to eccentric moment and axial shortening due to axial compression increased with time. The increasing rates of such long-term deformations rapidly decreased with increasing time.

- 2)
The axial shortenings of eccentrically loaded columns were the almost same as that of the concentrically loaded column. The axial shortenings of the eccentrically and concentrically loaded columns agreed well with the predictions based on the existing creep and shrinkage models of concrete specified in ACI 209R-92.

- 3)
The curvature or lateral displacement of the eccentrically loaded columns, increasing with time, were also predicted based on the age-adjusted effective modulus of concrete and the concrete creep and shrinkage models in ACI 209R-92. The predicted long-term lateral displacements agreed reasonably with the test results.

As investigated in this study, the sustained moment and resulting long-term deformation increases differential shortening between compression members under construction and ultimately changes member forces by redistributing loads. Thus when designing compression members subjected to high axial load, the effects of the sustained moment and long-term deformation on the behavior of the compression members should be considered.

## Notes

### Authors’ Contributions

TSE: Planning and Performing Experiments, Analyzing Experimental Results, and Drafting the Manuscript. CSK, XZ: Analyzing Experimental Results, and Revising the Manuscript. JYK: Performing Experiments, Analyzing Experimental Results, and Revising the Manuscript. All authors read and approved the final manuscript.

### Acknowledgements

This research was supported by a grant (Code No. 18CTAP-C129746-02, Land & Transport Technology Promotion Research Program) funded by the Ministry of Land, Infrastructure and Transport of Korea.

### Competing Interests

The authors declare that they have no competing interests.

### Availability of Data and Materials

The datasets used during the current study are available from the corresponding author on reasonable request.

### Funding

All experiment were financially supported by the Korea Agency for Infrastructure Technology Advancement (KAIA) funded by the Ministry of Land, Infrastructure and Transport (Grant No. 18CTAP-C129746-02).

### Publisher’s Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## References

- ACI Committee 209. (1992).
*Prediction of creep, shrinkage and temperature effects in concrete structures, ACI 209R-92*(p. 47). Farmington Hills: American Concrete Institute.Google Scholar - ACI Committee 318. (2014).
*Building code requirements for structural concrete and commentary, ACI 318-14*. Farmington Hills: American Concrete Institute.Google Scholar - Balaguru, P., & Nawy, E. G. (1982)
*Evaluation of creep strains and stress redistribution in RC columns*. ACI Special Publication, SP76-12, pp. 309–324.Google Scholar - Bradford, M. A. (1997). Service load analysis of slender reinforced concrete columns.
*ACI Structural Journal,**94*(6), 675–683.Google Scholar - European Committee for Standardization. (2004).
*Eurocode 2: Design of concrete structures—Part 1-1: General rules and rules for buildings, BS EN 1992-1-1: 2004*(p. 2004). London: British Standards Institute.Google Scholar - Fintel, M., Ghosh, S. K., & Iyengar, H. (1987).
*Column shortening in tall structure—prediction and compensation, EB108.01D*(p. 35). Skokie: Portland Cement Association.Google Scholar - Green, R., & Breen, J. E. (1969). Eccentrically loaded concrete columns under sustained load.
*Proceedings of ACI Journal,**66*(11), 866–874.Google Scholar - Kim, J. Y., & Abdelrazaq, A. K. (2009). Construction sequence analysis of the flat plate system in a high-rise building and its impact on the construction cycle.
*The Structural Design of Tall and Special Buildings,**18*(3), 341–349.CrossRefGoogle Scholar - Korean Concrete Institute (KCI). (2012).
*Design code for structural concrete, KCI 2012*. Seoul: Korean Concrete Institute.Google Scholar - Maru, S., Asfaw, M., Sharma, R. K., & Nagpal, A. K. (2003). Effect of creep and shrinkage on RC frames with high beam stiffness.
*Journal of Structural Engineering, ASCE,**129*(4), 536–543.CrossRefGoogle Scholar - McGregor, J. G. (1997).
*Reinforced concrete: Mechanics and design*(3rd ed., p. 938). New Jersey: Prentice Hall Inc.Google Scholar - Mickleborough, N. C., & Gilbert, R. I. (1991)
*Creep Buckling of uniaxially loaded reinforced concrete columns*. ACI Special Publication, SP129-03, pp. 39–53.Google Scholar - Park, R., & Paulay, T. (1975).
*Reinforced concrete structures*(p. 769). New York: Wiley.CrossRefGoogle Scholar - Tatsa, E. Z. (1989). Load carrying of eccentrically loaded reinforced concrete panels under sustained load.
*ACI Structural Journal,**86*(2), 150–155.Google Scholar - Viest, I. M., Elstner, R. C., & Hognestad, E. (1955). Sustained load strength of eccentrically loaded short reinforced concrete columns.
*Journal of the American Concrete Institute,**27*(7), 727–755.Google Scholar

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