9..energy

International Journal of Civil, Structural, Environmental and Infrastructure Engineering Research and Development (IJCSEIERD) ISSN 2249-6866 Vol. 2 Issue 4 Dec - 2012 87-100 © TJPRC Pvt. Ltd.,

ENERGY DISSIPATION IN LOW STRENGTH CONCRETE BRIDGE COLUMNS

ALI M. SYED, BASHIR ALAM & MOHAMMAD JAVED

Faculty member, Department of Civil Engineering, University of Engineering & Technology Peshawar, Peshawar 25120,

Pakistan

ABSTRACT

Low strength concrete is seen in many bridges in developing countries like Pakistan. The high seismic demand

warrants investigation to see how such bridges would dissipate energy in extreme loading that may occur due to

earthquakes. Two scaled bridge columns were fabricated with target strength of 2,400 psi. The columns were tested by

subjecting them to reverse cyclic quasi-static loading. Load and displacement data was recorded which was then used to

plot the hysteretic curves. The amount of energy that dissipated in every cycle and cumulative energy dissipation for each

column was calculated. This experimental study provided high value results regarding the energy dissipation and the

ultimate displacement capacity. Further, number of cyclic loading also affects the amount of energy dissipated and

maximum displacement of column.

KEY WORDS: Energy, Dissipation, Hysteresis, Bridge Column, Quasi-Static Loading

INTRODUCTION

The earthquakes cause the ground to move. The ground excitation imparts inertial forces to bridge structure. The

causative ground motion results in significant lateral loading (AASHTO LRFD, 2007) of the bridges and these forces are to

be resisted by the bridge columns. Single column bridges have only one load path and are most vulnerable due to lack of

redundancy (Syed, 2009). Seismic performance of reinforced concrete (RC) bridge columns depends on amount of energy

it can dissipate (Syed, 2009). When lateral loads act on the bridge, plastic hinge is formed in RC column due to forces that

exceed the elastic moment capacity (Syed, 2009). The formation of plastic hinge results in columns that are flexural

dominant (Syed, 2009). Bridge designs generally revolve around energy dissipation in a seismic event, which, if avoided,

requires uneconomical sections that may seem to be impracticable Syed (2009), Preistley et. al (1996). However, relying

on energy dissipation requires reasonable ductility to be present in plastic hinge zone without which the concept of energy

dissipation through the plastic hinge cannot be used (Preistley et. al, 1996).

The lateral motion of bridges due to ground shaking that causes the cracking and yielding in the plastic hinge zone

of columns, results in energy dissipation through hysteresis Kawashima (2006), Poljansek et al. (2009). The reverse cyclic

nature of inelastic deformation is the main cause of hysteresis in RC columns (Ming-Liang & Surendra, 1987). In single

column bridges that have a flexural dominant design, plastic hinge usually forms near the column base (Ming-Liang &

Surendra, 1987).

This paper presents laboratory testing of two scaled RC bridge columns that have solid circular section. A

comprehensive field survey was undertaken and it was revealed that many bridges in northern part of Pakistan had strength

below 2,500 pounds per square inch (psi) and some even had strength as low as 2,000 psi, Syed (2009), Ming-Liang &

Surendra, (1987). Pakistan lies in highly seismic active area with potential of large seismic events around Mw8 or above

(Bilham and Wallace, 2006), therefore a study of bridge column made of low strength concrete reinforced with mild steel

88 Ali M. Syed, Bashir Alam & Mohammad Javed

bars was undertaken to evaluate their capacity. It was decided to fabricate two columns of 2,400 psi target strength (Syed,

2009). An experimental program was devised to perform the reverse quasi-static cyclic testing in the laboratories of

Earthquake Engineering Center. The experimental investigations provided quantifiable results of the hysteretic energy that

the columns could dissipate.

DESCRIPTION OF TEST COLUMNS

Two test columns with target strength of 2,400 psi were prepared in the laboratory. They are named QSCT-2-003

and QSCT-3-004. These columns had solid circular cross-section with dead mass supported on top, and these represented

typical hammer-head bridge columns generally found in study areas in northern part of Pakistan. However, the authors

believe that similar columns do exist in other parts of the country and in other countries as well. Both the test columns

were similar in geometry and size, as shown in Figure 1 with similar materials as indicated in Table 1. The features of test

columns were deduced from the extensive field survey Syed (2009), Syed M. Ali et al, (2011), Naeem et al. (2005), EERI

(2006), Dellow et al. (2006) and Syed and Shakal (2007). The summary of field survey of bridges is presented in Table 2.

Scaling

After similitude analysis, scaling factors were finalized as presented in Table 3. During selection of scale factor,

the limitations that were considered were capacity of lab facilities (hydraulic actuator capacity, size of testing floors and

walls, capacity of sensors), cost and ease of construction. The side view of test column is presented in Figure 2 in which

geometric details can be seen. The top view of the test column can be seen in Figure 3.

Materials

As seen from Table 3 that scale factor for material properties Eλ of concrete and reinforcing steel was taken

unity; this indicates that no scaling was done for the material properties of the model concrete, also called micro-concrete.

This is generally recommended for inelastic testing (Reinhorn, 1992). The two columns QSCT-2-003 and QSCT-3-004

had target strength of 2400 psi (16.5 MPa). The mix design for micro-concrete is presented in Table 1 and the mechanical

properties of reinforcing steel used in test columns are provided in Table 4. The reinforcement detail of test column is

shown in Figure 4, whereas Figure 5 shows the details of rebar and confinement steel in cross sectional view.

Dead Mass

The required dead mass in test column was 19.24 tons shown in Figure 1. This mass would produce the same

level of stresses as found in prototype bridge column.

Fig. 1. Scaled RC Bridge Columns under Reverse Quasi-Static Cyclic Testing

Energy Dissipation In Low Strength Concrete Bridge Columns 89

Fig.2: Side View (Elevation) of Components and Geometric Details of the Model Bridge Column

Column base

2438

2133

914

762mm squaretop pedestal

305mm Ø

2438

Fig.3: To View of the Test Column with Modular Mass on Top

Fig.4: Reinforcement Details of Bridge Column and Base of Column in Elevation


26-7.37mm Ø rebar

5-1mmØ spiral @ 37.5mm pitch

Ø=305mm

Fig.5: Cross Section of Test Column Showing Reinforcement Details

EXPERIMENTAL PROGRAM

The reverse quasi-static cyclic testing to study the hysteretic energy dissipation was conducted in laboratories of

Earthquake Engineering Center of University of Engineering and Technology Peshawar. Hydraulic actuator of 50 ton

force capacity was used for lateral loading in longitudinal (North-South) direction. The corresponding displacements were

measured with the displacement transducers attached to the midpoint of pedestal on column top as seen in Figure 2. This is

centerline of lateral load applied by the hydraulic actuator.

Testing Rig

The test setup comprised of strong floor and strong wall used for anchoring the test specimen and hydraulic

actuator respectively as shown in Figure 1. The base of the column was anchored down with strong floor using four 44

millimeter diameter mild-steel bolts.

Instrumentation and Data Acquisition

Data was recorded for the lateral force applied and displacement of the end of hydraulic actuator. The data of the

two channels was recorded using UCAM-70 data acquisition system. In this study, the data was sampled at frequency of

around 1.5 Hz. It is important to recall here that the frequency of cyclic testing was around 0.0067 Hz, which shows that

the sampling frequency was well above the Nyquist-Shannon sampling frequency (Dally, Riley, & McConnell, 2004). The

data acquisition system with other equipment is shown in Figure 6.

Fig.6: Data Acquisition System with Allied Accessories


Testing Protocol

The tooth-saw loading waveform in reverse quasi-static testing was used with a frequency of 0.0067 Hz (150

seconds). Two testing schemes were used, Scheme-1 for first column QSCT-2-003 shown in Figure 7. In this scheme three

cycles per drift were applied until failure occurred at 4% drift and total of 37 cycles were applied. For all the cases the

testing was to be continued at least till 20% reduction in strength was observed, which means that during the test the

maximum force was monitored for each cycle until a cycle experiences a maximum force that is 80% of the maximum

force in the preceding cycles Kawashima (2006) and Poljansek et al. (2009). This point corresponds to a failure state or the

state beyond which performance is not acceptable.

Fig 7: Scheme-1, used for Testing Only First Column QSCT-2-003

Scheme-2 was used for the column QSCT-3-004 and is as shown in Figure 7. In this scheme 2 cycles per drift were used

with frequency of a cycle of 0.0067 Hz. In this scheme total of 13 cycles were applied.

Fig.8: Scheme-2, used for Testing the Remaining Three Columns

Sign Convention

A positive force is used to describe push of actuator which is acting in north direction on the column whereas

negative force is used for pull acting in south direction. In similar way, positive displacement is used for movement of

column in north direction and negative displacement is used to describe displacement in south direction.


EXPERIMENTAL RESULTS

The two test columns are discussed here one-by-one.

First Column QSCT-2-003

This column had strength of 2,420 psi cylinder strength when tested as per ASTM C39 (ASTM C39/ C39M,

2003) as described in Table 1. The testing began with one cycle of 0.1% drift which was followed by three cycles each of

0.2%, 0.4%, 0.5% and 0.75% drift. Minor hair line cracks were seen at around 0.5% drift level, which were so narrow that

they were hardly visible. At 1.0% drift, visible cracks appeared that are shown in Figure 9 for the north and south face of

the column.

Fig.9: Minor Cracks At 1% Drift of First Column QSCT-2-003

Following abbreviations are used in the discussion that follows in rest of the paper, Refer to Figure 10 for

elaboration. Pc = Force at initial cracking; Uc = Displacement at initial cracking; Pyo = Force at initial yield; Uyo =

Displacement at initial yield; Py = Force at yielding; Uy = Displacement at yielding; and Uu = Ultimate displacement at

80% of maximum restoring force

The maximum restoring force in this test was (+) 4.96 kips in north direction and (-) 6.76 kips in south direction.

From the analysis of hysteresis curves, it is seen that this point is the initial yield point, which means that at 1% drift (1.91

mm) the steel was at onset of yielding. Further from the analysis of data plotted for the displacement hysteresis curves it is

observed that initial cracking of concrete occurred at around 0.48% drift, initial yield at 1.0% and yield at 1.13% for the

force applied in north direction and for the force applied in south direction the cracking occurred at 0.50%, initial yield at

0.98% and yield at 1.10%, which is evident from the plot of backbone curve shown in Figure 10. The values for cracking,

initial yield and yield are provided in Table 5.


Fig.10: Minor Cracks at 1% Drift of First Column QSCT-2-003

From the load-deformation data the hysteresis curves were plotted and energy dissipated in each cycle was

calculated. Hysteresis curves for various stages of cyclic testing corresponding to 0.50%, 1.0%, 2.0%, 3.0%, 3.50% and

4.0% are shown in Figure 11.

Fig.11: Hysteretic Energy Dissipation Curves of First Column QSCT-2-003 at Various Drift Levels


It is noticed that energy dissipated per cycle increased with the increase in drift. The maximum energy dissipated

was in first cycle of 4.0% drift. It is further noticed that energy dissipation per cycle is more in first cycle than second or

third cycle and difference is less among the second and third cycle.

The energy dissipated per cycle is plotted in Figure 12, from this figure it is clear that energy dissipation starts at

around 1.0% drift which is onset of yielding. Here it is worth mention that energy dissipation happens when the system

yields, prior to that, in elastic system there is no hysteretic energy dissipation.

Fig.12: Hysteretic Energy Dissipation for Each Cycle of First Column QSCT-3-004

The cumulative energy dissipated in this column QSCT-2-003 is 174.2 kip-in and the numbers of cycles are 34

before failure at around 4.0% is reached.

Second Column QSCT-3-004

The main features of the testing were 2 cycles per drift until failure was observed. The loading scheme is shown

in Figure 8. This column had strength of 2,300 psi cylinder strength when tested as per ASTM C39 (ASTM C39/ C39M,

2003) as described in Table 1.

The testing began with one cycle of 0.1% drift which was followed by two cycles each of 0.25%, 0.50%, 1.0%,

2.0, 3.0% and 4.0% drift. Very thin hair line cracks appeared before 0.5% drift, which were so fine that they were not

clearly visible. However, at 1.0% drift, visible cracks appeared on north and south face of the column. From the hysteresis

curves, it is seen that 1% drift was the point of initial yield. Further from the analysis of data plotted for the displacement

hysteresis curves it is observed that for north direction force initial cracking of concrete occurred at around 0.35% drift,

initial yield at 1.0% and yield at 1.10% and for the force applied in south direction the cracking occurred at 0.50%, initial

yield at 1.0% and yield at 1.10%. The values for cracking, initial yield and yield are provided in Table 5.

From the load-deformation data the hysteresis curves were plotted and energy dissipated in each cycle was

calculated. It is noticed that energy dissipated per cycle increased with the increase in drift. The maximum energy

dissipated was in first cycle of 4.0% drift. It is further noticed that energy dissipation per cycle is more in first cycle than

second cycle.


The energy dissipated per cycle is plotted in Figure 13, from this it is clear that energy dissipation starts at around

1.0% drift. The cumulative energy dissipated in all cycles is 110.39 k-in. Here in this column it is important to note that

the numbers of cycles are 13 before failure which is observed around 4.0% drift.

Fig.13: Hysteretic Energy Dissipation for Each Cycle of Second Column QSCT-3-004.

RESULTS & DISCUSSIONS

In this paper reverse quasi-static cyclic testing of four low strength concrete columns is discussed. The first

column QSCT-2-003 with strength of 2,420 psi is tested using 3 cycles per drift level. This scheme has effect of subjecting

relatively large number of displacement cycles as compared to second scheme of 2 cycles per drift. Due to smaller

increments of drift it resulted in more number of cycles until the failure of column. In this column failure was seen in

second cycle of 4% drift which is the 37th

cycle. The energy dissipated in this column is the greatest among the entire four

test columns amounting to 174.2 k-in, which is due to the fact that the column has undergone greater number of cycles at

various increments of increasing drift beyond elastic range. The ultimate displacement in this column was 3.05% which is

least in all the four columns and this is also due to the fact that this column went under around three times more cyclic

demand.

The second column QSCT-3-004 had concrete strength of 2,300 psi and was tested using second scheme of

loading which comprised of 2 cycles per drift. The column had undergone 13 cycles of loading until failure at second

cycle 4% drift. The total energy dissipated was 110.39 k-in. The cycle of maximum energy dissipation was first cycle of

4% drift with 28.25 k-in energy dissipation. The overall energy dissipation was less than first column, around 37% less

and it can be attributed to less number of cycles to which this column was subjected in post elastic state. The ultimate

displacement in this column was 3.73% which is more than first column but is the highest among all the four columns.

CONCLUSIONS

This paper discusses the hysteretic energy dissipation in low strength concrete columns. From this study the following

conclusions are derived:

1. The hysteretic energy dissipation starts at around 1% drift in all columns which is onset of yielding.

2. The energy dissipation also depends on the displacement demand and number of cycles of reversals.


3. More energy is dissipated in first cycle of any drift level than subsequent cycles.

4. The failure in first columns with more number of reversals (that are 37) occurs earlier at 3.05% drift, whereas in

second column with less number of reversals (that are 13) occurs at higher drift level of 3.75%. This concludes that

displacement ductility is reduced due to more number of stress reversals.

5. The energy dissipation per cycle in first column with more reversals is less than the second column with lesser number

of reversals.

ACKNOWLEDGMENTS

The first author wishes to thank the Higher Education Commission (HEC) Islamabad for their PhD funding to

undertake this research. The author also thanks the UET administration for their facilitation for carrying out this research.

REFERENCES

1. AASHTO LRFD Standard, 2007, “Specifications AASHTO LRFD, Bridge Design Specification”, 4th

edition.

Washington D.C. (USA): The American Association of State Highway Officials”

2. Syed A. M. 2009, “Study of Energy Dissipation Capacity of RC Bridge Columns under Seismic Demand” Ph.D.

dissertation. NWFP University of Engineering and Technology Peshawar, Pakistan

3. M.J.N Priestley, F. Seible, G.M. Calvi, 1996, “Seismic Design & Retrofit of Bridges”, 1st Edition.

4. Kawashima, K. (2006). Seismic Design, Isolation and Retrofit of Bridge. Tokyo: Department of Civil Engineering

Tokyo Institute of Technology, Japan.

5. Poljansek, K., Perus, I., & Fajfar, P, 2009, “Hysteretic energy dissipation capacity and the cyclic to monotonic

drift ratio for rectangular RC columns in flexure”, Earthquake Engineering and Structural Dynamics, 38, 907-928.

6. Ming-Liang Wang1, Surendra P. Shah, 1987, “Reinforced Concrete Hysteresis Model Based on the Damage

Concept”, Earthquake Engineering & Structural Dynamics Volume 15, Issue 8, pages 993–1003.

7. Bilham R, Wallace K. Future Mw>8 earthquakes in the Himalaya: implications from the 26 Dec 2004 Mw=9.0

earthquake on India's eastern plate margin. CIRES and Geological Sciences, University of Colorado Boulder

2006. (http://cires.colorado.edu/~bilham/HimalayanEarthquakes/KangraCentenaryFinal.htm)

8. Dellow GD, Ali Q, Syed AM, Hussain S, Khazai B, Nisar A., 2006 “Preliminary Reconnaissance Report For The

Kashmir Earthquake Of 8 October 2005”. In: NZSEE Napier Conf., paper 31.

9. EERI 2006, “The Kashmir Earthquake of October 8, 2005: Impacts in Pakistan” Learning from Earthquakes,

Earthquake Engineering Research Institute, Oakland, California, 2005.

(http://www.eeri.org/lfe/pdf/kashmir_eeri_2nd_report.pdf)

10. Naeem A, Scawthorn C, Syed AM, Ali Q, Javed M, Ahmed I, et al., 2005 “First Report on the Kashmir

Earthquake of October 8, 2005” Learning from Earthquakes, Earthquake Engineering Research Institute, Oakland,

California, 2005. (http://www.eeri.org/lfe/pdf/kashmir_eeri_1st_report.pdf)

11. Reinhorn, A. M. (2008). “Lecture 2 - Modeling of Structures and Similitude” retrieved Dec 23, 2008, from

CIE616 - EXPERIMENTAL METHODS IN STRUCTURAL ENG.:


(http://civil.eng.buffalo.edu/CIE616/LECTURES/Lecture%202%20-

%20Modeling%20and%20Scaling/Slides%202%20%20Modeling%20and%20Scaling.pdf)

12. Syed A. M., Shakal, A. “Response to the Pakistan Earthquake of October 8, 2005” The National Academies 2007

(http//www7.nationalacademies.org/dsc/Quake_Report_2007.pdf)

13. Syed A. M., Khan N. A., Rahman S., Reinhorn A., M. 2011, “A Survey of Damages to Bridges in Pakistan after a

Major Earthquake of October 8, 2005”. Earthquake Spectra, 27: 947-970.

14. Dally, J. W., Riley, w. F., & McConnell, K. G, 2004, “Instrumentation for Engineering Measurements”, Second

edition, Singapore: John Wiley & Sons.

15. ASTM C 39/C 39M – 03, Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens,

ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.

Table 1: Identification of Columns, Target Strength and Mix Design of Concrete Used for Test Columns

Item

QSCT-2-

003 QSCT-3-004

Target concrete strength (psi) 2,400

Proportion by weight

Cement : Sand : Coarse Agg. 1:1.5:3 1:1.5:3

Water/Cement ratio 0.62 0.58

Average cylinder strength

achieved (psi) 2,420 2,300

Modulus of Rupture (psi) 595 595

Modulus of Elasticity (ksi*) 2,798 2,737

* ksi = kilo pounds per square inch

Table 2: Summary of Field Survey of Concrete Bridges in Pakistan

S.# Parameter Average Value *

1 Span 85 feet

2 Number of lanes 2

3 Column height 23.3 feet

4 Column diameter 4.27 feet

5 Rebar size 1 in

6 Rebar ratio 1.37%

7 Spiral bar size for confinement 0.39 in

8 Pitch of spiral 6 in

9 Rebar Grade 60 ksi

10 Concrete cylinder strength 2,400 psi

11 Load on column 683.43 kips

* in = inch; kips = kilo pounds; psi = pounds per square inch


Table 3: Summary of Scale Factors Used for Test Columns

Item Scale Factor

Required Provided

Length, l 4.0 4.0

Area, A 16.0 16.0

Moment of inertia, I 64.0 64.0

Linear displacement, D 4.0 4.0

Angular displacement, θ 1.0 1.0

Modulus of elasticity, E 1.0 1.0

Stress, σ 1.0 1.0

Specific mass for column only-static case, ρ 0.25 1.0

Poisson’s Ratio, ν 1.0 1.0

Strain, ε 1.0 1.0

Concentrated load, Q 16.0 16.0

Shear force, V 16.0 16.0

Moment, M 64.0 64.0

Mass on column top, m 16.0 16.0

Gravitational acceleration, g 1.0 1.0

Energy, e 64.0 64.0

Table 4: Mechanical Properties of Rebar Used in Test Columns

Parameter Value Rebar Value Confinement

Type Deformed Plain

Diameter 0.28 in 5 plain wires, each having 0.039 in

diameter

Number 26

Spiral Pitch - 1.5 in

Yield strength 52.94 ksi -

Ultimate

strength

70.34 ksi 89.05 ksi

% elongation 20.10% -


Table 5: Values for Cracking, Initial Yield and Yield for Column QSCT-2-003 and QSCT-3-004

QSCT-2-003 Value QSCT-3-004 Value

Item North / South Direction North / South Direction

Pc 3.70 kips / -4.45 kips 3.6 kips / -5.0 kips

Uc 0.48% (0.36 in) / -0.50% (-0.75 in) 0.35% (0.26 in) / -0.50% (-0.37)

Pyo 5.00 kips/ -6.80 kips 4.85 kips / -7.0 kips

Uyo 1.0% (0.75 in) / -0.98% (-0.74 in) 1.0% (0.75 in) / -1.0% (-0.75 in)

Py 6.49 kips / -6.76 kips 6.34 kips /-6.61 kips

Uy 1.13% (0.85 in) / -1.10% (-0.83 in) 1.10% (0.83 in)/ -1.10% (-0.83 in)

Uu 3.05% 3.73%

9..energy

Documents

concept of energy dissipation

cumulative energy dissipation

energy dissipated

scaled bridge columns

scaled rc bridge columns

plastic hinge zone of

single column bridges

bridge designs