seismic evaluation of al-najibiya bridge using …€¦ · with no change to the section dimension....

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International Journal of Civil Engineering and Technology (IJCIET) Volume 8, Issue 4, April 2017, pp. 550–558 Article ID: IJCIET_08_04_062

Available online at http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=8&IType=4

ISSN Print: 0976-6308 and ISSN Online: 0976-6316

© IAEME Publication Scopus Indexed

SEISMIC EVALUATION OF AL-NAJIBIYA

BRIDGE USING PUSHOVER ANALYSIS

Samir Abdul Baki Jabbar Al-Jassim

Department of Civil Engineering,

Engineering Collage, University of Basrah

ABSTRACT

Al-Najibiya bridge was designed to be constructed in the north of Basrah province

to connect Garmat Ali sub-district to Basrah city crossing Euphrates branch near it's

connection with Tigris to form Shat Al Arab river. The bridge consists of 7 spans with

a movable (turn over) part for navigation allowance with a total length of 250m. and a

width of 16m. The bridge was designed using a forced based design method (FBDM)

according to AASHTO 2011[1] specifications. During the implementation of the piles of

the bents, technical difficulties faced the contractor due to the high amount of steel

reinforcements which required a solution. The bridge owner decides to review the

design and reduce the amount of longitudinal rebars in the piles by an amount of 35%

with no change to the section dimension. This change eased the implementation but as

it believed that most of the reinforcement required by the seismic demands, a more

advanced seismic analysis is required to check the performance of the bridge due to the

design extreme earthquake. A pushover analysis of the bridge piers is performed to

evaluate their capacity compared to their seismic demand. Results reveal acceptable

performance of the bridge piers under the extreme design earthquake.

Key words: Pushover Analysis, Forced Based Design, Displacement Based Design,

Seismic Demand, Bridge Performance.

Cite this Article: Samir Abdul Baki Jabbar Al-Jassim, Seismic Evaluation of Al-

Najibiya Bridge Using Pushover Analysis. International Journal of Civil Engineering

and Technology, 8(4), 2017, pp. 550–558.

http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=8&IType=4

1. INTRODUCTION

Iraq is located in a relatively active seismic zone at the northern boundaries of the Arabian plate

along the Zagros-Touros mountain range. The general trend of this active zone is directly is

directly related to the general trend of the seismicity of the Alpine belt [3]. Up to date, there are

no specific national seismic regulations for analysis and design of structures and bridges. As

bridges are very important components of transportation network in any country and some of

them should remain functioning during disasters and emergency cases, hence all expected

extreme loading cases that may happen during the service life of the bridge should be

considered.



It was a common practice to design new bridges for single level of earthquake ground

motion. This ground motion often called the design earthquake represents the largest motion

that can be reasonably expected during the life of the bridge with a return period of about 500

years [16,20]. This design method is a called a force based design method. That is the force levels

correspond to the elastic response of the bridge to the design earthquake ground motion

represented by an acceleration response spectrum are calculated based on elastic stiffness

estimates. These elastic force levels are then divided by a force reduction factor representing

the assessed displacement ductility capacity. The bridge is then designed for this reduced force

levels [2,18]. This procedure has a lot of short comings as forces are poor indicators of damage

potential. The catastrophic events during the last 50 years reveal the inadequacy of the forced

base design procedure. The great engineering effort that was put to improve the seismic design

methods resulted in the development of the displacement based seismic design procedures. It

is a design philosophy based on the calculation of the required lateral resistance to achieve a

pre-determined displacement. The pre-determined displacement is based on the damage limits

deemed to be acceptable for each level of design limit state that are often defined by material

limits or limits required by non-structural components of the structure[5,10,13]. Thus the key

consideration in this method is the structural ductility which is the crucial element for the

survival of the bridges under severe earthquakes [4,11,23]. Seismic soil-structure interaction (SSI)

effect is one of the challenges in the evaluation of the seismic demand of bridges. It is

potentially a highly nonlinear phenomenon which cause the structural response to differ from

that of the ideal structure with a rigid base, therefore soil-structure interaction became a vital

consideration in the displacement based seismic analysis of bridges [9,12,17,19].

This paper presents the use of nonlinear static analysis (pushover analysis) as a tool for

displacement based seismic analysis of AL-Najibiya bridge to check the performance of bridge

piers to the design earthquake after the reduction in the amount of longitudinal steel

reinforcement of the piles. Nonlinear soil-structure interaction is included in the analysis.

2. DESCRIPTION AND SPECIFICATIONS OF THE BRIDGE

Al-Najibiya bridge is a 250m. in length consists of 7 spans as shown in figure1. It has a movable

(turn over) part between piers P4 to P6. The total span of the movable part is 80 meters to allow

for the navigation in the river according to the requirements of the Iraqi ports association. All

other spans are 34 meters. The deck is a composite (steel-concrete) structure includes 8 main

steel girders spacing at 2meters center to center, simply supported on neoprene pads (Isolators)

at each bent and abutment. The plate girders are connected transversely by top and bottom steel

I sections and angle sections for bracings as shown in figure 2. The steel frame is topped by a

20cm. reinforced concrete slab and 5cm. of asphalt concrete pavement layer. The piers of the

bridge are of three types, they are: Type I, for piers P2, P3 and P7. Type II, for piers P4 and P6

and type III for pier P5.

Piers type I include a reinforced concrete cross beam of (16 x 2 x 1.2m) over three circular

columns of 1.5 m. in diameter each spacing at 5 meters center to center reinforced

longitudinally with 36 rebars of 32mm. diameter and transversely with hoops of 12mm. bars at

200mm. spacing. The pile cap dimensions are (14 x 9 x 3m) resting on 6 cast in drilled hole

piles of 1.8m in diameter. Original reinforcement of the piles is 96 rebars of 32mm diameter

above river bed to pile cap and 64 rebars of 32mm below river bed to the end of the pile, this

reinforcement is modified to be 60 rebars of 32mm diameters all over the length of the piles.

Transvers the piles reinforced with spiral reinforcement with 10mm diameter bars at 150mm

pitch along the pile length plus hoops of 25mm diameter bars at 125mm spacing at the locations

of expected plastic hinges (pile to pile-cap connection and river bed). The piles have variable

length above river bed according to the depth of water (river profile), but all should penetrate

Seismic Evaluation of Al-Najibiya Bridge Using Pushover Analysis


to a depth of 30m below river bed. A section in the cross beam and column of this pier is shown

in figure 3.

Piers type II has a different cross beam as it accommodate the seating of the movable part

from one side and the fixed part from the other, as shown in figure 4. This beam supported on

3 circular columns of 2m in diameter and reinforced with 36 No. of 32mm diameter rebars

longitudinally and with hoops of 12mm diameter bars at 200mm transversely. The pile cap

dimensions are (16 x 9 x 3m) resting on 8 piles of 1.8m in diameter (similar to piles of type I).

Pier type III for the movable part of the bridge P5. This pier consists of a circular pile cap

of 14m diameter and 3m in thickness. A ring beam constructed directly over the pile cap to

carry the wheels of the super structure. The pile cap resting over 17 piles of 1.8m diameter

(similar to previous piles) distributed radially ender the pile cap. Figure 5 shows the pile cap

dimensions and layout of piles for each type of pier.

Figure 1 Al-Najibiya bridge longitudinal view

Figure 2 Section through the deck. Figure 3 Cross beam and column of type I

Pier



Figure 4 Cross beam and column of pier type II

Figure 4 Pile cap dimensions and piles layout for the three types of piers.

3. STRUCTURAL MODELING AND MATERIAL PROPERTIES

For the seismic evaluation of the sub-structure revised design, a static nonlinear analysis

(pushover analysis) is to be performed in both longitudinal and transverse direction for each

pier [6, 17]. To do so, a 3 dimensions structural modeling of the piers is employed in the

SAP2000 V17 program [8] with a concentrated plasticity model (plastic hinge). The columns,

piles and cap-beams are modeled as frame elements. Pile caps are modeled shell elements. The

formulation is based on cracked sections which is 40% of the gross sections. Soil-structure

interaction is included in the analysis by presenting the soil through a series of p-y, t-z and q-z

springs [2,7,12] with values conforming to the values used in the original design. The nonlinear

behavior of the columns and piles are modelled with P-M-M hinges (moments on two

perpendicular axes) since the axial load on them changes with respect to deformation. However

cap-Beams are not allowed to yield, a P-M (moment on one axis) plastic hinge model is used

for cap-beams to show that they will not yield. Yielding is allowed in the pile-cap to pile

interface since it is a reachable location for necessary retrofitting after the earthquake but no



yielding is allowed in the pile to soil interface since it is not reachable. P-δ effect included in

the analysis, and the FEMA 356 [10] coefficient method is used for estimating the target

displacement at the top of the cross-beam.

The material model used for the for the concrete is the Mander's model [14,15] as shown in

figure 5. For the steel reinforcement the Chai's strain hardening model [7] shown in figure 6 is

used. The properties of the steel and concrete used in the design and pushover analysis are

given in table 1.

Table 1 Material properties used in the analysis

Material Property Value

Reinforcing Steel

Yield stress fy 420 MPa

Modulus of Elasticity Es 200 GPa

Tensile strength fsu 650 MPa

Nominal Yield Strain εy 0.0021

Ultimate Tensile Strain εsb 0.12

Reduced Ultimate Tensile Strain εsu 0.087

On Set Strain Hardening εsh 0.0115

Poisson's Ratio 0.25

Concrete

Unconfined Compressive Strength fco 35 MPa

Unconfined Compressive Strain at the Maximum Stress εco 0.002

Unconfined Ultimate Compressive (Spalling) Strain εsp 0.005

Poisson's Ratio 0.2

Modulus of Elasticity Ec 30 GPa

Figure 5 Stress-strain curve of concrete- Mander model

Figure 6 Stress-strain curve for steel reinforcement –

Chai strain hardening model

4. SEISMIC HAZARD

Since no national seismic code is available, the recent spectral acceleration values given in the

(UFC 3-301-01) [24] are used. These values are for 10/50 events and should be modified to 5/50

events (with a return period of approximately 1000 years), the UFC provide formulas for the

required modification. The soil of the site is classified in class E, then the resulted design

acceleration-time response spectrum is shown in figure 7. Since the Sb1 = 0.9344g greater than

0.6g, then the bridge is in seismic performance zone 4 which corresponds to seismic design

category (SDC) D[22].



Figure 7 Design acceleration-time spectrum for the bridge.

5. PUSHOVER ANALYSIS, RESULTS AND DISCUSSION

The seismic design criteria by Caltrans (2013) [6] adopted a design procedure in which the

lateral strength of the system (size and reinforcement of the sub-structure sections) is assumed

at the beginning of the process. Then by means of displacement demand analysis and

displacement verification, it is confirmed that the bridge has an acceptable performance,

otherwise the strength is revised and the process repeated. In the demand analysis, the peak

inelastic displacement demands are estimated from a linear elastic response spectrum of the

bridge based on cracked sections stiffness. Then elastic peak displacements are converted to

peak inelastic displacements using an equal displacement approximation. Once the

displacement demands are estimated, the procedure requires the verification of the

displacement capacity of each pier by means of pushover analysis. Finally the sub-structure

elements are detailed to behave in a ductile manner (can deform in-elastically for several cycles

without significant degradation of strength or stiffness under the demands generated by the

extreme events) [25].

In this work the elastic dynamic analysis of the three types of piers are performed to

evaluate the structural period in the longitudinal and transverse directions. Target

displacements for both directions are calculated, and the resulted local displacement ductility

are also calculated and compared to the minimum capacity ductility required by Caltrans of µc

= 3. All the results are given in table 2.

Table 2 The evaluated structural periods, target displacements and local displacement ductility of

piles.

Pier type Direction Period in sec. Target displacement

in m.

Local displacement

ductility of piles

Type I Longitudinal 1.7643 0.498 1.89 < 3

Transverse 1.903 0.519 1.74 < 3

Type II Longitudinal 2.328 0.583 1.69 < 3

Transverse 2.502 0.662 1.52 < 3

Type III Both due to similarity 2.361 o.575 2.07 <3

Plastic hinges locations and their performance levels are shown in figures 8, 9 and 10 for

piers types I, II and III respectively. It is visible from these figures that all the plastic hinges

are located in the piles at the pile to pile-cap interface which can be allowed as it can be reached

for retrofitting. All the observed yielding (plastic hinges) correspond to a displacement ductility

demand ranging between 50% to 68% of the capacity ductility demand required by Caltrans,

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 1 2 3 4 5 6 7 8 9 10 11

Acc

ele

rati

on

g

Period Time in sec.



which can confirm the operation of the bridge to the emergency traffic (all plastic hinges are in

immediate occupancy level or less).

No yielding is observed at the pile to soil interface. And no yielding is observed in the pier's

columns or cross-beams.

Figure 8 Pier type I, plastic hinges location and their performance levels

Figure 9 Pier type II, plastic hinges location and their performance levels



Figure 10 Pier type III, plastic hinges location and their performance levels

6. CONCLUSIONS

The following conclusions can be drawn:-

1. Original design is over, and it seems that the bridge was designed to behave elastically under

the extreme earthquake, which is uneconomical.

2. The reduction of a 35% of the longitudinal piles reinforcement doesn't alter the bridge

importance as essential as resulted from the pushover analysis but enhance the implementation

and reduce the cost.

3. It is the author opinion that the seismic hazard consideration based on the UFC 3-301-01 gives

higher values than expected and therefore a national code based for earthquake based on

comprehensive studies is required.

4. This study is based on a pushover analysis with concentrated plasticity (plastic hinges) which

is an approximate method. A more reliable nonlinear dynamic analysis with distributed

plasticity model can be implemented if necessary.

REFERENCES

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State Highway and Transportation Officials, Washington, DC.

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Seismic Response of a Seismically Isolated Highway Pier", Journal of Civil Engineering

(IEB), 41 (20), pp 179-199.

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[5] ATC-40, 1996. "Seismic Evaluation and Retrofit of Concrete Buildings", APPLIED

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[7] Chai, Y.H., Priestley, M.J.N. and Seible, F., 1990. "Flexural Retrofit of Circular Reinforced

Bridge Columns by Steel Jacketing", Report No. SSRP-91-05, University of California,

San Diego, CA.

[8] CSI. SAP2000 V-17, 2014. "Integrated Finite Element Analysis and Design of Structures

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[11] Ger, J.F. and Cheng, F.Y., 2012. "Seismic Design Aids for Nonlinear Pushover Analysis of

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[16] MCEER/ATC Joint Venture, 2003. "Design Examples, Recommended LRFD Guidelines

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[23] SECOS, 1999. "Recommended Lateral Force Requirement and Commentary", Structural

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seismic evaluation of al-najibiya bridge using …€¦ · with no change to the section dimension....

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