EXPERIMENTAL TESTING OF SCALED BRIDGE MODEL SUBJECTED TO BI-DIRECTIONAL SEISMIC EXCITATION

[Universiti Teknologi Malaysia] [Faculty of Engineering, School of Civil Engineering]

Danish Tayyab Khorajia

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Overview

• Bridges, an important infrastructure, were designed using British Standards (lacks seismic resistant design) in Malaysia
• 1985 Maxico City Earthquake (8.1 Richter scale) with its epicenter 400km away from the main city ------Claims------ Seismic hazard to peninsular Malaysia (Kuala Lumpur) as Sumatran fault is just 300km away
• Wake up call for engineers from a locally originated earthquake (6 Richter scale) in Ranau Sabah region (June 2015, killed 18 lives & caused serious damage to many infrastructure)
• Need to investigate seismic performance of bridges in Malaysia

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Background

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• Euro code specification assess the seismic performance of a bridge by conducting nonlinear analysis using unidirectional input in the individual two orthogonal directions
• However, researchers have confirmed that structural response is highly influenced by the bi-directional seismic excitation

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• When post-elastic characteristics and safety against failure are of primary importance, experimental analysis is required
• Experimental testing of full scale prototype requires testing facilities with extremely high capacities
• Major problem in small-scale modelling of reinforced concrete structures is meeting similitude requirements (mainly material-driven)

Need to analyze the structural behavior of a bridge under bi-directional seismic excitation

Need to find a material that best achieve the desired similitude conditions

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Literature Review

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Literature Review (continued)

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Model material for concrete

Problem 1 Identified

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With increasing Aluminum content, compressive strength and tensile strength decreases

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• lower stiffness of micro-concrete result in distortion of strains in the model
• Higher tensile strength results delayed tensile cracking and less damage and degradation under cyclic loadings

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Objective 1: To experiment lightweight concrete as a model material for concrete in small scale modelling

Literature Review (continued)

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Bi-directional Seismic excitation

Problem 2 Identified

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• Most of these investigations remains analytical or computational
• Regardless of whether the analysis is computational or experimental, these were focused primarily on specific component of bridge

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Objective 2: To analyze structural behavior of a bridge segment (two piers & one deck) model through experimental testing on a shaking table under bi-directional seismic excitation

Scope of the research

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• A 1/25 scale bridge segment model constructed using lightweight concrete was subjected to number of time-history motion on a shaking table.

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• The function of bridge deck (constructed with a strong material) is to induce a diaphragm action.

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• The base of bridge model was constructed rigid, as the effect of soil while transfer of earthquake excitation from ground to structure is overlooked.

Methodology

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Objective 1

Objective 2

Methodology

• Core drilling machine was used to cut the concrete cylinders (size 50 × 100 mm) out of the blocks
• These cylinders were coated with cement slurry

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Establishing Material Properties

Methodology

• Universal testing machine was used for loading to obtain Ultimate compressive strength (f_c), modulus of elasticity (E) and Ultimate tensile strength (f_t)
• Stress and strain were continually monitored, printed, and plotted

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Establishing Material Properties

Methodology

• Single-span bridge model with a span length of 720 mm and deck width of 500 mm was selected
• Scale factor was approximately 1/25
• The actual bridge would have a span of 18 m and width of 12.5 m, which is well within realistic limits

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Model selection

Methodology

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Two vertically tapered I-shaped pier of scaled size as shown were constructed using lightweight concrete & coated with cement slurry

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These pier models were reinforced with four numbers of small scale reinforcement of diameter 3.2mm

• Wooden plywood of size 720 × 400 × 18 mm with stringers (resembling girders) glued and screwed to its bottom was used as deck
• Less emphasis was given to its similitude as its function was to produce diaphragm action

Model fabrication

Methodology

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Model fabrication (Complete assembly)

Base of the piers were submerged in a mortar block of size 450 × 240 × 150 mm (to ensure no soil effect)

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Bridge model was secured tightly on the connecting plate using the arrangement of bolts and nuts as shown

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Artificial mass was calculated using similitude laws and was applied on to the model

Methodology

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Experimental test setup

• Bridge model was oriented to create the effect of bidirectional seismic action on a unidirectional shaking table

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• Angle of 47º was to set the PGA of 0.135g (North-South) and 0.126g (East-West) as recorded from the Ranau Earthquake

Methodology

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Instrumentation and data acquisition

Artificial mass on to the model of 165 kg

Accelerometers installed along longitudinal & transverse direction

Accelerometer installed on the table along the axis of table

3D Accelerometer

Methodology

• Ranau earthquake (June 2015) time-history data was used as an input ground motion
• The scale model was shook until failure by increasing the acceleration according to the cases as listed in table
• Failure mode, crack pattern and natural frequency were recorded and observed

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Seismic loading/input

Methodology

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Complete setup

Result

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Mechanical properties of lightweight concrete

Force versus displacement

Stress versus strain

Result

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Mechanical properties of lightweight concrete

• Tensile strength of lightweight concrete is 8% of the compressive strength (desired)
• Stiffness of the material is too low.
• Lower elastic modulus of lightweight concrete or aerated concrete is helpful in reducing the artificial mass applied on the model

Result

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Damage observed after test

Visible surface cracks at the corners of the pier column after case 3

Damage near the flanges of piers after case 4

Result

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Damage observed after test

Reinforcement buckling at the corners of the pier column after case 5

Massive concrete spalling at column ends after case 6

Result

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Natural frequency

Reduction in natural frequency

along longitudinal axis

Reduction in natural frequency

along transverse axis

Conclusion

• Lower elastic modulus of lightweight concrete is helpful in reducing the artificial mass applied on the model
• Seismic resistant design using Euro code is in most cases sufficient. Only in structures with very different transverse stiffness Bi-directional analysis would be necessary (Cruz & Cominetti)
• Orienting the model on a shaking table can create the effect of two-directional earthquake on a unidirectional shaking table (Needs further analysis and verification)

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References

• Arias-Acosta, J. G., & Sanders, D. H. (2009). Shake Table Testing of Bridge Reinforced Concrete Columns under Combined Actions. In: Reno: University of Nevada at Reno.
• HACHEM, M. M., & MAHIN, S. A. DYNAMIC RESPONSE OF REINFORCED CONCRETE COLUMNS TO MULTIDIRECTIONAL EXCITATIONS.
• LI, Q.-w., FAN, J.-s., & NIE, J.-g. (2010). Effect of Directional Uncertainty of Earthquake Ground Motion on Structural Responses. Engineering Mechanics, 27(12), 135-140.
• Krawinkler, H., Harris, H. G., ACI Committee 444, M. o. C. S. S. o. D. M. o. C. S., Convention, A. C. I. A., & Convention, A. C. I. F. (1972). Dynamic Modelling of Concrete Structures: ACI.
• Mirza, M., Guedelhoefer, O., & Janney, J. (1972). Correlation of models and prototype structural elements and complete structures. Structural Concrete Models-A State-of-the-Art Report, McGill University, Montreal, 99-232.
• White, R. (1976). Modeling techniques for reinforced concrete structures. Department of Structural Engineering, Cornell University.
• Kim, W., El-Attar, A., & White, R. N. (1988). Small-scale modeling techniques for reinforced concrete structures subjected to seismic loads: National Center for Earthquake Engineering Research.

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Thank you

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Any question?