Seismic retrofitting for RC rigid frame viaducts with compression type braces

K. Yoshida1, N. Kita2, M. Okano2, M. Seki1

1JR-Central, Komai, Japan, 2Obayashi Corporation, Tokyo, Japan

Abstract

Quite a few structures were damaged by the Hyogo-ken Nambu earthquake (M7.2) on January 17th 1995, when some of the columns of railway RC rigid frame viaducts collapsed due to shear fracture and others were slightly damaged by flexural force. Since then the columns which were subject to shear failure have been retrofitted with steel jacketing. However, it has sometimes been difficult to apply this method if some obstructive walls are installed between the columns under a viaduct. A new device with a compression type brace has now been worked out for such cases. The device is made up of an X-type brace with a high resistance to compression and a dampener which absorbs external impact with shear panels in the middle. Also, instead of individually reinforc ing each of the columns, this method improves seismic performance of the whole viaduct through the installation of several damper braces on the four sides of a viaduct. For an investigation of the dynamic effectiveness of this retrofit, shaking table tests were carried out using the specimens which model the actual three-spanned RC rigid frame viaducts with eight columns. The main scale of each specimen was set at one-fifth actual size, while the cross section of a column was one-fourth that of actual size. Two types of the columns with a different height were prepared, one of which could meet shear fracture and the other of which flexural fracture. The two devices mentioned above were installed for each viaduct model; one was in the same direction of the bridge axis to which the other was perpendicular. The vibration was applied on the model specimens by means of a 5m×5m shaking table with a maximum load of 50 tons and a maximum horizontal acceleration of 3G. A seismic spectrum with Level-2 intensity, which is defined in the railway seismic design code, was selected for an input seismic wave. The specimens without any device installation were also used to investigate whether the process of shear or flexural failure could be simulated or not. The results showed that the retrofit improved the seismic performance by reducing the response

displacement for the viaducts of both fracture modes, which confirmed an adequate effectiveness against a seismic motion with Level-2 intensity. In addition the shaking tests using the specimens without retrofit could well simulate the similar fracture process observed in the earthquake in 1995; the specimen of a shear fracture type showed the total collapse of a bridge, while the specimen of a flexural fracture type was plasticized with the both ends of columns functioning as hinges.

Introduction

After some of the railway viaducts were damaged by Hyogo-ken Nambu earthquake on January 17th 1995, the columns have been retrofitted to avoid shear fracture. Shear reinforcement should be generally improved for the prevention of this failure, and practically steel jacketing has been widely adopted. The effectiveness of other methods using concrete jackets or composite materials such as fiberglass-epoxy or carbon fiver-epoxy has been clarified by tests and they have so far been applied in the field [1]. However it is sometimes difficult to apply above these jacketing reinforcement methods when the space under the viaducts is occupied by stores or offices. The author et al. proposed the application of steel compression-type braces as shown in Fig.1 for such cases [2]. Instead of an individual reinforcement of the columns, this method improves seismic performance of the whole viaduct by an installation of several damper braces on four sides of a viaduct. This advantage makes it possible to obtain required seismic performance for the viaducts some columns of which cannot be retrofitted due to the existence of an obstacle near them. The deformation process with this retrofitting method has so far been investigated by an FEM analysis and static cyclic loading tests. This paper reports the effectiveness of this method by carrying out shaking table tests using one-fifth scaled model viaducts with a fracture mode of shear failure or flexural failure. The vibration is applied on

the model specimens by means of a 5m×5m shaking table with a maximum load of 50 tons and a maximum horizontal acceleration of 3G [3]. A seismic spectrum with an intensity of Level-2 defined in the railway seismic design code [4] was selected for an input seismic wave. The specimens without any installations of the devices were also prepared to investigate whether the process of shear or flexural failure could be simulated or not.

The outline of the device

The device is constituted of a damper with four steel shear panels and X-shaped braces as shown in Fig. 1. In the event of an earthquake, shear yielding of the panels is caused by compressive axial force which acts on the brace, which eventually lets the damper soften the impact. Another feature is that when shear panels are deformed, the damper section, which has an identical shape as an RC frame, is deformed to form a parallelogram according to the deformation of the frame. In this process the braces under no compression are pushed toward their axis direction, and their edges are kept attached to the corners of the RC frame. The symmetric external impact results in the symmetric deformation as shown in Fig.2. Thus it is not necessary to use tensile anchors to fix the braces to the viaducts since only compressive force is applied to the braces by the lateral cyclic loading. This can avoid damaging the existing structures, by just making use of a simple connection, using non-shrinkage mortar, of the braces to the corners of a viaduct, where reinforcement bars usually have a congested arrangement. For a viaduct which has a fracture mode of shear failure, this method controls the response displacement in case of an earthquake, taking advantage of the high stiffness of the damper braces. In this case, a damper functions as a bumper after it plasticizes. On the other hand, for a viaduct with a fracture mode of flexural failure, flexural capacity and impulse absorption performance can be improved by an application of a dumper which can tolerate a still more deformation with the ductility ratio of 2 or 3.

Damper

Brace Shear panel

<Details of damper> Fig.1: Compression-type steel damper braces

Compressive force

Force of inertia Force of inertia

Compressive force Compressive force

Force of inertia Force of inertia

Compressive force

Fig.2: The resistance concept of compression-type damper braces

Shaking table tests

Shaking table tests have been carried out to investigate how the viaducts reinforced with above mentioned devices are affected by a seismic motion with level-2 intensity. Model viaducts which have a fracture mode of shear failure or flexural failure were prepared and used in the tests.

Model viaducts

The property of the real viaduct and the model viaduct is shown in Table 1, and the material used to produce the model viaduct is detailed in Table 2. A standard viaduct belongs to regular viaducts used for Tokaido Shinkansen, which has columns with a cross section of 600mm square and a height of 5.8m. This viaduct has three spans in longitudinal direction with eight columns, and has beams only at the top. The viaduct has a fracture mode of flexural failure, thus in order to simulate shear failure mode with this viaduct, the shear span was hypothetically shortened by reducing the height of the columns to 3.8m. The modeling scale was set as one-fifth taking into consideration the dimension of the shaking table. Here the correspondence in shear margin of a column between a standard and the model viaduct was considered to be the first priority. Thus the modeling scale was set one-fourth only for the cross section of columns taking into account several factors: the scale effect of shear capacity, the possibility of fracture caused by loss of bond between concrete and reinforcement, arrangement of shear reinforcement bars, and the workmanship in production of the model. This alteration would have caused discrepancy in stress intensity; however, it was well controlled by adjusting the amplitude of input wave and time scale. And the addition load which corresponds to the weight of a ballast track and trains is put on the slab of the model viaduct. The outline of the model viaducts is shown in Fig.3. The property of the damper brace is shown in Table 3. The dimension of a damper brace used for a standard viaduct is obtained by a conversion, based on identicalness in shape, of a damper used in a model viaduct.

Full size viaduct Scaled modelLongitudinal direction 6.0m 1.20mTransverse direction 5.2m 1.04m

Shear failure type 3.8m 0.76mFlexural failure type 5.8m 1.16m

600mm 600mm 150mm 150mm12-D32 16-D6

φ9@300 100mm φ2@60 20mmShear failure type 0.87 0.87

Flexural failure type 1.31 1.31

Hoop reinforcementShear strength to

flexural stregth ratio

Column sectionLongitudinal reinforcement

Span length

Height of column

Table 1: Properties of full size and scaled model viaduct

Reinforcement bar in colums Materials Yield strength Tensile strength Young's modulus

Longitudinal ( D6) SD295 403N/mm2 541N/mm2 1.99 x 105N/mm2

Hoop (φ2) SWRM6 205N/mm2 291N/mm2 1.97 x 105N/mm2

Concrete in columns Age of concrete Compressive strength Young's modulusCASE-1 (24-18-10N) 45days 30.1N/mm2 2.66 x 104N/mm2

CASE-2 (24-18-10N) 135days 31.0N/mm2 2.57 x 104N/mm2

CASE-3 (24-18-10N) 198days 31.7N/mm2 2.52 x 104N/mm2

Table 2: Material properties of scaled model

Shear failure type Flexural failure typeLength 40mm 160mmWidth 84mm 80mmLength 40mm 220mmWidth 79mm 110mm

4.7mm 4.6mm

52.8N/mm2 55.3N/mm2

Thickness of shear panel

Vertical shear panel

H75 75 9 9 [mm]

Horizontal shear panel

Dimension of braceCompressive strength of grout mortal

Table 3: Properties of damper braces in transverse direction

SurchargeSurcharge

CASE-1: Shear failure type model

SurchargeSurcharge

CASE-3: Flexure failure type model (CASE -2: Without retrofitting)

[mm]

Column section(CASE-1,2,3)

Fig.3: Outline of model viaducts

The law of similarity applied to input waves

When input seismic waves are concerned, based on a pushover analysis of real and model viaducts, the time scale was adjusted by multiplying Tm/T in equation (1) to a natural period derived form the gradient at yielding. Then the amplitude of acceleration was adjusted by multiplying am/a in equation (2) to the seismic coefficient at yielding. A spectrum wave for G4 ground was selected as an input wave according to the railway seismic design code [4]. Tm/T in equation (1) and am/a in equation (2) was 1/2.83 and 1.84 respectively. The shape of the wave the property of which was adjusted is shown in Fig.4.

GM

GM

TT

m

mm /= (1)

where T is natural period, M is mass, G is gradient at yielding, and m is the index which represents the model.

MP

MP

KhKh

aa y

m

ymmm /== (2)

where a is acceleration, Kh is seismic coefficient at yielding, Py is yield load.

-1500

-1000

-500

0

500

1000

1500

0 5 10 15 20 25 30

Time [s]

Acc

eler

atio

n [G

al]

Level2-Spec.II

max.1289[Gal]

max.-1261[Gal]-1500

-1000

-500

0

500

1000

1500

0 5 10 15 20 25 30

Time [s]

Acc

eler

atio

n [G

al]

Level2-Spec.I

max.759[Gal]

max.-843[Gal]

Fig.4: Seismic wave applied to experiment (G4, adjusted wave)

Test results

The shaking plans and the corresponding results are shown in Table 4 for the models which have a fracture mode of shear failure (CASE-1) and flexural failure (CASE-2, 3). L2-Spec.I, and then L2-Spec.II were applied to the models reinforced with the proposed devices in the direction of the axis and its perpendicular of the viaduct in both CASE-1 and CASE-3. After the series of shaking were applied to the model in CASE-1, the damper braces were removed, and then for the confirmation of fracture mode of the model viaducts, L2-Spec.I was applied to the model, which was supposed to have a fracture mode of shear failure. On the other hand for the confirmation of the fracture mode of flexural failure, L2-Spec.II was applied to the model of CASE-2 without any brace reinforcement. A CASE-2 specimen had the same condition of that of CASE-3 that had been reinforced with braces and had been exposed to the series of shaking shown in Table 4. The photo of the CASE-1 model which was retrofitted with the proposed devices is shown in Fig.5.

X direction Y direction(Before test) - - 6.63Hz 7.38HzL2 Spec. I Longitudinal (Y) 3.2mm 6.63Hz 7.25Hz Shear panel yieldedL2 Spec. I Transverse (X) 5.7mm 6.25Hz 7.25Hz Shear panel yieldedL2 Spec. II Longitudinal (Y) 3.5mm 6.25Hz 6.63HzL2 Spec. II Transverse (X) 5.4mm 6.00Hz 6.63HzL2 Spec. I Transverse (X) (28.6mm) - - Without retrofitting

(Before test) - - 9.25Hz 6.25HzL2 Spec. I Longitudinal (Y) 8.1mm 7.05Hz 4.50Hz Shear panel yieldedL2 Spec. I Transverse (X) 12.2mm 4.00Hz 4.63Hz Shear panel yieldedL2 Spec. II Longitudinal (Y) 8.5mm 4.13Hz 4.00HzL2 Spec. II Transverse (X) 13.8mm 3.50Hz 3.89Hz

(Before test) - - 5.00Hz 4.88HzL2 Spec. II Transverse (X) 133mm 1.38Hz 2.50Hz Without retrofitting

Max. responsedisplacement

CASE-2(Flexural)

Natural frequency after testRemarks

CASE-1(Shear)

CASE-3(Flexural)

Applied wave Shaking direction

Table 4: Summary of experiments

Fig.5: Model retrofitted with the proposed devices (CASE-1)

Shear failure type (Case-1)

Fig.6 shows the hysteresis curve and the displacement history obtained at the specimens with and without retrofitting by means of the proposed device when L2-Spec.I was applied in the transverse direction. These two figures indicate that the response displacement is greatly lowered by the reinforcement device. Shear failure was observed to be avoided and thus this retrofit method was demonstrated to be effective by this experiment. In the all cases of the models with this reinforcement, the response displacement was observed to be reduced, including several other shaking cases: L2-spec.I to longitudinal direction and L2-Spec.II to longitudinal and transverse direction.

Another feature observed is that in the cases of loading subsequent to the removal of the retrofitting devices, the shear capacity was lowered according to the increasing ductility after yielding, which was pointed out by Priestley [1]. After the flexural yielding, shear failure was observed when the displacement and the deformation angle reached 17.2mm and 1/44 respectively, which could well simulate the collapse of viaducts observed in Hyogo-ken Nambu earthquake.

Level2-Spec.I

Level2-Spec.I

Fig.6: Hysteresis curves and displacement histories in transverse direction (L2-Spec.I)

Flexural failure type (Case-2, 3)

Fig.7 shows the hysteresis curve and the displacement history obtained at the specimens with and without retrofitting by means of the proposed device when L2-Spec.II was applied in the transverse direction. Similarly observed in the cases of shear failure, the response displacement is greatly lowered by the reinforcement device as shown in Fig.7. The retrofitting lowered the response displacement and the residual displacement and it can be said that this experiment demonstrated the effectiveness of this retrofitting method. In the all cases of the models with this reinforcement, the response displacement and the residual displacement were lowered, including several other shaking cases: L2-spec.I to longitudinal and transverse direction and L2-Spec.II to longitudinal direction. In the models with flexural failure mode, the damper was designed so that it could tolerate the

displacement of RC viaduct frames as much as possible, thus the deformation of a damper was greater than that observed in the models with shear failure mode. The specimens without the proposed retrofitting showed the displacement of 133mm and the deformation angle of 1/9, and the capacity was lowered to 70% of the initial value. The deformation angle was observed great enough to be regarded as that generally found in fracture; however, the model specimen neither collapsed nor fell down. Fig.8 shows the typical damage phenomena of flexural failure: buckling of the reinforcement bars and spalling of the cover concrete.

Level2-Spec.II

Level2-Spec.II

Fig.7: Hysteresis curves and displacement histories in transverse direction (L2-Spec.II)

Fig.8: Flexural damage at column end (CASE-2)

Conclusions

The following is obtained by the shaking tests to investigate the effectiveness of the proposed compression type steel damper brace applied to railway RC rigid frame viaducts: 1) The application of the device can, against a seismic motion with level-2 intensity, lower the response

displacement and avoid the collapse of the RC viaducts of shear or flexural failure mode. This indicates the effectiveness of this retrofitting against a seismic motion with level-2 intensity.

2) The shaking tests for the model viaducts without any retrofitting well simulated the fracture process that had been observed in Hyogo-ken Nambu earthquake. The models with a fracture mode of shear failure collapsed by the expected fracture mode, and the ones with a fracture mode of flexural failure exposed the generation of plastic hinges at both edges of columns, which is usually observed in flexural failure.

3) Since the model viaducts of both fracture modes showed the expected pattern of failure, the assumption of the design and similarity law applied is considered to be reasonable. The series of the experiments above have demonstrated that the reinforcement of railway RC rigid frame viaducts with the proposed device is effective against earthquakes.

References

[1] M.J.N.Priestley, F.Seible, G.M.Calvi. “Seismic Design and Retrofit of Bridges.”, John Wiley & Sons, Inc., (1998).

[2] K. Yoshida, N. Kita, M. Okano, M. Seki. “ Seismic retrofitting method for railway viaducts using compression type steel damping braces.”, J. Struct. Engrg., JSCE, 50A, pp.551-558, (in Japanese), (2004).

[3] Shaking Table Group. “ Tri-axial shaking table (part2).”, Report of Obayashi Corporation Technical Reseach Institute, No.66, pp.117-120, (in Japanese) , (2003).

[4] Railway Technical Research Institute. “Seismic Design Code for Railway Structures.”, (1999).