Enhancement of Lateral Capacity of Damaged Non-ductile RC Frame Using Combined-Yielding Metallic...

�The Indian Concrete Journal ---- 2015

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The Indian Concrete Journal, ---- 2015, Vol. 89, Issue --, pp. 00-00

Enhancement of lateral capacity of damaged non-ductile RC frame using combined-yielding

metallic damper

Romanbabu M. Oinam and Dipti Ranjan Sahoo

Non-ductile reinforced concrete frames are vulnerable to severe damages or complete collapse under seismic excitations.

Several such deficient reinforced concrete frames are prevalent in the moderate-to-high seismic regions of India. In

order to improve their seismic performances, suitable strengthening schemes should be adopted. The use of passive

energy dissipation devices in the seismic strengthening technique is considered as an efficient and cost-effective solution.

In this study, a reduced-scale single-bay single-storey damaged RC frame specimen was retrofitted with the combined-

yielding metallic dampers. In addition, the damaged beam and columns were strengthened using steel elements. The

retrofitted frame was experimentally investigated under a constant gravity load and gradually-increased reversed-cyclic

loading. The test specimen was tested up to 6.0 % drift level. The RC frame exhibited better hysteretic response with

mild pinching. Unlike other metallic devices, combined-yielding metallic dampers was capable of dissipating hysteretic

energy through the flexure and shear yielding of metallic plates. The main parameters investigated are lateral strength,

ductility and hysteretic response Test results showed that the proposed strengthening technique significantly enhanced

the lateral strength and ductility.

Keywords: Non-ductile; ductile detailing; steel caging; retrofitting; lateral strength.

INTRODUCTION

Prior to the development of seismic codes in India, several concrete buildings have been constructed as per gravity load requirements. The main deficiencies in the earlier designs were insufficient shear reinforcement in joints, lack of splicing, less confinement on plastic regions, insufficient development length etc [1]. Many countries have used different strengthening techniques to enhance the overall capacity of all the existing non-earthquake resistant buildings which were impossible to replace. The strengthening techniques used can be divided into two

categories (a) local and (b) global. If the strengthening is done at the structural system level then it is called global strengthening and strengthening done at member level is called local strengthening. The techniques used for local level include, steel jacketing [2–5], concrete jacketing [6], steel caging [7, 8] and composite jacketing [9] and the ones in global level include , RC shear wall [10, 11], Steel plate shear wall [12], Steel brace [13, 14], and dampers [15]. In certain cases, the desired seismic performance of a RC structure may not be achieved by a single strengthening technique or, sometimes strengthening using a single configuration may be costly and labor-intensive. In such cases, both the strengthening techniques, i.e.,

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local and global modifications can be used to avoid underperformance of structure and economy loss [16]. In this study, combined metallic yielding damper (CMD) and steel caging are used together as a strengthening strategy. Steel caging will enhance lateral confinement of column and CMD will enhance flexural and shear capacity of the frame globally. This study explores the effectiveness of CMD in enhancing the seismic performance of a damaged non-ductile RC frame.

STReNgTheNINg TeChNIqUe

Steel caging

Commonly steel caging has been in use for RC columns and bridge piers as a local strengthening technique. Particularly, steel cage are used on columns because of its free accessibility and easiness in wrapping [17, 18]. This method is much simpler than other types of strengthening techniques. In steel caging, angle sections at four corners of the columns are provided and they are tied together using steel batten plates. The main objective of steel caging is to provide passive lateral confinement

on the strengthened member due to Poisson’s effect [2]. In steel cage, battens (horizontal member) take high tensile stresses, this happens due to internal crack propagation during axial compressive load application, and vertical steel angles provide uniform confinement along the height. The battens, there is no concrete confinement due to absence of confinement pressure.

Figure 1 shows clearly the arrangement of batten plates and angles as used for retrofitting a column. Same pattern has been followed for the present study.

COMbINeD MeTallIC yIelDINg DaMpeR

Generally, global strengthening technique enhances capacity of the structure globally in terms of lateral strength, stiffness and ductility. There are many types of global strengthening techniques and CMD is one of them.

CMD is a device, which dissipates the seismic energy through plastic hinge mechanism. It has both flexural and shear yielding characteristic under slow cyclic load [15]. It consists of two triangular shape flexural plates and one rectangular shape shear plate. Flexural plates and shear plates were kept transversely and longitudinally respectively in the loading directions. In between flexural and shear plates a gap was kept for the yielding the flexural plate and this system was bounded by two base plates [15]. Figure 2 shows the details of various components of CMD.

expeRIMeNTal pROgRaM

Description of RC bare frame

A single-storey single-bay reinforced concrete (RC) frame representing an interior bay of a prototype framed structure was considered as the test frame in this study and was casted. All the dimensions and percentage of reinforcement of test frame was simulated using 0.4 scale factor. The overall width and height of test frame was taken as 3200 mm and 2100 mm respectively. The cross sectional dimension of beam was taken as 160x130 mm, whereas both the columns dimension was 160x160 mm. To include the on slab effect on the test frame, a monolithic slab of 50 mm thick and 500 mm wide was


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cast over the beam [1]. Figure 3 shows the details of the geometric properties of the test frame. In representing the existing non-earthquake resistant building, ductile detailing was not considered. The frame was tested under cyclic load up to 3.5% lateral drift with constant gravity load [1]. This damaged frame was retrofitted using steel caging and CMD device and the same test was repeated up to 6.0% drift.

Description of retrofitted frame

The damaged RC bare frame was retrofitted using steel caging technique along with CMD device. Four numbers of Indian standard rolled angles of ISA 50x50x6 mm was used to fabricate the steel cage with intermediate battens provided at the rate of 330 mm center to center. Here mild steel plate of 50x5 mm was used as battens. At the beam column junction, steel collar was provided on column and steel cage was extended up to the end of the column. Indian standard rolled angles of ISA 60x60x8 mm angles were provided at the junction of slab and beam, and was extended from the actuator face up to ¾th of beam span to transfer the actuator load uniformly on to the test frame.

Combined metallic yielding damper (CMD) was installed at foundation level and it was supported by two bracing

systems. Two Indian square hollow section of 113.5 x 113.5 x 5.4 mm was used as brace and were connected to steel collar at the top of the column. For the CMD, 150 mm wide, 200 mm height and 6 mm thick mild steel plate was used as flexural plate; while 200 mm width, 200 mm height and 3.2 mm thick mild steel plate was used as shear plate. The flexural plate was welded on to the base plate, which was 12 mm thick at both sides. Compared to shear plate and flexural plate, shear plate was thinner than flexural plate so this size was not suitable for welding connection. Hence, 12 mm thick shear tabs were provided at both sides of the base plate using welding connection. Shear tabs and shear plate were connected using 8 numbers of 8 mm diameter high strength bolts. The bottom base plate was fixed as rigid a connection with foundation, while the upper base plate was connected to a supporting brace, allowing it to move longitudinally along the loading direction. The detail of retrofitting technique adopted in this study is as shown in Figure 4. All dimensions of various components of CMD used in this study is as per Table 1.

Table 1. Dimensions of various components of CMDFlexural plates Shear plates Base plates

150 x 200 x 6 200 x 200 x 3.2 500 x 160 x 12All dimensions are in millimeters



Test set-up

Lateral cyclic load was applied at beam level with the help of servo hydaulic actuator (250 kN capacity of 125 mm stroke) for upto 3.5% lateral drift on RC frame and 6.0% drift on retrofitted RC frame. Displacement history was referred from ACI Committee 374.1-05 [19] in addition with constant gravity loads. The gravity load was applied above the slab by putting concrete cubes.

A detail of displacement history is as shown in Figure 5. Both the frames were fixed on strong floor with the help of Indian hot rolled I-Sections and high strength bolts. Rigid steel frames were connected to the test frame as side support with the help of rollers. Main application of side support was to transfer the actuator load on frame in-plane. Due to this side support there is no free energy losses.

ReSUlT aND DISCUSSION

In this study, the following parameters were investigated from the specimens, (i) overall behaviors, (ii) hysteretic response, (iii) energy dissipation. Brief explanations of these parameters are given in the following sections.

Overall behavior

Initially there was no visible damage on both the specimens for smaller drift. Under these small drift, both

the specimens showed elastic nature. At 1.0 % drift, few micro cracks were noticed on the cover concrete of RC bare frame. These micro cracks became major cracks on column concrete, when displacement amplitude was increased to 1.4 % lateral drift. At 2.75 % drift, a single major crack (horizontal) was noticed on column face, which corresponded to ultimate load capacity of the frame in push direction. At 3.5% drift, crushing of concrete was observed at column base near the foundation. After this observation, the test was stopped and further higher drift level was not applied. From the overall observation of RC bare frames performance it showed that the plastic hinge was formed on both columns of top and bottom [1]. In case of retrofit frame, some micro cracks were noticed on the beam and on column face at 1.0% drift level. And in this drift level, CMD was behaving elastically. At 1.4 % drift, the shear plate was buckled (out of plane) on CMD and this was corresponding to the ultimate load carrying capacity (130.0 kN and 135.5 kN in tension and in compression respectively). The steel cage was left with no deformation for this drift. When displacement amplitude was increased to 1.75% drift, shear plate started tearing up from top right end, meanwhile shear cracks were observed at column bottom. Many minor cracks were noticed at beam column joints. At 2.2% drift, shear plate started tearing from top left end and micro cracks were noticed on foundation. Shear plate tore fully and got separated from upper the base plate at 2.75% drift. Major cracks on foundation were noticed at 3.5% drift. At 4.5%


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drift, the joint between damper and foundation became loose and concrete spalling was observed. At the first cycle of 6.0 % drift, the welding connection between brace and steel collar failed and the test was stopped thereafter. The performance of the retrofitted frame was excellent as compared to undamaged bare RC frame. In RC bare frame, pinching effect was observed tremendously while it was comparatively less in retrofitted frame. In general, concrete structure has a tendency for pinching effect, due to this behavior RCC structures dissipates its non-recoverable energy. Due to CMD device, pinching action was reduced in retrofit frame as compared to RC frame. Details of hysteretic behavior of retrofit frame are as shown in Figure 7 (b). Installation of CMD on test frame and buckle deformed shape of CMD is shown clearly in Figure 6.

Hysteretic response

Lateral load vs. displacement (Hysteresis) response of RC bare frame and retrofit frame is shown in Figure 7. Both the frames have same yield value of 0.5% (7.5 mm). At 2.75% drift, RC bare frame showed ultimate load carrying capacity in push direction. However, in the last cycle (3.5% drift) the load carrying capacity of RC bare frame was symmetrical in push and pull directions. And the values were noted as 56.7 kN and –57.0 kN in push and

pull direction. At the end of loading, there was not much difference between the load carrying capacity of last cycle and its previous cycle. Hence, there was less load degradation in hysteresis. The load resisted by retrofit frame was 130.0 kN and 135.5 kN in tension and in compression respectively at 1.4% lateral drift. At this drift shear plate was buckled up and suddenly load resisting capacity was reduced. Up to certain extend of 1.4% drift;



retrofit frame was dominated by shear behavior and later by flexural behavior. During the last cycle, the frame carried a load of 87.2 kN on push direction.

Figure 8 (a) shows backbone comparison of RC bare frame and retrofit frame. From this comparison, it can be observed that both the curves have symmetrical nature in push and pull directions. In case of RC bare frame, the curve is smooth and bi-linear. While retrofit frame

behaved in a trilinear form. Maximum load carrying capacity of RC bare frame was 56.63 kN and 56.87 kN in tension and compression respectively at 3.5% lateral drift, while retrofit frame showed 130.0 kN and 135.5 kN in tension and in compression respectively at 1.4% lateral drift. This showed that retrofit frame has higher load carrying capacity than the RC bare frame. As compared to RC bare frame, retrofit frame has 56.44% and 58.03% improved performances in tension and in compression respectively. It can also be seen that retrofit frame and RC bare frame can resist the displacement up to 6.0% and 3.5% drift respectively. From the backbone comparison it can also be observed that RC bare frame showed very low stiffness while retrofit frame showed very high stiffness.

Energy dissipation

Area under the hysteresis loop is known as energy dissipation for that particular specimen. Figure 8 (b) shows the comparison of cumulative energy dissipation for the test frames. Here cumulative energy of test frames were calculated for each drift. A big difference in dissipates energy was observed from the beginning. Particularly at drift level of 3.5%, RC bare frame dissipated energy at an amount 3.46 kNm while retrofit frame dissipation amounted to 16.3 kNm. It shows that retrofit frame has higher capacity of load resistance because of high energy dissipation in nonlinear range. Retrofit frame has 78.8 % higher energy dissipation than RC bare frame at a particular range of 3.5% drift. At end of the retrofit frame test, total amount of energy dissipated was 23.3 kNm.

CONClUSIONS

Overall performances of retrofit frame were much better than the RC bare frame because of lateral confinement of column and the contribution of CMD in terms of shear and flexural yield. Excess load on the structure was released through this yield mechanism as energy dissipated from it. Additionally, there was less pinching effect on retrofit frame than RC bare frame. This dissipated energy improved the overall behavior of the structure. Hence, it can be concluded that the proposed retrofitting technique (combination of local and global strengthening technique) has achieved its target performance and the energy dissipating device (CMD) yields perfectly and adds to the target performance. In future, it can also be used in


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many other local and global retrofitting techniques and with proper arrangement; it can lead to the most feasible system in terms of performance and cost.

This paper was first published in the proceedings of the Structural Engineering Convention 2014, IIT Delhi, December 22-24, 2014.

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This paper was first published in the proceedings of Structural Engineering Convention 2014, IIT Delhi, December 22 to 24, 2014.

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Romanbabu M. Oinam received his B.E (civil) from Manipur Institute of Technology (M.I.T) Takyelpat, Imphal; M.Tech. in earthquake engineering from National Institute of Technology (NIT) Silchar; pursuing his PhD at the Department of Civil Engineering, IIT Delhi. His research interests are reinforced concrete structures, earthquake resistant design of building; retrofitting of damage RC buildings; performance based design of structures, and large-scale testing of structures.

Dr. Dipti Ranjan Sahoo received his PhD in Civil Engineering from Indian Institute of Technology (IIT) Kanpur, India. He is an Assistant Professor in the Department of Civil Engineering, IIT Delhi, India. His research interests pertains to evaluation of seismic behaviour of RC, masonry and steel structures, fibre reinforced concrete, seismic strengthening of structures using supplemental devices, large-scale testing of structures, and performance-based seismic design. He is a recipient of Young Engineer Awards from Indian National Academy of Engineering (INAE) and Institute of Engineers India (IEI).

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