Retrofitting of RC Frame Structure Using Performance Based Friction Damper

Abstract— India suffered from many earthquakes over last some decades. Analysis of those earthquake showed that large number of existing structures needs to be upgrade for seismic actions. Performance based earthquake engineering is recent focus of researchers. Present study focused on the seismic retrofitting of existing structures which are previously designed for gravity loads only, by using performance based design philosophy of friction damper. Storey displacement, inter-storey drift, axial force, maximum moment, maximum shear force, base shear variation and effectiveness of dampers are observed for each model and in each direction for seven records of time history earthquakes. Results showed that response of structure are well within the limits of codes provisions. Thus friction damper can be used effectively for seismic retrofitting of structures. Keyword Retrofitting, Pushover analysis, Friction damper, Time history analysis, Inter-storey drift.


A. Modelling & Performance Evaluation
A typical frame of G+5 storey has selected for the present study. Two different plans are selected for purpose of retrofitting (Fig 1). Slab thickness considered as 100 mm and load has been transferred on beams as per yield line theory influence area. Stiff joint diaphragm has assigned to each model. Wall load calculated as per IS 875 part I unit weight recommendation has been equally distributed over beams. Live load and roof live load considered to be acting on both models is 2.5 kN/m2 and 1.5 KN/m2 respectively, according to IS 875 part II. Structures are assumed to be situated in seismic zone V (0.36), having a response reduction factor 3 and a structural importance factor as 1.5 as per IS-1893 2016 code. M20 grade of concrete and Fe415 grade of re-bars material has considered for modeling material. Confining reinforcement of 8mm diameter of FE-415 grade has been used. Plinth level considered 1.5 m from foundation level and typical storey height is considered as 3m for both the structures. As per IS-1893-2016, as live load is less than 3 kN/m 2 so mass source has defined for the 25% of live load. Modeling plan and details are given in tabe-1.
Retrofitting guidelines are provided by foreign codes ATC-40, FEMA-356 and ASCE 41-13. All of the above codes also provide software modeling guidelines for analysis purpose. Present study followed FEMA-356 modeling guidelines. Codes suggest nonlinear analysis for the structural evaluation. Proposed retrofitting methodology follows the following steps Step1-Analyse model for gravity loading only and design it for the 1.5(DL+LL) load combination as per IS456 2000. Friction damper is designed for axil force which is developed due to seismic lateral loading. Thus friction dampers will not take part to carry gravity load, therefore models are previously designed for gravity loads.
Step 2-To select of structural performance objectives-Fema-356 and ASCE 41 provided guidelines suggested that, structure should remain under immediate occupancy (IO) level for MCE earthquake.
Step 3-To perform pushover analysis on models for determination of capacity of structure and ultimately to decide that structure needs retrofitting or not. Present study follows ATC 40 procedure for pushover analysis results interpretation. Software generated pushover curve is base shear vs roof displacement, this curve needs to be converted into capacity spectrum i.e. Acceleration-Displacement Response Spectra (ADRS) using equations 1 & 2 given in ATC-40.
Latter, demand curve i.e. response spectra of IS1893 (2016) is converted into ADRS format with the help of equation 3. Both capacity spectra and demand spectra are overlapped on each other as shown in figure-2.
Reinforced concrete structure has 5% inherent damping. If IO level is below 5% damped demand spectra then capacity of structure is inadequate under the seismic loading. That defines structure needs to be retrofitted for seismic action. In our case both the models demand seismic retrofitting.

S S g (3)
Step 4-To identify required damping factor-Demand spectra for various damping ratio is plotted and overlapped with capacity spectra Fig 2. The curve for which IO level is above the damped demand spectra, is the required damping for design of friction damper. Effective required damping is found out for both models are given in TABLE . Step 5-Chaudhari and singh [11] gave the design procedure for the friction damper. Bottom storey slip load is given by equation given bellow, * Bottom storey slip load is distributed over the entire storeys as per the storey shear, using following formula, Step 6-Verification of retrofitting strategy-Non-linear time history analysis is performed on the retrofitted models. In present study 7 time history records were downloads from PEER data website. These records were match with response spectra of IS1893:2016 by using software Seismomatch. Matched records characteristic are shown in Table III   The yield force obtained in above step is use to find out the slip load. Yield force is distributed storey wise in accordance of inverted triangle of lateral load pattern and Normalize displacement vector at target performance level (IO). Four dampers are provided on each storey as per the guidelines of FEMA356. Connecting brace element of the damper has designed for tension and compression member as per IS800:2007 code. ISJC 100 has been used as bracing element for each damper. Table V gives the slip load at each damper on each storey. III. RESULTS AND DISCUSSION The nonlinear time history analysis for seven records has been performed on model A and Model B, before and after retrofitting. Detailed evaluation of structural response studied by comparing the various parameters such as storey displacement, storey drift, maximum axial force, shear force, moment, variation of base shear, floor acceleration and displacement with respect to time and etc. Fig 3 shows the damper assigned models.  Base shear is the function of seismic weight thus increase in base shear is recorded. Variation of shear force is quite fluctuating nature, in some records shear force before application of friction damper found maximum while in some cases shear force for without damper application found maximum. 3. Lateral roof displacement, roof acceleration and base shear: Performance of building for roof displacement and roof acceleration incorporated with dampers is the important parameter for analysis. Fig 5 shows the time variation plot. Variation of acceleration with respect to time shows that, friction damper controlled acceleration over the latter part of time. Friction damper system reduces the Roof displacement approximately by 70% as shown in graph. As noticed, displacement for the model incorporated with friction damper has successfully been reduced displacement at all-time instant. This provides the smooth transition and prevents sudden reversal of displacement load. Time varying plot of base shear depicts that, friction dampers are less significant to control base shear acting on structure. i.e. for our case it becomes 1.2% . All the time history records found storey drift below 1% after retrofitting. Effective reduction in inter-storey drift is found approx. 50%. Thus model A has been retrofitted successfully.

Hysteretic behaviour:
Tolvar studied the placement of damper and suggested that first storey damper is more effective than top storey damper. Fig shows plot for axial force as a function of displacement of friction damper. Bottom and top storey dampers yielded to their capacity and thus each storey damper is effective to control seismic action. Hysteretic loop of bottom storey is close to ideal behaviour of friction damper, as it shows the complete square shape. Top storey friction damper energy dissipation plot does not resemble to ideal shape of energy dissipation loop and stiff slip problem occurred. shows storey wise displacement for each time history record. Maximum percentage reduction in storey displacement for retrofitted frame model along x and y direction is found to be 60.58% & 63.56% respectively. Displacement controlling characteristic of friction damper can be effectively studied from this graph.

Hysteresis Loop
Top storey damper IV. CONCLUSION Friction dampers are the familiar and effective damping device for seismic retrofitting of structures. In this study proper and systematic step by step procedure is demonstrated for retrofitting of mid-rise building such that target performance objectives fulfil the codal provisions. It was observed that, after seismic retrofit inter-storey drift were reduced bellow the desired limit, axial force on column increased while maximum moment on flexural member decreased. Results found effective in controlling floor displacement. Based on the result obtained, it is concluded that models considered for study were retrofitted effectively by using friction dampers.