Effect of Increase of Reinforcement Near Beam – Column Joints in Performance Based Design of Low-RiseBuildings

PoleswaraRaoKovela , Balaji K.V.G.D , GopalRaju S.S.S.V and SrinivasaRao S Research Scholar, GITAM University, Visakhapatnam, Andhra Pradesh, India. E-mail: pr.kovela@gmail.com Professor of Civil Engineering, GITAM University, Visakhapatnam, Andhra Pradesh India. E-mail:balajigitam@gmail.com Associate Professor, Dept., of Civil Engineering, IIIT, Nuzividu, Andhra Pradesh India. E-mail: sssvgopalaraju@gmail.com Head of the Department of Civil Engineering, Government Women Polytechnic, Kakinada, Andhra Pradesh India. E-mail:srsingudasu@gmail.com


II. DESCRIPTION OF THE STRUCTURE:
Three structures i.e., four, five and six storied are considered to represent low-rise reinforced concrete building for study. The Plan and Elevation are shown in the Fig.1 and Fig.2 Column reinforcement and Beam reinforcement are obtained as per elastic analysis without considering the seismic / wind forces and shown in Fig.3. Modeling approach: Analysis have been performed using ETABS, which is a widely used structural designprogramme for static and dynamic analysis of the structures. A three dimensional model of each structure is created to carryout nonlinear statistic analysis. Beams and columns elements are modeled as nonlinear frame elements with plasticity by defining plastic hinges at both ends of column / beam members. The beam and column are divided into three parts i.e., central half part and quarter part on either side. The ¼ of the span of the beam and ¼ th height of the column on either side of the beam &column joint are considered for strengthening by increase of reinforcement. The reinforcement in beams and column near joints for a length of 0.25 L is increased by 25%, 50% and 75% for each model for this study. The initial reinforcement in columns and beams is arrived based on the elastic analysis considering only gravity loads. The equivalent reinforcement is provided duly adopting selective combination of different dia of reinforcement barsThe % of increase is with reference to the initial reinforcement provided based on elastic analysis. The reinforcement provided in beams and columns is shown in Table Iand II. The nonlinear hinges are provided on either side of the quarter part and either side of the central half part of beams and columns to allow the failure mechanism in hinges in the central beam and column also, when reinforcement in members near joints is increased. Six Nos. of non linear hinges are provided for each column and beam as shown in figure4.  ETABS provides default hinge properties and considered PMM hinges for column and M3 hinges for beams. Once the structure is modeled with section properties, steel contents and the loads on it, defaults hinges properties (as per table 10.8 [6] ) will be assigned to the members.

Properties of the Building:-
The default hinge properties of the beam depends upon the ratio of , and column depends up on the ratio of , where as p = Ratio of tension reinforcement, p bal = Reinforcement ratio producing balanced strain conditions,p ' = Ratio of compression reinforcement, V = Design shear force at section concurrent with moment,bw = Web width, d = Distance from extreme compression fiber to centroid of tension reinforcement,f = Compression strength of concrete, P = Axial force in concrete column, Ag = Gross area of the column, s = Spacing of shear reinforcement.
The non linear hinge properties of the beam depend up on longitudinal and transverse reinforcement and where as the non linear hinge properties of the column depend up the transverse reinforcement only. It is assumed that the shear reinforcement confirms the required spacing provision in modeling the non linear hinge prosperities.

Pushover analysis:
The push over analysisconsists of the application of gravity loads and representative lateral load pattern. The building is subjected to gravity loading and simultaneous lateral loading.
There are sixteen models generated for each 4 stories, 5 stories and 6 stories buildings.One of it is the basicmodel with required reinforcement for gravity loading and other fifteen models are with combination of changes in the beam and column reinforcement i.e., 25%, 50% and 75% increase. The pushover curve for displacement in 'x' direction and 'y' directionverses base shear are obtained.

III. RESULTS AND DISCUSSIONS
The three Models 4, 5 and 6 storey are analyzed by using "ETABS"software and the results i.e., the maximum displacement versus base shear values are tabulated as below. (Table III to Table VIII)

STOREY
The following observations are made from the results obtained.
1) The base shear capacity of the RCC building is considerably increased by increasing the reinforcement in beams near beam -column joints. 2) (a) The base shear capacity of the building in 'X' direction is increased upto 30% by increase of 75% reinforcement in beams and columns near joints. The increase is reduced gradually from 4 th story to 6 story building. (b) The base shear capacity of the building in 'Y' direction is increasedupto 43%. The increase is reduced gradually from 4 story and 6 story. 3) (a)The base shear capacity in 'x' direction is increased upto 25% for increase of 75% of only beam reinforcement. (b)The base shear capacity in 'y' direction is increased upto 39% for increase of 75% only beam reinforcement near joints. 4) The increase of base shear capacity in both 'X' and 'Y' direction is very negligible when only column reinforcement is increased. 5) In general the increase in base shear capacity in 'Y' direction is more when comparative to the increase in X directions. It may be mainly due to the more stiffness of the building in 'X' direction when comparative to the 'Y' direction. The reasons for the negligible increase in base shear capacity when only column reinforcement near joints is increased may be due to the fact that (i) The model is designed such that strong column and weak beam principle is followed.
(ii) The hinge properties of the column do not depend on the longitudinal reinforcement of the column IV. CONCLUSIONS 1) The increase in the reinforcement in column joints has negligible impact in base shear capacity of the building 2) The increase in the reinforcement of beam joints has substantial impact on the base shear capacity for the low raise buildings (4 to 6 stories which are commonly adopted).
3) The increase in base shear capacity in Y direction (weaker direction) is more. 4) The increase in base shear capacity due to increase of reinforcement at joints is slightly decreasing with increase of number of floors. 5) The seismic design of low rise buildings for severe earthquake zone also can be more economically designed by this performance based nonlinear static pushover analysis method.