15th Symposium on Earthquake Engineering Indian Institute of Technology, Roorkee December 11-13, 2014
Paper No. A126
RETROFITTING OPEN GROUND STOREY BUILDING WITH MASONRY WALLS IN GUWAHATI CITY
Ritukesh Bharali1 Bhargob Deka2 and Jayanta Pathak3 1
Research Assistant, Assam Engineering College,
[email protected] 2
Graduate, Assam Engineering College,
[email protected] 3
Professor, Assam Engineering College,
[email protected]
ABSTRACT
There has been phenomenal increase in construction of mid-rise multistoried apartment and commercial buildings since the year 1995-96 in Guwahati city. The Urban Local Bodies (ULB) that were engaged in development control were not yet ready for handling the phenomenal rise in multistoried construction buildings during the decade 1990–2000. Several hundreds of multistoried buildings with open ground storey for parking were added to the city‟s building stock without reliable lateral load-resisting systems such as the provisions of shear walls, appropriate bracings systems etc. A significant number of such buildings have collapsed during 1999 Turkey, 1999 Taiwan, 2001 Bhuj and 2003 Algeria earthquakes. A study has been made to reduce the seismic vulnerability of these mid-rise buildings with open ground storey by introducing masonry walls in strategic locations in the ground storey as a cost-effective retrofitting option. Masonry infill walls of varying thickness are modeled and placed at strategic
locations at the ground level, keeping in view of the required space for existing parking facilities. Comparative analyses of seismic response of model and existing buildings have been carried out and the results of static non linear analyses are examined to characterize the effect on few response parameters for various possible locations of masonry walls. It has been observed that, introducing appropriately positioned masonry infill in open ground storey of existing building increases the lateral stiffness substantially and enhances the lateral load capacity of the structure as a whole, thereby preventing collapse. The study shows that strategically introduced masonry infill provides an economic, quicker and reliable option in comparison to highly interventional options viz. RC shear walls or steel bracings, to reduce the vulnerability of existing open ground storied multistoried buildings. Keywords: Drift, Masonry, Open Storey, Response, Seismic, Retrofitting
INTRODUCTION The increase in urban population in advent of urbanization in many cities has led to the upsurge of construction of multi-storied buildings in both residential and commercial sector. The housing demand for the ever-increasing population in the city of Guwahati has fuelled the demand for multistoried apartment buildings. During the last decade, most of these buildings came up in limited plot area with much open space for parking. Owing to high cost of land, small plot areas, the trend of open ground storey (OGS) for parking has become an unavoidable feature in such constructions. It is important to understand the response of such building stock in the event of an earthquake to reduce their vulnerability. These types of buildings are found behave as an inverted pendulum, translating back and forth, in the event of an earthquake and the columns in the open ground storey are highly susceptible to severe damage due to large inter-storey drifts, and may often lead to collapse when displacement demand is high. Several earthquakes in the past (1999 Turkey, 1999 Taiwan, 2001 Bhuj and 2003 Algeria) has shown the consistent weak performances of these buildings. An attempt has been made in this paper to understand the vulnerability of such buildings considering the geometric properties, wall thickness and material properties of such buildings constructed in the last decade in Guwahati. The study has explored the opportunity to reduce the vulnerability of these existing open ground storied buildings by introducing masonry infill at strategic locations in the open ground storey, as a simple, economic, non-interventional retrofitting measure.
LITERATURE REVIEW Masonry infill is synonymous with RC framed buildings, all over the world. They are used to fill in the voids between vertical and horizontal resisting members. However, dubbed as non-structural elements, they are mostly not designed nor their response is considered in computation of lateral stiffness of the structure. Earlier studies [Tassios (1984), CEB (1994)] concluded that, the presence of infill leads to a decrease in shear force in the columns, as “non-structural” infill takes part in resistance to seismic forces. Murty and Jain (2000) carried out experimental studies to understand the effect of masonry infill on RC frame models. They found infill to have a very beneficial influence on the building, as it increases its strength, stiffness, overall ductility and energy dissipation. Furthermore, the deformation and ductility demand was seen to have decreased. However, they also indicated the detrimental effects of infill, such as soft storey and torsion, which are of great concern and needs to be addressed with expertise.
Various researchers explained the reduction in lateral stiffness of frames with infill due to the presence of openings. Benjamin and Williams (1958), Mallick and Garg (1971), Ginnakus et al. (1987) stated the reduction to be 70-80% for an opening percentage of 20-30%. Asteris (2003) concluded that, the case of an infill frame with soft ground storey could lead to higher shear forces in the columns as compared to the bare frame analysis. Reconnaissance study of the Bhuj earthquake by EERI credited the soft storey at ground level among the major factors to have triggered mass destruction in terms of lives and property. Indian Code of Practice 1893-2002 provides two formulae for computation of time period, for the case of bare RC frame and a RC frame with infill present. The need for accurate modeling of masonry infill led to numerous experimental and analytical studies. Polyakov (1960) proposed first to model infill as an equivalent diagonal pin-jointed strut in analyzing steel frame structures. Smith (1962) proposed the modeling of infill as equivalent struts, considering elastic theory and proposed that, the effective width of such struts should be a function of infill stiffness with respect to that of the bounding frame. Stafford Smith (1969) developed some empirical curves to relate stiffness parameters to effective width of a diagonal strut. Mainstone (1971) proposed an empirical relation between effective width of strut and Smith‟s stiffness parameter. Pauley and Priestly (1992) recommended a width of 0.25 times the diagonal length for modeling of equivalent concentric diagonal struts. FEMA 273 (1997) specifies that masonry infill can be modeled as equivalent struts placed concentrically or eccentrically. It uses the relative stiffness of the infill to frame stiffness developed by Smith along with Mainstone‟s approach for equivalent width of strut. FEMA 306 (1999) has also reestablished the reliability of FEMA 273 method of equivalent strut. The literature review establishes infill as vital structural elements providing lateral resistance to building frame, and absence of infill at any level renders a soft storey condition to a structure. The current research work therefore, attempts in utilizing advantage of masonry infill as lateral load resisting system in reducing seismic vulnerability of open ground storey building in Guwahati city.
METHODOLOGY A G+4 idealized building representing a sample of building stock in the city building and an existing building of medium irregularity, designed conforming to Indian Standard Codes of practice was modeled in SAP2000. The grade of concrete and steel were M20 and Fe415 respectively. The modeling of both the buildings involved was threefold: Case I (Bare Frame) Building modeled as bare frame with wall loads taken as uniformly distributed loads and slabs (125mm thickness) modeled as thick shell element. The thickness of the wall was taken as 5 inches (127 mm) as per the common practice in Guwahati city. The Mander‟s confined model was used for defining beam and column sections. Diaphragms were assigned at each level. The bare frame model of idealized building frame is shown in Fig. 2. Case II (Open Ground Storey) Infill in the upper storey was modeled as equivalent concentric diagonal pin-jointed compression struts using the FEMA 273 guidelines. Struts were placed in locations, where the there are 100% infill or infill with openings (in form of doors, windows, ventilations) less than 30%. The idealized building frame model with equivalent struts as infill but with OGS is shown in Fig. 3.
Modeling of Equivalent Struts (FEMA 273) The stiffness and strength contribution of the infill walls are to be considered by modeling it as Equivalent Strut having pin-jointed at the beam to column juncture of the confining frame. The Equivalent Struts are modeled with similar properties as infill walls as shown in Table 1. The masonry infill panel will be represented by an equivalent diagonal strut of width, a, and net thickness tw as shown in Figure 1. The width (Weff) of equivalent struts are calculated as per the following relations (Eq. 1&2) Weff = 0.175(hH)-0.4(H2+L2)1/2 h = [{Ewtwsin(2)}/4EcIcHin]1/4
(1) (2)
Where Ew and Ec are the moduli of elasticity of the infill wall and the concrete (i.e. the frame material) respectively, = arctan(H/L) is the inclination of the diagonal, tw is the thickness of the infill wall, and I c is moment of inertia of the column of the frame, whereas Hin, H and L are the net height of the infill wall, the storey height, and the bay length of the frame. The thicknesses of the equivalent struts are the same as the thicknesses of the walls.
Fig. 1: FEMA 273 Method Designing Equivalent Strut (Source: ERDC/CERL TR-02-01) Table 1: Properties of Infill Walls INFILL MATERIAL PROPERTIES Compressive Strength Modulus of Elasticity Shear Modulus Thermal Coefficient Poisson‟s Ratio
VALUES 4.63 MPa 2200 MPa 1018 MPa 0.0000081/.C 0.2
Case III (Masonry Retrofitted buildings) Masonry infill of varying thickness (5 inches and 10 inches) were used to retrofit the open ground storey of the building as a cost-effective and non-interventional measure as case III. The infill was placed strategically with infill at the upper storey contributing to lateral stiffness were continued at the ground
level. Some infill were also added at strategic locations at ground level, in order to maintain the symmetry of plan, to prevent torsional modes. For each of these cases, hinges were assigned to the frame elements as per the FEMA 356 method, which is incorporated in the SAP2000 itself. A pushover analysis was carried out till a monitored roof displacement of 50 mm and 70 mm in the idealized building and existing building respectively.
Fig. 2: Idealized building (Bare frame)
Fig. 4: Idealized building (Retrofitted)
Fig. 3: Idealized building (OGS)
Fig. 5: Existing building (Retrofitted)
In all the above cases static lateral loads were defined as per IS 1893:2002 for seismic zone V. It has been observed that, in all of the cases of G+4 storied building frames, the normalized spectral acceleration in scenarios considering bare frame and considering infill differ by small margins. Hence, there is no significant change in the lateral force pattern in both the cases. In reality, however, the situation is quite different, as the displacement demand in the ground floor is large, which is often not met.
Fig. 6: Ideal Moment-Rotation Curve Figure 6 shows the ideal moment rotation curve with all the hinge levels, for pushover analysis carried out in SAP2000.
RESULTS AND DISCUSSION
Horizontal Base Shear (KN)
The non-linear static pushover analysis for the open ground storey condition in the idealized buildings showed the formation of hinges at Immediate Occupancy level (Fig. 8). This condition renders a building vulnerable to seismicity. During seismic shakings, the hinge level may rise up to Life Safety (LS), Collapse Prevention (CP) and finally Collapse (C). On retrofitting the structure by introducing masonry infill of 5 inch and 10 inch thickness at given locations of open ground storey [Fig. 8 (b & c) ], the hinges are not formed and shows a level below IO level. The horizontal base shear increases when masonry infill are considered as a structural element in analysis (Fig. 7).
9000 8000 7000 6000 5000 4000 3000 2000 1000 0
Bare Frame
Open Ground Storey
0
0.02
0.04
0.06
0.08
Roof Displacement (m)
Model Building with 5 inch infill retrofit Model Building with 10 inch infill retrofit
Fig. 7: Static Pushover curve for idealized building
The interstorey drift in the ground storey columns for open ground storey building (25.8 mm) was found to be greater than that of the bare frame (17.9 mm). The interstorey drifts in the ground storey columns
were significantly reduced to 7.2 mm and 4.7 mm after retrofitting with 5 inch and 10 inch infill at the ground level.
(a)
(b)
(c)
Fig. 8: Hinge formation for Idealized building (a) Open ground storey, (b) Retrofitted with 5 inch masonry infill, (c) Retrofitted with 10 inch masonry infill
The 10 inch infill proved to be a better retrofitting option, as lesser number of hinges were observed in this case, which can be seen clearly from Fig. 8 (a, b and c).
The non-linear static pushover analysis for the existing building showed a considerable increase in the lateral resistance of the building with masonry infill as structural element. It has been observed that, for a given value of roof displacement, calculated horizontal base force is more for open ground storey (with masonry infill in upper levels) condition than bare frame condition. This establishes masonry infill as vital structural element contributing to lateral stiffness. However, on introducing retrofitting with 5 inch and 10 inch masonry infill at ground level, there is further increase in the horizontal base shear as shown in Fig. 9, which justifies the strategic introduction of masonry infill given location at the ground storey. Horizontal Base Force (KN)
6000 5000 bare frame
4000 3000 2000 1000 0 -1000 0
0.02
0.04
0.06
0.08
Open Ground Storey Frame with 5 inch infill retrofit Frame with 10 inch infill retrofit
Roof Displacement (m)
Fig. 9: Static Pushover Curve for Existing building
It has been observed clearly in the existing building analysis that, the horizontal base shear is higher in case of open ground storey model compared to bare frame model because of the increase in stiffness. The interstorey drift for bare frame was found to be 3.2 mm at the ground level, and that of the open ground storey frame was found to be 8.4 mm. However, the increase in lateral stiffness is also accompanied by an increase in interstorey drift. The analysis has shown that, continuing the masonry infill contributing to the lateral stiffness at upper storeys to the ground level, along with some strategically placed infill to reduce torsion, leads to a significant decrease in the interstorey drift along with an increase in lateral stiffness (5.6 mm and 4.4 mm for 5 inch and 10 inch infill respectively). The decrease in interstorey drift was found to be larger for 10 inch infill (47.62%) as compared to that of 5 inch infill (33.33%) as retrofitting option. In the existing building with open ground storey model, the hinges of LS and C level were formed at 60 mm and 68 mm roof displacement respectively (Fig. 10). It has been observed that, on retrofitting the existing structure by introducing masonry infill of 5 inch and 10 inch thickness at given location of open ground storey, the decrease in inter-storey drift at ground level led to a reduction in hinges level to IO Immediate Occupancy (Fig. 11&12). Therefore, it is observed that, there is considerable improvement of the behavior of the building against lateral load both in terms of seismic resistance and reduction of inter storey drift with strategically placed masonry infill wall in the ground storey.
Fig. 10: Hinge level (Existing building, OGS)
Fig. 11: Hinge level (Existing buiding, Retrofitted with 5 inch retrofit)
Fig 12: Hinge level (Existing buiding, Retrofitted with 10 inch infill)
CONCLUSION The rapidly increasing population in the Guwahati city has considerably increased the demand for housing in form of multi-storeyed apartments. The most of these buildings in Guwahati city constructed in the last decade have open ground storey as parking, which renders a soft ground storey condition. Most of these buildings are designed as bare frame without considering the masonry infill as structural elements. The study explored the opportunity to make this large mid rise building stock less vulnerable to earthquake by introducing a cost effective and quicker retrofitting solution in the form of masonry infill walls in the open ground storey at strategic locations. The study was carried out on few typical idealized building models and then expanded to the analysis of an existing G+4 storeyed building in Guwahati. The results from the study is encouraging and it has been found that, introduction of masonry infill at strategic locations at the ground level, enhances the performance of the structure in terms of increased lateral stiffness/ resistance and significantly reduced interstory drift. The 10 inch infill performed better as a retrofit measure as compared to that of 5 inch infill. The current research establishes masonry infill as an economic, non-interventional retrofitting measure to enhance the seismic performance of buildings, which can be implemented with lesser skill and in shorter time span compared to interventional retrofitting technique.
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2.
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3.
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