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SEISMIC STRUCTURAL PERFORMANCE INDEX EVALUATION OF EXISTING REINFORCED CONCRETE BUILDINGS by Andres W.C. Oreta, Benito M. Pacheco and William T. Tanzo* ABSTRACT: A method for evaluating the basic structural performance index, Io, of existing reinforced concrete (RC) buildings is presented. The index, which is evaluated using a Level-1 screening procedure by the Architectural Institute of Japan (AIJ), with some modification, may be applied to medium and low-rise buildings up to six stories. The index of the building may be used as a measure of the expected seismic performance of a building. KEYWORDS: Seismic Capacity, RC Building, Earthquake, Metro Manila 1. INTRODUCTION The problem of possible hazards brought about by earthquakes such as building collapse and loss of lives, especially in major urban metropolis like Metro Manila, must be addressed by city planners, building officials and structural engineers. Although our present structural design codes may have incorporated special provisions for the earthquake resistant design of new buildings, there is still a danger for possible collapse or life-threatening damage in existing buildings, especially the old and deteriorated ones. Buildings with large occupancy such as schools and buildings which are essential after an occurrence of an earthquake such as hospitals and other emergency service facilities must be checked urgently on their expected performance against major earthquakes. Civil and structural engineers must address the need of assessing the seismic safety of buildings, possibly for retrofitting or strengthening. Given the large number of existing buildings in the metropolis, it is clear that seismic upgrading cannot be advanced simultaneously. It is thus important that a brief method of seismic screening of buildings be done to determine which buildings are at greatest risk, and hence must be given priority in detailed evaluation and possible retrofitting. This paper presents one method of evaluating the seismic performance of reinforced concrete (RC) buildings by ocular inspection and simple computations. 2. PROCEDURE FOR BRIEF SEISMIC DIAGNOSIS The Japanese standard for evaluating the seismic performance of existing RC buildings, especially low and medium-rise buildings up to six stories, uses a seismic index, Is, for judging whether a building has sufficient capacity or not. This method has been used by the Architectural Institute of Japan (AIJ) in the inspection of the safety of existing buildings in past earthquakes such as the 1980 Miyagi-Ken-Oki Earthquake, 1990 Philippine Earthquake and the 1995 Kobe Earthquake. The most general form of the equation for computing Is was given by Umemura (1980) as (1) I s = Io ⋅ SD ⋅T ⋅G * Presented at the 10th ASEP International Convention, May 23-24, 2003, Manila Pavilion, pp. 1-11 1 where Io = basic structural performance index calculated by the ultimate strength, ductility, number of story and story level considered, SD = structural design index to modify Io due to degree of irregularity of the building shape and the distribution of stiffness, T = time index to modify Io due to the degree of deterioration of strength and ductility, G = local geology index to modify Io. The Io-index is the dominant value in the computation of Is. The indices SD, T and G, are reduction factors less than or equal to 1.0. In the latest 1995 publication of the International Institute of Seismology and Earthquake Engineering (IISEE), Hirosawa et al (1995) gives guidelines on appropriate values for SD and T, while the G-index is not included in the seismic index equation. The Is-index is calculated at each floor in each direction of the building. This value corresponds to the maximum elastic response shear coefficient that the floor can resist. A larger value of Is indicates a higher seismic performance of the building. The seismic safety of the buildings is determined by comparing the computed Is with a critical value. Nakano (1986) stated that an index larger than about 0.6 (using Level-2 screening) should be required in order to survive a severe earthquake, the intensity of which is nearly equal to Tokachi-Oki or Miyagi-Ken-Oki earthquakes. The critical value of 0.6 is thus recommended as determined from correlation studies from damaged buildings of past severe earthquakes in Japan. Buildings designed by the current Japanese design codes have Is more than 0.6. The Japanese standard has three levels of screening procedures - the simple Level-1 screening based on the horizontal strength of the building to a more detailed Level-3 screening based on the failure mechanism of building frames. The higher level screening procedure is expected to give a more reliable estimate of the seismic performance of buildings. The present study adopts the AIJ Level-1 screening procedure with some modification. Since the structural performance Io-index is the most important value in the seismic index computation, this study focuses on the computation of Io-index. Moreover, although the Io-index can be computed for each story level, the Io-index at the ground floor level, being the most critical level, will be used. 2.1 Classification of vertical elements The structural performance Io-index for Level 1 screening is calculated from the horizontal strength of the vertical elements of the building, based on the sum of the horizontal cross-sectional areas of columns and shear walls and on their respective average unit strengths. Columns are first classified based on the ratio of the clear height to dimension of column (h/d) as long columns, short columns or extremely short columns. Similarly, shear walls are also grouped as Wall-Type 1, 2 or 3. Wall-Type 1 has one boundary column or no peripheral column. Wall-Type 2 has a peripheral column, while Wall-Type 3 has two boundary columns. Refer to Table 1. The unit shear stress given in Table 1 corresponds to minimum values obtained in experimental data of RC elements in Japan. A compressive strength of 200 kg/cm2 (2,800 psi or about 20 MPa) is used in determining the unit shear stress. 2 Table 1. Type of Elements and Shear Stresses Element Section Property Shear Stress (kg/cm2) kPa Long Column h/d > 6 7 687 Short Column 2 <= h/d <= 6 10 981 h/d < 2 15 1472 Wall - Type 1 10 981 Wall - Type 2 20 1962 Wall - Type 3 30 2943 Extremely Short Column 2.2 Strength Index The C–index or strength index of each group is calculated by Eqns. 2-a, b and c. C c = (∑ τ c Ac + ∑ τ s As ) / W (2-a) C e = ∑ τ e Ae / W (2-b) C w = (∑ τ 1 A1 + ∑ τ 2 A2 + ∑ τ 3 A3 ) / W (2-c) where Cc = strength index of long and short columns Ce = strength index of extremely short columns Cw = strength index of walls τc = shear stress of long columns in kPa τs = shear stress of short columns in kPa τe = shear stress of extremely short columns in kPa τ1, τ2, τ3 = shear stress of walls of type 1, 2 and 3, respectively A = cross-sectional area of corresponding columns and walls in m2 W = building weight above the story under consideration in kN 2.3 Story Index The story index (β) indicates the ratio of the response base shear coefficient and the i-th story response shear coefficient and is given as β = (n + 1)/(n + i). Here, n is the total number of stories and i is the story level under consideration. At the ground floor level (i=1), the value of β = 1. 3 2.4 Structural Performance Index The structural performance index considers the contribution of the vertical elements in resisting lateral forces through the strength index. Incorporated in the computation of this index are the ductility index and strength reduction factors of the vertical elements. For a building without extremely short columns (Ce = 0), the index is computed as I1 = β (0.7C c + C w ) ⋅1.0 or I1 = β (C c ) ⋅1.0 (3) Τhe second equation in Eq. 3 is used only if there are also no walls (Cw = 0). For a building with extremely short columns (Ce ≠ 0), either Eq. 3 or Eq. 4 below will be used depending on a specified condition. I 2 = β (0.5C c + 0.7C w + C e ) ⋅ 0.8 (4) Umemura (1980) stated the following condition. If the failure of the extremely short columns results to the fatal damage of the building, then use Eq. 4, otherwise use Eq. 3. This condition is subject to various interpretations depending on the judgement of the engineer. However for simplicity, it is assumed here that a value of Ce ≥ 0.5 will result to fatal damage of the building. Based on some numerical simulations, when Ce = 0.5, about 30% of the columns are extremely short, and failure of 30% of the columns may lead to possible collapse. Hence when Ce < 0.5, the index is computed using Eq. 3. Notice that Eq. 3 does not include Ce even if it is not equal to zero. If the number of extremely short columns is minimal, Eq. 3 assumes that these extremely short columns fail first and only the long and short columns and the walls will be left to resist the shear forces. The values 0.7 and 0.5 in Eq.3 and Eq. 4 are strength reduction factors due to displacement compatibility. The values 1.0 and 0.8 are ductility indices. Notice that the first equation of Eq. 3 considers only 70% contribution of the long and short columns compared to 100% contribution of the walls. However, when there are no walls, then we let the columns carry 100% of shear forces as shown in the second equation. Eq. 4, on the other hand reflects the contribution of the various elements with the extremely short columns resisting 100%, followed by walls, 70% and the long and short columns, 50%. Long and short columns and walls are considered more ductile than extremely short columns as reflected by the factors 1.0 in Eq. 3 and 0.8 in Eq. 4. 2.5 Concrete Hollow Blocks (CHB) Infill Walls The present study recognizes the effect of CHB or masonry infill on the seismic behavior of frames. The true influence of infill on frames will be to stiffen the infilled frames relative to the other frames. The proportion of the total seismic shear transmitted by the infilled frames will increase because of the increased stiffnesses of these frames relative to the other frames. The stiffening due to infill results to a reduction of the effective clear height of the columns - at the top and bottom ends of this height, the plastic hinges are developed. The higher shear force will be developed, however, at the expense of ductility (Paulay and Priestley 1992). This increase in shear force may be considered in the present study by simply reducing the effective clear height of the 4 columns, which may result to the column being classified to a shorter column with a larger shear stress as shown in Table 1. In the present method, a long column with more than 30% infill will be re-classified as a short column. A short column with more than 70% infill will be re-classified as an extremely short column. 2.6 Procedure of Level 1 Screening The general procedure for Level 1 Screening of RC buildings is summarized below: 1) Ocular inspection of the building is first done to gather information about the building such as the floor area, number of stories, type of framing and flooring system, location and dimensions of columns and shear walls, infill CHB walls and the clear height of floor to ceiling. 2) A simple diagrammatic plan, similar to Figure 1, is drawn for the ground floor showing the main lateral force resisting system which consists of columns, shear walls and infill walls. The lateral force resisting frames are defined for both x and z axes. A B C D E F G H I J K L 5.8 m 5 2.9 m 2.8 m 4 4.7 m 3 2 1 5.57 m (typical) [email protected] = 73.2 m 4 Story Bldg. Floor Area = 1010 sq m All columns = 0.5 m x 0.6 m Figure 1. Ground Floor Plan of an Inspected Building 5 3) Using the spreadsheet program, data are inputted in Tables 2, 3 and 4. Inputted in Table 2 are the dimensions of the column cross-sections and the number of columns of the same crosssection for each frame. The average percentage of CHB infill per frame is also inputted. In Table 3, the required data are the shear wall dimensions (length and thickness) and the type of shear wall (Type 1, 2 or 3). The data for Table 4 are used for the computation of the seismic dead load or weight of the building. The data needed are the floor area, estimated dead load per unit area that includes the weight of floor slab, floor finish, ceiling, partitions and other permanent loads. The clear height and the average column dimension are also necessary input data. Table 2. Input for Columns Frames along X-direction Column Section A Column Section B Label b (m) d (m) No. b (m) d (m) No. 1 0.5 0.6 10 2 0.5 0.6 4 3 0.5 0.6 10 4 0.5 0.6 13 5 0.5 0.6 13 Frames along Z-direction Column Section A Column Section B Label b (m) d (m) No. b (m) d (m) No. A 0.5 0.6 3 B,C 0.5 0.6 6 D 0.5 0.6 5 E,F 0.5 0.6 8 H 0.5 0.6 4 J 0.5 0.6 4 M 0.5 0.6 2 G,I 0.5 0.6 8 K,L 0.5 0.6 8 Column Section C b (m) d (m) No. % CHB Infill 20 90 10 10 70 Column Section C b (m) d (m) No. % CHB Infill 100 0 50 0 40 0 100 80 40 6 Table 3. Input for Shear Walls Frames along X-direction Label Wall Section A w (m) t (m) No. Type w (m) 5 5.57 0.3 2 1 1 5.57 0.3 1 1 4' 5.57 0.3 1 1 Frames along Z-direction Label Wall Section A w (m) t (m) No. Type w (m) D' 4.7 0.3 1 1 G' 5.8 0.3 1 1 L' 5.8 0.3 1 1 F 2.9 0.3 1 1 G 2.9 0.3 1 1 Wall Section B t (m) No. Type w (m) Wall Section C t (m) No. Type Wall Section B t (m) No. Type w (m) Wall Section C t (m) No. Type 2.54 0.5 h/d 5.1 Table 4. Input for Weight Computation WEIGHT COMPUTATION Level 6 5 4 3 2 1 Add'l Loads Load 1 Load 2 Load 3 Load 4 Area (m2) height of 1st level , h (m) = average col. dimension (m) = DL (kPa) w (kN) 1011 1011 1011 1011 Description Storage/warehouse 25% LL Partitions Equipment Others Total Weight at Ground Floor 4 5 5 5 0.00 0.00 4042.88 5053.60 5053.60 5053.60 6 5 4 3 2 1 0 h 0.00 19203.68 7 4) Based on the input data, the columns and shear walls are grouped automatically. Columns are grouped as long columns, short columns or extremely short columns based on the h/d ratio and % CHB infill. Shear walls, on the other hand, are grouped as Wall Type 1, 2 or 3. The sums of the cross-sectional areas of the columns and walls per group are then computed for two directions (x and z) as shown in Table 5. Table 5. Grouping of Columns and Walls SUMMARY OF SHEAR AREAS (m2) OF COLUMNS AND WALLS Frames along X -direction Label COLUMNS WALLS Long Short Extreme Type 1 Type 2 1 0.00 3.00 0.00 3.34 0.00 2 0.00 0.00 1.20 1.67 0.00 3 0.00 3.00 0.00 1.67 0.00 4 0.00 3.90 0.00 0.00 0.00 5 0.00 0.00 3.90 0.00 0.00 6 0.00 0.00 0.00 0.00 0.00 7 0.00 0.00 0.00 0.00 0.00 8 0.00 0.00 0.00 0.00 0.00 9 0.00 0.00 0.00 0.00 0.00 10 0.00 0.00 0.00 0.00 0.00 Total 0.00 9.90 5.10 6.68 0.00 1 2 3 4 5 6 7 8 9 10 Frames along Z -direction Label COLUMNS Long Short 0.00 0.00 0.00 1.80 0.00 1.50 0.00 2.40 0.00 1.20 0.00 1.20 0.00 0.00 0.00 0.00 0.00 2.40 0.00 0.00 Total 0.00 10.50 Extreme 0.90 0.00 0.00 0.00 0.00 0.00 0.60 2.40 0.00 0.00 3.90 Type 1 1.41 1.74 1.74 0.87 0.87 0.00 0.00 0.00 0.00 0.00 6.63 WALLS Type 2 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Type 3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Type 3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 8 5) Table 6 shows the results of the computation of the strength indices, Cc, Ce and Cw based on the total area and the average strength per group and the total weight of the building at ground level. The structural performance indices, Io, for both directions are then computed using the applicable formula. The smaller value of Io is then used as the structural performance index of the building. Table 6. Strength Index and Structural Performance Index Computation INDEX CALCULATED FOR GROUND FLOOR Element Type 19204 w(kN) = COL COL COL WALL WALL WALL COL COL COL WALL WALL WALL LONG SHRT E.SHRT TYP1 TYP2 TYP3 LONG SHRT E.SHRT TYP1 TYP2 TYP3 X X X X X X Z Z Z Z Z Z 0.0 Direction of EQ Area (m2) S. Stress (kN/m2) S. Force (kN) Strength Index: C 0.0 9.9 5.1 6.7 0.0 0.0 687 981 1472 981 1962 2943 0 9712 7505 6557 0 687 0 0 10.5 3.9 6.6 0.0 0.0 981 1472 981 1962 2943 10301 5739 6504 0 Cc Ce Cw Cc Ce Cw 0.51 0.39 0.34 0.54 0.30 0.34 I1 I2 Index wrt X & Z Index 0.70 N/A 0.70 0 0.71 N/A 0.71 0.7 3. NUMERICAL APPLICATION To understand the factors which affect the value of the Io –index, the numerical procedure is applied to the building with the ground floor plan shown in Figure 2. Assuming a dead load of 5 kPa and %CHB infill less than 30%, the Io –index is computed for the building using column dimensions of 0.3 m and 0.4 m for different number of stories. A B 5.0 m D C 4.0 m 5.0 m 3 5.0 m Clear height = 2.5 Square columns 2 Z 2.0 m 1 X Figure 2. Ground Floor Plan of a Building 9 ( D L = 5 kN , 0% CHB infill) No. of Stories 6 d = 0.30 m d = 0.40 m 5 4 3 2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Seism ic Structural Perform ance Index Figure 3. Index vs. No. of Stories for the Building in Figure 2. Figure 3 shows the Io –index for the building in Figure 2 for different number of stories and for column dimensions of 0.3 m and 0.4 m. Observe that the index decreases with an increase of the number of stories because of the increase in the weight of the building being supported by the columns. The building with a 0.4 m x 0.4 m columns has a significantly larger index than the building with 0.3 m x 0.3 m columns. For the building with 0.3 m x 0.3 m columns, the index at two stories is 0.8 and was reduced to 0.25 at six stories, while for the building with 0.4 m x 0.4 m columns, the index is reduced from 1.35 at two stories to 0.40 to six stories. Based on the example given, it is important that the dimensions of the columns and walls must be accurately measured in the site and compared with the plans (if available). Moreover, the engineer must reasonably estimate the building weight based on the inspection of the materials used in the floors, partitions and roof of the building. The present Level-1 AIJ screening procedure is presently being applied to existing RC buildings in Metro Manila. Results of the brief diagnosis of existing RC buildings using this method will be presented in a future report. Further studies in the future (including post-earthquake damage inspections) should also aim to check the limitations of the method. 10 REFERENCES Nakano, Y. (1986). “Seismic Capacity of Existing Reinforced Concrete Buildings,” Proc. 7th Japanese Earthquake Engineering Symposium, pp. 2041-2046. Hirosawa, M., Sugano, S. and Kaminosono, T. (1995). Essentials of Current Evaluation and Retrofitting for Existing and Damaged Buildings in Japan, Japan International Cooperation Agency (JICA), Japan Paulay, T. and Priestley, M. (1992). Seismic Design of Reinforced Concrete and Masonry Buildings. Sect. 7.4, pp. 584-594, Wiley and Sons, Canada. Umemura, H. (1980). “A Guideline to Evaluate Seismic Performance of Existing Medium- and Low-Rise Reinforced Concrete Buildings and Its Application,” Proc. 7WCEE, pp. 505-512. ABOUT THE AUTHORS Andres W.C. Oreta, D. Eng., M. ASEP, M.PICE Associate Professor in Civil Engineering De La Salle University, 2401 Taft Ave., Manila Tel/Fax: (632) 5240563, Tel. (632) 5244611 loc. 226, Email: [email protected] or [email protected] Benito M. Pacheco, PhD, PE, F.ASEP, M.PICE, M.ASCE President and Co-founder of Vibrametrics, Inc. Technology Resource Group Professorial Lecturer, Dept. of Civil Engineering, UP-Diliman, QC Special Lecturer in Earthquake Engineering, PUP, Manila Tel/Fax: (632) 4260044, Email: [email protected] William T. Tanzo, D.Eng. Vice-President and Co-founder of Vibrametrics, Inc. Technology Resource Group Associate Professor, Dept. of Civil Engineering, UP Los Banos, College, Laguna Professorial Lecturer, Dept. of Civil Engineering, DLSU-Manila Tel/Fax: (632) 4260044 or (6349) 5365614 Email: [email protected] ACKNOWLEDGEMENT The support of the engineers and staff of Vibrametrics, Inc. in the preparation of the paper and slide presentation is deeply appreciated. 11