BUILDINGS
T.OKADA
Institute of Industrial Science, University of Tokyo, Tokyo, Japan
Demolition and Reuse of Concrete. Edited by Erik K.Lauritzen. © 1994 RILEM.
Published by E & FN Spon, 2–6 Boundary Row, London SE1 8HN. ISBN 0 419 18400 7.
Abstract
For the rehabilitation of existing buildings, it is necessary to evaluate the structural capacity before and after rehabilitation. In this paper, the basic concept of the guideline to evaluate the seismic capacity of existing reinforced concrete buildings, the guideline for seismic strengthening and their application to existing buildings in Japan are described. The decision criteria to screen vulnerable buildings is also described.
Keywords: Seismic Capacity Evaluation, Reinforced concrete, Building, Seismic Strength, Ductility, Rehabilitation
1 Introduction
A rehabilitation of buildings has been made mostly when buildings, building components, or materials were physically deteriorated with age. However, there is a trend to rehabilitate rather new buildings. For example, the recent development of the earthquake engineering requires that the seismic safety of existing buildings must be reevaluated and increased, if necessary. To increase the seismic safety, the structural rehabilitation; strengthening, is necessary. Another example is the rehabilitation due to the change of the use of the building. The reform of the educational system, which is a recent trend in Japan, is requiring the remodel of existing school buildings associated with structural renovation. These non-physical reasons could be called “deterioration of software”, while the physical reasons “deterioration of hardware”.
A large number of reinforced concrete buildings have been designed and constructed in Japan since 1920’s according to the seismic codes requiring rather high level of seismic capacity. However, the recent experience of earthquake damage and the current knowledge in earthquake engineering suggest that some of the existing buildings do not
have sufficient seismic capacity.
Since 1968 Tokachi-Oki Earthquake, the importance to develop the methodology to evaluate seismic capacity of existing buildings, as well as to revise the existing seismic codes, had been strongly recognized, and various methodologies were proposed [(1), (2), (3)]. In order to unify them, the guideline for evaluation of seismic capacity of existing reinforced concrete buildings [(4)] was developed in 1977 by the special committee at the Japan Building Disaster Prevention Association under the sponsorship of the Ministry of Construction, Japanese Government. The author was the chairman of the task committee to draft the guideline. The guideline for strengthening of existing buildings estimated vulnerable by the evaluation guideline was also developed [(5)]. The guidelines have been widely used and applied to many existing buildings. And they have also been used for estimating reserve seismic capacity of earthquake damaged buildings.
The purpose of this paper is to describe 1) basic concept of the gruideline to evaluate seismic capacity, 2) seismic capacity of buildings damaged due to past severe earthquakes, 3) seismic capacity of existing buildings, 4) decision criteria to screen sound buildings and to strengthen vulnerable buildings and basic concept to strengthen vulnerable buildings.
2 Basic Concept of The Guideline for Seismic Capacity Evaluation of Existing Reinforced Concrete Buildings
The guideline can be used to evaluate the seismic capacity of existing reinforced concrete buildings and consists of three different level procedures; first, second and third level procedures. The first level procedure is the simplest, but most conservative of the three, while the basic concept is common for all three. In the guideline, the unified seismic performance index of structure (Is) up to six stories is evaluated by the following equation at each story and to each direction;
The standard values of the G-, SD- and T-indices are 1.0.
The Is-index corresponds to the level of response acceleration normalized by the gravity which causes damage to the building. Therefore, the building can be
Is=Eo·G·SD·T
where, Eo = basic structural index calculated by ultimate horizontal strength, ductility, number of stories and story level considered. At the first story, the Eo-index is basically estimated by:
Eo=(Ultimate Based Shear Coefficient)×(Ductility) G = local geological index to modify the Eo-index.
SD = structural design index to modify the Eo-index due to the grade of the irregularity of the building shape and distribution of stiffness.
T = time index to modify the Eo-index due to the grade of the deterioration of strength and ductility.
approximately judged whether it is safe or not according to the earthquake level expected at the building site and the fundamental period of the building.
In order to calculate the Is-index, any of the first, the second and the third level screening procedure may be used.
i) The First Level Screening Procedure
Eo-index is approximately calculated from the horizontal strength of the building, based on the sum of the horizontal cross sectional areas of columns and walls and on their average unit strength.
SD-index is evaluated by the eight items on the shape of the building both in plan and section.
T-index is evaluated, based on the age of the building and the visible distortion and cracks in columns and walls.
ii) The Second Level Procedure
Eo-index is calculated by the ultimate horizontal strength, failure modes and ductility of columns and walls with assumption of rigid and strong beam and floor system.
SD-index is evaluated by horizontal stiffness distribution and vertical mass and stiffness distribution in addition to the results in the first level evaluation procedure.
T-index is evaluated by the grade of structural cracking, distortion, changes in quality and deterioration of the building.
iii) The Third Level Procedure
Eo-index is calculated by the ultimate horizontal strength, failure modes and ductility of columns and walls, based on failure mechanism of frames, considering the strength of beams and overturning of walls.
SD-index and T-index may be taken as the same values as used in the second level screening procedure.
3 Seismic Capacity of Earthquake Damaged Buildings
An example of the damage ratio of low-rise reinforced concrete buildings due to past severe earthquake in Japan is shown in Table 1. Most of them were three to four story buildings. The damage ratio including heavy and medium damage in the intensity VIII–
IX zone by the modified Mercalli scale was about 10% in each earthquake and the ratio of heavy damage was less than 5% [(6)]. These ratios were same in other earthquakes[(7), (8)].
In order to estimate their seismic capacity, the Is-indices were calculated by the guideline as shown in Fig. 1.
In the Fig. 1, the Is-indices of thirty reinforced concrete buildings subjected to 1968 Tokachi-Oki Earthquake, 1978 Miyagi-ken-Oki Earthquake and 1978 Izuoshima-Kinkai Earthquake are shown [(9)]. The abscissa expresses the Is-indices of the east to west direction of the buildings and the ordinate the Is-indices of the north to south direction.
Numerals show ID-numbers of the buildings and a couple of points connected by broken line shows upper and lower bounds of the index of the bullding. The buildings with
Is-index of more than 0.6 by the second level screening procedure were not damaged and most of the buildings with Is-index of less than 0.4 were damaged.
A similar trial was done for the buildings in the city of Mexico which experienced the 1985.9.19–20 Mexico Earthquake as shown in Fig. 2. Seismic capacities of seven types of apartment houses at Tlaltelolco, two college buildings, two secondary school buildings and an office building were evaluated by the evaluation standard [(10)]. According to increase of the Is-indices, the number of damaged buildings decreases and Is-index of about 0.4 is a border between damage and non-damage.
Fig. 1 IS-indices by Second Level Screening Procedure vs. Earthquake Damage In Japan [Ref. (9)]
Fig. 2 IS-indices by Second Level Screening Procedure vs. Earthquake Damage in Mexico [Ref. (10)]
Fig. 3 Distribution of IS-indices of Existing-Buildings [Ref. (9)]
Fig. 4 Distribution of IS-indices [Ref. (11)]
Fig. 5 Distribution of ET-indices [Ref. (11)]
4 Seismic Capacity of Existing Buildings
Since the guideline was published, much effort to apply it to existing buildings and to find out vulnerable buildings has been done by Japanese engineers.
For an example, in Shizuoka Prefecture where a severe earthquake is predicted to occur in near future, the guideline has been applied to more than four thousand public buildings and about four hundreds of them have already been strengthened or demolished.
Fig. 3 shows the distribution of seismic capacity of about 700 existing buildings in Shizuoka Prefecture, where the Is-indices to both directions of each building are considered [(11)]. Most of them were designed and constructed before the code revision in 1970. As shown in the figure, the distribution of the Is-indices can be approximated by a log-normal probability density function.
By the guideline, the building with enough seismic capacity is screened by the equation (1).
The ET-index expresses the decision criteria depending upon the level of ground acceleration, soil condition, number of stories and type of failure [(9), (11)]. An example of the ET-index In the lowest seismic zone in Shizuoka Prefecture is shown in Table 2, where the input acceleration to the basement of building on 0.4 sec. ground soil is
Is≥ET
(1)
assumed as 23% of the gravity (0.23g). The acceleration in the highest seismic zone is double of that in the lowest zone.
The ET-index was determined by the consideration of non-linear earthquake response of the idealized structural models and the damage experience. For the different level of the ground acceleration, the ET-index is considered proportional to the ground acceleration.
5 Decision Criteria for Screening and for Strengthening Vulnerable Buildings
The building satisfying the equation (1) may avoid a damage. However, even if the equation (1) is not satisfied, it does not always mean the building must be strengthened.
Because, the equation (1) is considered to give an enough condition to judge the safety of the building. Fig. 1 shows such tendency well. If all buildings with Is-indices less than ET-indices were unsafe, the damage ratios In past earthquakes would be greater than the ratios shown in Table 1. Therefore, a different decision criteria should be used for strengthening. Table 3 shows the decision criteria for strengthening proposed for school buildings [(12)].
In order to verify the concept used in the criteria in Table 3, a reliability based analysis on the seismic safety of existing buildings and damaged buildings was done. Fig. 4 is a schematic expression of distribution of the Is-indices of existing and damaged buildings.
Fig. 4-(a) is showing the distribution when the ET-index is deterministic, while Fig. 4-(b) is showing the probabilistic characteristics of ET-index. The hatched part in the Fig. 3 shows the histogram of
Table 1 Damage Ratio due to 1978 Miygi-ken-Oki Earthquake [Ref. (6)]
Table 2 ET-indices for Maximum Ground Acceleration of 0.23g [Ref. (9)]
Table 3 Decision Criteria for Strengthening of School Buildings [Ref. (10)]
Fig. 6 Concepts of Seismic Strengthening [Ref. (13)]
Fig. 7 Strengthening by Walls or Braces [Ref. (5)]
Fig. 8 Detail to provide R/C Wall in Existing Frame [Ref. (13)]
Fig. 9 Detail of Connection between Wall and Existing Frame [Ref. (13)]
Fig. 10 Detail of Steel Brace for Strengthening of Existing R/C Frame [Ref.
(13)]
the Is-indices of damaged buildings shown in Fig. 1, where a modification is employed so that the number of the damaged buildings becomes 10% of the total number of buildings.
The shape of the Fig. 3 is similar to Fig. 4-(b). It suggests the ET-index may be considered to be probabilistic. Defining P1 and PET which represent density functions of Is-index of existing buildings and ET-index, respectively, the damage ratio V is determined by
Setting
The term of Vp2 may be considered to represent the frequency of Isindices of damaged buildings shown in Fig. 3. Substituting the function p1 in Fig. 3 and the density of hatched part In Fig. 3 Into the equation (3), we obtain the probabilistic density of ET -indices as shown in Fig. 5.
Assuming the normal distribution, we obtain the probabilistic density function of ET -indices as shown in Fig. 5 The curve in Fig. 3 is obtained by the equation (3), where
(2)
(3)
p1 function in Fig 3, and pET function in Fig. 5 are used.
Fig. 11 Strengthening of Columns to Increase Ductility [Ref. (5)]
6 Basic Concept for Strengthening of Vulnerable Buildings
A building judged that strengthening is necessary should be strengthened as soon as possible to prevent earthquake damage even if it has not experienced severe earthquake.
A vulnerable building lacks enough strength or enough ductility or sometimes both of them. Therefore, the purpose of strengthening is to provide (1) additional strength, (2) additional ductility or (3) both additional strength and ductility. These concepts are illustrated in Fig. 6. Most popular method to increase strength is to provide reinforced concrete shear walls or steel braced frames into existing framing system as shown in Fig.
7. As shown in Fig. 8, anchor bolts are provided at the existing beams and columns, wall reinforcing bars provided and then, concrete is cast. In order to prevent a splitting shear failure at the connection of wall and frame, spiral reinforcement is often used. Special grouting is also used at the connection as shown in Fig. 9. When the soil condition is not so good, the steel braced frame is sometimes used to minimize the increase of the building weight and to increase the strength as shown in Fig. 10. In order to increase the ductility of the building, various techniques to enclose existing column by steel plate or by reinforced concrete jacketing have been developed as shown in Fig. 11. Gaps are
usually provided both at the top and the bottom of the column, to prevent the increase of bending capacity and to increase only the shear capacity, which is expected to increase ductility.
7 Concluding Remarks
Evaluation of seismic capacity of existing buildings and the strengthening if necessary are very important to mitigate earthquake hazard. In this paper, recent trends on this problem in Japan are reported. The author wishes the methodologies described here is applied to the existing buildings not only in Japan but also in other countries with a proper modification.
8 References
(1) Hirosawa, M. (1973), Proposal on Standard to Judge Seismic Capacity of Existing R/C Buildings, Kenchiku Gijutsu (in Japanese).
(2) Architectural Institute of Japan (1975), Method to Evaluate Seismic Safety of R/C School Buildings and Method of Strengthening (in Japanese).
(3) Okada, T. and Bresler, B. (1976), Strength and Ductility Evaluation of Low-Rise Reinforced Concrete Buildings-Screening Method-, EERC Report No.76–1, Univ. of California, Berkeley.
(4) Japan Building Disaster Prevention Association (1977), Guideline for Evaluation of Seismic Capacity of Existing Reinforced Concrete Buildings (in Japanese).
(5) Japan Building Disaster Prevention Association (1977), Guideline for Strengthening of Existing Reinforced Concrete Buildings (in Japanese).
(6) Architectural Institute of Japan (1980), Report on Damage due to 1978 Miyagi-ken-Oki Earthquake (In Japanese).
(7) Building Research Institute (1965), Damage on Buildings due to 1964 Niigata Earthquake, Report of Building Research Institute, No.42 (in Japanese).
(8) Architectural Institute of Japan (1968), Report on Damage due to 1968 Tokachi-Oki Earthquake (in Japanese).
(9) Umemura, H., Okada, T. and Murakami, M. (1980), Seismic Judgment Index Values for Guideline for Evaluation of Seismic Capacity of R/C Buildings, Proceedings of Annual Convention of Architectural Institute of Japan (in Japanese).
(10) Okada, T. et al. (1986), Seismic Capacity of Reinforced Concrete Buildings which suffered 1985 Mexico Earthquake in Mexico City, Part 1–part 13, Proceedings of the Annual Convention of Architectural Institute of Japan (in Japanese).
(11) Okada, T. (1983), Seismic Capacity and Strengthening of Reinforced Concrete Buildings, Proceedings of Panel Discussion for Strengthening of Existing Reinforced Concrete Buildings, Japan Concrete Institute (in Japanese).
(12) Murakami, M. and Okada, T. (1981), Evaluation and Judgment of Seismic Safety of R/C School Buildings, Japan Building Disaster Prevention Association (in Japanese).
(13) Japan Concrete Institute (1984), Handbook for Strengthening of Concrete Structures, Editor: T.Okada, Gihodo-Shuppan.