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Seismic diagnosis for rehabilitation and upgrading of steel gymnasiums
EngineeringStructures,Vol.
Plh S0141-0296(97)00017-5
ELSEVIER
20, Nos 4-6, pp. 533-539, 1998 © 1997 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0141-0296/98 $19.00 + 0.00
Seismic diagnosis for rehabilitation and upgrading of steel gymnasiums K. O h i Institute of Industrial Science, University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo 106, Japan
K. T a k a n a s h i Faculty of Engineering, Chiba University, 1-33 Yayoi-cho, lnage-ku, Chiba 263, Japan
In this paper we describe an improvement of an existing method of seismic diagnosis for steel gymnasiums used in Japan. This improvement is made so that the results of diagnosis match the actual seismic damage of steel educational facilities (mostly gymnasiums) caused by the great Hanshin-Awaji earthquake, which occurred on 17 January 1995. The strategies for rehabilitation and upgrading including decision making as to which structure, where in the structure and how much to improve, are determined in consideration of the results of seismic diagnosis for the present (or damaged) state as well as the retrofitted state of the structure. In this paper, seismic performance indices are calculated for the pre-damaged state of the educational facilities by use of the existing seismic diagnosis method and compared with the actual damage observed. An index of seismic performance should have strong negative correlation with the damage level observed in the facilities after severe earthquakes. The index used herein is calculated from two other indices, one for load carrying capacity and one for deformability. The latter index is similar in its role to the R-factor in the US and the q-factor in the European seismic regulations. By revising the manner of calculation of the deformability index only, the correlation between the seismic performance index and the damage level has been much improved. © 1997 Elsevier Science Ltd.
Keywords: steel frame, seismic diagnosis, damage level, retrofit planning, upgrading planning
1.
were school gymnasiums and made recommendations to local governments and engineers in charge of rehabilitation planning as to whether the damaged facilities should be rehabilitated or demolished. L2 During this advisory mission, the working group also collected documents related to the design of these facilities. In this paper we propose improving the existing seismic diagnosis method through comparison between actual damage levels and the results of seismic diagnosis. Such a technique of seismic diagnosis is applied not only to the present state of the structure including the damaged state, but also to the upgraded or retrofitted state. The results and the process of such a diagnosis provide information helpful in the decision-making in the planning of rehabilitation and upgrading of existing structures.
Introduction
After the great Hanshin-Awaji earthquake, MESSC (the Ministry of Education, Science, Sports and Culture, Japanese Government) commissioned AIJ (the Architectural Institute of Japan) to coordinate a research program on seismic performance of educational facilities during this earthquake. University professors were also asked to participate in this program, to inspect damaged educational facilities and to advise on technical matters for development of a rehabilitation plan for these facilities. Two working groups were organized to carry out this research project: one focused on reinforced concrete structures and the other on steel structures. The latter group inspected about 30 steel or partially steel educational facilities, about half of which
533
534
Seismic diagnosis for rehabilitation and upgrading of steel gymnasiums: K. Ohi and K. Takanashi
2. Observed damage of steel or partially steel educational facilities The number of steel or partially steel facilities analyzed during the advisory mission, 26, is about one-tenth of the number of reinforced concrete (R/C) school buildings, about 240. Most of the steel or partially steel facilities which were damaged are relatively old, having been built in the 1960s and 1970s, as shown in Figure 1. It should be noted that the Japanese national seismic regulations, the Enforcement Order of Building Standard Law, were revised in 1981, the same year in which checking of the ultimate strength to withstand severe earthquakes and application of special strength requirements to the design of steel joints to ensure the yielding of the members at gross-section were initiated. The steel educational facilities built in the 1980s are scarcely damaged. The following types of damage were observed: ( 1) Concrete cracks and loosened anchor bolts at the joints between steel members and R/C portions including footings were frequently observed, as shown in Photo 1. (2) Breaking at brace joints was observed in steel gymnasiums as shown in Photos 2 and 3. (3) Cracks or breaking at steal beam-to-column welded joints in a middle-rise steel building used for classrooms was observed as shown in Photo 4. (This type of damage was not observed in steel gymnasiums.) (4) Pre-cast concrete roof beams fell down as shown in Photo 5. (This is out of the scope of this paper, because it was not a steel structure, but the most dangerous damage related to gymnasiums.) (5) Non-structural damage such as falling of ceilings and displacement of floor-supporting posts were observed as shown in Photo 6. The observed damage was classified into five levels termed 'complete collapse', 'heavy damage', 'moderate damage', 'slight damage', and 'no structural damage', according to the existing classification rule for damage levels after earthquakes 3. These terms were defined mainly based on the magnitude of the permanent set of story drift angle and the significance of other structural failures detected by inspection, such as breaking at joints and buckling of members. Table 1 shows the correspondence of the terms for damage levels with the drift angle and the structural failure detected. When the permanent set of story drift angle and the structural failure correspond to different damage levels, the higher level is adopted as a rule. For example, the damage Frequency I0 No structural damage
IllfllIT[ITI
1960-1969
1970-1979
1 1
1980-1989
Slight damage Moderate damage Heavy damage
After 1989
Year when built
Figure 1 Frequency of damage levels and year when built
level is recognized as 'moderate damage' if breaking at one joint is detected, while the permanent set of drift angle remains around 1/150. If the permanent set of story drift angle exceeds 1/30, that is, the level of 'heavy damage' is assigned to the structure, its rehabilitation is usually considered difficult due to both technical and economic reasons. When the permanent set exceeds 1/20, the advisory group recommended demolition of the structure without further investigation or cost study. Twenty-two facilities, 16 damaged and 6 undamaged, are dealt with in this paper. The frequencies for each damage level are plotted against the year when built and the use of the facilities in Figures 1 and 2, respectively, and the locations of the facilities are shown in Figures 3 and 4, together with their damage levels.
3. Seismic diagnosis method for rehabilitation and/or upgrade planning Some local governments, as well as some associations in Japan related to disaster prevention or building engineering, have prepared documents to specify methods of seismic diagnosis for existing buildings. During the advisory mission, the seismic diagnosis method t developed by MESSC in 1995, which is based on the recommendation 4 made by AIJ to MESSC in 1990, was used to analyze the damaged steel educational facilities in rehabilitation planning for them. This version of seismic diagnosis is referred to as the 1995 version hereafter. The procedure recommended by the MESSC advisory group is very similar to the procedure described in Section 8.5, 'Assessment in the case of damage', of the 15th draft 5 of the revision of ISO-2394, except that a deterministic seismic diagnosis is adopted instead of probabilistic reliability assessment. In the seismic diagnosis methods developed recently, an index of seismic performance denoted by Is is comonly calculated based on the actual state of existing buildings. This index is equivalent to the ratio of the elastic base-shear that a building can withstand with inelastic deformation, but no fatal damage to the required elastic base-shear. In the case of a single-story structure, the index is expressed by Is = Eo/(Z. Rt. Co)
( 1)
where Eo is an index of structural seismic resistance given by equation (2). This index is equivalent to the elastic baseshear coefficient that the structure can withstand with inelastic deformation, but no fatal damage; (Z. Rt. Co) is the required elastic base-shear coefficient, or the load effect stipulated in the code; Z is the zone factor considering seismic activity. The values 0.8-1.0 are assigned in Japan, except Okinawa. For most of Honshu Island including the Hanshin-Awaji district, Z = 1.0 is assigned; Rt is the design response spectrum normalized to 1.0 at maximum; Co is the reference value of elastic base-shear coefficient. The current seismic regulation requires Co = 1.0 for newly designed buildings. Thus, the standard value of (Z. Rt. Co) is 1.0 for low-rise buildings such as gymnasiums located in the zone of highest seismicity. In this standard case, Is = Eo. Eo = Qu . F / W
(2)
where Qu is the ultimate load carrying capacity of the struc-
Seismic diagnosis for rehabilitation and upgrading of steel gymnasiums: K. Ohi and K. Takanashi
535
Table 1 Damage level
Permanent set of drift angle
Structural failure detected Cannot resist gravity any more
Complete collapse Heavy damage
exceeds 1/30"
More than 50% of braces broken More than 20% of beam-to-column joints broken Severe local buckling
Moderate damage
exceeds 1/100
Brace broken Bean-to-column joint broken Local buckling
Slight damage
around 1/150
Yielding Buckling of slender braces Slipping at high-strength bolt friction joint
No structural damage
None
No damage in main load-carrying system
*The MESSC advisory grouo recommended demolition of steel gyms without further investigation if a permanent set greater than 1/20 is detected.
Type of use
I No structuraldamage
I
Slight damage Moderate damage Heavy damage
1 1 Classrooms
Gymnasiums
/ 0
1
2
3
4
5
6
7
8
9
10
11
12
Frequency
//
j•ik///e•6
Heavy damage &Moderatedamage A ASlight damage ONe structuraldamage
/
,/~/
(
(
Figure 2 Frequency of damage levels and type of use
Figure4 Steel educational facilities analyzed in Awaji area
A
o
O
o
A ASlight damage @No structuraldamage Figure3 Steel educational facilities analyzed in Hanshin area ture in terms of base shear strength; F is an index of deformability or inelastic" energy absorption capacity, which has a similar meaning to the R-factor in the US or the q-factor in the European seismic regulations. It should be noted that the R-factor and the q-factor are applied to the yield strength of a frame when the first plastic hinge is formed, but the F-factor is applied to the ultimate strength of the frame at its collapse mechanism. Thus, usually smaller values are assigned to the F-factor compared with the R or q-factor. W is the weight of the structure.
q = ls/(F.St)
or
q.St = Qu/(W.Z. Rt. Co)
(3)
In the standard case that (Z. Rt. Co) is equal to 1.0, q.St = Q u / W where q is the index of load carrying capacity. The term q.St is equal to the shear coefficient capacity in the standard case and this index is used to control the minimum shear coefficient capacity denoted by St. The value of 0.25 is assigned to St for steel structures.
536
Seismic diagnosis for rehabilitation and upgrading of steel gymnasiums: K. Ohi and K. Takanashi
The F-factor is specified with respect to the type of failure as well as deformability of a structural element. In the case that structural elements with different F-values are supposed to fail, the averaged value with the weight of contribution to Qu is assumed to represent the system F-factor. F-factors specified in the 1995 version are mostly the inverse of the Ds factor (0.25-0.5) stipulated in the current seismic regulations for newly designed buildings. The largest value of F is then 4.0 (=1/0.25) for a moment resisting frame with very ductile members. While the maximum value of Ds-factor is 0.5 (that is, F = 2.0) in designing steel structures, but the 1995 version specifies slightly stringent value in seismic diagnosis, F = 1.82 (=1/0.55), for the premature failure at steel joints, which have insufficient strength to develop yielding at gross area of cross-section of members.
The possible failure mode detected in the calculation of these indices in seismic diagnosis is also helpful to determine where to strengthen the structure in the planning of upgrading. Figure 5 shows a schematic flow diagram of planning for rehabilitation and/or upgrading. In Japan, a national law to promote upgrading of the seismic resistance of public buildings became effective on 25 December 1995. This law strongly recommends and encourages public organizations to carry out seismic diagnosis and to upgrade the seismic resistance of buildings for public use. Accordingly, a technical manual 6 for upgrading steel buildings was published by JBDPA (the Japan Building Disaster Prevention Association) and JSSC (the Japan Society of Steel Construction) in January 1996, in which several structural details for upgrading are recommended.
Assumepresentstate based on inspection
Assumepossibleactions and improvedstructuraldetails ~ Assume 1) Reduceweight improved 2) Repairdamagedportions state 3) Strengthen 4) Increasedeformability 5) Add members
J
SeismicDiagnosis Calculate Is (Seismicperformanceindex)
.. °
. °-'''°'''"
Failuremodei n f o r m . a t i o i / ~
SYe,!w • m-I ~~u%m~;ndl~ad
"
er"
Act2: s
Figure5 Flow diagram of planning for rehabilitation and/or upgrading
Seismic diagnosis for rehabilitation and upgrading of steel gymnasiums: K. Ohi and K. Takanashi • Heavy damage •Moderate damage : Slight damage o Nu structural damage
• Heavy damage •Moderate damage -, Slight damage o No structural damage
1'0 f 0.9 ~" 0.8 "~ 0.7 0.6 - ~
1.0 0.9~ 0.8
z~ I - 07 s-
• 0"71 ~ ,~= .= O.b l- X ~ s = 0 . 7
]Low possibility of collapse
-
o
• •
O
~
0.4I~ " 03~..~. 1,=0.3
4. C o r r e l a t i o n b e t w e e n c a l c u l a t e d indices a n d a c t u a l d a m a g e levels The indices calculated for steel educational facilities in the pre-damaged state by the 1995 version are plotted with actual damage levels on the F-q.St plane and Is-q.St plane in Figures 6 and 7, respectively. In the 1995 version, the following criterion is adopted for judgment on the possibility of collapse and the need for strengthening or upgrading:
(1) Is>=0.7
and q.St>=0.25 The possibility of collapse is low. Urgent actions are not needed, but s;trengthening or upgrading to Is> = 1.0 is recommended. (2) I s < 0 . 3 or q.St < 0.125 The possibility of collapse is high. Urgen actions including demolition and rebuilding are needed. (3) Otherwise the possibility of collapse is not negligible and strengthening or upgrading is needed.
Domains corresponding to these judgments are also indicated on the F-q.St plane and the Is-q.St plane in Figures 6 and 7. Most damaged cases are in the domain with Is < 0.7, but the correlation between the index and the damage level is weak. The main reason is that the F-factors specified in the 1995 version do not change clearly according to deformability and no value less than 1.82 is specified. The following improvements with respect to the F-factor are proposed and checked using the same data: (a) in the case of premature joint failure due to insufficient
Is = 0.7
0.4
0.3 b 0.2
~,9
l
,•
o ,
q'St = 0A25
0.1 -High possibi.lity of eollaps.e 0.6 0.9 1.2 Is (Seismic p e r f o r m a n c e index)
Figure 7 S t r e n g t h i n d e x a n d s e i s m i c p e r f o r m a n c e on the 1995 version diagnosis
. q.st=o:5 I 3.5
4.0
Figures 8 and 9 show the results obtained using the improved F-factors and it is found that the correlation between the judgment domain and the damage levels is better than that based on the 1995 version. Of course, three factors: (i) the definitions of damage levels; (ii) the seismic diagnosis method; and (iii) the criterion for judgment on the need for strengthening or upgrading, shall be discussed together in relation to the improvement of the process of upgrading. The above-mentioned proposal for the F-factors is made under the assumption that the existing definitions of damage levels, the judgment criterion and the framework of the seismic diagnosis method are adopted as they are. 5.
Conclusion and future research needs
The results of a correlation study are presented, where calculated seismic performance indices are compared with actual levels of damage observed in steel or partially steel educational facilities observed after the great HanshinAwaji earthquake. These results are interpreted and used to facilitate increases in the reliability of the seismic diagnosis method that can be used in the upgrade planning with respect to seismic resistance of existing facilities as well as the rehabilitation planning for damaged facilities. In order to strengthen the correlation between the results of seismic
0.9-
"~ 0.8 -
index based
0.7 1.0 -
0.6-
~
q-St = 0.125 0.3
-.
strength, the F-factor is decreased to 1.30 from 1.82 in the 1995 version; (b) in the case of brace yielding without joint failure, the F-factor is increased to 2.20 from 2.00 in the 1995 version.
..
o
q'St=0.25
~
[
• Heavy damage •Moderate damage "Slight damage oNe structural damage
Low possibility of collapse
0.6 =~ 05 -
. ~
Figure8 Strength index and d e f o r m a b i l i t y index based on i m p r o v e d diagnosis
• Heavy damage • Moderate damage zxSlight damage eNd structural damage
I s = 0.3
.
0.1 V High possibility of collapse 01 I I I I 1.0 1.5 2.0 2.5 3.0 F (Deformability index)
4.0
1995 v e r s i o n d i a g n o s i s
"~ 0.8 _
Q o
02 t-
Figure 6 S t r e n g t h i n d e x a n d d e f o r m a b i l i t y i n d e x b a s e d o n t h e
1.0 0.9
Low possibility of~ollapse
o.5 -
O
0.4o • v q.St = 0.25 r~ 0.3 ~ . ~ . ~ 0 . 3 • t~ • ~. 0.2 ~ " " ~ - - . - . . a . . _ ~ zx •• q-St = 0.125 0.1 -High possibility of collapse 0 J I I I I 1.0 1.5 2.0 2.5 3.0 3.5 F (Deformability index)
0.7 -
537
Low possibility of collapse
Is• = 0.3
•oO
o
0.5-
--
Is = 0.7
0.4-
••
0.3 -
• •
o
•o
o
•o
q.St = 0.25
0.2 . ~•~• • q.St = 0.125 0.1 -High possibility of c o l l a p ] ~ _ _ ~ 0
0.3
0.6 0.9 1.2 Is ( S e i s m i c p e r f o r m a n c e index)
1.5
Figure 9 Strength index and seismic p e r f o r m a n c e index based on i m p r o v e d diagnosis
538
Seismic diagnosis for rehabilitation and upgrading o f steel gymnasiums: K. Ohi and K. Takanashi
Figure 13 Cracks at welded beam-to-column joint
Figure 10 Cracks of R/C bracket supporting truss beam Figure 14 Falling of pre-cast concrete roof beams
Figure 11 Breaking at the joint of circular tube braces
Figure 15 Displacement of concrete posts supporting gymnasium floor
Figure 12 Angle braces broken near gusset plate
diagnosis and the damage levels anticipated, the following modifications of the current deformability index, the F-factor, are recommended: (a) the F-factor corresponding to joint failure shall be reduced; (b) the F-value corresponding to brace yielding without joint failure shall be increased. The proposed improvements have been fully reflected in the 1996 revision of the Standard of Seismic Diagnosis for Steel Gymnasiums (MESSC5-9607) 7 and the standard is now being used in the planning of upgrading of seismic resistance of steel gymnasiums at schools. Further research on the following items is needed, based on the damage
Seismic diagnosis for rehabilitation and upgrading of steel gymnasiums: K. Ohi and K. Takanashi observed after the great Hanshin-Awaji earthquake and other destructive earthquakes not only for steel gymnasiums, but also for other types of steel buildings. (1) The current seismic provisions in the Building Standard Law of Japan require the level of Co = 1.0 as a standard value for earthquake resistant design of new buildings. The uncertainty due to possible differences between the design and the actual structure may be reduced in the case of seismic diagnosis based on inspection, and then the target index of structural seismic resistance adopted in planning of rehabilitation and/or upgrading milght be smaller than 1.0, as 0.7 is assumed in this paper. On the other hand, for gymnasiums and other buildings for public use, an appropriate importance factor as a refuge after a future destructive earthquake shall be considered in the target index. (2) Structural details recommended for upgrading are not always based on data obtained in experiments. Experimental studies are needed to check the seismic performance of recommended structural details, for instance, by sub-structure pseudo-dynamic tests under severe earthquake conditions.
Acknowledgements The authors gratefully acknowledge Professors K. Inoue, M. Tada, M. Nakashima, T. Nakamura, K. Kawaguchi, K.
539
Morita, K. Kohzu and Dr Kubodera, the members of Steel Structure Working Group of the Subcommittee on Seismic Performance of School Buildings, AlL for their efforts to collect the data on structural damage in the MESSC advisory mission.
References 1 Subcommittee on Seismic Performance of School Buildings, '1995 Report to MESSC on seismic performance of educational facilities,' Architectural Institute of Japan, Tokyo, March 1995 2 Ohi, K., Takanashi, K. et al. 'An advisory mission for repair planning of steel educational facilities damaged by the great Hanshin-Awaji earthquake,' SE1SAN-KENKYU, 1995, 47( 11 ), 542-545 3 Japan Building Disaster Prevention Association, 'Recommendation for damage level identification and rehabilitation of steel buildings damaged by earthquakes,' Tokyo, February 1991 4 Subcommittee on Upgrading of Seismic Resistance of Steel School Buildings, '1990 Report to MESSC on upgrading of seismic resistance of steel school buildings,' Architectural Institute of Japan, Tokyo, March 1990 5 ISO/TC98/SC2/WGI, 15th draft of Revision of ISO-2394: 'General principles on reliability for structures,' May 1995 6 Japan Building Disaster Prevention Association and Japan Society of Steel Construction, 'A construction manual for upgrading of seismic resistance of steel buildings,' Giho-do, Tokyo, February 1996 7 Ministry of Education, Science and Culture, Japanese Government, 'Standard of seismic diagnosis for steel gymnasiums (MESSC59607),' July 1996
Plh S0141-0296(97)00017-5
ELSEVIER
20, Nos 4-6, pp. 533-539, 1998 © 1997 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0141-0296/98 $19.00 + 0.00
Seismic diagnosis for rehabilitation and upgrading of steel gymnasiums K. O h i Institute of Industrial Science, University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo 106, Japan
K. T a k a n a s h i Faculty of Engineering, Chiba University, 1-33 Yayoi-cho, lnage-ku, Chiba 263, Japan
In this paper we describe an improvement of an existing method of seismic diagnosis for steel gymnasiums used in Japan. This improvement is made so that the results of diagnosis match the actual seismic damage of steel educational facilities (mostly gymnasiums) caused by the great Hanshin-Awaji earthquake, which occurred on 17 January 1995. The strategies for rehabilitation and upgrading including decision making as to which structure, where in the structure and how much to improve, are determined in consideration of the results of seismic diagnosis for the present (or damaged) state as well as the retrofitted state of the structure. In this paper, seismic performance indices are calculated for the pre-damaged state of the educational facilities by use of the existing seismic diagnosis method and compared with the actual damage observed. An index of seismic performance should have strong negative correlation with the damage level observed in the facilities after severe earthquakes. The index used herein is calculated from two other indices, one for load carrying capacity and one for deformability. The latter index is similar in its role to the R-factor in the US and the q-factor in the European seismic regulations. By revising the manner of calculation of the deformability index only, the correlation between the seismic performance index and the damage level has been much improved. © 1997 Elsevier Science Ltd.
Keywords: steel frame, seismic diagnosis, damage level, retrofit planning, upgrading planning
1.
were school gymnasiums and made recommendations to local governments and engineers in charge of rehabilitation planning as to whether the damaged facilities should be rehabilitated or demolished. L2 During this advisory mission, the working group also collected documents related to the design of these facilities. In this paper we propose improving the existing seismic diagnosis method through comparison between actual damage levels and the results of seismic diagnosis. Such a technique of seismic diagnosis is applied not only to the present state of the structure including the damaged state, but also to the upgraded or retrofitted state. The results and the process of such a diagnosis provide information helpful in the decision-making in the planning of rehabilitation and upgrading of existing structures.
Introduction
After the great Hanshin-Awaji earthquake, MESSC (the Ministry of Education, Science, Sports and Culture, Japanese Government) commissioned AIJ (the Architectural Institute of Japan) to coordinate a research program on seismic performance of educational facilities during this earthquake. University professors were also asked to participate in this program, to inspect damaged educational facilities and to advise on technical matters for development of a rehabilitation plan for these facilities. Two working groups were organized to carry out this research project: one focused on reinforced concrete structures and the other on steel structures. The latter group inspected about 30 steel or partially steel educational facilities, about half of which
533
534
Seismic diagnosis for rehabilitation and upgrading of steel gymnasiums: K. Ohi and K. Takanashi
2. Observed damage of steel or partially steel educational facilities The number of steel or partially steel facilities analyzed during the advisory mission, 26, is about one-tenth of the number of reinforced concrete (R/C) school buildings, about 240. Most of the steel or partially steel facilities which were damaged are relatively old, having been built in the 1960s and 1970s, as shown in Figure 1. It should be noted that the Japanese national seismic regulations, the Enforcement Order of Building Standard Law, were revised in 1981, the same year in which checking of the ultimate strength to withstand severe earthquakes and application of special strength requirements to the design of steel joints to ensure the yielding of the members at gross-section were initiated. The steel educational facilities built in the 1980s are scarcely damaged. The following types of damage were observed: ( 1) Concrete cracks and loosened anchor bolts at the joints between steel members and R/C portions including footings were frequently observed, as shown in Photo 1. (2) Breaking at brace joints was observed in steel gymnasiums as shown in Photos 2 and 3. (3) Cracks or breaking at steal beam-to-column welded joints in a middle-rise steel building used for classrooms was observed as shown in Photo 4. (This type of damage was not observed in steel gymnasiums.) (4) Pre-cast concrete roof beams fell down as shown in Photo 5. (This is out of the scope of this paper, because it was not a steel structure, but the most dangerous damage related to gymnasiums.) (5) Non-structural damage such as falling of ceilings and displacement of floor-supporting posts were observed as shown in Photo 6. The observed damage was classified into five levels termed 'complete collapse', 'heavy damage', 'moderate damage', 'slight damage', and 'no structural damage', according to the existing classification rule for damage levels after earthquakes 3. These terms were defined mainly based on the magnitude of the permanent set of story drift angle and the significance of other structural failures detected by inspection, such as breaking at joints and buckling of members. Table 1 shows the correspondence of the terms for damage levels with the drift angle and the structural failure detected. When the permanent set of story drift angle and the structural failure correspond to different damage levels, the higher level is adopted as a rule. For example, the damage Frequency I0 No structural damage
IllfllIT[ITI
1960-1969
1970-1979
1 1
1980-1989
Slight damage Moderate damage Heavy damage
After 1989
Year when built
Figure 1 Frequency of damage levels and year when built
level is recognized as 'moderate damage' if breaking at one joint is detected, while the permanent set of drift angle remains around 1/150. If the permanent set of story drift angle exceeds 1/30, that is, the level of 'heavy damage' is assigned to the structure, its rehabilitation is usually considered difficult due to both technical and economic reasons. When the permanent set exceeds 1/20, the advisory group recommended demolition of the structure without further investigation or cost study. Twenty-two facilities, 16 damaged and 6 undamaged, are dealt with in this paper. The frequencies for each damage level are plotted against the year when built and the use of the facilities in Figures 1 and 2, respectively, and the locations of the facilities are shown in Figures 3 and 4, together with their damage levels.
3. Seismic diagnosis method for rehabilitation and/or upgrade planning Some local governments, as well as some associations in Japan related to disaster prevention or building engineering, have prepared documents to specify methods of seismic diagnosis for existing buildings. During the advisory mission, the seismic diagnosis method t developed by MESSC in 1995, which is based on the recommendation 4 made by AIJ to MESSC in 1990, was used to analyze the damaged steel educational facilities in rehabilitation planning for them. This version of seismic diagnosis is referred to as the 1995 version hereafter. The procedure recommended by the MESSC advisory group is very similar to the procedure described in Section 8.5, 'Assessment in the case of damage', of the 15th draft 5 of the revision of ISO-2394, except that a deterministic seismic diagnosis is adopted instead of probabilistic reliability assessment. In the seismic diagnosis methods developed recently, an index of seismic performance denoted by Is is comonly calculated based on the actual state of existing buildings. This index is equivalent to the ratio of the elastic base-shear that a building can withstand with inelastic deformation, but no fatal damage to the required elastic base-shear. In the case of a single-story structure, the index is expressed by Is = Eo/(Z. Rt. Co)
( 1)
where Eo is an index of structural seismic resistance given by equation (2). This index is equivalent to the elastic baseshear coefficient that the structure can withstand with inelastic deformation, but no fatal damage; (Z. Rt. Co) is the required elastic base-shear coefficient, or the load effect stipulated in the code; Z is the zone factor considering seismic activity. The values 0.8-1.0 are assigned in Japan, except Okinawa. For most of Honshu Island including the Hanshin-Awaji district, Z = 1.0 is assigned; Rt is the design response spectrum normalized to 1.0 at maximum; Co is the reference value of elastic base-shear coefficient. The current seismic regulation requires Co = 1.0 for newly designed buildings. Thus, the standard value of (Z. Rt. Co) is 1.0 for low-rise buildings such as gymnasiums located in the zone of highest seismicity. In this standard case, Is = Eo. Eo = Qu . F / W
(2)
where Qu is the ultimate load carrying capacity of the struc-
Seismic diagnosis for rehabilitation and upgrading of steel gymnasiums: K. Ohi and K. Takanashi
535
Table 1 Damage level
Permanent set of drift angle
Structural failure detected Cannot resist gravity any more
Complete collapse Heavy damage
exceeds 1/30"
More than 50% of braces broken More than 20% of beam-to-column joints broken Severe local buckling
Moderate damage
exceeds 1/100
Brace broken Bean-to-column joint broken Local buckling
Slight damage
around 1/150
Yielding Buckling of slender braces Slipping at high-strength bolt friction joint
No structural damage
None
No damage in main load-carrying system
*The MESSC advisory grouo recommended demolition of steel gyms without further investigation if a permanent set greater than 1/20 is detected.
Type of use
I No structuraldamage
I
Slight damage Moderate damage Heavy damage
1 1 Classrooms
Gymnasiums
/ 0
1
2
3
4
5
6
7
8
9
10
11
12
Frequency
//
j•ik///e•6
Heavy damage &Moderatedamage A ASlight damage ONe structuraldamage
/
,/~/
(
(
Figure 2 Frequency of damage levels and type of use
Figure4 Steel educational facilities analyzed in Awaji area
A
o
O
o
A ASlight damage @No structuraldamage Figure3 Steel educational facilities analyzed in Hanshin area ture in terms of base shear strength; F is an index of deformability or inelastic" energy absorption capacity, which has a similar meaning to the R-factor in the US or the q-factor in the European seismic regulations. It should be noted that the R-factor and the q-factor are applied to the yield strength of a frame when the first plastic hinge is formed, but the F-factor is applied to the ultimate strength of the frame at its collapse mechanism. Thus, usually smaller values are assigned to the F-factor compared with the R or q-factor. W is the weight of the structure.
q = ls/(F.St)
or
q.St = Qu/(W.Z. Rt. Co)
(3)
In the standard case that (Z. Rt. Co) is equal to 1.0, q.St = Q u / W where q is the index of load carrying capacity. The term q.St is equal to the shear coefficient capacity in the standard case and this index is used to control the minimum shear coefficient capacity denoted by St. The value of 0.25 is assigned to St for steel structures.
536
Seismic diagnosis for rehabilitation and upgrading of steel gymnasiums: K. Ohi and K. Takanashi
The F-factor is specified with respect to the type of failure as well as deformability of a structural element. In the case that structural elements with different F-values are supposed to fail, the averaged value with the weight of contribution to Qu is assumed to represent the system F-factor. F-factors specified in the 1995 version are mostly the inverse of the Ds factor (0.25-0.5) stipulated in the current seismic regulations for newly designed buildings. The largest value of F is then 4.0 (=1/0.25) for a moment resisting frame with very ductile members. While the maximum value of Ds-factor is 0.5 (that is, F = 2.0) in designing steel structures, but the 1995 version specifies slightly stringent value in seismic diagnosis, F = 1.82 (=1/0.55), for the premature failure at steel joints, which have insufficient strength to develop yielding at gross area of cross-section of members.
The possible failure mode detected in the calculation of these indices in seismic diagnosis is also helpful to determine where to strengthen the structure in the planning of upgrading. Figure 5 shows a schematic flow diagram of planning for rehabilitation and/or upgrading. In Japan, a national law to promote upgrading of the seismic resistance of public buildings became effective on 25 December 1995. This law strongly recommends and encourages public organizations to carry out seismic diagnosis and to upgrade the seismic resistance of buildings for public use. Accordingly, a technical manual 6 for upgrading steel buildings was published by JBDPA (the Japan Building Disaster Prevention Association) and JSSC (the Japan Society of Steel Construction) in January 1996, in which several structural details for upgrading are recommended.
Assumepresentstate based on inspection
Assumepossibleactions and improvedstructuraldetails ~ Assume 1) Reduceweight improved 2) Repairdamagedportions state 3) Strengthen 4) Increasedeformability 5) Add members
J
SeismicDiagnosis Calculate Is (Seismicperformanceindex)
.. °
. °-'''°'''"
Failuremodei n f o r m . a t i o i / ~
SYe,!w • m-I ~~u%m~;ndl~ad
"
er"
Act2: s
Figure5 Flow diagram of planning for rehabilitation and/or upgrading
Seismic diagnosis for rehabilitation and upgrading of steel gymnasiums: K. Ohi and K. Takanashi • Heavy damage •Moderate damage : Slight damage o Nu structural damage
• Heavy damage •Moderate damage -, Slight damage o No structural damage
1'0 f 0.9 ~" 0.8 "~ 0.7 0.6 - ~
1.0 0.9~ 0.8
z~ I - 07 s-
• 0"71 ~ ,~= .= O.b l- X ~ s = 0 . 7
]Low possibility of collapse
-
o
• •
O
~
0.4I~ " 03~..~. 1,=0.3
4. C o r r e l a t i o n b e t w e e n c a l c u l a t e d indices a n d a c t u a l d a m a g e levels The indices calculated for steel educational facilities in the pre-damaged state by the 1995 version are plotted with actual damage levels on the F-q.St plane and Is-q.St plane in Figures 6 and 7, respectively. In the 1995 version, the following criterion is adopted for judgment on the possibility of collapse and the need for strengthening or upgrading:
(1) Is>=0.7
and q.St>=0.25 The possibility of collapse is low. Urgent actions are not needed, but s;trengthening or upgrading to Is> = 1.0 is recommended. (2) I s < 0 . 3 or q.St < 0.125 The possibility of collapse is high. Urgen actions including demolition and rebuilding are needed. (3) Otherwise the possibility of collapse is not negligible and strengthening or upgrading is needed.
Domains corresponding to these judgments are also indicated on the F-q.St plane and the Is-q.St plane in Figures 6 and 7. Most damaged cases are in the domain with Is < 0.7, but the correlation between the index and the damage level is weak. The main reason is that the F-factors specified in the 1995 version do not change clearly according to deformability and no value less than 1.82 is specified. The following improvements with respect to the F-factor are proposed and checked using the same data: (a) in the case of premature joint failure due to insufficient
Is = 0.7
0.4
0.3 b 0.2
~,9
l
,•
o ,
q'St = 0A25
0.1 -High possibi.lity of eollaps.e 0.6 0.9 1.2 Is (Seismic p e r f o r m a n c e index)
Figure 7 S t r e n g t h i n d e x a n d s e i s m i c p e r f o r m a n c e on the 1995 version diagnosis
. q.st=o:5 I 3.5
4.0
Figures 8 and 9 show the results obtained using the improved F-factors and it is found that the correlation between the judgment domain and the damage levels is better than that based on the 1995 version. Of course, three factors: (i) the definitions of damage levels; (ii) the seismic diagnosis method; and (iii) the criterion for judgment on the need for strengthening or upgrading, shall be discussed together in relation to the improvement of the process of upgrading. The above-mentioned proposal for the F-factors is made under the assumption that the existing definitions of damage levels, the judgment criterion and the framework of the seismic diagnosis method are adopted as they are. 5.
Conclusion and future research needs
The results of a correlation study are presented, where calculated seismic performance indices are compared with actual levels of damage observed in steel or partially steel educational facilities observed after the great HanshinAwaji earthquake. These results are interpreted and used to facilitate increases in the reliability of the seismic diagnosis method that can be used in the upgrade planning with respect to seismic resistance of existing facilities as well as the rehabilitation planning for damaged facilities. In order to strengthen the correlation between the results of seismic
0.9-
"~ 0.8 -
index based
0.7 1.0 -
0.6-
~
q-St = 0.125 0.3
-.
strength, the F-factor is decreased to 1.30 from 1.82 in the 1995 version; (b) in the case of brace yielding without joint failure, the F-factor is increased to 2.20 from 2.00 in the 1995 version.
..
o
q'St=0.25
~
[
• Heavy damage •Moderate damage "Slight damage oNe structural damage
Low possibility of collapse
0.6 =~ 05 -
. ~
Figure8 Strength index and d e f o r m a b i l i t y index based on i m p r o v e d diagnosis
• Heavy damage • Moderate damage zxSlight damage eNd structural damage
I s = 0.3
.
0.1 V High possibility of collapse 01 I I I I 1.0 1.5 2.0 2.5 3.0 F (Deformability index)
4.0
1995 v e r s i o n d i a g n o s i s
"~ 0.8 _
Q o
02 t-
Figure 6 S t r e n g t h i n d e x a n d d e f o r m a b i l i t y i n d e x b a s e d o n t h e
1.0 0.9
Low possibility of~ollapse
o.5 -
O
0.4o • v q.St = 0.25 r~ 0.3 ~ . ~ . ~ 0 . 3 • t~ • ~. 0.2 ~ " " ~ - - . - . . a . . _ ~ zx •• q-St = 0.125 0.1 -High possibility of collapse 0 J I I I I 1.0 1.5 2.0 2.5 3.0 3.5 F (Deformability index)
0.7 -
537
Low possibility of collapse
Is• = 0.3
•oO
o
0.5-
--
Is = 0.7
0.4-
••
0.3 -
• •
o
•o
o
•o
q.St = 0.25
0.2 . ~•~• • q.St = 0.125 0.1 -High possibility of c o l l a p ] ~ _ _ ~ 0
0.3
0.6 0.9 1.2 Is ( S e i s m i c p e r f o r m a n c e index)
1.5
Figure 9 Strength index and seismic p e r f o r m a n c e index based on i m p r o v e d diagnosis
538
Seismic diagnosis for rehabilitation and upgrading o f steel gymnasiums: K. Ohi and K. Takanashi
Figure 13 Cracks at welded beam-to-column joint
Figure 10 Cracks of R/C bracket supporting truss beam Figure 14 Falling of pre-cast concrete roof beams
Figure 11 Breaking at the joint of circular tube braces
Figure 15 Displacement of concrete posts supporting gymnasium floor
Figure 12 Angle braces broken near gusset plate
diagnosis and the damage levels anticipated, the following modifications of the current deformability index, the F-factor, are recommended: (a) the F-factor corresponding to joint failure shall be reduced; (b) the F-value corresponding to brace yielding without joint failure shall be increased. The proposed improvements have been fully reflected in the 1996 revision of the Standard of Seismic Diagnosis for Steel Gymnasiums (MESSC5-9607) 7 and the standard is now being used in the planning of upgrading of seismic resistance of steel gymnasiums at schools. Further research on the following items is needed, based on the damage
Seismic diagnosis for rehabilitation and upgrading of steel gymnasiums: K. Ohi and K. Takanashi observed after the great Hanshin-Awaji earthquake and other destructive earthquakes not only for steel gymnasiums, but also for other types of steel buildings. (1) The current seismic provisions in the Building Standard Law of Japan require the level of Co = 1.0 as a standard value for earthquake resistant design of new buildings. The uncertainty due to possible differences between the design and the actual structure may be reduced in the case of seismic diagnosis based on inspection, and then the target index of structural seismic resistance adopted in planning of rehabilitation and/or upgrading milght be smaller than 1.0, as 0.7 is assumed in this paper. On the other hand, for gymnasiums and other buildings for public use, an appropriate importance factor as a refuge after a future destructive earthquake shall be considered in the target index. (2) Structural details recommended for upgrading are not always based on data obtained in experiments. Experimental studies are needed to check the seismic performance of recommended structural details, for instance, by sub-structure pseudo-dynamic tests under severe earthquake conditions.
Acknowledgements The authors gratefully acknowledge Professors K. Inoue, M. Tada, M. Nakashima, T. Nakamura, K. Kawaguchi, K.
539
Morita, K. Kohzu and Dr Kubodera, the members of Steel Structure Working Group of the Subcommittee on Seismic Performance of School Buildings, AlL for their efforts to collect the data on structural damage in the MESSC advisory mission.
References 1 Subcommittee on Seismic Performance of School Buildings, '1995 Report to MESSC on seismic performance of educational facilities,' Architectural Institute of Japan, Tokyo, March 1995 2 Ohi, K., Takanashi, K. et al. 'An advisory mission for repair planning of steel educational facilities damaged by the great Hanshin-Awaji earthquake,' SE1SAN-KENKYU, 1995, 47( 11 ), 542-545 3 Japan Building Disaster Prevention Association, 'Recommendation for damage level identification and rehabilitation of steel buildings damaged by earthquakes,' Tokyo, February 1991 4 Subcommittee on Upgrading of Seismic Resistance of Steel School Buildings, '1990 Report to MESSC on upgrading of seismic resistance of steel school buildings,' Architectural Institute of Japan, Tokyo, March 1990 5 ISO/TC98/SC2/WGI, 15th draft of Revision of ISO-2394: 'General principles on reliability for structures,' May 1995 6 Japan Building Disaster Prevention Association and Japan Society of Steel Construction, 'A construction manual for upgrading of seismic resistance of steel buildings,' Giho-do, Tokyo, February 1996 7 Ministry of Education, Science and Culture, Japanese Government, 'Standard of seismic diagnosis for steel gymnasiums (MESSC59607),' July 1996