NIST GCR 10-917-6 Concrete Model Building Subtypes Recommended for Use in Collecting Inventory Data
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Concrete Model Building Subtypes Recommended for Use in Collecting Inventory Data
Disclaimers: The policy of the National Institute of Standards and Technology is to use the International System of Units (metric units) in all of its publications. However, in North America in the construction and building materials industry, certain non-SI units are so widely used instead of SI units that it is more practical and less confusing to include measurement values for customary units only. Any opinions, findings, conclusions, or recommendations expressed in this publication do not necessarily reflect the views of the National Institute of Standards and Technology. Additionally, neither NIST nor any of its employees make any warranty, expressed or implied, nor assume any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, product, or process included in this publication. This report was prepared under Contract SB1341-07-SE1029 between the National Institute of Standards and Technology and the National Institute of Building Sciences. The statements and conclusions contained in this report are those of the authors and do not imply recommendations or endorsements by the National Institute of Standards and Technology.
NIST GCR 10-917-6
Concrete Model Building Subtypes Recommended for Use in Collecting Inventory Data
Prepared for:
The National Institute of Standards and Technology Building and Fire Research Laboratory
Gaithersburg, Maryland 20899-8600
Contract SB1341-07-SE1029
May 2010
National Institute of Standards and Technology
Patrick D. Gallagher, Director
Table of Contents Executive Summary .................................................................................................................... 7 Chapter 1, Introduction ................................................................................................................ 9 Background .......................................................................................................................... 9 Purpose of This Study ......................................................................................................... 10 Chapter 2, Development of Subtypes for Older Concrete Buildings ........................................ 13 Definition of Older Concrete Buildings .............................................................................. 13 Process Used in This Study ................................................................................................. 14 Background of Use of Model Building Types and Other Inventory Classification Systems ....................................................................... 14 Relevant Concrete Building Attributes ............................................................................... 17 Chapter 3, Conclusions and Recommendations ........................................................................ 19 General ................................................................................................................................ 19 Basic Inventory Data........................................................................................................... 19 Enhanced Inventory ............................................................................................................ 20 Attributes Requiring Engineering Review/Evaluation ....................................................... 20 Summary of Recommendations .......................................................................................... 20 Chapter 4, Description of Recommended Occupancy Groups .................................................. 23 General ................................................................................................................................ 23 Occupancy Descriptions ..................................................................................................... 23 Warehouse/Manufacturing ........................................................................................... 23 Small Commercial ........................................................................................................ 24 Large Commercial ........................................................................................................ 25 Residential Dormitory (Apartment/Condominium) ..................................................... 25 Residential Hotel/Motel ............................................................................................... 26 Residential Mid-/High-Rise ......................................................................................... 26 Institutional/Monumental/Academic ............................................................................ 27 Parking Garage ............................................................................................................. 27 Assembly ...................................................................................................................... 28 Essential Services ......................................................................................................... 28 Hospital ........................................................................................................................ 28 Schools K-12 ................................................................................................................ 29 Other ............................................................................................................................. 29 Chapter 5, Description of Typical Structural System Families ................................................. 31 General ................................................................................................................................ 31 Structural Frame Families ................................................................................................... 31 Concrete Moment Frame Family (C1) ......................................................................... 31 Concrete Shear Wall (Bearing Wall Systems) Family (C2b) ....................................... 33 Concrete Shear Wall (Gravity Frame Systems) Family (C2f) ..................................... 35 Concrete Frames with Infill Masonry Shear Walls Family (C3) .................................. 37 Precast Concrete Family (PC) ...................................................................................... 39 Chapter 6, Targeted Performance-Based Design Research Suggested by This Study .............. 43
6 Concrete Model Building Subtypes
References ................................................................................................................................. 45 Study Participants ...................................................................................................................... 47 List of Tables Table 1, Level of Effort to Obtain Relevant Concrete Building Attributes ............................... 18 Table 2, Recommended Attributes for Collection of Concrete Building Inventory Data ......... 22
Concrete Model Building Subtypes 7
EXECUTIVE SUMMARY This study was conducted by the National Institute of Building Sciences’ Building Seismic Safety Council (BSSC). The goal was to develop standardized categories for older concrete buildings that incorporate more detail than those included in the standard set of Federal Emergency Management Agency (FEMA) Concrete Model Building Types – namely, C1-frames, C2-shear walls, C3-concrete frames with masonry infill, and PC2- precast frames. The recommended “subtypes” generally can be identified without extensive engineering evaluation and therefore can document the inventory of older concrete buildings in an effort to better quantify how many structures might require mitigation. The identification of these subtypes also contributes to determination of the highest priority research needed to enable rapid implementation of performance-based seismic design for mitigation. For initial collection of inventory data, building age, height, and occupancy should be documented. Data on additional attributes that affect risk of high damage levels or collapse would require engineering evaluation and cannot be collected reliably using typical inventory-collection techniques; however, additional attributes that may be important for identifying a high-risk concrete building are discussed. The primary attribute suggested for initial classification is occupancy and 13 different occupancies are recommended. The structural systems typically, but not exclusively, used for each occupancy are described as are potential seismic deficiencies. Also described are the many variations that can affect the risk levels of each of the FEMA Concrete Model Building Types (C1, C2, C3, and PC2) including strength, stiffness, configuration irregularities, and gravity frame type. These descriptions are organized into Structural Type Families based on the four primary model building types. Without further understanding of the collapse mechanisms of older concrete buildings, building subtypes that can reliably distinguish seismic risk levels cannot be established.
8 Concrete Model Building Subtypes
Concrete Model Building Subtypes 9
Chapter 1 INTRODUCTION
Background As a class, older concrete buildings are likely to include a significant number of buildings at risk of significant damage and collapse in earthquake shaking. Certain combinations of configuration characteristics, coupled with the potential for brittle behavior in key elements, can produce these collapse conditions. This group of buildings generally is considered to be second only to unreinforced masonry bearing wall (URM) buildings as a high seismic risk in both the United States and worldwide. Risk mitigation programs for older concrete buildings have lagged behind those aimed at URM buildings, possibly because this building category is less homogeneous. The variation in configuration and framing systems in older concrete buildings results in a class of buildings that presents a wide range of risk. The building type is difficult to identify. Further, methods to efficiently identify high-risk members of the type are not currently available. The cost of typical retrofit procedures and the associated disruption are perceived to be much greater than for the URM class of buildings. Recently, however, interest in the development of risk mitigation programs for older concrete buildings has grown, spurred by their consistently poor performance in other countries and knowledge that the category includes buildings with high occupancies and/or importance in this country. The Concrete Coalition (Comartin, 2008) was formed by the Earthquake Engineering Research Institute (EERI) to promote further understanding of these buildings and to encourage mitigation programs. The National Science Foundation’s NEES Research Program awarded a Grand Challenge to the Pacific Earthquake Engineering Research Center (PEER, 2006) to increase basic understanding of the building type. Interest in this building type also is demonstrated by the formation and activity of the first American Concrete Institute (ACI) committee (ACI 369) to consider older concrete buildings. The first step in achieving mitigation of the risk from this building type is to improve understanding of the existing conditions as reflected by the total number of buildings in the class, the uses and occupancies of those buildings, and the range of configurations and structural systems employed. Accordingly, collection of inventory data is a first step for both the PEER Grand Challenge (with collection of inventory in Los Angeles) and the Concrete Coalition (with collection of inventory in California’s high seismic regions for the State of California). Insurance industry and emergency management efforts to classify buildings in order to estimate risk and losses have been under way for more than 35 years (FEMA 1989; FEMA 1994); however, more attention has been paid to this subject since the mid-1980s. A multitude of classification schemes exist but there is no agreed-upon standard. The design of classification schemes depends on available data sources, regional construction characteristics, and ultimate use of the data as well as the resources available to collect and
10 Concrete Model Building Subtypes
analyze the data. Although each scheme has some distinct features and different numbers of classes, all the schemes generally are based on similar assumptions and, as such, have many commonalities. Existing model building type schemes typically attempt to characterize the variation in performance (or damage) of different structures subjected to earthquake loading. Classification schemes include factors such as structural system, construction materials, building size or height, and the extent to which seismic loads and detailing have been included in design and construction. However, it is important to note that in all schemes the initial classification can be made without specific-building level engineering evaluation. The Federal Emergency Management Agency (FEMA) developed the classification system currently in most widespread use as part of its overall program to mitigate the seismic risk posed by existing buildings and has been formalized in several FEMA publications (FEMA, 1992; FEMA, 1994; FEMA, 2002). This set of structural descriptions, called Model Building Types (MBT), is similar to other systems previously used. It also was adopted for use in HAZUS, FEMA’s loss-estimation software (FEMA, 1994). Unfortunately, however, the system provides only a very coarse separation of concrete building types that does not adequately distinguish between levels of risk among buildings or allow a reliable categorization of individual buildings using economical inventory collection methods. A subgrouping of older concrete buildings that both identifies relative life safety risk and allows for collection of inventory data without engineering evaluation would offer an important and efficient first step to communities that want to reduce their seismic risk. A basic assumption for this study is that the identification of concrete building subtypes should not require building-specific engineering evaluation. A better understanding of the characteristics of the existing inventory of older concrete buildings also will inform research necessary to advance the accuracy and usefulness of performance-based design in this area. Purpose of This Study The purpose of this study is described in the project scoping statement as follows:
Older concrete buildings are generally considered the most dangerous building type as a group, but the class includes a wide variety of structural systems and configurations, not all of which are in the high-risk category. To achieve effective and efficient mitigation, the building class must be broken down into smaller subclasses that present more consistent seismic risk and that can be more readily identified. The identification of these subclasses will also allow determination of the highest priority research to enable rapid implementation of performance-based seismic design for mitigation. The goal of this effort is to identify a sub-grouping system for older concrete buildings and to provide more robust information about the specific sub-building types that need to be researched.
Efforts to inventory older concrete buildings to date have not used any standard system to subcategorize the group and, in many cases, have not subcategorized the buildings at all.
Concrete Model Building Subtypes 11
This project seeks to balance the relatively small level of effort needed to collect basic inventory (at the lowest level, the number of buildings) with the larger effort and cost of drawing review and engineering evaluation – ranging on the lowest level from FEMA 154 (FEMA, 2002) to the highest level represented by methods in ASCE 31 (ASCE, 2003) and ASCE 41 (ASCE, 2006).
12 Concrete Model Building Subtypes
Concrete Model Building Subtypes 13
Chapter 2 DEVELOPMENT OF SUBTYPES FOR
OLDER CONCRETE BUILDINGS Definition of Older Concrete Buildings For the purpose of this study, the age of “older concrete buildings” is defined by the “milestone” code years given in Table 3 of ASCE 31, Seismic Evaluation of Existing Buildings (ASCE, 2003). This table lists the editions of the various U.S. building codes that can be considered to yield seismically adequate buildings needing no further evaluation. For concrete buildings, the key provisions first appeared in the 1976 Uniform Building Code (ICBO) and equivalent editions of other U.S. codes used are identified in the ASCE 31 table. Thus, “older concrete buildings” in this study are taken as those that were designed to earlier codes and are referred to in this study as “pre-milestone” designs. Concrete has been used alone in U.S. buildings in many configurations, including a wide variety of both gravity and lateral systems, as well as in combination with structural steel and wood. Characteristics of older inadequately reinforced concrete buildings that make them, as a class, high risk are:
• Brittle failure modes and/or cyclic degradation in lateral-force-resisting elements and
• Shear failures that lead to shortening and loss of capacity in a gravity-load-
supporting element. Those buildings that contain concrete but are unlikely to exhibit these failure modes should not be considered part of this class even if it is impossible to make such distinctions while collecting inventory data. In addition, tilt-up buildings (with much of their gravity load supported on independent steel columns and lateral performance typically controlled by diaphragm connections) and composite buildings (with structural steel embedded within concrete columns and beams) should not be considered part of this class. Lift-slab buildings, although not common, are another special case. One dangerous failure mode is loss of support at the steel-column-to-flat-slab connection, which could be controlled by brittle concrete behavior. The lateral system for these buildings is typically a concrete core that, when improperly detailed, can exhibit serious degradation. Therefore, although mostly supported on steel columns, such a building is likely to suffer a brittle concrete failure and should be included in the class. Steel framed buildings with concrete shear walls also are a special case. In pre-milestone buildings, it is expected that shear walls will be minimal and, although the initial response of the lateral system will mirror concrete behavior, the collapse potential is much different. It is recommended that these buildings not be included in an inventory of older concrete
14 Concrete Model Building Subtypes
buildings; however, if local construction practices are known to produce buildings of this type that perform poorly, they can easily be included. Process Used in This Study A small workshop was convened with representatives from the various regions of the country with moderate to high seismicity. Each attendee described the typical types, ages, and characteristics of older concrete buildings in his/her region. Both the PEER Grand Challenge and the Concrete Coalition were represented. Characteristics of concrete buildings including occupancy, height, lateral-force-resisting system, gravity support system, and configuration deficiencies (soft story, torsional layouts) were identified and discussed. Several categorization schemes were assessed to determine their applicability to inventory collection and the potential to differentiate risk. Workshop participants agreed that the characteristics of a given concrete building that may indicate a high risk of collapse typically will not be available during collection of inventory data unless engineering evaluation to at least the level of detail of rapid visual screening (FEMA 154) is employed. Based on the workshop discussions, the general method recommended herein was developed, proposed to the workshop participants, and discussed and refined as described in Chapter 3. Background of Use of Model Building Types and Other Inventory Classification Systems FEMA Model Building Types (MBT) Several sets of standard structural types have been created to describe the building inventory of the United States. These model building type classification systems initially were developed for the purposes of assigning fragility relationships to inventories of buildings for loss estimation in ATC 13 (ATC, 1985). Studies of the existing U. S. inventory during development of ATC 14, Evaluating the Seismic Resistance of Existing Buildings (ATC, 1987) identified a large variety of construction types and subtypes, but identified 15 primary lateral-force-resisting systems that could be used to group evaluation considerations. This classification system included only five types of concrete building: C1 – Concrete moment-resisting frame buildings C2 – Concrete shear wall buildings C3 – Concrete frame buildings with unreinforced masonry infill walls PC1 – Tilt-up buildings (primarily West Coast style tilt-ups with exterior bearing walls) PC2 – Precast concrete frame buildings ATC 14 was later adapted for use in the FEMA series as FEMA 178, NEHRP Handbook for Seismic Evaluation of Buildings (FEMA, 1992a). This set of building types has
Concrete Model Building Subtypes 15
subsequently been used extensively in other FEMA documents related to existing buildings (FEMA 154, 1988; FEMA 227, 1992b; FEMA 156, 1995) and has become known as the FEMA Model Building Types. Recently, the concrete group of buildings was expanded in FEMA 547, Techniques for Seismic Rehabilitation of Existing Buildings (FEMA, 2007), to better differentiate between buildings that are primarily composed of bearing walls and those that have a few walls within a gravity frame structure. Specifically, concrete shear wall buildings (Building Type C2) were split into two groups, those with essentially complete gravity frames (Building Type C2f) and those primarily using bearing walls (Building Type C2b). FEMA 154, Rapid Visual Screening The importance of quickly and efficiently identifying high-risk buildings was formally recognized in FEMA’s original action plan to mitigate risks from existing buildings developed in 1985 (FEMA, 1985). Implementation of that portion of FEMA’s plan resulted in Rapid Visual Screening of Buildings for Potential Seismic Hazards in 1988, updated in 2002 (FEMA, 2002). The method is based on a first-level categorization of a building into a FEMA Model Building Type and a second-level identification of risk characteristics such as age (to determine design code), height, configuration irregularities, and site soil type. The final result is a numerical score intended to be used to rank buildings by relative risk. Although it was originally intended as a screening method that could be completed in the field, the data judged to be minimally necessary for estimation of relative risk for most engineered structures could not be recognized at the site. Except for wood (W1 and W2), tilt-up (PC1) and URM buildings, the identification of model building type, which is dependent on the material and type of lateral-force-resisting system in the building, typically requires review of construction drawings. In addition, vertical and plan configuration irregularities identified in the field often prove to be structurally insignificant. The FEMA 154 process also collects occupancy data that are potentially useful to community planners and risk managers. Occupancy data often are related to structural types, and guidance for these relationships is given in Tables D4 through D7 of the document. The occupancy classes included are assembly, commercial, government, historic, industrial, office, residential, and school. In addition, the tables estimate the number of occupants and notes apparent exterior falling hazards. FEMA 154 has been used extensively both with and without a drawing review but most commonly to collect general inventory information and to gauge a community’s overall risk rather than to identify high-risk individual buildings. The FEMA 154 developers concluded that subcategorization beyond the MBT should be dependent on several variables, the weighting and combining of which required numerical scoring. The need to combine characteristics to identify risk, even on the rapid visual screening level, indicates that the number of subcategories needed to represent all the
16 Concrete Model Building Subtypes
combinations of significant characteristics and deficiencies of concrete buildings would be large and impractical. Further, experience with rapid visual screening indicates that even the most basic structural characteristics of buildings can seldom be identified without a level of effort approaching evaluation (engineering review of drawings and/or detailed field review of each building). HAZUS Emergency operations planners at the local, state, and federal levels use the HAZUS earthquake loss estimation methodology to estimate building damage, casualties, and monetary losses, thus making the HAZUS model building types one of the more widely used classification schemes. Unfortunately, this doesn’t provide any additional insight into the ideal classification scheme for this study because, as indicated in the HAZUS Technical Manual (2003), the HAZUS model building types are based on the FEMA model building types that were described in the previous section. The HAZUS scheme includes the same five concrete classes as the standard FEMA set described above. HAZUS refines the FEMA MBTs by accounting for the effect of building height on structural capacity and response by subdividing buildings into low-rise (1 to 3 stories), mid-rise (4 to 7 stories), and high-rise (8+…
Disclaimers: The policy of the National Institute of Standards and Technology is to use the International System of Units (metric units) in all of its publications. However, in North America in the construction and building materials industry, certain non-SI units are so widely used instead of SI units that it is more practical and less confusing to include measurement values for customary units only. Any opinions, findings, conclusions, or recommendations expressed in this publication do not necessarily reflect the views of the National Institute of Standards and Technology. Additionally, neither NIST nor any of its employees make any warranty, expressed or implied, nor assume any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, product, or process included in this publication. This report was prepared under Contract SB1341-07-SE1029 between the National Institute of Standards and Technology and the National Institute of Building Sciences. The statements and conclusions contained in this report are those of the authors and do not imply recommendations or endorsements by the National Institute of Standards and Technology.
NIST GCR 10-917-6
Concrete Model Building Subtypes Recommended for Use in Collecting Inventory Data
Prepared for:
The National Institute of Standards and Technology Building and Fire Research Laboratory
Gaithersburg, Maryland 20899-8600
Contract SB1341-07-SE1029
May 2010
National Institute of Standards and Technology
Patrick D. Gallagher, Director
Table of Contents Executive Summary .................................................................................................................... 7 Chapter 1, Introduction ................................................................................................................ 9 Background .......................................................................................................................... 9 Purpose of This Study ......................................................................................................... 10 Chapter 2, Development of Subtypes for Older Concrete Buildings ........................................ 13 Definition of Older Concrete Buildings .............................................................................. 13 Process Used in This Study ................................................................................................. 14 Background of Use of Model Building Types and Other Inventory Classification Systems ....................................................................... 14 Relevant Concrete Building Attributes ............................................................................... 17 Chapter 3, Conclusions and Recommendations ........................................................................ 19 General ................................................................................................................................ 19 Basic Inventory Data........................................................................................................... 19 Enhanced Inventory ............................................................................................................ 20 Attributes Requiring Engineering Review/Evaluation ....................................................... 20 Summary of Recommendations .......................................................................................... 20 Chapter 4, Description of Recommended Occupancy Groups .................................................. 23 General ................................................................................................................................ 23 Occupancy Descriptions ..................................................................................................... 23 Warehouse/Manufacturing ........................................................................................... 23 Small Commercial ........................................................................................................ 24 Large Commercial ........................................................................................................ 25 Residential Dormitory (Apartment/Condominium) ..................................................... 25 Residential Hotel/Motel ............................................................................................... 26 Residential Mid-/High-Rise ......................................................................................... 26 Institutional/Monumental/Academic ............................................................................ 27 Parking Garage ............................................................................................................. 27 Assembly ...................................................................................................................... 28 Essential Services ......................................................................................................... 28 Hospital ........................................................................................................................ 28 Schools K-12 ................................................................................................................ 29 Other ............................................................................................................................. 29 Chapter 5, Description of Typical Structural System Families ................................................. 31 General ................................................................................................................................ 31 Structural Frame Families ................................................................................................... 31 Concrete Moment Frame Family (C1) ......................................................................... 31 Concrete Shear Wall (Bearing Wall Systems) Family (C2b) ....................................... 33 Concrete Shear Wall (Gravity Frame Systems) Family (C2f) ..................................... 35 Concrete Frames with Infill Masonry Shear Walls Family (C3) .................................. 37 Precast Concrete Family (PC) ...................................................................................... 39 Chapter 6, Targeted Performance-Based Design Research Suggested by This Study .............. 43
6 Concrete Model Building Subtypes
References ................................................................................................................................. 45 Study Participants ...................................................................................................................... 47 List of Tables Table 1, Level of Effort to Obtain Relevant Concrete Building Attributes ............................... 18 Table 2, Recommended Attributes for Collection of Concrete Building Inventory Data ......... 22
Concrete Model Building Subtypes 7
EXECUTIVE SUMMARY This study was conducted by the National Institute of Building Sciences’ Building Seismic Safety Council (BSSC). The goal was to develop standardized categories for older concrete buildings that incorporate more detail than those included in the standard set of Federal Emergency Management Agency (FEMA) Concrete Model Building Types – namely, C1-frames, C2-shear walls, C3-concrete frames with masonry infill, and PC2- precast frames. The recommended “subtypes” generally can be identified without extensive engineering evaluation and therefore can document the inventory of older concrete buildings in an effort to better quantify how many structures might require mitigation. The identification of these subtypes also contributes to determination of the highest priority research needed to enable rapid implementation of performance-based seismic design for mitigation. For initial collection of inventory data, building age, height, and occupancy should be documented. Data on additional attributes that affect risk of high damage levels or collapse would require engineering evaluation and cannot be collected reliably using typical inventory-collection techniques; however, additional attributes that may be important for identifying a high-risk concrete building are discussed. The primary attribute suggested for initial classification is occupancy and 13 different occupancies are recommended. The structural systems typically, but not exclusively, used for each occupancy are described as are potential seismic deficiencies. Also described are the many variations that can affect the risk levels of each of the FEMA Concrete Model Building Types (C1, C2, C3, and PC2) including strength, stiffness, configuration irregularities, and gravity frame type. These descriptions are organized into Structural Type Families based on the four primary model building types. Without further understanding of the collapse mechanisms of older concrete buildings, building subtypes that can reliably distinguish seismic risk levels cannot be established.
8 Concrete Model Building Subtypes
Concrete Model Building Subtypes 9
Chapter 1 INTRODUCTION
Background As a class, older concrete buildings are likely to include a significant number of buildings at risk of significant damage and collapse in earthquake shaking. Certain combinations of configuration characteristics, coupled with the potential for brittle behavior in key elements, can produce these collapse conditions. This group of buildings generally is considered to be second only to unreinforced masonry bearing wall (URM) buildings as a high seismic risk in both the United States and worldwide. Risk mitigation programs for older concrete buildings have lagged behind those aimed at URM buildings, possibly because this building category is less homogeneous. The variation in configuration and framing systems in older concrete buildings results in a class of buildings that presents a wide range of risk. The building type is difficult to identify. Further, methods to efficiently identify high-risk members of the type are not currently available. The cost of typical retrofit procedures and the associated disruption are perceived to be much greater than for the URM class of buildings. Recently, however, interest in the development of risk mitigation programs for older concrete buildings has grown, spurred by their consistently poor performance in other countries and knowledge that the category includes buildings with high occupancies and/or importance in this country. The Concrete Coalition (Comartin, 2008) was formed by the Earthquake Engineering Research Institute (EERI) to promote further understanding of these buildings and to encourage mitigation programs. The National Science Foundation’s NEES Research Program awarded a Grand Challenge to the Pacific Earthquake Engineering Research Center (PEER, 2006) to increase basic understanding of the building type. Interest in this building type also is demonstrated by the formation and activity of the first American Concrete Institute (ACI) committee (ACI 369) to consider older concrete buildings. The first step in achieving mitigation of the risk from this building type is to improve understanding of the existing conditions as reflected by the total number of buildings in the class, the uses and occupancies of those buildings, and the range of configurations and structural systems employed. Accordingly, collection of inventory data is a first step for both the PEER Grand Challenge (with collection of inventory in Los Angeles) and the Concrete Coalition (with collection of inventory in California’s high seismic regions for the State of California). Insurance industry and emergency management efforts to classify buildings in order to estimate risk and losses have been under way for more than 35 years (FEMA 1989; FEMA 1994); however, more attention has been paid to this subject since the mid-1980s. A multitude of classification schemes exist but there is no agreed-upon standard. The design of classification schemes depends on available data sources, regional construction characteristics, and ultimate use of the data as well as the resources available to collect and
10 Concrete Model Building Subtypes
analyze the data. Although each scheme has some distinct features and different numbers of classes, all the schemes generally are based on similar assumptions and, as such, have many commonalities. Existing model building type schemes typically attempt to characterize the variation in performance (or damage) of different structures subjected to earthquake loading. Classification schemes include factors such as structural system, construction materials, building size or height, and the extent to which seismic loads and detailing have been included in design and construction. However, it is important to note that in all schemes the initial classification can be made without specific-building level engineering evaluation. The Federal Emergency Management Agency (FEMA) developed the classification system currently in most widespread use as part of its overall program to mitigate the seismic risk posed by existing buildings and has been formalized in several FEMA publications (FEMA, 1992; FEMA, 1994; FEMA, 2002). This set of structural descriptions, called Model Building Types (MBT), is similar to other systems previously used. It also was adopted for use in HAZUS, FEMA’s loss-estimation software (FEMA, 1994). Unfortunately, however, the system provides only a very coarse separation of concrete building types that does not adequately distinguish between levels of risk among buildings or allow a reliable categorization of individual buildings using economical inventory collection methods. A subgrouping of older concrete buildings that both identifies relative life safety risk and allows for collection of inventory data without engineering evaluation would offer an important and efficient first step to communities that want to reduce their seismic risk. A basic assumption for this study is that the identification of concrete building subtypes should not require building-specific engineering evaluation. A better understanding of the characteristics of the existing inventory of older concrete buildings also will inform research necessary to advance the accuracy and usefulness of performance-based design in this area. Purpose of This Study The purpose of this study is described in the project scoping statement as follows:
Older concrete buildings are generally considered the most dangerous building type as a group, but the class includes a wide variety of structural systems and configurations, not all of which are in the high-risk category. To achieve effective and efficient mitigation, the building class must be broken down into smaller subclasses that present more consistent seismic risk and that can be more readily identified. The identification of these subclasses will also allow determination of the highest priority research to enable rapid implementation of performance-based seismic design for mitigation. The goal of this effort is to identify a sub-grouping system for older concrete buildings and to provide more robust information about the specific sub-building types that need to be researched.
Efforts to inventory older concrete buildings to date have not used any standard system to subcategorize the group and, in many cases, have not subcategorized the buildings at all.
Concrete Model Building Subtypes 11
This project seeks to balance the relatively small level of effort needed to collect basic inventory (at the lowest level, the number of buildings) with the larger effort and cost of drawing review and engineering evaluation – ranging on the lowest level from FEMA 154 (FEMA, 2002) to the highest level represented by methods in ASCE 31 (ASCE, 2003) and ASCE 41 (ASCE, 2006).
12 Concrete Model Building Subtypes
Concrete Model Building Subtypes 13
Chapter 2 DEVELOPMENT OF SUBTYPES FOR
OLDER CONCRETE BUILDINGS Definition of Older Concrete Buildings For the purpose of this study, the age of “older concrete buildings” is defined by the “milestone” code years given in Table 3 of ASCE 31, Seismic Evaluation of Existing Buildings (ASCE, 2003). This table lists the editions of the various U.S. building codes that can be considered to yield seismically adequate buildings needing no further evaluation. For concrete buildings, the key provisions first appeared in the 1976 Uniform Building Code (ICBO) and equivalent editions of other U.S. codes used are identified in the ASCE 31 table. Thus, “older concrete buildings” in this study are taken as those that were designed to earlier codes and are referred to in this study as “pre-milestone” designs. Concrete has been used alone in U.S. buildings in many configurations, including a wide variety of both gravity and lateral systems, as well as in combination with structural steel and wood. Characteristics of older inadequately reinforced concrete buildings that make them, as a class, high risk are:
• Brittle failure modes and/or cyclic degradation in lateral-force-resisting elements and
• Shear failures that lead to shortening and loss of capacity in a gravity-load-
supporting element. Those buildings that contain concrete but are unlikely to exhibit these failure modes should not be considered part of this class even if it is impossible to make such distinctions while collecting inventory data. In addition, tilt-up buildings (with much of their gravity load supported on independent steel columns and lateral performance typically controlled by diaphragm connections) and composite buildings (with structural steel embedded within concrete columns and beams) should not be considered part of this class. Lift-slab buildings, although not common, are another special case. One dangerous failure mode is loss of support at the steel-column-to-flat-slab connection, which could be controlled by brittle concrete behavior. The lateral system for these buildings is typically a concrete core that, when improperly detailed, can exhibit serious degradation. Therefore, although mostly supported on steel columns, such a building is likely to suffer a brittle concrete failure and should be included in the class. Steel framed buildings with concrete shear walls also are a special case. In pre-milestone buildings, it is expected that shear walls will be minimal and, although the initial response of the lateral system will mirror concrete behavior, the collapse potential is much different. It is recommended that these buildings not be included in an inventory of older concrete
14 Concrete Model Building Subtypes
buildings; however, if local construction practices are known to produce buildings of this type that perform poorly, they can easily be included. Process Used in This Study A small workshop was convened with representatives from the various regions of the country with moderate to high seismicity. Each attendee described the typical types, ages, and characteristics of older concrete buildings in his/her region. Both the PEER Grand Challenge and the Concrete Coalition were represented. Characteristics of concrete buildings including occupancy, height, lateral-force-resisting system, gravity support system, and configuration deficiencies (soft story, torsional layouts) were identified and discussed. Several categorization schemes were assessed to determine their applicability to inventory collection and the potential to differentiate risk. Workshop participants agreed that the characteristics of a given concrete building that may indicate a high risk of collapse typically will not be available during collection of inventory data unless engineering evaluation to at least the level of detail of rapid visual screening (FEMA 154) is employed. Based on the workshop discussions, the general method recommended herein was developed, proposed to the workshop participants, and discussed and refined as described in Chapter 3. Background of Use of Model Building Types and Other Inventory Classification Systems FEMA Model Building Types (MBT) Several sets of standard structural types have been created to describe the building inventory of the United States. These model building type classification systems initially were developed for the purposes of assigning fragility relationships to inventories of buildings for loss estimation in ATC 13 (ATC, 1985). Studies of the existing U. S. inventory during development of ATC 14, Evaluating the Seismic Resistance of Existing Buildings (ATC, 1987) identified a large variety of construction types and subtypes, but identified 15 primary lateral-force-resisting systems that could be used to group evaluation considerations. This classification system included only five types of concrete building: C1 – Concrete moment-resisting frame buildings C2 – Concrete shear wall buildings C3 – Concrete frame buildings with unreinforced masonry infill walls PC1 – Tilt-up buildings (primarily West Coast style tilt-ups with exterior bearing walls) PC2 – Precast concrete frame buildings ATC 14 was later adapted for use in the FEMA series as FEMA 178, NEHRP Handbook for Seismic Evaluation of Buildings (FEMA, 1992a). This set of building types has
Concrete Model Building Subtypes 15
subsequently been used extensively in other FEMA documents related to existing buildings (FEMA 154, 1988; FEMA 227, 1992b; FEMA 156, 1995) and has become known as the FEMA Model Building Types. Recently, the concrete group of buildings was expanded in FEMA 547, Techniques for Seismic Rehabilitation of Existing Buildings (FEMA, 2007), to better differentiate between buildings that are primarily composed of bearing walls and those that have a few walls within a gravity frame structure. Specifically, concrete shear wall buildings (Building Type C2) were split into two groups, those with essentially complete gravity frames (Building Type C2f) and those primarily using bearing walls (Building Type C2b). FEMA 154, Rapid Visual Screening The importance of quickly and efficiently identifying high-risk buildings was formally recognized in FEMA’s original action plan to mitigate risks from existing buildings developed in 1985 (FEMA, 1985). Implementation of that portion of FEMA’s plan resulted in Rapid Visual Screening of Buildings for Potential Seismic Hazards in 1988, updated in 2002 (FEMA, 2002). The method is based on a first-level categorization of a building into a FEMA Model Building Type and a second-level identification of risk characteristics such as age (to determine design code), height, configuration irregularities, and site soil type. The final result is a numerical score intended to be used to rank buildings by relative risk. Although it was originally intended as a screening method that could be completed in the field, the data judged to be minimally necessary for estimation of relative risk for most engineered structures could not be recognized at the site. Except for wood (W1 and W2), tilt-up (PC1) and URM buildings, the identification of model building type, which is dependent on the material and type of lateral-force-resisting system in the building, typically requires review of construction drawings. In addition, vertical and plan configuration irregularities identified in the field often prove to be structurally insignificant. The FEMA 154 process also collects occupancy data that are potentially useful to community planners and risk managers. Occupancy data often are related to structural types, and guidance for these relationships is given in Tables D4 through D7 of the document. The occupancy classes included are assembly, commercial, government, historic, industrial, office, residential, and school. In addition, the tables estimate the number of occupants and notes apparent exterior falling hazards. FEMA 154 has been used extensively both with and without a drawing review but most commonly to collect general inventory information and to gauge a community’s overall risk rather than to identify high-risk individual buildings. The FEMA 154 developers concluded that subcategorization beyond the MBT should be dependent on several variables, the weighting and combining of which required numerical scoring. The need to combine characteristics to identify risk, even on the rapid visual screening level, indicates that the number of subcategories needed to represent all the
16 Concrete Model Building Subtypes
combinations of significant characteristics and deficiencies of concrete buildings would be large and impractical. Further, experience with rapid visual screening indicates that even the most basic structural characteristics of buildings can seldom be identified without a level of effort approaching evaluation (engineering review of drawings and/or detailed field review of each building). HAZUS Emergency operations planners at the local, state, and federal levels use the HAZUS earthquake loss estimation methodology to estimate building damage, casualties, and monetary losses, thus making the HAZUS model building types one of the more widely used classification schemes. Unfortunately, this doesn’t provide any additional insight into the ideal classification scheme for this study because, as indicated in the HAZUS Technical Manual (2003), the HAZUS model building types are based on the FEMA model building types that were described in the previous section. The HAZUS scheme includes the same five concrete classes as the standard FEMA set described above. HAZUS refines the FEMA MBTs by accounting for the effect of building height on structural capacity and response by subdividing buildings into low-rise (1 to 3 stories), mid-rise (4 to 7 stories), and high-rise (8+…
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