Primary Author: Andrew Charleson (February 2011)

Introduction

This section reviews several existing structural taxonomies as part of the process of developing a taxonomy for the GEM project. Before commencing the review it must be noted that each building type has to be defined adequately to satisfy the information and analysis requirements of GEM. Therefore the structural taxonomy is just one of several taxonomies that together will contain all the relevant data about a particular building type. For example, as well as the pivotal structural taxonomy, other taxonomies need to cover issues related to; general building information, non-structural elements, occupancy type and intensity, construction aspects, retrofit work, etc., and even photographs and/or drawings. Given the possibility of extending GEM beyond buildings to other built forms there will need to be another and in fact an initial taxonomy that can be expanded to include not only buildings but bridges, tunnels, dams, wharves, tanks, towers, and other non-building construction.

The taxonomies reviewed below cover just structural aspects. They have in general been developed to describe and classify building structures in terms of seismic resistance and response. They have all been developed since 1985 and can be expected to have improved upon earlier similar taxonomies. Note that the taxonomies have been presented in chronological sequence.

The approach taken in this section follows that of Porter (2005) whose review of existing taxonomies focuses on those detailing with nonstructural components. In his review, Porter evaluates each taxonomy against a range of criteria in order to identify the most appropriate taxonomy to build upon.

The primary requirements of the proposed structural taxonomy are listed below. Table 1 shows the extent to which each taxonomy meets the criteria. Following that graphical summary, each of the reviewed taxonomies are briefly commented upon.

Requirements of the Proposed Structural Taxonomy

The criteria by which the existing taxonomies are assessed are listed below:

1. Distinguishes differences in seismic performance. The taxonomy distinguishes earthquake-resistant versions of structural systems from non-earthquake resistant version, including the " before" and "after" states of common seismic retrofits and between ductile and non-ductile systems.

2. Observable. Two people examining the same structural system in the field or using data obtained from the field should independently assign the same taxonomic group based solely on the text definition of the taxonomic group.

3. Complete. The taxonomy must include all engineering features relevant to the global seismic performance of a building structure. As mentioned above it is recognized that there will be a need for additional taxonomies to capture all aspects of the seismic performance and losses for an entire building, including building dimensions and nonstructural components. The structural taxonomy must contain sufficient attributes to meet the needs of the end users of GEM.

4. Simple and collapsible. The taxonomy should have as few groups as possible, while still meeting the other requirements. It is also desirable to define common combinations and relative quantities of structural systems so that fragility or vulnerability functions can be created by aggregating the fragilities or vulnerabilities of detailed components, while still distinguishing say, differences in ductility, design or retrofit alternatives. A taxonomy is judged to be collapsible if taxonomic groups can be combined and the resulting combinations still distinguish differences in seismic performance.

5. Nearly exhaustive. Within practical limits almost every structural system can be sensibly assigned to a taxonomic group.

6. Familiar to engineering practitioners and architects. It is desirable that engineers and architects be familiar with the taxonomic system, particularly to readily and accurately identify structural attributes. If the new taxonomic system corresponds readily to an existing taxonomic system, it can give users access to existing data. Engineers and architects should be familiar with the nomenclature to be defined to avoid ambiguity.

7. Currently treats non-buildings. Built forms other than buildings need to be included in the taxonomy sometime in the future. These include constructions like dams, bridges and tunnels.

8. Extensible to other hazards. In the future the GEM model may include other natural hazards such as floods, hurricanes and volcanic eruptions.

9. User-friendly. The taxonomy should be straightforward, intuitive, and as easy to use as possible by both those collecting data, those arranging for its analysis and the end users.

10. International in scope. As far as possible the taxonomy should be appropriate for any region of the world. It should not privilege any one region but be technically and culturally acceptable to all regions.

A comparison of the existing structural taxonomies is presented in Table 1.

The value of this comparison is to identify the taxonomy with the greatest potential for development in order to satisfy GEM requirements. If a simple scoring system is used the SYNER-G taxonomy emerges as the one with the greatest potential to be further developed.

Comments and Discussion on the Reviewed Taxomonies

ATC-13 (ATC 1985)

• A pioneering effort to develop a facility classification scheme for California, including engineering classification and social function classification.
• Key engineering characteristics considered in developing the classification include construction material, soil conditions, foundation, height, structural framing system, configuration, structural continuity, design and construction quality, age, and proximity to other structures.
• Engineering classification contains 78 classes of structures, 40 of which are buildings and 38 are other structure types (bridges, storage tanks, towers, etc.); 11 structure categories contain two or three height ranges. It would be advantageous to uncouple height from the structural taxonomy.
• Not collapsible.
• Uses a labelling scheme which consists of letters and symbols (slash "/" and dash "-") to identify facility classes
• California-focused and embedded assumptions that are often not valid nor relevant internationally (similar to HAZUS).

FEMA 154 (FEMA 1988)

• One of the advantages of FEMA 154 is its simplicity, consisting of only 15 structure types. However, the disadvantage is that most of the structure type definitions are too broad. For example, there are only 2 classes for wood buildings, 5 classes for steel buildings, 3 classes for reinforced concrete, 2 classes for precast concrete, and 3 classes for masonry buildings.
• Most classes address only the vertical structural system - type of diaphragm (rigid/flexible) was considered only for reinforced masonry buildings.
• Description of structural classes is very detailed and includes illustrations of structural systems and their components, which is very helpful for sidewalk surveys of buildings.
• US-focused.

EMS-98 (Grunthal 1998)

• One of the advantages of EMS-98 is its simplicity, consisting of only 15 structure types. However, the disadvantage is that most of the structure type definitions are too broad.
• Only variation in the seismic performances of RC frames and walls are able to be distinguished. They are defined as "without earthquake resistant design", "with moderate level of earthquake resistant design", and "with high level of earthquake resistant design".
• All steel and timber structures are covered under a single type which does not give the opportunity to distinguish between say ductile and non-ductile steel structures.

WHE (EERI 2000)

• The World Housing Encyclopedia database captures far more than structural information about a building. Amongst the information it holds about a building includes, architectural, socio-economic, vulnerability, construction, insurance and strengthening aspects.
• Detailed structural information can be selected from 14 house construction types and 45 sub types. Gravity and lateral load resisting systems can be independently assigned to a building.
• There are 20 options for floors and roofs, and 18 for foundations.
• Some structural types without seismic resisting features, like RC frames, are itemized but others like shear walls and braced frames are not, so there is a certain lack of rigour. Also, as designed it is not collapsible.
• Quite a lot of the information entered in a descriptive manner rather than through pick lists.
• One very attractive feature is that it contains photographs of each building type.
• It contains a lot of nonstructural information pertinent to seismic performance this is the most comprehensive of all building taxonomies.

Coburn and Spence (2002)

• Divided into non-engineered and engineered buildings. So not clear where pre-engineered buildings fall.
• Building types are listed beginning with the most vulnerable through to those least vulnerable.
• Many vulnerability parameters, other than the main structural classification and building type are listed, but are not included in the classification. These parameters need to be included in the final taxonomy.

HAZUS (FEMA 2003)

• Building types are based on the classification system of FEMA 178 (FEMA 1992) and the classes are divided into height ranges.
• Contains 36 structural categories in total, including 9 with three height ranges to choose from (low-rise, mid-rise and high-rise). It would be advantageous to uncouple height from the structural taxonomy and capture it in a general building taxonomy.
• Relatively simple but not designed to be collapsible.
• U.S. focussed and embedded assumptions that are often not valid nor relevant internationally. For example, assumptions made of concrete strengths and ductility capabilities are based on U.S. conditions.
• Some materials and construction technologies are missing, e.g. earthen or stone construction.
• Extending the taxonomy to include, for example, configuration aspects and revealing assumptions like the degree of ductility etc. would require many more structural types, making the taxonomy very cumbersome.

CEZ (CEZ 2003)

• Developed for the Californian insurance industry with an emphasis on fire performance.
• No differentiation between gravity and lateral load resisting systems.
• Does not capture the main factors that predict seismic performance.
• U.S. construction types only.

Gunel and Ilgin (Gunel and Ilgin 2007)

• For modern high-rise buildings only
• Six structural systems form the classification system of which five are not included in any other taxonomies. Just three materials including composite (RC + steel) construction

PAGER-STR (Jaiswal and Wald 2008)

• Most comprehensive taxonomy developed to date
• Captures most of the key structural aspects that affect seismic performance but there are some missing. For example, factors like concrete strength, provision of ductile detailing, and configuration irregularities are very important in predicting seismic behaviour. To some extent the way it differentiates between say, ductile and non-ductile frames, it makes allowance in a generic fashion for the factors above.
• Simple and collapsible.
• International coverage. It contains a breadth of structural types that are found outside the more developed countries.
• Difficulty in extending the taxonomy. If it is desirable to be more specific about ductility and configuration issues then the number of possible structural types increases rapidly and the taxonomy quickly becomes cumbersome.
• The more modern structural systems, like RC structures are subdivided into three building heights; 1-3 story, 4-7 story and 8+ story. It is possible to simplify the taxonomy if the building height or number of stories can be uncoupled from the structural taxonomy and entered in another table or facet, say under the heading of "General Building Information".

SYNER-G (2011)

• This taxonomy for the European region is currently under development by a number of collaborators.
• The only taxonomy reviewed that is non-hierarchical. Currently it consists of fifteen facets or lists of categories. The number of facets will need to be increased in order to capture all the vulnerabilities and other data which GEM requires, but this can be easily achieved.
• The existing structure of the database would benefit from some reorganisation.
• The taxonomy with the potential for greatest degree of completeness and the most flexibility.

CEQID (Lee, Pomonis, So, and Spence, 2011)

• The Cambridge Earthquake Impact Database (CEQID) which contains damage data from more than 70 studies covering more than 600 locations in 53 earthquakes occurred in the 20th century, has accumulated almost 300 building classes in its system.
• The building class descriptions in the CEQID include the following parameters: i) main construction material (e.g. adobe, brick, reinforced concrete); ii) structural system (e.g. steel moment resisting frame, shear wall); secondary attribute details (e.g. walls, floors, roofs); age or age reference (e.g. 1941-56, pre-1941, post-1976, pre-code, modern code); height (e.g. 2 to 3 floors, 4 to 10 stories), and occupancy type (e.g. rural, residential).
• Across all regions, the current building classification in CEQID exhibits three types of inconsistencies:

1. Inconsistencies across the format of the building class label, or how the descriptor components are shorthanded;
2. Inconsistencies between building class descriptions and building class labels; and
3. Inconsistencies in how the descriptor components are delimited.

Summary

Several relevant structural classification systems were each rated for their suitability for the GEM project. The SYNER-G taxonomy is considered the most suitable given the inclusion of all the features that GEM requires. Some additional structural types and other factors affecting seismic performance may need to be added and minor reorganisation of the order of the facets and the contents within them is required.

References

ATC, 1985. ATC-13, Earthquake Damage Evaluation Data for California, Applied Technology Council, Redwood City, CA, 492 pp.

CEZ, 2003. California Earthquake Zoning and Probable Maximum Loss Evaluation Program: An Analysis of Potential Insured Earthquake Losses from Questionnaires Submitted to The California Department of Insurance by Licensed Property/Casualty Insurers in California for 1997 to 2001. California Department of Insurance, Los Angeles, California.

Coburn, A. W. and Spence, R., 2002. Earthquake Protection. John Wiley & Sons.

EERI, 2000. World Housing Encyclopedia. www.world-housing.net [viewed 10 January 2011].

FEMA, 1988. FEMA 154: Rapid Visual Screening of Buildings for Potential Seismic Hazards: A Handbook, Federal Emergency Management Agency, Washington, DC (also known as ATC-21) - note second edition published in 2002.

FEMA, 1992. FEMA 178: NEHRP Handbook for the Seismic Evaluation of Existing Buildings. Federal Emergency Management Agency, Washington, DC.

FEMA, 2003. HAZUS-MH MR4 Technical Manual, Federal Emergency Management Agency. http://www.fema.gov/plan/prevent/hazus/hz_manuals.shtm[viewed 10 January 2011]

Grunthal, G. (ed.), 1998. European Macroseismic Scale 1998 (EMS-98). Cahiers du Centre Europeen de Geodynamique et de Seismologie 15, Centre Europeen de Geodynamique et de Seismologie, Luxembourg.

Gunel, M. H. and Ilgin, H. E., 2007. A Proposal for the Classification of Structural Systems of Tall Buildings. Building and Environment, Elsevier, Vol.42, p.2667-2675.

Jaiswal, K.S., and Wald, D.J., 2008. Creating a Global Building Inventory for Earthquake Loss Assessment and Risk Management, U.S. Geological Survey Open-File Report 2008-1160, 103 p.

Lee,W.V., Pomonis, A., So, E., and Spence, R. (2011). Existing Building Stock Classification in the Cambridge Earthquake Impact Database (CEQID)., GEM Foundation, GEM Technical Report 2011-X, 23 pp.

Porter, K. A., 2005. A taxonomy of building components for performance-based earthquake engineering, Pacific Earthquake Engineering Research Center, PEER 2005/03, Berkeley, Calif, 58 p.

SYNER-G, 2011. SYNER-G Taxonomy (under development). Systemic Seismic Vulnerability and Risk Analysis for Buildings, Lifeline Networks and Infrastructures Safety Gain programme.