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FIGURE 9.5 Schematic of Division 88 URM retrofitting techniques for URM.

FIGURE 9.5 Schematic of Division 88 URM retrofitting techniques for URM.

Furthermore, Division 88 required that unreinforced masonry be positively anchored to floor and roof diaphragms with anchors spaced not more than 6 ft apart. There were also parapet height limitations, based on wall thickness. Continuous inspection was also required on the retrofitting work. These retrofitting measures are shown schematically in Figure 9.5.

Alternatives to these specific provisions were also possible. Division 88 was renamed Chapter 88 in the 1988 City of Los Angeles Code. In addition to masonry bearing walls, veneer walls constructed before October 6, 1933, were included. This edition also added Section 8811 ("Design Check — Compatibility of Roof Diaphragm Stiffness to Unreinforced Masonry Wall Out-of-Plane Stability'').

At the time of the Northridge Earthquake, it is believed that essentially all URM buildings in the city of Los Angeles had their parapets either removed or laterally braced. Unconfirmed reports indicate that in the city of Los Angeles, about 80% of URM buildings had been retrofitted to comply with Division 88; however, the percentage was reported to be considerably lower in other cities in the Los Angeles area.

9.5.4.2 Other Retrofitting Guidelines

The NEHRP, in conjunction with the FEMA, has produced a series of documents dealing with the seismic evaluation and retrofitting of structures, including masonry structures:

1. FEMA 172 (1992), Handbook of Techniques for the Seismic Rehabilitation of Existing Buildings, provides a general list of retrofitting techniques.

2. FEMA 178 (1992) presents an overall method for engineers to identify buildings or building components that present unacceptable risks in case of an earthquake.

3. FEMA 273 (1997) and 274 (1997), NEHRP Guidelines and Commentary for the seismic rehabilitation of buildings, provide code-type procedures for the assessment, evaluation, analysis, and rehabilitation of existing building structures.

4. FEMA 306 (1998), Evaluation of Earthquake-Damaged Concrete and Masonry Wall Buildings, Basic Procedures Manual, provides guidance on evaluation of damage and on performance analysis and includes newly formulated Component Damage Classification Guides and Test and Investigation Guides. The procedures characterize the observed damage caused by an earthquake in terms of the loss in building performance capability.

5. FEMA 307 (1998), Evaluation of Earthquake-Damaged Concrete and Masonry Wall Buildings, Technical Resources, contains supplemental information, including results from a theoretical analysis of the effects of prior damage on single-degree-of-freedom mathematical models, additional background information on the Component Damage Classification Guides, and an example of the application of the basic procedures.

6. FEMA 308 (1998), The Repair of Earthquake-Damaged Concrete and Masonry Wall Buildings, discusses the technical and policy issues pertaining to the repair of earthquake-damaged buildings and includes guidance on the specification of individual repair techniques and newly formulated Repair Guides.

7. FEMA 356 (2000), Prestandard and Commentary for the Seismic Rehabilitation of Buildings, is an attempt to encourage the use of FEMA 273 and to put the guidelines of that document into mandatory language.

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