Glendale (PDF) - Hazard Mitigation Web Portal - State of California

Natural Hazards Mitigation PlanCity of Glendale, CaliforniaSection 6 – Earthquakesstructures may have unreinforced masonry such as bricks, clay tiles and terra cotta tiles as claddingor infilling. Cladding in newer buildings may be glass, infill panels or pre-cast panels that may failand generate a band of debris around the building exterior (with considerable threat to pedestrians inthe streets below). Structural damage may occur if the structural members are subject to plasticdeformation which can cause permanent displacements. If some walls fail while others remain intact,torsion or soft-story problems may result. Overall, modern steel frame buildings have been expectedto perform well in earthquakes, but the 1994 Northridge earthquake broke many welds in thesebuildings, a previously unanticipated problem.Buildings are often a combination of steel, concrete, reinforced masonry and wood, with differentstructural systems on different floors or different sections of the building. Combination types thatare potentially hazardous include: concrete frame buildings without special reinforcing, precastconcrete and precast-composite buildings, steel frame or concrete frame buildings with unreinforcedmasonry walls, reinforced concrete wall buildings with no special detailing or reinforcement, largecapacity buildings with long-span roof structures (such as theaters and auditoriums), largeunengineered wood-frame buildings, buildings with inadequately anchored exterior cladding andglazing, and buildings with poorly anchored parapets and appendages (FEMA, 1985). Additionaltypes of potentially hazardous buildings may be recognized after future earthquakes.Mobile homes are prefabricated housing units that are placed on isolated piers, jackstands, ormasonry block foundations (usually without any positive anchorage). Floors and roofs of mobilehomes are usually plywood, and outside surfaces are covered with sheet metal. Mobile homestypically do not perform well in earthquakes. Severe damage occurs when they fall off theirsupports, severing utility lines and piercing the floor with jackstands.In addition to building types, there are other factors associated with the design and construction ofthe buildings that also have an impact on the structures’ vulnerability to strong ground shaking.Some of these conditions are discussed below:• Building Shape - A building’s vertical and/or horizontal shape can be important. Simple,symmetric buildings generally perform better than non-symmetric buildings. During anearthquake, non-symmetric buildings tend to twist as well as shake. Wings on a buildingtend to act independently during an earthquake, resulting in differential movements andcracking. The geometry of the lateral load-resisting systems also matters. For example,buildings with one or two walls made mostly of glass, while the remaining walls are made ofconcrete or brick, are at risk. Asymmetry in the placement of bracing systems that provide abuilding with earthquake resistance, can result in twisting or differential motions.• Pounding - Site-related seismic hazards may include the potential for neighboring buildingsto "pound," or for one building to collapse onto a neighbor. Pounding occurs when there islittle clearance between adjacent buildings, and the buildings "pound" against each other asthey deflect during an earthquake. The effects of pounding can be especially damaging if thefloors of the buildings are at different elevations, so that, for example, the floor of onebuilding hits a supporting column of the other. Damage to a supporting column can result inpartial or total building collapse.Damage to the region’s critical facilities and infrastructure need to be considered and planned for.Critical facilities are those parts of a community's infrastructure that must remain operational afteran earthquake. Critical facilities include schools, hospitals, fire and police stations, emergencyoperation centers, and communication centers. Plate H-12 shows the locations of the City’s firestations, police stations, schools, and other critical facilities. A vulnerability assessment for thesefacilities involves comparing the locations of these facilities to the hazardous areas identified in the2006 PAGE 6 - 32