1812 Structural Damage

Every day, regions of high seismicity experience many small earthquakes. However, structural damage does not usually occur until the magnitude approaches 5.0. Most structural damage during earthquakes is caused by the failure of the surrounding soil or from strong shaking. Damage also results from surface ruptures, the failure of nearby lifelines, or the collapse of more vulnerable structures. We consider these effects secondary because they are not always present during an earthquake. However, when there is a long surface rupture such as that which accompanied the 1999 Ji Ji, Taiwan, earthquake, secondary effects can dominate.

Since damage can mean anything from minor cracks to total collapse, categories of damage have been developed as shown in Table 18.1. These levels of damage give engineers a choice for the performance of their structure during earthquakes. Most engineered structures are designed only to prevent collapse. This is not only to save money, but also because as a structure becomes stronger it attracts larger forces. Thus, most structures are designed to have sufficient ductility to survive an earthquake. This means that elements will yield and deform but they will be strong in shear and continue to support their load during and after the earthquake.

As shown in Table 18.1, the time that is required to repair damaged structures is an important parameter that weighs heavily on the decision making process. When a structure must be quickly repaired or must remain in service, a different damage state should be chosen.

During large earthquakes the ground is jerked back and forth, causing damage to the element whose capacity is furthest below the earthquake demand. Figure 18.3 shows that the cause may be the supporting soil, the foundation, weak flexural or shear elements, or secondary hazards such as surface

TABLE 18.1 Categories of Structural Damage

Damage state

Functionality

Repairs required

Expected outage

None (preyield) (1)

No loss

None

None

Minor/slight (2)

Slight loss

Inspect, adjust, patch

<3 days

Moderate (3)

Some loss

Repair components

<3 weeks

Major/extensive (4)

Considerable loss

Rebuild components

< 3 months

Complete/collapse (5)

Total loss

Rebuild structure

>3 months

Soil Foundation Flexure Shear 1

damage damage damage damage Faulting

FIGURE 18.3 Common types of damage during large earthquakes.

Soil Foundation Flexure Shear 1

damage damage damage damage Faulting

FIGURE 18.3 Common types of damage during large earthquakes.

faulting or failure of a nearby structure. Damage also frequently occurs due to the failure of connections, from large torsional moments, from tension and compression, buckling, pounding, etc.

In this chapter structural damage as a result of soil problems, structural shaking, and secondary causes will be discussed. These types of damage illustrate the most common structural hazards that have been seen during recent earthquakes.

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