Continuous and Redundant Load Paths

DOE-STD-1020-2002

floor and shear wall or braced frame construction on upper floors (e.g., Olive View Hospital, San

Fernando, CA earthquake, 1971 and Imperial County Services Building, Imperial Valley, CA

earthquake, 1979). In addition, adjacent structures should be separated sufficiently so that they

do not hammer one another during seismic response.

Continuous and Redundant Load Paths - Earthquake excitation induces forces at all

points within structures or equipment of significant mass. These forces can be vertical or along

any horizontal (lateral) direction. Structures are most vulnerable to damage from lateral

seismic-induced forces, and prevention of damage requires a continuous load path (or paths)

from regions of significant mass to the foundation or location of support. The designer/evaluator

must follow seismic-induced forces through the structure (or equipment or distribution systems)

into the ground and make sure that every element and connection along the load path is adequate

in strength and stiffness to maintain the integrity of the system. Redundancy of load paths is a

highly desirable characteristic for earthquake-resistant design. When the primary element or

system yields or fails, the lateral forces can be redistributed to a secondary system to prevent

progressive failure. In a structural system without redundant components, every component

must remain operative to preserve the integrity of the structure. It is good practice to incorporate

redundancy into the seismic-resisting system rather than relying on any system in which distress

in any member or element may cause progressive or catastrophic collapse.

In some structures, the system carrying earthquake-induced loads may be separate from

the system that carries gravity loads. Although the gravity load carrying systems are not needed

for lateral resistance, they would deform with the rest of the structure as it deforms under lateral

seismic loads. To ensure that it is adequately designed, the vertical load carrying system should

be evaluated for compatibility with the deformations resulting from an earthquake. Similarly,

gravity loads should be combined with earthquake loads in the evaluation of the lateral force

resisting system.

Detailing For Ductile Behavior - In general, for earthquakes that have very low

probability of occurrence, it is uneconomical or impractical to design structures to remain within

the elastic range of stress. Furthermore, it is highly desirable to design structures or equipment

in a manner that avoids low ductility response and premature unexpected failure such that the

structure or equipment is able to dissipate the energy of the earthquake excitation without

unacceptable damage. As a result, good seismic design practice requires selection of an

appropriate structural system with detailing to develop sufficient energy absorption capacity to

limit damage to permissible levels.

Structural steel is an inherently ductile material. Energy absorption capacity may be

achieved by designing connections to avoid tearing or fracture and to ensure an adequate path for

a load to travel across the connection. Detailing for adequate stiffness and restraint of

compression braces, outstanding legs of members, compression flanges, etc., must be provided to

avoid instability by buckling for relatively slender steel members acting in compression.

Furthermore, deflections must be limited to prevent overall frame instability due to P-delta

effects. SAC document recommendations of AISC following Northridge earthquake must be

complied with.

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