Analytical Treatment of Energy Dissipation and Absorption

DOE-STD-1020-2002

Uncertainties. There are uncertainties in the soil properties and parameters used for

SSI analysis. Therefore, a relatively wide variation of soil properties is recommended such that

a conservative structure response calculation may be expected. An acceptable method to account

for uncertainties in the SSI analysis, as given in ASCE 4-98 Section 3.3.1.7, is that the soil shear

modulus shall be varied between the best estimate value times , and the best estimate value

divided by (1+Cv), where Cv is a coefficient of variation. In general, a (1+Cv) value of about 1.0

covers uncertainties in soil properties. If there are sufficient site soils data, it is permissible to

evaluate soil property uncertainty by probabilistic techniques. The minimum value of Cv shall be

0.50.

C.4.4

Analytical Treatment of Energy Dissipation and Absorption

Earthquake ground shaking is a limited energy transient loading, and structures have

energy dissipation and absorption capacity through damping and through hysteretic behavior

during inelastic response. This section discusses simplified methods of accounting for these

modes of energy dissipation and absorption in seismic response analyses.

C.4.4.1 Damping

Damping accounts for energy dissipation in the linear range of response of structures

and equipment to dynamic loading. Damping is a term utilized to account for various

mechanisms of energy dissipation during seismic response such as cracking of concrete, slippage

at bolted connections, and slippage between structural and nonstructural elements. Damping is

primarily affected by:

Type of construction and materials used.

The amount of nonstructural elements attached.

The earthquake response strain levels.

Damping increases with rising strain level as there are increased concrete cracking and

internal work done within materials. Damping is also larger with greater amounts of

nonstructural elements (interior partitions, etc.) in a structure that provide more opportunities for

energy losses due to friction. For convenience in seismic response analyses, damping is

generally assumed to be viscous in nature (velocity-dependent) and is so approximated.

Damping is usually considered as a proportion or percentage of the critical damping value,

which is defined as that damping which would prevent oscillation in a system excited by an

initial perturbation.

Chapter 2 reports typical structural damping values for various materials and

construction (Refs. C-5, C-15, C-16, and C-17) for three different response levels. Response

Level 3 values correspond to strains beyond yielding of the material, and, they are recommended

for usage along with other provisions of this document for design or evaluation seismic response

analyses of Performance Category 3 and higher SSCs. Post-yielding damping values are judged

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