How do you select and apply dynamic load factors for different types of soil-structure interaction?

1 DLF definition and purpose

A DLF is a ratio of the peak response of a structure with SSI to the peak response of the same structure without SSI. A DLF can be greater than one, meaning that SSI increases the dynamic response, or less than one, meaning that SSI decreases the dynamic response. The purpose of using DLFs is to simplify the analysis and design of structures subject to dynamic loading by incorporating the effects of SSI into equivalent static loads or displacements.

2 DLF classification and sources

DLFs can be classified into two main categories: kinematic and inertial. Kinematic DLFs account for the relative motion between the soil and the structure due to the propagation of seismic waves through the soil. Inertial DLFs account for the interaction between the soil and the structure due to the inertia forces generated by the structural vibration. DLFs can be obtained from different sources, such as analytical solutions, numerical methods, experimental tests, or empirical formulas. The choice of the source depends on the accuracy, complexity, and availability of the DLFs for a given SSI problem.

3 DLF selection criteria

The selection of DLFs for a specific SSI problem depends on several factors, such as the type and shape of the structure, the soil properties and conditions, the frequency and direction of the dynamic loading, and the design codes and standards. Generally, kinematic DLFs should be used for rigid or flexible structures in relation to soil stiffness, while inertial DLFs should be used for flexible or slender structures in relation to soil mass. For structures with both rigid and flexible components, combined kinematic and inertial DLFs should be used. Additionally, it is important to select DLFs that are compatible with the soil model and loading model used in the analysis or design of the structure, as well as those that are consistent with applicable design codes and standards.

4 DLF application methods

When applying DLFs to a SSI problem, the method chosen depends on the form and sign of the DLFs as well as the type of analysis or design performed. If the DLFs are positive and greater than one, multiply dynamic loads or displacements by the DLFs to get equivalent static loads or displacements. If the DLFs are positive and less than one, divide structural stiffness or damping by the DLFs to get equivalent reduced stiffness or damping. When the DLFs are negative, use their absolute value with opposite signs and apply them as described above. For complex DLFs, use the real part for equivalent static loads or displacements, and use the imaginary part for equivalent reduced stiffness or damping.

5 DLF examples and references

To illustrate how to select and apply DLFs for different types of SSI problems, here are some examples and references to consult. Gazetas et al. (2016) provides kinematic DLFs for piles and foundations under seismic loading, while Veletsos and Meek (1974) covers inertial DLFs for buildings under seismic loading. Kavvadas and Gazetas (1993) offers combined kinematic and inertial DLFs for pile-supported structures under seismic loading, Prakash and Puri (1988) provides DLFs for retaining walls under seismic loading, and Mylonakis et al. (1997) gives DLFs for embedded structures under seismic loading.

6 Here’s what else to consider

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