2/1/2014 Soil structure interaction - Wikipedia, the free encyclopedia

http://en.wikipedia.org/wiki/Soil_structure_interaction 1/2

Soil structure interactionFrom Wikipedia, the free encyclopedia

Most of the civil engineering structures involve some type of structural element with direct contact with ground.When the external forces, such as earthquakes, act on these systems, neither the structural displacements nor theground displacements, are independent of each other. The process in which the response of the soil influencesthe motion of the structure and the motion of the structure influences the response of the soil is termed as soil-

structure interaction (SSI).[1]

Conventional structural design methods neglect the SSI effects. Neglecting SSI is reasonable for light structuresin relatively stiff soil such as low rise buildings and simple rigid retaining walls. The effect of SSI, however,becomes prominent for heavy structures resting on relatively soft soils for example nuclear power plants, high-

rise buildings and elevated-highways on soft soil.[2]

Damage sustained in recent earthquakes, such as the 1995 Kobe Earthquake, have also highlighted that theseismic behavior of a structure is highly influenced not only by the response of the superstructure, but also by the

response of the foundation and the ground as well.[3] Hence, the modern seismic design codes, such as

Standard Specifications for Concrete Structures: Seismic Performance Verification JSCE 2005 [4] stipulate thatthe response analysis should be conducted by taking into consideration a whole structural system includingsuperstructure, foundation and ground.

Effect of soil structure interaction on structural response

It has conventionally been considered that soil-structure interaction has beneficial effect on the seismic responseof a structure. Many design codes have suggested that the effect of SSI can reasonably be neglected for the

seismic analysis of structures.[5][6] This myth about SSI apparently stems from the false perception that SSIreduces the overall seismic response of a structure, and hence, leads to improved safety margins. Most of thedesign codes use oversimplified design spectra, which attain constant acceleration up to a certain period, andthereafter decreases monotonically with period. Considering soil-structure interaction makes a structure moreflexible and thus, increasing the natural period of the structure compared to the corresponding rigidly supportedstructure. Moreover, considering the SSI effect increases the effective damping ratio of the system. The smoothidealization of design spectrum suggests smaller seismic response with the increased natural periods and effectivedamping ratio due to SSI. With this assumption, it was traditionally been considered that SSI can convenientlybe neglected for conservative design. In addition, neglecting SSI tremendously reduces the complication in theanalysis of the structures which has tempted designers to neglect the effect of SSI in the analysis.

This conservative simplification is valid for certain class of structures and soil conditions, such as light structuresin relatively stiff soil. Unfortunately, the assumption does not always hold true. In fact, the SSI can have adetrimental effect on the structural response, and neglecting SSI in the analysis may lead to unsafe design for

both the superstructure and the foundation.[7]

Detrimental effects of SSI

Using rigorous numerical analyses, Mylonakis and Gazetas [7] have shown that increase in natural period ofstructure due to SSI is not always beneficial as suggested by the simplified design spectrums. Soft soil sedimentscan significantly elongate the period of seismic waves and the increase in natural period of structure may lead tothe resonance with the long period ground vibration. Additionally, the study showed that ductility demand cansignificantly increase with the increase in the natural period of the structure due to SSI effect. The permanentdeformation and failure of soil may further aggravate the seismic response of the structure.

2/1/2014 Soil structure interaction - Wikipedia, the free encyclopedia

http://en.wikipedia.org/wiki/Soil_structure_interaction 2/2

When a structure is subjected to an earthquake excitation, it interacts with the foundation and the soil, and thuschanges the motion of the ground. Soil-structure interaction broadly can be divided into two phenomena: a)kinematic interaction and b) inertial interaction. Earthquake ground motion causes soil displacement known asfree-field motion. However, the foundation embedded into the soil will not follow the free field motion. Thisinability of the foundation to match the free field motion causes the kinematic interaction. On the other hand, themass of the super-structure transmits the inertial force to the soil causing further deformation in the soil, which is

termed as inertial interaction.[2]

At low level of ground shaking, kinematic effect is more dominant causing the lengthening of period and increasein radiation damping. However, with the onset of stronger shaking, near-field soil modulus degradation and soil-pile gapping limit radiation damping, and inertial interaction becomes predominant causing excessivedisplacements and bending strains concentrated near the ground surface resulting in pile damage near the ground

level.[2]

Observations from recent earthquakes have shown that the response of the foundation and soil can greatlyinfluence the overall structural response. There are several cases of severe damages in structures due to SSI in

the past earthquakes. Yashinsky [8] cites damage in number of pile-supported bridge structures due to SSIeffect in Loma Prieta Earthquake in San Francisco in 1989. Extensive numerical analysis carried out by

Mylonakis and Gazetas [7] have attributed SSI as one of the reasons behind the dramatic collapse of HanshinExpressway in 1995 Kobe Earthquake.

References

1. ^ Tuladhar, R., Maki, T., Mutsuyoshi, H. (2008). Cyclic behavior of laterally loaded concrete piles embeddedinto cohesive soil, Earthquake Engineering & Structural Dynamics, Vol. 37 (1), pp. 43-59

2. ̂a b c Wolf, J. P. (1985). Dynamic Soil-Structure Interaction. Prentice-Hall, Inc., Englewood Cliffs, NewJersey

3. ^ Mylonakis, G., Gazetas, G., Nikolaou, S., and Michaelides, O. (2000b). The Role of Soil on the Collapse of18 Piers of the Hanshin Expressway in the Kobe Earthquake, Proceedings of 12th World Conference onEarthquake Engineering, New Zealand, Paper No. 1074

4. ^ Japan Society of Civil Engineers. Standard Specifications for Concrete Structures – 2002: SeismicPerformance Verification. JSCE Guidelines for Concrete No. 5, 2005

5. ^ ATC-3(1978). Tentative Provisions for the Development of Seismic Regulations of Buildings: A CooperativeEffort with the Design Profession, Building Code Interests, and the Research Community, National Bureau ofStandards, Washington DC

6. ^ NEHRP (1997). Recommended provisions for seismic regulations for new buildings and other structures,Part 1 and 2, Building Seismic Safety Council, Washington DC

7. ̂a b c Mylonakis, G. and Gazetas, G. (2000a). Seismic soil structure interaction: Beneficial or Detrimental?Journal of Earthquake Engineering, Vol. 4(3), pp. 277-301

8. ^ Yashinsky, M. (1998). The Loma Prieta, California Earthquake of October 17, 1989 – Highway Systems,Professional Paper 1552-B, USGS, Washington

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