Soil-structure interaction
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Ground–structure interaction (SSI) consists of the interaction between
Most of the civil engineering structures involve some type of structural element with direct contact with ground. When the external forces, such as
Conventional structural design methods neglect the SSI effects. Neglecting SSI is reasonable for light structures in 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
Effect of (Soil-structure interaction) SSI and SSI provisions of seismic design codes on structural responses
It is conventionally believed that SSI is a purely beneficial effect, and it can conveniently be neglected for conservative design. SSI provisions of seismic design codes are optional and allow designers to reduce the design base shear of buildings by considering soil-structure interaction (SSI) as a beneficial effect. The main idea behind the provisions is that the soil-structure system can be replaced with an equivalent fixed-base model with a longer period and usually a larger damping ratio.[5][6] Most of the design codes use oversimplified design spectra, which attain constant acceleration up to a certain period, and thereafter decreases monotonically with period. Considering soil-structure interaction makes a structure more flexible and thus, increasing the natural period of the structure compared to the corresponding rigidly supported structure. Moreover, considering the SSI effect increases the effective damping ratio of the system. The smooth idealization of design spectrum suggests smaller seismic response with the increased natural periods and effective damping ratio due to SSI, which is the main justification of the seismic design codes to reduce the design base shear when the SSI effect is considered. The same idea also forms the basis of the current common seismic design codes such as ASCE 7-10 and ASCE 7-16. Although the mentioned idea, i.e. reduction in the base shear, works well for linear soil-structure systems, it is shown that it cannot appropriately capture the effect of SSI on yielding systems.[7] More recently, Khosravikia et al.[8] evaluated the consequences of practicing the SSI provisions of ASCE 7-10 and those of 2015 National Earthquake Hazards Reduction Program (NEHRP), which form the basis of the 2016 edition of the seismic design standard provided by the ASCE. They showed that SSI provisions of both NEHRP and ASCE 7-10 result in unsafe designs for structures with surface foundation on moderately soft soils, but NEHRP slightly improves upon the current provisions for squat structures. For structures on very soft soils, both provisions yield conservative designs where NEHRP is even more conservative. Finally, both provisions yield near-optimal designs for other systems.
Detrimental effects
Using rigorous numerical analyses, Mylonakis and Gazetas [9] have shown that increase in natural period of structure due to SSI is not always beneficial as suggested by the simplified design spectrums. Soft soil sediments can significantly elongate the period of seismic waves and the increase in natural period of structure may lead to the resonance with the long period ground vibration. Additionally, the study showed that ductility demand can significantly increase with the increase in the natural period of the structure due to SSI effect. The permanent deformation and failure of soil may further aggravate the seismic response of the structure.
When a structure is subjected to an
At low level of ground shaking,
Observations from recent
Design
The main types of foundations, based upon several building characteristics, are:
- Isolated plinths(currently not feasible)
- Plinths connected by foundations beams
- Reverse beams
- A plate(used for low-quality grounds)
The filing of foundations grounds takes place according to the mechanical properties of the grounds themselves: in Italy, for instance, according to the new earthquake-proof norm – Ordinanza 3274/2003 – you can identify the following categories:
- Category A: homogeneous rockformations
- Category B: compact granularor clayey soil
- Category C: quite compact granular or clayey soil
- Category D: not much compact granular or clayey soil
- Category E: surfacelayer grounds (very low quality soil)
The type of
For further information about the various ways of building foundations see
Both grounds and structures can be more or less deformable; their combination can or cannot cause the
For instance, suppose there are two buildings that share the same high stiffness. They stand on two different soil types: the first, stiff and rocky—the second, sandy and deformable. If subjected to the same seismic event, the building on the stiff ground suffers greater damage.
The second interaction effect, tied to mechanical properties of soil, is about the lowering (sinking) of foundations, worsened by the seismic event itself, especially about less compact grounds. This phenomenon is called soil liquefaction.
Mitigation
The methods most used to mitigate the problem of the ground-structure interaction consist of the employment of the before-seen isolation systems and of some ground brace techniques, which are adopted above all on the low-quality ones (categories D and E). The most diffused techniques are the
References
- ^ Tuladhar, R., Maki, T., Mutsuyoshi, H. (2008). Cyclic behavior of laterally loaded concrete piles embedded into cohesive soil, Earthquake Engineering & Structural Dynamics, Vol. 37 (1), pp. 43-59
- ^ a b c Wolf, J. P. (1985). Dynamic Soil-Structure Interaction. Prentice-Hall, Inc., Englewood Cliffs, New Jersey
- ^ Mylonakis, G., Gazetas, G., Nikolaou, S., and Michaelides, O. (2000b). The Role of Soil on the Collapse of 18 Piers of the Hanshin Expressway in the Kobe Earthquake, Proceedings of 12th World Conference on Earthquake Engineering, New Zealand, Paper No. 1074
- ^ Japan Society of Civil Engineers. Standard Specifications for Concrete Structures – 2002: Seismic Performance Verification. JSCE Guidelines for Concrete No. 5, 2005
- ^ ATC-3(1978). Tentative Provisions for the Development of Seismic Regulations of Buildings: A Cooperative Effort with the Design Profession, Building Code Interests, and the Research Community, National Bureau of Standards, Washington DC
- ^ NEHRP (1997). Recommended provisions for seismic regulations for new buildings and other structures, Part 1 and 2, Building Seismic Safety Council, Washington DC
- S2CID 110609192.
- .
- ^ a b Mylonakis, G. and Gazetas, G. (2000a). Seismic soil structure interaction: Beneficial or Detrimental? Journal of Earthquake Engineering, Vol. 4(3), pp. 277-301
- ^ Yashinsky, M. (1998). The Loma Prieta, California Earthquake of October 17, 1989 – Highway Systems, Professional Paper 1552-B, USGS, Washington