SIMULATION OF A SLIDING LAYER BETWEEN FOUNDATION AND SPATIAL FOUNDATION PLATFORM. SYSTEM APPROACH TO APPLICATION SEISMIC ISOLATION IN SEISMIC CONSTRUCTION

ABSTRACT

Traditional seismic isolation devices are compared with an unconventional device in the form of a sliding layer under a continuous spatial foundation platform. The efficiency of the sliding layer at large dynamic displacements of the weak base is confirmed by the keyword.

АННОТАЦИЯ

Традиционные устройства сейсмоизоляции сравниваются с нетрадиционным устройством в виде скользящего слоя под платформой непрерывного пространственного фундамента. Эффективность скользящего слоя при больших динамических смещениях слабого основания подтверждена ключевым словом.

Keywords: keyword, Scad, dynamic, seismic

Ключевые слова: ключевое слово, скад, динамика, сейсмика

Traditional and non-traditional approaches

It should be noted that traditional seismic isolation devices, in including seismic isolating supports, have a significant common drawback: they divide the whole system "building foundation" into separate parts, which leads to a weakening of the system in favor of seismic isolation of a certain part of this system. In this case, mutual displacements arise between the insulated and non-insulated parts, which must be identified by kinematic calculation, and to limit these mutual displacements, it is necessary to install dampers that dissipate the energy of seismic action.

This situation is created, in our opinion, because seismic isolation is applied (as if mechanically introduced) in the structures of buildings that are designed for non-seismic areas, without fundamental understanding of the new system, including the foundation, especially for complex soil conditions. As an example, we can point to a solution in which a pile foundation with a grillage is separated from the superstructure by a layer of sand. The integrity of the "building-foundation" system is violated with possible undesirable consequences. It is possible to formulate some general fundamental provisions that increase the efficiency of seismic isolation of buildings and structures. The building (construction), together with the foundation, must form a single integral spatial multi-connected system, which, even when separated from the base, must remain geometrically unchanged. And in the event of an accidental violation of one (or several) local connections, the possibility of spatial redistribution of efforts should be made to prevent the global collapse

The seismic isolation device should refer to this entire system, and not to its separate part. An example of such a constructive solution can be a building (structure), combined with a solid spatial foundation platform, between which and the leveled base there is a sliding layer that reduces friction. In this case, a powerful seismic wave slips under the platform, the level of large horizontal seismic actions (including asymmetric, torsional, etc.) on the platform and thus on the topside is significantly reduced.

The integrity and multi-connection of buildings with the foundation make it possible to perceive vertical shocks as well. In this case, possible horizontal displacements will take place not between individual parts of the building (integrity is not violated), but between the system (“building-foundation”) and the foundation. Small (on the order of a few centimeters) displacements may be allowed in the planning of the territory, and to limit large displacements, stops are installed (dampers, return devices, etc.).

Thus, the sliding layer forms a seismic isolation protective device that does not violate the integrity of the "building-foundation" system. Other possible types of protective seismic isolating (screen) devices that are outside the “building-foundation” system should be pointed out, for example, the construction of trenches (ditches) across the dynamic impact.

Evaluation of the effectiveness of seismic isolation in the form of a sliding layer between the spatial foundation platform (SFP) and the foundation

The calculation scheme "building-spatial foundation platform" on a sliding layer is a rather complex nonlinear model due to friction and possible kinematic movements (as a result of overcoming friction).

To determine the resulting seismic isolating effect, one can consider the stage of operation when friction is overcome. In this case, an auxiliary scheme can be used, in which the horizontal (tangential) connections between the PFP and the base are removed or significantly weakened, i.e. only vertical (normal) links remain significant. Such approach models the state of the sliding layer when the friction is overcome.

Such a model can be implemented, for example, with the help of vertical rods in the nodes of a finite element mesh, which have a very high longitudinal stiffness and negligible bending stiffness, i.e. transmission of normal forces with little shear resistance (slippage). In this way, the main idea of the sliding layer will be implemented: the horizontal displacements of the base will not be transmitted to the PFP and the superstructure.

The studies were carried out for a model of a 5-storey frame building, the design scheme of which was taken in the form of a square in terms of spatial frame with nodal concentrated masses (section of columns 0.4x0.4m2, crossbars 0.6x0.3m2; material - concrete E = 3.25 104 MPa, µ = 0.2), located on a solid foundation slab measuring 16x16x0.5m3 (Picture 1).

Figure 1. Spatial design scheme of a 5-storey frame building on the PFP

The calculations of the spatial frame for the horizontal vibration load Р(t) = P0 sin(θt) applied in the nodes of the upper layer of the subgrade (nodal mass m= 20 t, P0 = 294 kN, θ = 20 rad/s) were carried out using the SCAD software Office taking into account the elastic properties of the soil base according to the spatial model of an elastic weightless half-space. Category III soil (E =11 MPa, µ=0.3) is represented by a spatial array 50x50x26m

3 . Volumetric finite elements are used. The lower plane of the soil mass is fixed motionlessly.

Two models of the sliding layer were considered. In the first one, the sliding layer was modeled by vertical metal rods (length 0.1 m, square section 0.05x0.05 m2), pivotally connected to the foundation slab and soil base with the exception of horizontal connections between base plate and foundation. At the same time, non-stretching horizontal bonds were imposed at the corner points of the foundation slab to ensure geometric invariability. In the second model, the sliding layer was modeled by rod elements (0.5 m long, with almost zero bending stiffness - EJ = 0.098 kN m2), rigidly connected at the nodes of the finite element mesh with the foundation slab and base.

As can be seen from the calculation results presented in the form of bending moment diagrams in Fig. 2, both sliding layer models provide a good seismic isolation effect. So, for the 1st model of the sliding layer, the largest bending moments in the columns and crossbars of the frame turned out to be 10.23 and 12.09 kN m (Picture. 2, b); for the 2nd model of the sliding layer - 9.53 and 12.97 kN m (Picture. 2, c), which is on average 40 times less than for the frame on the PFP without the sliding layer (Picture. 2, a)

Figure 2. Diagrams of bending moments (kN m) in the middle frame from the action of horizontal harmonic load Р(t): a – frame on PFP without sliding layer; b, c - frame on PFP with a sliding layer according to the 1st and 2nd calculation models

It should be noted that for the 1st sliding layer model, the horizontal movements of the foundation slab, with the frame located on it, we got practically zero (0.003 mm) compared to the displacements of the upper soil layer (10 mm), and for the 2nd model of the sliding layer they are equal to 0.3 mm. That is, under seismic action, the slab, especially for the 1st model of the sliding layer, practically does not move in the horizontal direction compared to the points of the subgrade.

At the same time, the horizontal displacement of the top of the building for the frame on a solid foundation slab was 36.6 mm, and for the frame on a slab with a sliding layer, 0.11 and 0.6 mm, respectively, for the first and second models of the sliding layer.

The obtained results show that the sliding layer between the PFP connected to the topside and the subgrade significantly reduces the forces in the topside. It should be noted that the device of the sliding layer is well combined with foundations in the form of a solid slab (SFP), which makes it possible to build buildings and structures on soft soils [10–17].

Similar calculations performed at other frequencies of dynamic impact confirm the effectiveness of the use of a seismic insulating layer, and for construction not only on soft soils (category III), but also on other soils.

Conclusion

Note that it is not advisable to divide the seismically insulating building (together with the foundation) into parts, thereby creating conditions for the mutual displacement of these parts. Such seismic isolation weakens the building. This is a significant drawback of traditional seismic isolation. It is advisable to build "closed type buildings", creating a seismic isolation (sliding layer) between the foundation slab and the foundation. Then only displacements of the building with the foundation as an integral system along relation to the base. At the same time, the forces in the upper structure are significantly reduced with large dynamic effects on the base.

References

1. CH RK EN 1998-1:2004/2012 DESIGN OF STRUCTURES RESISTANT TO SEISMICITY
2. EUROCODE 8: DESIGN OF STRUCTURES FOR EARTHQUAKE RESISTANCE – PART 5: FOUNDATIONS, RETAINING STRUCTURES AND GEOTECHNICAL ASPECTS
3. SP RK 5.01-102-2013 "FOUNDATIONS OF BUILDINGS AND STRUCTURES" (WITH AMENDMENTS AND ADDITIONS AS OF 03/18/2021)