Nonlinear time-history analysis of the Evolution Tower resting on cellular raft due to earthquake loads

Most traditional structural design methods ignore the impacts of soil–structure interaction (SSI). Under seismic loading, SSI is seen as helpful since it causes the system to dampen more and extends the lateral fundamental period. According to recent studies, SSI can be hazardous, and ignoring its influence could lead to an unsafe design for both the superstructure and the foundation, especially for structures built on soft soil [1, 2]. Mylonakis and Gazetas [2] observed that SSI amplified structures' responses during different earthquakes despite potential dampening rises. They found that the earthquake particularly affected buildings with 10–12 stories, whose vibration duration increased due to the SSI from approximately 1.0 s to almost 2.0 s resting on soft clay. Several authors have made an effort to investigate how the SSI affects the seismic response of buildings. Veletsos [3], Nair [4], and Bielak [5] performed preliminary studies in this field.

The analysis of tall buildings with fixed bases is unsuccessful in accurately reflecting the seismic reaction, according to Yingcai [6], Lu [7], Julio et al. [8], Priyanka et al. [10], and Tahghighi, Mohammadi [14] that considered the interaction between soil and piles while examining the seismic behavior of framed structures. Anand et al. [9] studied how RCC structures respond to earthquakes under different soil conditions with and without shear walls. Xiao Lu et al. [11] quantitatively evaluated the effects of SSI on seismic reactions at Shanghai tower by using comparable static loads, response spectra, and time histories. Shehata et al. [12] studied the impacts of SSI on multi-story raft-supported buildings. Ebadi et al. [13] investigated the effects of seismic soil–pile–structure interaction on superstructure seismic responses; several numerical simulations were run on two types of superstructures and six types of piled-raft foundations. Sahil et al. [15] examined the soil–structure interaction effect on a four-story RC building resting on hill slopes. Kaddah et al. [16] studied the response of midrise concrete frame structures resting on silty sandy soil using ABAQUS software. Hokmabadi et al. [17, 18] investigated the SSPSI phenomena by conducting shaking table tests on different structures, and results showed that SSI amplifies lateral deflections and inter-story drifts in structures supported by floating pile foundations, but reduces lateral displacements due to reduced rocking components. Nguyen et al. [19] stated that design engineers must carefully consider shallow foundation size and interaction parameters for safe, cost-effective seismic design in buildings. Reza et al. utilized the wavelet transform methodology for investigating the seismic response of soil–structure systems and to reduce the cost of calculations for the incremental dynamic analysis [20‐24].

Reza et al. studied the improvement of high-rise buildings seismic performance considering SSI using outrigger systems and the optimization of its location [25, 26], or by using different mass-tuned dampers [27, 28].

As considering the soil-foundation system in the analysis of high-rise buildings leads to lengthening the vibration period because of the buildings base flexibility and also leads to increase the sway along the buildings height, a lot of research is concerned with applying different methods to decrease or control at least the lateral deformation along towers during earthquakes. This paper presents the study of using cellular rafts instead of solid rafts under high-rise buildings. Dynamic analyses of both 52-story reinforced concrete (RC) twisting and regular towers are carried out using time-history analysis under seismic loads. Both towers are analyzed in two cases: using a solid raft on piles as the tower foundation considering soil–structure interaction (SSI) and the same case but replacing a solid raft with a cellular one. All rafts are made of reinforced concrete (RC) through multi-layer soil. The only difference between both towers is the shape of the elevation. All analyses are carried out using finite-element software (Midas GTS NX). The twisting tower is a simulation of the Evolution Tower in Moscow. The behavior of using cellular rafts instead of solid ones against seismic loads is discussed by showing the dynamic response difference between the twisting tower and the regular tower with the same dimensions and properties.

The method used to analyze the interaction between soils and structures in this research is the direct method, which will be illustrated below.

3D models are created using the finite element program (Midas GTS NX), which is verified in this type of analysis by Shaaban et al. [29]. A 52-story RC twisting tower and a regular tower with the same dimensions and properties are the main subjects of a nonlinear time-history analysis. The twisting tower is a simulation of the Evolution Tower in Moscow, which is an office skyscraper with a reinforced concrete frame. The structure rises 246 m and has 52 stories above ground. The Evolution Tower has a circular core in the center and a square plan overall. The creative construction utilizes twisted square floor slabs with an octagonal-shaped central core and eight columns to support a vertical reinforced concrete frame. The building's central core is surrounded by floors that are constantly rotated at a fixed angle of 3°, creating a clockwise twist from the bottom to the top of the structure of around 135° [30] and [31], as shown in Figs. 3 and 4. Soil and raft are modeled as 10-node pentahedral solid elements. Beams and columns are modeled as beam elements with six degrees of freedom on each node. Piles are modeled as beam elements with interface elements to simulate friction with the soil, as shown in Fig. 2.

The building structure's material is considered as reinforced concrete (RC), and for nonlinear time-history analysis, geometric nonlinearity for the structure is considered (Figs. 3, 4). RC properties are introduced in Table 1. The cross-sectional details are shown in Table 2.

Scaled time histories of the El-Centro, Northridge, Kobe, Chichi, Friuli, Kocaeli, and Loma earthquakes are used for nonlinear time-history study. Scaling was conducted using the Egyptian Code (ECP-201) for these records [32] (New Mansoura Zone). The seismic features of this zone are the 2nd seismic zone with ground design acceleration (ag = 0.125 g). The soil is classed as (C). The target response spectrum due to ECP-201 is shown in Fig. 5, and the response spectrum curve for the seven earthquake records is shown in Fig. 6. The seven earthquake records after being scaled to the target response spectrum are introduced in Fig. 7. Rayleigh damping was adopted to take damping into consideration during the time-history analysis with coefficients of (α = 0.108535398 and β = 0.0049548014), which are calculated using the two fundamental periods of the building (T₁ = 5.458918 s and T₂ = 0.3301475 s) from the following equations:where \(\omega\) is the natural circular frequency, \(\xi\) is the damping ratio which is assumed as 5%.

A square R.C. raft measuring 47.0 × 47.0 m and 3.5 m in depth supports both towers. This raft was supported by 34.0 m-long, 1 m-diameter piles, as shown in Fig. 8. For this model, a soil profile was considered as taken by [33]. This profile can be considered for the chosen zone for the analysis (New Mansoura, Egypt), as shown in Fig. 9.

The Mohr–Coulomb model, widely regarded as the most widely used constitutive model in soil media modeling, was utilized to characterize the material nonlinearity for soil. Friction and cohesiveness are the only two strength parameters used to explain plastic behavior [34].

To achieve optimal soil dimensions, absorbing borders are added at a suitable distance from the structure to mimic energy radiation. Gosh and Wilson [35] revealed that the distance between the foundation center and the soil HL border should be three to four times the base radius, and the distance between the foundation center and the VL border should be two to three times the foundation radius, in order to obtain fairly minimal reflexive wave effects. Moreover, Rayhani and El-Naggar [36] showed that furthermost seismic activity amplification occurs in the first 30 m of the soil profile and that increasing the soil border from 5 to 10 times the structure's breadth only produces a 5% difference in output. Moreover, there are a lot of studies in that point like Kumar et al. [37] and Visuvasam and Chandrasekaran [38]. Therefore, the soil dimensions were taken at 245 m × 245 m × 60 m.

A cellular raft is composed of a configuration of two-way interlocking foundation beams with a suspended slab on top and a ground-bearing slab beneath. I sections are often created by combining the upper and lower slabs with the beams. The large slab is effectively divided into continuous, tiny panels that span both directions by the intersecting beams, as shown in Fig. 10.

The advantages of the cellular raft are discussed in more detail by Williams and Pellissier [39] and include: Different decrease percentages in raft volume are studied for determining the optimal gaps dimensions in the cellular raft, which are (5–50% with 5% increment) from the solid raft dimensions (47.0 m × 47.0 × 3.5 m), then the findings are reviewed and led to:

Time-history analysis is performed on the different models used for the study taking into consideration the previously mentioned earthquake records. Results of the analysis showed that:

Effect of using cellular raft on the twisting tower (due to El-Centro as an example):

Effect of using cellular raft on the regular tower (due to El-Centro as an example):

Table 5 shows a statistical analysis for all results obtained from time-history analyses under the three earthquake records. Results showed that:

This research investigated the effect of using a cellular raft in twisting building analysis for seven earthquake records matched to the ECP-201 response spectrum and comparing the cellular raft effect between the twisting tower and a regular one with the same structural properties. Midas GTS NX considers full 3D models for all scenarios. Based on the findings and discussions, it can be concluded that: