Research Article  Open Access
Effect of Soil Classification on Seismic Behavior of SMFs considering SoilStructure Interaction and NearField Earthquakes
Abstract
Seismic response of a structure is affected by its dynamic properties and soil flexibility does not have an impact on it when the bottom soil of foundation is supposedly frigid, and the soil flexibility is also ignored. Hence, utilizing the results obtained through fixedbase buildings can lead to having an insecure design. Being close to the source of an earthquake production causes the majority of earthquake’s energy to reach the structure as a longperiod pulse. Therefore, nearfield earthquakes produce many seismic needs so that they force the structure to dissipate output energy by relatively large displacements. Hence, in this paper, the seismic response of 5 and 8story steel buildings equipped with special moment frames (SMFs) which have been designed based on typeII and III soils (according to the seismic code of IranStandard 2800) has been studied. The effects of soilstructure interaction and modeling of the panel zone were considered in all of the two structures. In order to model radiation damping and prevent the reflection of outward propagating dilatational and shear waves back into the model, the vertical and horizontal Lysmer–Kuhlemeyer dashpots as seen in the figures are adopted in the freefield boundary of soil. The selected near and farfield records were used in the nonlinear timehistory analysis, and structure response was compared in both states. The results obtained from the analysis showed that the values for the shear force, displacement, column axial force, and column moment force on typeIII soil are greater than the corresponding values on typeII soil; however, it cannot be discussed for drift in general.
1. Introduction
Over the past years, various design rules and seismic analysis for steel framed structures have been highly developed. Although most of the recent seismic design codes have taken up many of these developments, they are still described by some restrictions, with two of them being the most critical ones [1].
The ﬁrst limitation has to do with the soilstructure interaction (SSI) phenomenon, which is disregarded by several seismic codes along with the regular everyday structural practice, presuming that lack of this phenomenon causes conservative outcomes [2–6]. In the related articles, several investigations approve the great impacts of SSI [7–11]. In this way, observations and examinations from real samples and also analyses have demonstrated that structures based on ﬂexible soil display importantly other seismic responses than that of structures setup on solid rock caused by the substitution of the dynamic features of the system. The ﬂexibility of soil resulted in a decline of stiffness, an increase in deformation, and to a rise in whole damping as a result of the radiation damping in the soil medium [12–14]. In addition, it should be taken into account that soft soil also affects the frequency content of the approaching seismic signals. Plus, the SSI phenomenon can result not only in positive but also in damaging consequences [15–17].
The second one with current seismic codes and design practice has to do with the traditional selection of farﬁeld seismic ground motions to measure seismic design loads. This being said that a small number of norms and codes do consider, quantitatively and qualitatively, the seismic design of structures subjected to nearfield records, for instance, FEMA P750 [18]. Moreover, the powerful ground motions recorded near the fault zone and at stations placed in the direction of the fault rupture are qualitatively various from the usual farﬁeld records [19].
The impacts of nearfield and farfield ground motions on civil engineering structures have been studied in several past researches [20–24]. For the same ductility factor, the nearfield ground motions inflict a greater strength demand than the farfield motions for SDF (single degree of freedom) systems [19]. Characterization of forwarddirectivity pulse motion of nearfield earthquakes was named along with the fundamental parameters that contain amplitude (peak ground velocity), velocity pulse period, and the number of considerable cycles [25, 26].
In the direct method of analysis of soilstructure interaction (SSI), modeling both the underlying soil and the structure is needed exactly as they are to be analyzed with each other at the same time. The structure is a bounded medium and can be modeled to almost any practical level of validity at this time. On the other hand, two major barriers appear at the time of modeling the soil: its unlimited dimensions and its nonlinear behavior from very small strains. To eliminate the obstacle of limitlessness of soil medium, the soil is usually made restricted to a rigid bedrock at the bottom and two vertical artiﬁcial boundaries on the two sides. Based on the soil material behavior, in SSI analysis, usually the soil is presumed to act in a linear way but the stiffness and damping attributes of soil are altered to be steady with the moderate strain level in each layer. This approach is recognized as the equivalent linear method (ELM). Although the ELM has the significant benefit of highly simplifying the SSI analysis, it may not be appropriate to be used for the soil in the proximity of foundation, where the strain level is too high for the ELM to be adequately precise. However, this fact is often neglected and the ELM is applied for the whole soil medium [27].
Ghandil and Behnamfar [28] in their currently published article resolved the ELM barriers and suggested the nearfield method as an altered ELM with an additional decline of the soil shear modulus in the nearfield zone that led to accuracy in adopting the ELM throughout. The current research establishes this new method as it can be suitable for a variety of structures and includes the impacts of various vibrational modes of the structure. In the first part of this article, a group of dimensionless parameters describing the corresponding properties of structure and soil consisting of the stiffness ratio (s), the slenderness ratio (h), and the mass ratio (m) were chosen, and a comprehensive parametric study was accomplished to specify the Bull Earthquake Eng. 123 nearfield zone dimensions and dynamic properties. Next, for calculating the nearfield properties, semianalytical relations were suggested as functions of the dimensionless parameters [28]. Raychowdhury and Singh [27] examined the consequence of nonlinear soilstructure interaction (SSI) on the seismic responses of lowrise steel momentresisting frame (SMRF) structures. In the situations where the soil below the foundation was presumed to be a system of closely spaced, independent, nonlinear spring elements, a Winklerbased method was selected. Nonlinear dynamic analyses and static pushover analysis were done on a 3story SMRF building, and the efficiency of the structure was measured through a variety of force and displacement demand parameters. It was noticed that incorporation of nonlinear SSI results in a rise in story displacement demand and a drastic decrease in base moment, base shear, and interstory drift demands, showing the significance of its consideration towards achieving an economic, yet safe, seismic design.
In accordance with the performancebased design concept, Gerami and Abdollahzadeh [29] demonstrated the capability of steel moment frames exposed to nearfield ground motions. In this regard, Dimakopoulou et al. [30] noticed the consequence of modeling hypothesis on the seismic response of structures under nearfield records. The impact of forward directivity on collapse risk was also assessed by Champion and Liel [31]. In addition to that, Rahgozar et al. [32] managed a research on the behavior of selfcentering braced frames which were subjected to nearfield ground motions.
This paper generally evaluates the seismic behavior of steel structures with a special moment frame system. This structural system was selected because of the engineers’ expectation of high plasticity on the one hand and attention to the nature of the nearfield earthquakes with progressive direction taking on the other. In this type of records, the structure does not have enough time to employ all its plasticity against the pulselike behavior of these records due to the pulselike behavior of the earthquake. In other words, these records act like a strong pulse in a short period on the structure, whereas we have high expectations from the special moment frame system. Moreover, the panel zone that has a significant impact on attenuation is modeled in order to adapt the models more to actual examples. Furthermore, the effects of soilstructure interaction (SSI) and the importance of changes created in the structure’s seismic response due to the analysis of this behavioral set are generally nonnegligible. These effects may increase or decrease the seismic response of structures or their other seismic parameters under the force of the earthquake. These effects, in turn, depend on freefield dynamic properties of the structure (including the main vibration period and attenuation) and flexibility of the abutment. Similarly, it is possible that these changes cause changes in the force of the structural components (decrease or increase) and affect their safety, efficiency, or durability [15].
In Iran, the design code of resistant structures against earthquake (Standard No. 2800) [33] classified the bottom soil of foundation into four categories including types I, II, III, and IV based on the shear wave velocity (V_{s}) at a depth of 30 m (Table 1).

The structures in this research are located in Tehran, Iran, whose soil is normally of type II or type III according to the Iranian national building codes (Standard No. 2800). In comparison with soil type II, soil type III leads to obtaining a larger base shear for designing the structure in accordance with standard 2800; thus, sections will be larger. As a result, sometimes designers assume the soil has the structure type III regardless of soil mechanics reports when designing structures in typeII soil and design their structures based on this assumption. They consider this a more confident decision. However, the results of the present study prove this belief wrong.
2. Numerical Modeling
2.1. Selecting the Prototype Building Model
In this study, 5 and 8story buildings are chosen. Figure 1 displays the studied architectural plan of the building with three bays, which are 5 m each, in each direction. The floortofloor height is equal to 3 m.
The lateral resisting systems are the special momentresisting frames in X and Y directions. The buildings are residential in very highrisk zones on soil types II and III and are loaded and designed according to Iranian national building codes (Standard No. 2800). The dead and live loads of all stories are presumed to be 650 kg/m^{2} and 200 kg/m^{2}, respectively. For the roof of the building, the loads are also equal to 450 kg/m^{2} and 150 kg/m^{2}, respectively. The buildings with fixed bases are designed according to ACI318–05 in ETABS software. Therefore, the schematic view of the reference frames is depicted in Figure 2.
(a)
(b)
The crosssections of the beams and columns for the second for the 5 and 8story frames are shown in Tables 2 and 3.
 
∗European standard profiles. 
 
∗European standard profiles. 
The panel zone deforms primarily in shear due to the opposing moments in the columns and beams. The panel zone was explicitly modeled using the method of Gupta and Krawinkler [34] as a rectangle composed of eight very stiff elastic beamcolumn elements with one rotational spring to represent shear distortions in the panel zone. The Bilin material imitates the modified IbarraMedinaKrawinkler deterioration model with a bilinear hysteretic response [35].
The studied structures were modeled with elastic beamcolumn elements connected by rotational springs to represent the structure’s nonlinear behavior. The springs follow a bilinear hysteretic response based on the Modified Ibarra Krawinkler Deterioration Model [36].
2.2. Modeling SoilStructure Interaction
In this study, the direct method is employed for modeling SSI. Based on this method, structure and soil are modeled concurrently, and their responses are determined by analyzing the SSI system in each time step.
It should be noted that modeling of infinite soil media in soilstructure interaction plays a vital role. In the direct method of interaction analysis between soil and structure, the soil is modeled by the finite element method and the boundary conditions are implemented around the soil body. In this method, in addition to considering the geometrical damping, foundation burying and soil strata in horizontal and vertical directions may easily be modeled in the analysis [37]. In this method, there is a boundary limit obligation; thus, in order to consider the effect of energy distribution and its simulation, in direction of finite element boundaries, the primary boundaries are considered instead, in this research. These types of boundaries do not absorb energy, and for reduction of reflexive wave’s effects, the distance between the structure and the boundaries must be increased [38].
For this purpose, modeling and obtaining the nonlinear dynamic responses of the SSI system are performed in the opensource finite element software OpenSees [35]. The finite element model of the SSI system adopted in this study is depicted in Figure 3.
The soil domain with the total length of 100 m and the depth of 30 m is modeled using isoparametric fournoded quadrilateral finite elements with two degrees of freedom per node. It is also assumed that the conditions of this domain are considered as plane strain with a constant crossplane thickness equal to the interframe distance. The modified pressureindependent multiyield surface J_{2} plasticity model shown in Figure 4 is adopted as the constitutive model of the soil domain. Furthermore, some parameters of soil depend on the shear wave velocity of soil, V_{s}. In this study, the parameter V_{s} for the soil types II and III is considered 400 and 275 m/s, respectively.
(a)
(b)
In order to model radiation damping and prevent the reflection of outwardpropagating dilatational and shear waves back into the model, the vertical and horizontal Lysmer–Kuhlemeyer dashpots as seen in Figure 4 are adopted in the freefield boundary of soil. The dashpots are modeled based on using a zero length element and the viscous uniaxial material. It is noted that more details of the Lysmer–Kuhlemeyer dashpot can be found in [40]. Furthermore, a raft foundation is considered as rigid and the connection between the soil and the structure is obtained using common nodes and appropriate constrains. The constraints are applied by equal DOF command in X and Y directions in order to ensure equal displacements for both the soil and the foundation of structure.
To observe the radiation damping by performing a numerical test, the nonlinearity in the soil and structure as well as any viscous damping simulated by the Rayleigh approach was temporarily eliminated, keeping only the viscous dashpots at boundaries. At this stage, a dynamic load is applied to the structure for a limited duration and the structure and soil response are observed for a certain time after the load ends. Since the structure and the soil are linear, and viscous damping is not included (temporarily), the energy inputted to the system is only dissipated through radiation; after the input ends, you should observe the elastic oscillation of the structure that is damped (displacements tend to vanish) due to geometric effects (radiation damping).
Past studies show that if the distance of the structure center to the soil finite element model boundaries is within 34 times of the foundation radius in the horizontal direction and 23 times of the foundation radius in the vertical direction, the influences of the reflexive waves are negligible [37, 41]. In other words, the radiation condition is satisfied by considering nonreflecting boundaries simulated through dashpots, which means the model has included the radiation damping. Furthermore, there is hysteretic damping in the soil from the use of a nonlinear model.
As the typical damping ratios of the soils are between 3% and 10% [42], a damping ratio of 5% is applied. Also, for the structure, 5% damping ratio is considered.
The fundamental period of the designed frames with and without the SSI effects is shown in Table 4. As seen from Table 4, SSI effects are considered as the fundamental period of the designed frames.

Modeling the steel SMRF is considered using the lumped plasticity approach. In this approach, the beams and columns of the steel SMRF are modeled with elastic beamcolumn elements connected by zero length elements which serve as rotational springs to represent the nonlinear behavior of the structure. The rotational springs at the member ends follow a bilinear hysteretic response based on the Ibarra–Medina–Krawinkler (IMK) model [36]. Figure 5 represents the properties of the IMK model. As shown in Figure 5, using five parameters expressed in [43] can be completely taken into account for the nonlinearity in the model.
In the steel SMRF, the panel zone is deformed due to the shear force which is produced by the opposing moments in the beams and columns. To capture the deformation, the panel zone is considered in this study and explicitly modeled using the approach of Gupta and Krawinkler [34]. Based on this approach, a rectangular region is adopted and composed of eight very stiff elastic beamcolumn elements with one zero length element which serves as a rotational spring to represent shear distortions in the panel zone (see Figure 6).
3. NearField Earthquakes of Fault Undergoing Forward Directivity
In nearfield earthquakes, the geometry position of fault relevant to the considered place is important beside the rupture mechanism and kind of faulting. The amplitude of this pulse depends on the directivity of rupture propagation to the site. Since the rupture propagation velocity is nearly equal to the velocity of shear wave propagation, if the fault rupture spreads to the considered place, the waves in a shortterm period will reach to the place, causing a pulse with high amplitude and long period, which is called forwardeffect directivity. Now, if the rupture is in a direction of getting away from the site, the waves will scatteredly reach the spot which is defined backward directivity, whereas the rupture directivity neither close to the location nor far is called neutral directivity [44]. Pulse motion can also be produced by permanent displacement of ground caused by surface rupture.
What has been examined in this paper is the effect of pulse caused by forward directivity that, according to the obtained results of previous researches, exerts the most extensive damage to the structure [45, 46]. This angle is small in nature and is shown in Figure 7.
3.1. Selecting of Ground Motions
The random nature of earthquakes has been a challenging problem in the seismic evaluation of structures and has converted this evaluation to a probabilistic problem. Hence, in this study, 14 real ground motion records are selected from the PEERNGA database. The properties of these records are indicated in Tables 5 and 6.


It is noted that the ground motion records shown in Tables 5 and 6 are selected based on (i.e., bedrock condition) in order to obtain the ground motion records based on the soil types II and III according to Iranian national building code [33].
In timehistory analyses, choosing a suitable record in order to evaluate structures is of utmost importance. Hence, in this study, 14 real ground motion records are selected from the PEERNGA database. The properties of these records are indicated in Tables 5 and 6. In this research, 7 earthquake records related to nearfield undergoing forward directivity and 7 records of farfield earthquakes have been selected in order to analyze nonlinear timehistory analyses. Farfield earthquakes have been recorded in a distance of 60 km from the fault. The magnitude of these records ranges from 6.5 to 7.2. The magnitude of nearfield earthquakes ranges from 6.61 to 7.51, being recorded in a distance of 10 km from fault. It is noted that the ground motion records shown in Tables 5 and 6 are selected based on (i.e., bedrock condition).
It should be noted that, in the direct approach, the whole system of soil and structure should be vibrated, so ground motions are applied to the bedrock, which is assumed to be at a certain depth under the soil system (Figure 8). Hence, it is clear that the ground motions are related to the bedrock. When the soil under the structure is considered as the type III, in the modeling process, the properties of this type of soil (e.g., shear wave velocity) are assigned. For example, in [38], regarding the selection of the ground motion, the soil type I (based on EC8) was selected in SSI analysis.
The ground motions were scaled such that the average value of their square root of the sum of the squares (SRSS) spectra does not fall below 1.4 times the Standard Design Spectra for periods of 0.2T second to 1.5T seconds, where T is the fundamental period of vibration [33]. Figure 9 shows the scaling procedure for the 8story building under nearfield records.
4. Evaluation of Seismic Response of Structures
One of the most important steps in seismic design is the estimation and control of structure’s drift, causing stable increase of the whole building during an earthquake. In addition, confining lateral displacement of the structure decreases the damage of nonstructural elements such as mechanical equipment and architectural elements.
Moreover, the relative displacement of the stories can be a criterion for the calmness of the dwellers against displacement originated from wind load as well as structural damage. Investigations have revealed that the angle value of relative displacement of the stories can demonstrate the performance of structural and nonstructural elements in a way that if this angle is 0.001, the nonstructural damages would be possible; if the value of this parameter reaches 0.007, the possibility of nonstructural damages would be certain and structural damages would be possible. On the other hand, if the angle value of relative displacement exceeds 0.015, the nonstructural damages will occur and structural damages would be possible. Another reason to confine drift is secondary effects such as P∆, particularly in the nearfield (Naeim, F (2001)).
After investigating the results obtained from nonlinear timehistory analysis, it was specified that in nearfield earthquakes, the maximum story drift ratio in typeII soil in 5 and 8story buildings are 0.025 and 0.024, respectively (under the effect of Sanfernando record), and in typeIII soil are 0.024 and 0.019 (under the effect of Loma Prieta Los Gatos record). As shown, the maximum drift ratio in 5story structure, constructed on typeII soil, was 5% more than the maximum drift ratio in typeIII soil. Also, the maximum drift ratio in 8story building in typeII soil was 23.2% greater than the maximum drift ratio in typeIII soil. The results obtained from the two mentioned structures indicated that the maximum drift imposed to the structures in typeII soil is greater than that in typeIII soil which is set to augment as the number of stories increase. The changes of maximum drift ratio in the mentioned structures can be seen for all records in Figure 10.
(a)
(b)
In Table 7, it has been shown that, on average, the maximum drift ratio of 5 and 8story structures in typeII soil is equal to the average maximum drift ratio of 5 and 8story structures on typeIII soil.

Another critical parameter in structure design is the shear force of the stories. According to the obtained results, it can be said that the maximum shear force of stories subjected to nearfield earthquakes in 5 and 8story buildings on typeII soil are 123.38 kN (under Loma Prieta Gilroy Array record) and 297.81 kN (under San Fernando record). Also, this parameter for 5 and 8story buildings on typeIII soil is 125.49 kN and 332.48 kN, respectively. Based on these results, the maximum shear force in a 5story building on typeII soil is 2% lower than the maximum shear force on typeIII soil. Also, for 8story building, this discrepancy is approximately 11%. The classification effect of the site’s soil on the base shear force of structure is insignificant in a 5story building, whereas by changing the soil type from II to III, the base shear force increases in 8story buildings.
According to Table 8, it can be said that, on average, the maximum shear force of the stories in 5 and 8story buildings on typeII soil are 3% and 11% less than the 5 and 8story buildings on typeIII soil. Shear force changes of the stories can be observed in Figure 11.

(a)
(b)
The third important evaluated parameter is the displacement of the stories. The maximum displacements of stories subjected to nearfield earthquakes in 5 and 8story structures on typeII soil are 303.3 mm and 326.7 mm, respectively. In 5 and 8story structures, the maximum displacements of stories have increased and these values are 334.7 mm and 387.7 mm. In 5 and 8story buildings in both type soils Loma Prieta Gilroy Array record and San Fernando record resulted in these values. As shown, the maximum displacements of 5 and 8story structures on typeII soil are 10% and 8% lower than the amount of this parameter on typeIII soil. The results show that the site change from typeII soil to typeIII soil imposes higher displacement on stories. In Figure 10, the maximum and average displacements have been given.
Based on Figure 12, the maximum and average displacements of the 8story structure on typeII soil are greater than those of typeIII soil.
(a)
(b)
According to Table 9, the average maximum displacement of 5story structure on typeII soil in relations to typeIII soil has been increased to about 13%, whereas this value in 8story buildings on typeII soil compared with typeIII soil has decreased to around 12%.

Of the other seismic parameters evaluated in this paper are axial forces and moments in columns.
The maximum axial force produced in columns subjected to nearfield earthquakes in 5 and 8story structures on typeII soil is 11.12 kN and 21.56 kN. On the other hand, the maximum axial force created in the columns of 5 and 8story buildings on typeIII soil is 12.63 kN and 22.84 kN, respectively. This value in 5 and 8story buildings on typeII soil occurred in C12 (the position of the columns in the frames is shown in Figure 13) and C13 columns under the Landers record. Also, for typeIII soil, this parameter for 5 and 8story structures occurred in C12 and C13 columns under San Fernando record. It can be concluded that the maximum axial force of the columns in the 5story structure on type II soil is 12% lower than that in a 5story structure on typeIII soil. This was also seen for 8story buildings. The maximum axial force caused on typeII soil was 6% lower than that on typeIII soil. According to this, the maximum axial force produced in columns of the structure is weaker than the corresponding values in typeIII soil.
(a)
(b)
The maximum moment produced in columns of 5 and 8story structures subjected to nearfield earthquakes on typeII soil that occurred under Loma Prieta Gilroy Array record is 9.29 kN·m and 11.72 kN·m. The maximum moment created in columns of 5 and 8story structures on typeIII soil which was obtained under the Loma Prieta Los Gatos record is 11.63 kN·m and 14.90 kN·m. According to the obtained results, it can be said that the maximum moment created in a 5story building on typeII soil is 21.2% lower than that in typeIII soil. Also, this value for the 8story building is about 22.2%. A comparison of the results reveals that the maximum moment in both 5 and 8story structures on typeIII soil is stronger than the corresponding values on typeII soil.
According to Table 10, it can be said the average maximum axial forces of 5 and 8story structures on type II are 14% and 20% lower than the average maximum axial forces on typeIII soil, respectively. Also, the average maximum produced moment in a 5story building on typeII soil is 17% lower than the created moment on typeIII soil. This also is 24.2% for typeIII soil.

4.1. Seismic Evaluation of Structures Subjected to FarField Earthquakes
After investigating obtained results of nonlinear timehistory analysis subjected to farfield earthquakes records, it was specified that the maximum drift ratios in 5 and 8story structures on typeII soil are 0.014 and 0.012 (under the San Fernando earthquake record), respectively. These values are 0.02 and 0.01 on typeIII soil which occurred under the effect of San Fernando earthquake record. In 5story structure on typeII soil, the maximum drift ratio is 30% lower than the maximum drift ratio on typeIII soil. This value for the 8story structure is 20% greater than that on typeIII soil.
Figure 14 gives information about drift changes in the mentioned structures for all records.
(a)
(b)
According to Table 11, on average, the maximum drift ratios of 5 and 8story structures on typeII soil are equal to the maximum drift ratios on typeIII soil.

San Fernando records reveal that the maximum shear force of stories subjected to farfield earthquakes in 5 and 8story structures on typeII soil are 105.15 kN and 257.34 kN, respectively. These values on typeIII soil are 112.40 kN and 278.34 kN (under the San Fernando record). Based on the data, it can be expressed that the maximum shear force in 5story buildings on typeII soil is 7% lower than that on typeIII soil; the figure that increased to 8% in 8story buildings. Site’s soil classification changing from type II to III makes the base shear force of both structures augmented.
Figure 15 illustrates the changes in the shear force of the structure. Table 12 shows that, on average, the maximum shear forces of 5 and 8story structures on type II are 6% and 5% lower than those on typeIII soil.
(a)
(b)

The maximum displacements of stories’ subjected to farfield earthquakes in 5 and 8story buildings on typeII soil are 74.15 mm and 95.03 mm, respectively. In 5 and 8story structures constructed on typeIII soil, these values increased and were 120.8 mm and 102.56 mm (under San Fernando record), respectively. As it can be seen, the maximum values of displacement in 5 and 8story structures on typeII soil are 39% and 7% lower than those on typeIII soil. Figure 16 manifests the comparison of the maximum changes and average displacement of stories.
(a)
(b)
According to Table 13, it can be said that the average maximum of stories’ displacement in 5 and 8story structures on typeII soil in relation to typeIII soil is about 29% more and around 7% less, respectively.

The maximum axial force produced in columns subjected to the farfield earthquake in 5 and 8story buildings on typeII soil are 9.4 kN and 16.80 kN. These values are 10.90 kN and 18.35 kN on typeIII soil, respectively. Meanwhile, the maximum values of axial force in 5 and 8story structures on typeIII soil occurred in the C13 column. However, in typeII soil, this occurred in C12 and C13 columns for 5 and 8story structures, respectively (San Fernando record in types II and III). It can be concluded that the maximum axial force of columns in the 5story structure on typeII soil is 14% lower than that on typeIII soil, whereas for 8story building, this value for typeII soil is 9% lower than that in typeIII soil. The results show that the maximum produced axial force in columns on typeII soil is weaker than the corresponding values on typeIII soil.
The maximum caused moment in the columns of 5 and 8story buildings subjected to farfield earthquakes on typeII soil are 6.73 kN·m and 7.94 kN·m respectively. Meanwhile, for typeIII soil, in 5 and 8story structures, these parameters are 7.34 kN·m and 8.13 kN·m respectively (San Fernando record in types II and III). Based on the obtained results, it can be said that the created moment on typeII soil for the 5story structure is 8% lower than that on typeIII soil. Also, this value in 8story buildings on typeII soil is 25% lower than that on typeIII soil.
According to the given results in Table 14, it can be said that the average maximum axial force in 5 and 8story structures on typeII soil are 21% and 10% lower than the average maximum axial force in typeIII soil. On the other hand, the average maximum axial force in the 5story structure on typeII soil is 32% lower than that on typeIII soil. This value on typeII soil in the 8story buildings is 4% lower than that on typeIII soil.

To summarize the results, in Table 15 and Table 16, the comparative results between near and farfield ground motions are listed.


The careful examination of the values in Table 15 and Table 16 reveals the following:(i)The analysis of the maximum drift ratio values of structures built on typeII soil shows that the ratio of the maximum drift ratio of structures under the influence of nearfield earthquakes to that under the influence of farfield earthquakes increases 1.8 and 1.94 times, respectively, in 5 and 8storey structures with typeII soil. Assessing the maximum drift values of structures constructed on typeIII soil indicates that the ratio of the maximum drift under the influence of nearfield earthquakes to that under the influence of farfield earthquakes increases 1.17 and 1.97 times, respectively, in 5 and 8storey structures with typeIII soil. Thus, in both soil types, the maximum drift of structures is greater under the influence of nearfield earthquakes with a progressive directiontaking than that under the influence of farfield earthquakes.(ii)The maximum displacement values of structures constructed on typeII soil indicates that the ratio of the maximum structure displacement under the influence of nearfield earthquakes to that under the influence of farfield earthquakes is 4 and 3.75, respectively, in 5 and 8storey structures with typeII soil. The maximum displacement values of structures constructed on typeIII soil indicate that the ratio of the maximum displacement under the influence of nearfield earthquakes to that under the influence of farfield earthquakes increases 2.77 and 3.78 times, respectively, in 5 and 8storey structures with typeIII soil. The maximum axial force on the columns of structures constructed on typeII soil indicates that the ratio of the maximum axial force on columns under the influence of nearfield earthquakes to that under the influence of farfield earthquakes increases 1.18 and 1.28 times, respectively, in 5 and 8storey structures with typeII soil. The examination of the maximum axial force on the columns of structures constructed on typeIII soil indicates that the ratio of the maximum axial force under the influence of nearfield earthquakes to that under the influence of farfield earthquakes increases 1.16 and 1.24, respectively, in 5 and 8storey structures with typeIII soil.(iii)The maximum moment values in the columns of structures constructed on typeII soil indicate that the ratio of the maximum moment in columns under the influence of nearfield earthquakes to that under the influence of farfield earthquakes increases 1.38 and 1.47 times, respectively, in 5 and 8storey structures with typeII soil. The examination of the maximum moment of the columns of both structures constructed on typeIII soil indicates that the ratio of the maximum moment under the influence of nearfield earthquakes to that under the influence of farfield earthquakes is 1.58 and 1.83, respectively, in 5 and 8storey structures with typeIII soil.(iv)The comparison of the base shear force in a 5storey structure with typeII soil under the influence of nearfield and farfield earthquakes indicates that the base shear force of this structure is 1.17 times higher under the influence of a nearfield earthquake than under the influence of a farfield earthquake. Similarly, the ratio in an 8storey structure with typeII soil is equal to 1.16. A similar trend reveals that these ratios are 1.12 and 1.19 in 5 and 8storey structures on typeII soil.
5. Conclusions
Design regulations of resistant structures against earthquake in Iran classify the type of soil into 4 groups, types I, II, III, and IV. In this paper, the difference of seismic behavior of steel special moment frames constructed on types II and III soils subjected to far and nearfield earthquakes with forward directivity has been studied. In order to acquire comprehensive results, the effects of soilstructure interaction and panel zone in modeling of the frames have been considered. To model soil structure, the interaction direct method was utilized. According to classification of HAZUSMH MR5 [49] regulation, 5 and 8story buildings are considered as medium and highrise buildings. The following results were obtained by undertaking nonlinear timehistory analysis for modeling the aforementioned structures:(i)The results obtained from the structures subjected to nearfield earthquakes with forward directivity show that the maximum drift in 5story buildings on typeII soil in relation to typeIII soil has increased around 3%. This parameter in 8story buildings on typeII soil has augmented about 23% in relation to typeIII soil. On the other hand, in analyzing structures subjected to farfield earthquakes, a different trend can be seen in the sense that the maximum drift in 5story buildings on typeII soil was decreased approximately 30% in comparison with the obtained values for typeIII soil. But, similar to nearfield earthquakes, in 8story structures on typeII in proportion to typeIII soil, this value declined about 20%.(ii)Applying 7 nearfield earthquake records with forward directivity showed that the maximum difference between shear force value in the 5story structures on typeII and 5story structures on typeIII soil is insignificant, around 2%. This comparison revealed that the maximum shear force created in 8story buildings on typeIII soil is 11% greater than the value obtained for typeIII soil. Moreover, applying 7 farfield earthquakes records, it can be said that the maximum base shear force in a 5story building on typeIII soil in relation to the 5story structure on typeII soil had increased to about 7%. This comparison for the 8story building was about 8%.(iii)Studying the obtained results demonstrate that in both far and nearfield earthquakes, the maximum base shear force has been obtained for typeIII soil.(iv)Investigating the average maximum displacement of stories subjected to 7 far and nearfield earthquake records makes apparent that the average maximum displacement of stories in 5story structures on typeII soil in relation to typeIII soil has increased to 13%. This comparison for 8story buildings shows that this parameter has decreased to around 12% for typeII soil compared with typeIII soil. Furthermore, the results obtained from applying 7 farfield earthquake records to the 4 structures showed that the maximum displacement of the stories in 5story building on typeII in relation to typeIII soil has increased to around 29%, whereas for 8story building, this parameter has decreased to about 7%.(v)Evaluating the average maximum axial force caused in columns of the four structure subjected to nearfield earthquakes shows that the average maximum axial forces in columns of 5 and 8story buildings on typeII soil are 14% and 20% lower than the average maximum axial forces in 5 and 8story buildings on typeIII soil, respectively. Following that, the average maximum axial forces subjected to farfield earthquakes in 5 and 8story buildings on typeII soil are 21% and 10% lower than the values obtained on typeIII soil. Overall, the average maximum axial forces produced on typeIII soil subjected to far and nearfield earthquakes are greater than the corresponding values on typeII soil.(vi)By assessing the maximum moment created in columns in each of the 4 structures subjected to nearfield earthquakes, it can be said that the maximum moment of the columns in 5 and 8story structures on typeII soil are 17% and 24% lower than the maximum moment in 5 and 8story structure on typeIII soil, respectively. Following that, the average maximum moment in 5 and 8story structures in typeII soil are 32% and 4% lower than the average maximum moment in 5 and 8story buildings on typeIII soil, respectively. On the whole, the average maximum produced moment in structures on typeIII soil subjected to far and nearfield earthquakes are greater than the corresponding values in structures on typeII soil.(vii)In general, the drift, displacement, base shear force, moment, and axial force of columns in both types of soil are greater under the influence of nearfield earthquakes compared with farfield earthquakes.
Data Availability
No data were used to support this study.
Conflicts of Interest
The authors declare that they have no conflicts of interest regarding the publication of this paper.
Acknowledgments
This research was supported by a grant (18TBIPC14431501) from Technology Business Innovation Program (TBIP) funded by the Ministry of Land, Infrastructure and Transport of Korean government.
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Copyright
Copyright © 2018 Shahrokh Shahbazi et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.