Seismic Comparison of Hybrid Steel–Reinforced Concrete and Conventional Frames

Abstract

Conventional buildings made of reinforced concrete (r/c) or steel are practically encountered daily in common construction practice. Current regulations offer complete guidance on the seismic design and dimensioning of typical structures made of the same structural material throughout. Nevertheless, in the case of a structure constructed with r/c structural elements at the lower part and steel structural elements at the upper part, forming a so-called hybrid steel–r/c building is common. The present regulations do not address hybrid buildings in design or dimensioning. This study aims to fill this gap in the literature by comparing the seismic performance of 3D hybrid buildings to conventional r/c and steel buildings. Three sets of buildings are designed and dimensioned, namely r/c buildings, steel ones, and hybrid steel–r/c ones. The considered r/c, steel, and hybrid models are subjected to the same strong ground excitations using a nonlinear time history analysis, considering the potential impact of the excitation orientation. Especially for hybrid models, two limit interconnection conditions are dealt with, characterized here as a “fixed” or “fixed-pinned” support of the steel part upon the r/c one. Unitless parameters are selected to compare the seismic response diagrams to determine the most detrimental structural effect. The advantages and disadvantages of r/c, steel, and hybrid buildings are comparatively discussed in terms of seismic resilience, noting that a hybrid configuration provides a promising alternative for seismic performance compared to typical constructions, thus providing enhanced possibilities in structural design. The analysis results show that fewer structural failures occur for hybrid buildings compared to conventional ones when subjected to the same earthquake excitations. The findings suggest that hybrid buildings could be a viable solution for practical construction projects, particularly in seismic-prone areas.

1. Introduction

Nowadays, one can often observe in everyday construction routines that a building form comprises lower levels of reinforced concrete (r/c) stories and higher ones made of steel, named “hybrid” steel–r/c buildings [1]. The hybrid structural approach offers several advantages, such as facilitating vertical expansion in reconstruction, repair, structural strengthening, or changes in architectural utilization. In reality, it is common for a structure to be developed based on the benefits and drawbacks of different materials due to construction needs. Hybrid structures leverage the benefits of both materials, i.e., reinforced concrete provides stiffness and strength at the lower levels, while steel allows for reduced weight and faster construction at the higher levels. However, addressing challenges such as construction complexity, structural material compatibility, and potential cost implications is essential. Despite their practical applications, the present seismic standards [2,3,4,5,6,7] ignore the combination of different structural materials along the building height while providing full detailed guidelines for conventional buildings, i.e., the use of a single structural material in the entire building, such as reinforced concrete [2] or steel [3]. The current investigation aims to systematically evaluate the structural response of 3D hybrid models to traditional ones when subjected to the same near-fault ground excitations that the existing research has not addressed.

Numerous studies have been conducted on the earthquake planning and evaluation of structures by one material conforming to existing regulations [2,3,4,5,6,7], such as reinforced concrete (e.g., Fardis [8], Faifar and Krawinkler [9], Penelis and Kappos [10], Zameeruddin and Sangle [11], Lu et al. [12], and Otani [13]) or structural steel (e.g., Papagiannopoulos et al. [14], Gioncu and Mazzolani [15], Kato and Akiyama [16], and Li et al. [17]). Extensive research articles, experiments, and practical studies on reinforced concrete or steel structures are available.

The practical construction case of a lower building part made with reinforced concrete and an upper part made with structural steel, based on empirical rules and practical engineering experience, is often seen since no relative regulation is applicable. Limited studies are available regarding the creation and seismic evaluation of hybrid steel–r/c structures. Maley et al. [18] checked and found available traditional design methods applicable for plane frames with a change in the structural material along the building’s height, referred to as “mixed” frames, even though they are not included in design codes. Fanaie and Shamlou examined the response modification factors of various mixed frames [19]. Also, the “response reduction factor” was explored by Bhattarai and Shakya [20] regarding steel–r/c frames.

Currently, the investigated hybrid buildings have similar plans and dimensions for the reinforced concrete component accompanied by the steel component, both belonging to the primary structural system, to avoid confusion with smaller and lighter non-seismic building parts referred to as “secondary structures” [21]. Kiani et al. [22,23] studied the height-wise position of the transition story in mixed multistory frames using fragility curves. Ghanbari et al. [24] evaluated the reaction of “mixed” frames subjected to “mainshock-aftershock” ground motions. Askouni and Papagiannopoulos [25,26,27,28] studied the attitude of mixed 3D frames subjected to strong “near-fault” [25] excitations. Askouni [1] assessed the responses of hybrid buildings under recurrent ground motions— where the reinforced concrete part was assessed by referring to a past structure in which the upper structural steel part is supported—and considered a modern design, which is a building form widely seen in renovation and structural upgrade projects.

Li et al. [29] created an innovative, simplified algorithm for estimating the lateral loads for mixed structures. Kaveh and Ardebili suggested using an algorithm approach for the optimal design of mixed structures with given cost terms [30]. Kiani et al. [31] proposed seismic factors for mixed “concrete/steel buildings”. A “damage detection” procedure in plane “mixed frames” was performed using a “wavelet analysis” by Pnevmatikos et al. [32]. From the previously mentioned research studies, an interest arises in investigating selected cases of the hybrid building form. However, a systematic comparison of hybrid steel–r/c structures to typical r/c or steel structures has not yet been performed.

Hybrid structures comprise two different structural materials in height-wise story distribution, where the current code [4,5,6,7] does not supply a value-damping ratio, which is necessary for structural analysis, leaving a research gap. Papagiannopoulos [33] suggested the modal-damping ratio values for plane mixed steel–r/c building frames. Farghaly [34] provided diagrams for the estimation of an “equivalent” damping ratio for mixed “building systems”. Sivandi-Pour et al. [35,36] supplied a math technique for the calculation of a single “damping ratio” for “mixed buildings”, which was verified by Sivandi-Pour et al. [37] and successfully applied in recent research studies [1,25,26,27,28]. Liu et al. [38,39] investigated the application of rate-independent damping devices in tall base-isolated buildings.

The previously mentioned research works refer to structures resting on an ideal rigid soil assumption [4,5,6,7], as usually considered for typical buildings. In addition, Askouni [40] assessed the contribution of ground deformability on the hybrid frames’ demeanor as opposed to the inflexible soil by checking the effect of a variable excitation angle as raising interest in various works [41,42,43,44]. Dwivendi et al. [45] and Larijani and Tehrani [46] investigated the effect of earthquake orientation. Apart from the mentioned research works, a scientific gap is obvious in examining the dynamic nonlinear response of hybrid structures.

Thus far, previous studies have not addressed the comparison between ordinary reinforced concrete or steel buildings and hybrid steel–r/c structures, even for the usual rigid soil assumption. This study fills this gap by focusing on this novel investigation. Specifically, it presents the first systematic examination of the seismic nonlinear behavior of hybrid symmetric 3D models compared to conventional ones, considering the effects of varying near-fault excitation angles. This study employs practical, simplified 3D models with symmetrical plans designed according to the current codes for conventional RC and steel structures while using reasonable approximations for hybrid forms. This approach ensures that the findings broadly apply to various buildings. This study identifies the most critical response patterns through dynamic earthquake analyses of the selected models, emphasizing the importance of comparing hybrid structures to traditional ones. The necessity of this seismic comparison is further elaborated in the following sections.

2. Considered Buildings and Seismic Analysis

Simple geometrical 3D models are chosen for investigation here to serve the purpose of this work, which is comparing the seismic performance of hybrid buildings to conventional ones. The selected analysis models have the same geometric configuration, with an identical symmetrical plan across all levels; however, they are made of different structural materials, i.e., reinforced concrete, steel, or a combination of reinforced concrete for the lower part and steel for the upper part. The number of stories is selected from two to six to represent typical small- to medium-rise buildings. Selected 3D models are subjected to strong earthquakes to evaluate their seismic performance by dynamic analysis, which offers more accurate results than other analysis types [47,48]. The analysis results are evaluated, given the effect of the structural material combined with the most detrimental structural behavior. The findings are numerically justified by story horizontal deformation, story acceleration, and the maximum base shear that is evaluated through dimensionless parameters following current standards [2,3,4,5,6,7] and relative research works [49,50], as explained in the following section. Although various response variables can be found in the literature, in the current investigation, only the selected parameters are preferred so as to maintain the focus of this work on comparative seismic comparisons of the structural response of various 3D models because of the varying material effect. This study excludes asymmetries in planning or loading to avoid additional nonlinearities and focuses on the influence of different structural materials on seismic response. The following section provides a precise description of the model and analysis methods.

This investigation considers three series of buildings: a series of conventional r/c 3D frames, one of the traditional steel 3D frames, and a series of hybrid steel–r/c frames. Conventional and hybrid buildings are assigned similar geometrical characteristics and loadings for comparison.

The design includes five r/c frames with two to six stories, representing typical reinforced concrete buildings, following the current Eurocode standards [2,4,5,6,7]. The first story of all r/c frames has a height of 4.0 m, while all upper floors have a 3.0 m elevation (Figure 1). The r/c frame’s plan has dimensions of 15.0 × 15.0 m², comprising three identical 5.0 m spans in both horizontal directions. The choice of material strength depends on the structural demand, ensuring a balance between safety and efficiency. The materials are selected according to standards such as the Eurocodes [2,3,4,5,6,7], considering factors like durability, environmental resistance, and cost-effectiveness. The structural material is concrete grade C25/30, which is reinforced with steel B500c, according to Eurocode 2 [2]. At each story, r/c slabs are considered with a 0.15 m thickness, behaving as an inflexible diaphragm, carrying a passive weight of 5.0 kN/m² and an active weight of 2.5 kN/m² [51]. Following the applicable guidelines [2,3,4,5,6,7], the design of the r/c buildings considers typical buildings with an importance factor of 1.0, a zone spectrum acceleration of 0.36 g (where g refers to the gravity acceleration), a ductility class medium (DCM), a viscous damping ratio of 5%, and a design spectrum of I type and ground type C. The analysis assumes minimal impact from deformable ground effects and neglects them, and the maximum behavior factor is considered 3.9 for moment-resisting frames (MRFs), following the applicable code [4,5,6,7]. The seismic loadings on both lateral orientations follow the 0.30 rule of [4,5,6,7], with an accidental eccentricity of 0.05 [4,5,6,7]. The earthquake planning and check of the concrete buildings include the damage restriction for “brittle non-structural elements” [4,5,6,7]. The component size is selected based on structural load calculations, including dead, live, and seismic loads.

Table 1. Dimensions of vertical and horizontal structural elements of reinforced concrete models.

R/C Building (Figure 1a)
Story	Columns			Beams
	Section (m2)	Axial Bars	Stirrups	Section (m2)	Axial Bars	Stirrups
1	0.50 × 0.50	8 Ø 26	Ø 8/10	0.25 × 0.60	8 Ø 18	Ø 8/10
2	0.40 × 0.40	8 Ø 18	Ø 8/10	0.25 × 0.50	6Ø 14, 2Ø 8	Ø 8/10
R/C building (Figure 1b)
Story	Columns			Beams
	Section (m2)	Axial Bars	Stirrups	Section (m2)	Axial Bars	Stirrups
1	0.60 × 0.60	16 Ø 20	Ø 8/10	0.25 × 0.60	8Ø18	Ø 8/10
2	0.50 × 0.50	8Ø20, 4Ø14	Ø 8/10	0.25 × 0.60	8Ø18	Ø 8/10
3	0.50 × 0.50	8Ø20, 4Ø14	Ø 8/10	0.25 × 0.60	8Ø18	Ø 8/10
R/C building (Figure 1c)
Story	Columns			Beams
	Section (m2)	Axial Bars	Stirrups	Section (m2)	Axial Bars	Stirrups
1	0.60 × 0.60	16Ø20	Ø 8/10	0.25 × 0.70	8Ø20, 4Ø14	Ø 8/10
2	0.60 × 0.60	8Ø20, 8Ø16	Ø 8/10	0.25 × 0.70	8Ø20, 4Ø14	Ø 8/10
3	0.50 × 0.50	8Ø20, 4Ø16	Ø 8/10	0.25 × 0.60	8Ø18	Ø 8/10
4	0.50 × 0.50	8Ø20, 4Ø16	Ø 8/10	0.25 × 0.60	8Ø18	Ø 8/10
R/C building (Figure 1d)
Story	Columns			Beams
	Section (m2)	Axial Bars	Stirrups	Section (m2)	Axial Bars	Stirrups
1	0.70 × 0.70	8Ø22, 16Ø20	Ø 8/10	0.25 × 0.75	8Ø20, 4Ø14	Ø 8/10
2	0.70 × 0.70	16Ø20	Ø 8/10	0.25 × 0.70	8Ø20, 4Ø14	Ø 8/10
3	0.70 × 0.70	16Ø20	Ø 8/10	0.25 × 0.70	8Ø20, 4Ø14	Ø 8/10
4	0.70 × 0.70	16Ø20	Ø 8/10	0.25 × 0.70	8Ø18	Ø 8/10
5	0.50 × 0.50	12Ø18	Ø 8/10	0.25 × 0.60	8Ø18	Ø8/10
R/C building (Figure 1e)
Story	Columns			Beams
	Section (m2)	Axial Bars	Stirrups	Section (m2)	Axial Bars	Stirrups
1	0.75 × 0.75	16Ø22, 8Ø18	Ø 8/10	0.25 × 0.75	8Ø20, 4Ø14	Ø 8/10
2	0.75 × 0.75	16Ø22	Ø 8/10	0.25 × 0.75	8Ø20, 4Ø14	Ø 8/10
3	0.70 × 0.70	16Ø20	Ø 8/10	0.25 × 0.70	8Ø20, 4Ø14	Ø 8/10
4	0.70 × 0.70	16Ø20	Ø 8/10	0.25 × 0.70	8Ø20, 4Ø14	Ø 8/10
5	0.50 × 0.50	8Ø20, 8Ø14	Ø 8/10	0.25 × 0.70	8Ø18	Ø 8/10
6	0.50 × 0.50	8Ø20, 8Ø14	Ø 8/10	0.25 × 0.60	8Ø18	Ø 8/10

lists the final structural dimensioning of the r/c buildings based on the design process.

As a second step, the steel 3D buildings, as presented in Figure 2, are designed with the same geometrical assumptions and loadings as the previous r/c buildings. However, for the steel buildings, the structural steel grade is S355 [3], while the behavior factor has the maximum value of 4.0 for steel MRFs following [3], while the analysis considers composite slabs, respectively. The alignment of metal columns is depicted in Figure 3, aiming to form a robust outer frame.

Table 2. Dimensions of columns and beams of the steel frames.

Steel Frames	Steel Columns	Steel Beams
Two-Story Steel Frames (Figure 2a)	HEM 300 *	IPE 300 *
Three-Story Steel Frames (Figure 2b)	HEM 400 *	IPE 400 *
Four-Story Steel Frames (Figure 2c)	HEM 550 *	IPE 450 *
Five-Story Steel Frames (Figure 2d)	HEM 600 *	IPE 500 *
Six-Story Steel Frames (Figure 2e)	HEM 650 *	IPE 550 *

* The cross-section of the steel columns, as well as the steel beams, is the same across all stories concerning each steel building.

shows the dimensioning of steel columns and beams of the steel buildings, following the same loadings of the previous r/c buildings and design assumptions [4,5,6,7] to provide an objective evaluation by the sense of seismic endurance.

In the third step, the hybrid steel–r/c models combine the previous conventional r/c and steel models. The two-story hybrid model consists of the first story of reinforced concrete with a 4.0 m height, with an upper steel story of 3.0 m height (Figure 3a). Two r/c stories, the first with a 4.0 m and the second with a 3.0 m height, support a steel story with a 3.0 m height while forming the hybrid three-story model (Figure 3b). Similarly, three r/c stories, with a 4.0 m height for the first one and a 3.0 m height for the second and third ones, uphold a top steel story of 3.0 m, creating, in this way, the four-story hybrid model (Figure 3c). In a similar manner, the five- and six-story hybrid models consist of three and four corresponding r/c lower stories and two steel upper ones (Figure 3d,e), where the first story has a 4.0 m height, and all upper ones have a 3.0 m height. The layout of the hybrid models has the same length and width of 15.0 m, comprising three identical spans of 5.0 m each at both horizontal directions, similar to the conventional models.

For comparison purposes, the hybrid models’ reinforced concrete and the structural steel have the same quality as the corresponding r/c and typical steel models. The steel columns are justified to create a strong outer form, as shown in Figure 4a, similar to the steel buildings. Figure 4b presents a simple schematic plan of the considered models. The dimensioning of the hybrid models is completed individually for the steel and concrete portions, with maximum values of 4.0 and 3.9, which are appropriate [4,5,6,7] for the usual rigid soil assumption [4,5,6,7]. Composite slabs of a 0.15 m thickness act as an inflexible diaphragm at each story of the hybrid models.

Table 3. Dimensions of vertical and horizontal structural members, hybrid steel–r/c buildings.

Hybrid Building (Figure 3a)
Story	Material	Columns			Beams
		Section (m2)	Axial bars	Stirrups	Section (m2)	Axial Bars	Stirrups
1	Reinforced concrete	0.50 × 0.50	8Ø 22	Ø 8/10	0.25 × 0.60	8Ø 18	Ø 8/10
2	Structural steel	HEA360			IPE270
Hybrid Building (Figure 3b)
Story	Material	Columns			Beams
		Section (m2)	Axial bars	Stirrups	Section (m2)	Axial Bars	Stirrups
1	Reinforced concrete	0.55 × 0.55	16Ø20	Ø 8/10	0.25 × 0.60	8Ø20, 8Ø10	Ø 8/10
2	Reinforced concrete	0.50 × 0.50	8Ø20, 8Ø10	Ø 8/10	0.25 × 0.60	8Ø18	Ø 8/10
3	Structural steel	HEA360			IPE270
Hybrid Building (Figure 3c)
Story	Material	Columns			Beams
		Section (m2)	Axial bars	Stirrups	Section (m2)	Axial Bars	Stirrups
1	Reinforced concrete	0.60 × 0.60	16Ø20	Ø 8/10	0.25 × 0.70	8Ø20, 8Ø10	Ø 8/10
2	Reinforced concrete	0.60 × 0.60	8Ø20, 8Ø16	Ø 8/10	0.25 × 0.70	8Ø20, 8Ø10	Ø 8/10
3	Reinforced concrete	0.50 × 0.50	8Ø20, 8Ø10	Ø 8/10	0.25 × 0.60	8Ø18	Ø 8/10
4	Structural steel	HEA360			IPE270
Hybrid Building (Figure 3d)
Story	Material	Columns			Beams
		Section (m2)	Axial bars	Stirrups	Section (m2)	Axial Bars	Stirrups
1	Reinforced concrete	0.70 × 0.70	8Ø22, 16Ø20	Ø 8/10	0.25 × 0.70	8Ø20, 8Ø16	Ø 8/10
2	Reinforced concrete	0.70 × 0.70	16Ø20	Ø 8/10	0.25 × 0.70	2Ø20, 3Ø10	Ø 8/10
3	Reinforced concrete	0.70 × 0.70	8Ø20, 8Ø10	Ø 8/10	0.25 × 0.60	8Ø18	Ø 8/10
4	Structural steel	HEΒ500			IPE360
5	Structural steel	HEΒ500			IPE300
Hybrid Building (Figure 3e)
Story	Material	Columns			Beams
		Section (m2)	Axial bars	Stirrups	Section (m2)	Axial Bars	Stirrups
1	Reinforced concrete	0.70 × 0.70	32Ø20	Ø 8/10	0.25 × 0.70	8Ø20, 8Ø10	Ø 8/10
2	Reinforced concrete	0.70 × 0.70	16Ø20	Ø 8/10	0.25 × 0.70	8Ø18	Ø 8/10
3	Reinforced concrete	0.70 × 0.70	16Ø20	Ø 8/10	0.25 × 0.70	8Ø18	Ø 8/10
4	Reinforced concrete	0.70 × 0.70	16Ø20	Ø 8/10	0.25 × 0.70	8Ø18	Ø 8/10
5	Structural steel	HEA500			IPE400
6	Structural steel	HEA500			IPE440

provides the final dimensioned sections of the vertical and horizontal structural members of the hybrid structures. Comparing Table 1 and Table 3, the reinforced concrete columns of the hybrid models tend to have slightly smaller cross-section dimensions than the concrete buildings. The steel columns and beams of the hybrid frames have smaller sections than the ones, respectively, for the steel frames (Table 2 and Table 3).

The interaction between steel and concrete components is crucial in defining their overall structural behavior in hybrid structures. Two extreme boundary conditions are incorporated to account for this interaction, complying with the findings of Refs. [1,25,26,27,28,40]. The “fixed support” condition of the steel columns represents a fully restrained connection, enabling complete moment transfer in both horizontal directions. In contrast, the “fixed-pinned” connection ensures stability by allowing fixation along the minor sectional axis while permitting a pinned connection along the primary sectional axis [25,26,27,28]. Table 3 provides the detailed specifications and modeling considerations for the boundary conditions of the hybrid models for examination.

SAP2000 software [52] performs modal analysis on the three series of investigated models—r/c, steel, and hybrid.

Table 4. Modal characteristics of hybrid and conventional buildings.

Building Stories	Mode Number	R/C Buildings		Steel Buildings		Hybrid Buildings: Fixed Case		Hybrid Buildings: Fixed-Pinned Case
		Period (S)	Participating Mass Percentage	Period (S)	Participating Mass Percentage	Period (S)	Participating Mass Percentage	Period (S)	Participating Mass Percentage
Two stories	1	0.414	93%	0.525	96%	0.399	91%	0.429	83%
	2	0.414	93%	0.412	96%	0.399	91%	0.429	83%
	3	0.375	93%	0.407	91%	0.356	92%	0.394	80%
Three stories	1	0.469	89%	0.616	96%	0.506	89%	0.522	85%
	2	0.469	89%	0.446	96%	0.451	89%	0.474	85%
	3	0.427	89%	0.436	96%	0.425	90%	0.451	82%
Four stories	1	0.546	87%	0.711	95%	0.559	86%	0.571	83%
	2	0.546	87%	0.488	95%	0.559	86%	0.571	83%
	3	0.494	87%	0.463	95%	0.504	86%	0.519	82%
Five stories	1	0.569	85%	0.795	95%	0.581	80%	0.601	76%
	2	0.569	85%	0.552	95%	0.581	80%	0.601	76%
	3	0.517	85%	0.508	95%	0.522	81%	0.545	75%
Six stories	1	0.650	80%	0.863	95%	0.498	80%	0.509	82%
	2	0.650	80%	0.603	95%	0.498	80%	0.509	82%
	3	0.589	80%	0.54	95%	0.449	80%	0.465	83%

presents the main modal characteristics of the 15 models, listing the “modal period” and “participating mass ratios” [53] for each model’s three primary modes. The modal periods of the steel buildings are more significant than the corresponding values for the r/c and hybrid ones (Table 4). In the hybrid modes, the modal periods have relatively close values for the two extreme supports—” fixed” and “fixed-pinned”—with slightly greater values observed for the “fixed-pinned” case compared to the “fixed” one (Table 4). Regarding the three-, four-, and five-story models, the hybrid models present intermediate values of the modal periods compared to the corresponding values for the concrete and steel ones. The one- and six-story hybrid models present smaller values of the modal periods than the conventional models, as observed in Table 4.

Intense near-fault seismic events are chosen as inputs at the “nonlinear time history (NLTH)” [25] assessment of the conventional and the hybrid models by using SAP2000 software [52]. The seismic records are downloaded from the database of [54] with the main characteristics, as presented in

Table 5. Seismic near-fault event records in the analysis.

Plotline Number	Location of the Seismic Event	Year	Recording Station	Seismic Event Duration (S)	Mw	PGA (g)
1	“San Fernando”—USA	1971	Pacoima Dam	20.48	6.6	1.17/1.08
2	“Tabas”—Iran	1978	Tabas	63.48	7.1	0.93/1.10
3	“Landers”—USA)	1992	Lucerne Valley	48.05	7.3	0.81/0.73
4	“Kefalonia”—Greece	2014	Lixouri	67.74	6.1	0.67/0.60
5	“Cape Mendocino”—USA	1992	Petrolia	60.00	6.9	0.66/0.59
6	“Kobe”—Japan	1995	Takatori	41.15	6.9	0.61/0.62
7	“Loma Prieta”—USA	1989	Los Gatos	25.05	7.0	0.56/0.61
8	“Chi-Chi”—Taiwan	1999	TCU 052	90.01	7.6	0.50/0.36
9	“Superstition Hills”—USA	1987	Parachute Test Site	22.40	6.5	0.45/0.38
10	“Northridge”—USA	1994	Sylmar Converter St.	28.48	6.7	0.37/0.58
11	“Imperial Valley”—USA	1979	El Centro Array 6	36.90	6.5	0.34/0.46

, such as the event location, event year, recording station name, seismic event duration, “moment magnitude (M_w)”, and the “peak ground acceleration (PGA)” [1] in gravity acceleration “g” units [3]. The seismic motions are selected based on their magnitude and near-fault conditions to evaluate the seismic structural response under unfavorable circumstances. Selected recorded accelerograms have moment magnitude values in the range of 6.1~7.6, representing strong ground motions according to Hanks and Kanamori [55] and Kanamori [56], as well as being found helpful for comparison in the analysis of hybrid models [25]. Site conditions are categorized according to Eurocode 8 [4,5,6,7], considering ground type C as the mentioned design assumptions, which affect the amplification and response characteristics. More information on the detailed earthquake characteristics and site conditions appears in the database of [54]; however, this information does not appear here to maintain the study’s focus on the seismic structural comparison between the hybrid steel–r/c models and traditional ones.

Using the software Seismospect [57], the seismic event spectra are calculated and plotted in Figure 5, as well as the Eurocode 8 design spectrum [4,5,6,7] for the considered design assumptions of the examined models to ensure compatibility with the selected building design assumptions. As in Figure 5, the seismic event spectra have a range of values close and higher than the “design spectrum” [4,5,6,7], so these earthquake data are suitable for assessing the chosen buildings’ seismic reaction. The values of the first “modal period” [53] of the investigated buildings are within the critical value range of the earthquake spectra and the Eurocode 8 [4,5,6,7] design spectrum, so the chosen seismic records shall be helpful for investigating the seismic structural responses under adverse conditions.

In the seismic analysis, the input excitation angle alters the structural reaction within existing studies, e.g., [41,42,43,44,45,46]. Concerning the symmetrical building plan, the current selected excitation angles are 0° and 90° on X and Y, the primary lateral directions (Figure 4).

For the needs of nonlinear dynamic analysis, the “damping ratio” [53] value is 5% for the steel buildings and 7% for the structural steel ones, following [53]. The current codes do not refer to the determination of the damping ratio’s numerical value for the hybrid buildings. In contrast, one value is necessary to avoid wasting the laborious NLTH analysis performance time. The Sivandi-Pour et al. approach [35,36] is adopted here to calculate a single damping ratio value for each hybrid building, where the resulting values for the hybrid buildings are 0.0457 for the two-story one, 0.0433 for the three-story one, 0.0363 for the four-story one, 0.0231 for the five-story one, and 0.0214 for the six-story one.

The analysis model constructed by SAP2000 [52] simulates the performance of r/c elements under elastoplastic conditions during variable loading by employing pointed hinges on the two ends of the structural elements because of seismic excitation. These elastoplastic point hinges incorporate an essential mechanistic simulation, chosen due to the straightforward incorporation into the used analysis software [39] and compliance with the established theoretical principles and verification methods [2,4,5,6,7,8,9,10,52] to maintain this study’s interest in the seismic comparison of conventional and hybrid buildings’ responses. By the use of the elastoplastic point hinges at r/c members, I assessed the strength decrease, a 5% “post-yield hardening ratio”, a “backbone moment-rotation curve” via code ASCE 41-17 [58], and restrictions of the rotation of the hinges following the “seismic performance levels” of [59]. The reduced stiffness of the concrete elements is regarded as 0.5 [2,4,5,6,7] while integrating the “modified Takeda hysteresis model” [2,52] into the analysis model to include the stiffness degradation and strength deterioration [2,52,58]. A potential shear failure is checked by using the appropriate regulations [2,4,5,6,7]. Similarly, for steel elements, nonlinear “point hinges” [14,15,52] are applied at the two element ends, assuming a 2% strain hardening, the respective “hinge rotation” limits for steel sections regarding the “seismic performance levels” [3,58], and all necessary mechanistic properties [3,52,58]. The aforementioned simplified approximation of the nonlinearities of r/c or steel structural elements is included in the analysis to focus on the current study’s aim, which is to compare the seismic performance of the hybrid buildings to typical ones without involving the complex material mechanical models found in various research articles.

Finally, a significant number of 440 NLTH analysis cases are performed for conventional r/c and steel models (Figure 1 and Figure 2) and hybrid models (Figure 3) for both considered interconnections of the concrete component, which supports the steel one. This is calculated as a math product of 20 (referring to the model number) times 2 (for seismic directions of 0° and 90°) times 11 (referring to the number of seismic events in Table 5). The suggested response outcomes are shown and commented on subsequently.

3. Results and Discussion

This chapter sorts and reviews the NLTH analysis results for all typical and hybrid steel–r/c models, using qualitatively dimensionless factors to compare their earthquake responses, as mentioned in the following:

• The “interstorey drift ratio (IDR)” [59] on both horizontal global directions is represented and comparatively discussed according to the respective performance code limits [59], which are “1%” regarding “Immediate Occupancy (IO)”, “2%” regarding “Life Safety (LS)” and “4%“ regarding the “Collapse Prevention (CP)” levels concerning r/c MRFs; while for steel MRFs, the respective limit values are “0.7%” for “IO”, “2.5%” for “LS” and “5%” for the ”CP” levels [59]. The IDR plots present their distributions along the height of each building.
• The amount of “peak floor acceleration” [1] divided by “peak ground acceleration PFA/PGA” [1,25,60] is presented graphically for the examined buildings along the X-Y axes for each earthquake input.
• The normalized base shear ratio, called here for brevity reasons the “FX ratio”, denotes the fraction of the highest base shear absolute value divided by each building’s weight along the X-axis while being used for a comparative evaluation of typical r/c or steel and hybrid models. The “FY ratio” means the corresponding ratio along the Y direction.

For the reader’s convenience, this investigation divides the analysis response plots into subsections according to the number of stories for the r/c, steel, and hybrid steel–r/c buildings. Interest is raised in the maximum response parameters to examine each building’s form by the most detrimental aspect while maintaining a focus on the current investigation’s aim. Especially for the hybrid buildings, the presented plots are given in each case for the most unfavorable response results, comparing the two boundary support conditions “fixed” or “fixed-pinned” accordingly. The results focus on comparing the seismic performance of conventional and hybrid buildings without analyzing the interconnection effect for hybrid frames, which Refs. [25,26,27,28] discuss.

3.1. Two-Story Buildings

For the two-story r/c buildings, the IDR-X has greater values at the first-story level than the other levels (Figure 6a), up to 1.95%, within the LS limit [59]. In the second-story steel buildings (Figure 6b), the IDR-X increases more at the second story than the other stories, with a maximum value of 2.95%, which is within the CP level [59]. The IDR-X of the hybrid building with a “fixed-pinned” interconnection (Figure 6c) has a maximum value of 2% at the first story and 4.1% at the second story within the CP level [59]. For the steel frame, the IDR-plotline for the earthquake of Kobe with 90° is omitted because it exceeds the limits of [59], showing structural failure, while this effect does not appear in the r/c and hybrid frames.

The IDR-Y of the r/c model is analogous to the IDR-X, displaying the highest number of 2.6% for the first story (Figure 7a). The IDR-Y has an upper limit of 3.9% for the reinforced concrete and 3% for the hybrid buildings (Figure 7b,c). The IDR tends to have greater values at the Y-axis than the X-axis for the considered buildings (Figure 6 and Figure 7). The maximum IDR-X for the two-story hybrid buildings is greater by 39% than the respective one for r/c (Figure 6 and Figure 7). The IDR-Y for the hybrid building has the most outstanding value, smaller by 26% for the steel building and greater by 15% for the r/c one (Figure 6 and Figure 7).

The PFA/PGA-X rises with the height of the considered buildings by up to 2.45 for the r/c frame, 3.7 for the steel one, and 2.45 for the hybrid one, as in Figure 8. The PFA/PGA-Y has the most significant values at the building’s top (Figure 9), up to 3.5 for the reinforced concrete building, 4.2 for the steel, and 3.7 for the hybrid one. Observing Figure 8 and Figure 9, the PFA/PGA tends to have similar values for the hybrid and r/c buildings while presenting a bigger value by 50% for the steel ones compared to other cases on the X-axis and correspondingly by 13~20% on the Y-axis.

The base shear ratio “FX ratio” presents 4.5 as the top value for the two-story steel building and a minimum of 1.69 for the r/c ones (Figure 10). The FX ratio has very close values for the two interconnection types of hybrid structures, shown in Figure 10 as 1.86 for the steel part’s “fixed” support on the r/c one and 1.92 for the “fixed-pinned” support.

As observed in Figure 11, the base shear ratio “FY ratio” presents the most significant value of 3.5 for the steel building and the smallest one of 2.3 for the r/c one, with intermediate values for the hybrid buildings as 2.4 for the “fixed” and 2.3 for the “fixed-pinned” support. The base shear ratio for the hybrid buildings tends to have values close to the ratio for the r/c buildings (Figure 10 and Figure 11).

3.2. Three-Story Buildings

For the three-story r/c buildings, the IDR-X has a maximum value of 2.9% within the CP level [59] while tending to have greater values on the first floor (Figure 12a). Concerning the steel buildings, the IDR-X scores tend to be higher at the second-story level, with a maximum value of 3.6% within the CP level [59] (Figure 12b). For the hybrid steel–r/c buildings, the IDR-X displays more significant values on the first floor than the second and third ones, with a maximum of 2.2% (Figure 12c), while presenting smaller values than, respectively, for the conventional buildings.

The plot of the IDR-Y scores for the three-story r/c buildings (Figure 13a) resembles the IDR-X plot’s form with the highest value, 2.1%, within the CP level [59]. The IDR-Y plot of the steel buildings presents excellent values at the first-story level, with a maximum of 4.6%, and smaller ones at the second and third stories, even by ¼ times (Figure 13b). For the hybrid three-story buildings, the IDR-Y presents more significant values at the first story, up to 2.9%, and is relatively smaller at the second story, up to 2.5%, while at the third story, this is up to 2.3% (Figure 13c).

The IDR in both directions has greater values for the steel three-story buildings than the r/c and hybrid ones (Figure 12 and Figure 13). Exceedance of the permissible IDR limits [59] was noticed at the r/c three-story building for the earthquake of Tabas 0°, at the steel building for the earthquake of Kobe 90°, and at the hybrid building with “fixed” support for the earthquake of Loma Prieta 0°.

As displayed in Figure 14 and Figure 15, the PFA/PGA ratio rises with the building elevation for the considered three-story structural cases in the X and Y directions, with greater values at the building’s top. The PFA/PGA-X increases up to 2.7 for the r/c building and 3.2 for the steel and the hybrid buildings (Figure 14). As plotted in Figure 15, the PFA/PGA-Y raises to 3.2 for the reinforced concrete building, 4.1 for the steel one, and 3.4 for the hybrid building. Comparing Figure 14 and Figure 15, the PFA/PGA tends to have a slightly greater range of values at the Y-axis instead of the X-axis while presenting greater values when considering the steel building, smaller values for the concrete one, and intermediate values regarding the hybrid cases.

Figure 16 shows that the base shear ratio, the “FX ratio”, has similar values in the range of 1.76~1.85 for the r/c and hybrid three-story buildings, while a more considerable value of up to 3.75 is seen for the steel buildings, which is almost double the value than the respective one for the other cases. The base shear ratio, the “FY ratio”, is plotted in Figure 17, showing a reduced value of 1.82 for the r/c building, a greater 2.49 for the steel building, and an intermediate of 2.04~2.05 for the hybrid models with both interconnection types.

3.3. Four-Story Buildings

The maximum IDR-X value for the r/c four-story buildings is noticed at the first story as 2.5%, which is within the CP level [59] (Figure 18a). The maximum IDR-X occurs at the second story of the steel buildings at 1.8% in the LS level [59] (Figure 18b). The maximum IDR-X value of the hybrid buildings with a fixed interconnection is 3.5% at the first-story level (Figure 18c), which is similar to the r/c buildings.

The IDR-Y displays maximum values of 2.4% at the r/c buildings, 4.9% at the steel ones, and 2.4% at the hybrid steel–r/c ones, where all presented IDR values are within the CP level [59] (Figure 19). The form of IDR-X and IDR-Y plots is quite similar for the hybrid and r/c four-story buildings, as displayed in Figure 18 and Figure 19.

The IDR limits [59] are surpassed, showing structural failure, at the r/c four-story model for the excitations of Tabas 90° and Northridge 90°, at the steel model for Loma Prieta 90° and Kefalonia 90°, and the hybrid model considering both interconnection types, for the excitations of Landers 0°, Loma Prieta 90°, and Kefalonia 90°, so these IDR plotlines are omitted.

The “PFA/PGA” varies relative to the rise of the considered four-story buildings in the X and Y directions, as seen in Figure 20 and Figure 21. The “PFA/PGA-X” presents the highest value of 3.1 at the top of the r/c and hybrid buildings (Figure 20a,c). The PFA/PGA-X rises to 3.7 at the highest point of the steel frame (Figure 20b), which is a more excellent value by 19.3% than, respectively, for the r/c and hybrid buildings (Figure 20a,c).

Similarly, the PFA/PGA-Y increases to 3.2 for the r/c and hybrid four-story buildings (Figure 21a,c), which is very close to the top score in the X direction. At the hybrid steel–r/c building, the PFA/PGA-Y increases intensely at the first story, up to 4.1, and has the highest value of 4.8 at the top (Figure 21b), which is bigger by 50% than the respective number for the r/c and hybrid buildings.

As shown in Figure 22, the base shear ratio “FX ratio” has a reduced value of 1.74 for the r/c four-story buildings, a maximum of 3.44 for the steel ones, and 2.16~2.17 for the hybrid ones. In the Y direction, the base shear ratio rises to 1.98 for the steel buildings, shows moderate values close to 1.7 for the hybrid buildings, and a minimum value of 1.33 for the r/c buildings (Figure 23). The base shear ratio tends to have more significant values in the X direction than the Y, by 31% for the r/c frames, 70% for the steel ones, and 8.5% for the hybrid frames (Figure 22 and Figure 23).

3.4. Five-Story Buildings

The displayed IDR-X plotlines of the r/c five-story building (Figure 24a) demonstrate that maximum IDR-X values occur at the first story, up to 2.2%. Similarly, at the hybrid steel–r/c building with a fixed-pinned interconnection, the maximum IDR-X values are observed at the first story at 3.4% (Figure 24c). The IDR-X diagram of the steel building shows increased values at the second-story level, even up to 5%, which is the CP level’s peak [59] (Figure 24b).

The IDR-Y of all five-story models presents greater values at the first story (Figure 25), up to 2.7 for the r/c model, 3.9% for the steel one, and 2% for the hybrid one, which are within the permissible limits of the CP level [59]. At both axes, higher IDR values for the steel model are noted compared to the r/c and hybrid ones.

For the five-story buildings, the IDR plotlines are omitted, as follows, because of violations of the allowed norms [59]: regarding the r/c frames for the earthquakes of Tabas 0° and Kobe 90°; regarding the steel models under the earthquakes of Landers 90°, San Fernando 90°, Loma Prieta 90°, Chi-Chi 90°, Tabas 0°, Superstition Hills 90°, Northridge 90°, and Imperial Valley 90°; regarding the hybrid models with fixed support for San Fernando 90° and Tabas 0°; regarding the hybrid models with the fixed-pinned backing for the earthquakes of San Fernando 0°, Loma Prieta 0°, and Imperial Valley 90°. Obviously, the steel models exhibit more structural failures.

The PFA/PGA-X ratio increases floatingly along the model’s elevation, with greater values at the rooftop (Figure 26). The hybrid model top reaches the highest PFA/PGA-X ratio at 4.2 (Figure 26c), while the r/c model top has the lowest at 3.6 (Figure 26a). The steel model (Figure 26b) shows intermediate values. Similarly, the PFA/PGA-Y plots fluctuate and increase along the building height in Figure 27, up to 4.6 for the hybrid model, 3.7 for the concrete one, and 3.2 for the steel model.

Along the X direction, the base shear ratio is shown (Figure 28) to have the lowest value for the r/c building, as 1.7; the greatest for the r/c building, as 3.2; and intermediate for the hybrid buildings with both considered interconnections, as 2.3~2.5. However, in Figure 29, the Y-axis base shear ratio displays the highest value for the hybrid buildings, up to 1.9 and 1.6 for the r/c and steel buildings.

3.5. Six-Story Buildings

At the six-story r/c building, the IDR-X value reaches its highest at the first story, measuring 3.1% within the CP level [59], where the IDR-X seems to reduce with the increase in the building height above the first story (Figure 30a). At the steel building in Figure 30b, the maximum IDR-X is displayed at the second story as 2%, within the LS level [59], where the IDR-X tends to decrease along the building height.

Similarly, the IDR-Y of the concrete building shows a maximum value of 2% at the first story (Figure 31a) and smaller values above the first story. At the steel building, the IDR-Y has the highest value, 4.2%, inside the CP stage [59].

At the hybrid steel–r/c building with a fixed-pinned connection, the greatest IDR-X is observed at the first story, up to 2.8%, within the CP level [59]. In the hybrid model, the IDR-X, above the first story, decreases with the increase in the building height, close to almost zero, as 0~0.3% values at the fourth story, which is the floor of the steel part linkage with the concrete one. Then, above the fourth floor of the hybrid six-story building, the IDR-X tends to have greater values, up to 1.5%, at the building’s top (Figure 30c). Analogously, the IDR-Y shows a maximum value at the first story, close to 3%, a more minor value at the interconnection story, at 0~0.5%, and an intermediate value close to 2.1% at the building’s top (Figure 31c).

A surpass of the allowable IDR restrictions [59] is noticed at the r/c six-story building under the earthquake of Superstition Hills 90°; at the steel model under the Landers 0°, San Fernando 90°, Loma Prieta 90°, Chi-Chi 90°, Kefalonia 90°, Tabas 90°, and Northridge 90° earthquakes; at the hybrid models with a fixed interconnection for the earthquakes of Loma Prieta 90° and Northridge 90°; and for the hybrid models with a fixed-pinned interconnection for the excitation of Tabas 90°. Consequently, more building failures are found for the steel six-story building, less for the r/c one, and a small number for the hybrid six-story building.

At the r/c and steel six-story buildings, the PFA/PGA-X ratio increases variably in building height up to 3 and 4, respectively (Figure 32a,b). At the hybrid steel–r/c building, the PFA/PGA-X ratio tends to increase variably close to 3.9 up to the interconnection story, and then for the upper stories, and it tends to decrease to 0.9~2.9 (Figure 32c) variably.

Along the Y-axis, the PFA/PGA ratio of the r/c building tends to increase variably along the building height up to 3.5~3.7 (Figure 33a). At the six-story steel building, the PFA/PGA-Y ratio varies increasingly up to the range of 4.3~4.5 (Figure 33b). The PFA/PGA-Y ratio of the hybrid building (Figure 33c) presents a similar form to the one for the X-axis, with greater values up to 3.4 at the fourth story and smaller values at the building’s top to 2.4.

As presented in Figure 34, the base shear ratio, the “FX ratio”, has values in the range of 2.15~2.17 for the hybrid six-story buildings, slightly smaller than 2.05 for the r/c one, and higher than 2.8 for the steel building. As plotted in Figure 35, the base shear ratio, the “FY ratio” of the hybrid buildings, displays values of 2.2~2.3, similar to those in the X direction. The minimum base shear ratio, the “FY ratio”, is presented for the steel six-story building as 1.3, while an intermediate value of 1.5 occurs for the r/c building (Figure 35). As displayed in Figure 10, Figure 11, Figure 16, Figure 17, Figure 22, Figure 23, Figure 28, Figure 29, Figure 34, and Figure 35, the normalized base shear may present values exceeding 1, which means an increased base shear force because of the influence of near-fault ground motions. This phenomenon is consistent with previous research on structures subjected to near-fault earthquakes [61,62,63] that often induce high-intensity seismic demands.

4. Conclusions

In this original study, conventional r/c, steel, and hybrid steel–r/c steel buildings are designed and dimensioned according to the applicable codes [2,3,4,5,6,7]. The hybrid and typical buildings are subjected to strong near-fault ground excitations, considering angle orientations of 0° and 90°. “Nonlinear time history analysis” [1] is accomplished at the three series of examined frames, incorporating an appropriate mechanical model for the elastoplastic demeanor of the r/c sections of the structural members and steel ones. In addition, for the hybrid steel–r/c frames, two boundary interconnection types are examined concerning the “fixed” or “fixed-pinned” backing of the structural steel portion on the reinforced concrete part. As an initial investigation in the literature, the analysis compares earthquake reactions among the 3D hybrid frames and the typical r/c and steel frames, using the latter as a reference point. This investigation limits its findings to regular building structures with symmetrical in-plan configurations and does not consider the possible effect of ground deformability. The comparative presentation of the response results and plot used unitless factors to ensure objective findings on structural resilience, as noticed subsequently.

Using hybrid building materials is cost-effective due primarily to the smaller cross-sectional dimensions of structural elements. Compared with traditional reinforced concrete (r/c) and steel buildings, hybrid models adopt less material while preserving structural integrity, which may reduce material costs and construction times. This advantage emphasizes hybrid construction’s economic and practical benefits, making it an attractive choice for maximizing performance and affordability in modern building design.

The findings of the 3D NLTH analysis show that hybrid buildings demonstrate greater structural resilience compared to conventional steel and reinforced concrete similar buildings. The smaller number of structural failures suggests that the hybrid building form may enhance overall safety and durability. Additionally, the increased exceedance of the inter-story drift ratio (IDR) limits in steel buildings highlights potential challenges in their design, emphasizing the need for improved lateral stiffness strategies. These results underscore the advantages of hybrid construction in mitigating structural vulnerabilities, making it a promising approach for future engineering applications.

The inter-story drift ratio (IDR) distribution shows crucial structural performance features across building types. The highest IDR values in reinforced concrete (r/c) and hybrid buildings are concentrated on the first level, showing that these structures deform more near the base. In contrast, steel buildings have an increased IDR on the second floor, indicating an individual deformation profile that may need unique design approaches. Furthermore, conventional steel and r/c structures exhibit a greater IDR in the Y-axis, whereas hybrid buildings have a more balanced reaction in both horizontal directions, emphasizing their structural stability. The more significant fluctuation in the IDR for steel buildings, notably during the collapse prevention (CP) performance stage, highlights their increased susceptibility to severe deformations. These findings illustrate the importance of specialized engineering approaches to enhance steel structural resilience further and establish hybrid systems’ effectiveness in minimizing excessive lateral displacement under strong earthquakes.

Steel buildings have a higher dimensionless PFA/PGA ratio than the reinforced concrete (r/c) or hybrid models, with hybrid buildings showing slightly higher respective values than the r/c ones. The latter shows that steel structures experience more significant floor accelerations concerning strong ground motions, which could harm occupant comfort and the function of non-structural components. Also, regarding all structures analyzed under the selected near-fault earthquakes, the PFA/PGA ratio is generally higher along the Y-axis than the X-axis, showing directional sensitivity in seismic response. These findings emphasize the significance of considering the building type and the directional effects in the structural design for earthquake resilience.

The dimensionless base shear ratio is generally higher in steel buildings than in reinforced concrete (r/c) and hybrid structures, implying that steel frames must resist greater lateral forces. In contrast, r/c and hybrid buildings have similar base shear ratios, reflecting similar seismic force distribution. These research results show the impact of material characteristics and structural systems on seismic load resistance, pointing out the importance of specialized design techniques to optimize the performance of the hybrid building form.

By reviewing the structural failures, allowable inter-story drift ratio (IDR) restrictions, and overall seismic response, the hybrid steel–reinforced concrete buildings appear to be an appealing alternative to traditional constructions. In addition, based on the observed response parameters, hybrid steel–r/c buildings have advantages in resisting near-fault ground motions compared to traditional ones. The improved efficiency and balanced seismic behavior of the hybrid building’s form pose it as an accessible option for modern practical construction, providing both structural resilience and practical advantages.

Future research could enhance connectivity mechanisms between the steel and reinforced concrete components to optimize the hybrid structure design and improve load transfer efficiency. In addition, research into innovative materials, such as high-performance concrete or shape-memory alloys, could boost seismic resilience. Using computational optimization models incorporating machine learning techniques could indicate optimal hybrid design configurations for particular seismic zones.

Ongoing studies might broaden this study by investigating hybrid constructions under varied earthquake intensities, soil characteristics, and loading scenarios. Experimental validation of the numerical conclusions through large-scale testing would also provide further information on hybrid building performance. In addition, life-cycle cost analysis and sustainability investigations could better understand hybrid construction’s longer-term advantages. The hybrid steel–reinforced concrete system may be further optimized by addressing these challenges, making it a more durable and efficient option for modern seismic-resistant building designs.