1. Introduction
In recent decades, the number of high-rise buildings has increased all over the world. They are designed and built using innovative structural systems and lighter materials. So, these structures tend to be more flexible with lower damping than previous constructions. However, these buildings are more sensitive to dynamic excitation caused by strong earthquake loads in comparison to older structures. The majority of tall structures are prone to seismic loads and vibrations, and, hence, it is essential to design these structures appropriately. The mass, stiffness, natural period, and damping coefficient are the main dynamic characteristics of these structures. If a structure has a lower natural frequency, then it vibrates significantly under seismic loading. In addition, the structural design response is considered an important factor in the design of tall structures [
1]. There has been a significant amount of research on tall buildings with various characteristics such as vertical irregularity [
2], irregular plans [
3], dual structural systems [
4,
5], and buildings with a basement [
6]. In addition, newly-developed techniques to enhance the seismic performance of RC structures, such as using shape memory alloys with superelastic behavior instead of conventional steel bars, have been investigated in several research studies [
7,
8,
9,
10]. From a structural design perspective, this type of building requires concrete with superior properties to withstand the high stresses due to the increasing weight of buildings in addition to the lateral loads effect. Nowadays, conventional and high-strength concrete, which has a compressive strength range from 40 to 70 MPa, is commonly used in the design of these buildings. In general, several research studies have categorized concrete based on its compressive strength into conventional (20 to 50 MPa), high-strength (50 to 80 MPa), high performance (80 to 150 MPa), and ultra-high-performance (150 to 220 MPa) concrete [
11,
12,
13]. Conventional concrete is slowly being replaced with a new, more densified type of concrete called ultra-high-performance concrete (UHPC). This new type of concrete is able to possess more advanced properties in terms of compressive strength, durability, tensile strength, ductility [
14,
15,
16,
17,
18], the placement of concrete, its brittleness, and the life span of the material. UHPC is considered suitable for bridge decks, piers, strengthening and repair applications, high ductility designs, and elements for blast protection [
14,
19,
20,
21,
22]. Some other UHPC applications include beam shells [
23], column shells [
24], and closure joints between prefabricated bridge deck elements [
25,
26]. Moreover, UHPC has a lower porosity and moisture content compared to other concrete types, which assists in structural rehabilitation [
16,
18,
26,
27,
28]. However, one of the main limitations of UHPC is that it is perceived as a costly material due to its highly-densified structure. Also, UHPC requires high-temperature curing and fine quartz powder, increasing the associated cost and energy consumption [
29]. In addition, Liu et al. reported that UHPC loses most of its strength at high temperatures (higher than 500 °C) [
30]. However, fire proofing could be implemented in such cases to improve the fire resistance of UHPC. Thus, this paper compares the seismic performance and analyzes the cost of different models of a 60-story building developed using high-strength concrete and UHPC. In addition, this paper utilizes ductile coupled shear walls to resist the applied seismic loading. Shear walls are usually used as the lateral force resistant system due to their high in-plane stiffness. Almost all shear walls tend to have openings due to the architectural requirements of the building. These openings split the shear walls into two or more slender walls connected using coupling beams. The main advantages of utilizing a coupled shear wall system include the reduction of moment due to the coupling action, the dissipation of seismic energy through the coupling beams over the stories, and a higher lateral stiffness compared to the sum of its individual wall piers [
31]. Moreover, hybrid coupled shear walls consisting of reinforced concrete shear walls with steel coupling beams are considered a suitable and appropriate alternative to reinforced concrete shear walls [
31,
32].
Recently, several studies compared different techniques used to optimize the seismic response of RC structures. In other words, using alternative materials such as FRP bars instead of conventional steel reinforcement led to a significant improvement in different aspects, as reported in [
33,
34,
35,
36,
37,
38,
39,
40,
41,
42,
43,
44,
45,
46,
47,
48,
49,
50,
51,
52,
53,
54]. Furthermore, cost-analyses were reported in different seismic applications [
55,
56,
57,
58,
59,
60,
61,
62,
63,
64,
65,
66,
67]. They mentioned a significant reduction in costs in addition to the enhancement of the performance of structural elements [
68,
69,
70].
Despite the advanced behavior of UHPC in the construction and design industry, no guidance yet exists for the ways in which a designer can optimize the use of UHPC in structural designs. Numerous studies have investigate UHPC [
14,
26,
71,
72,
73,
74,
75,
76,
77,
78,
79,
80,
81,
82,
83,
84,
85]. Barnett et al. investigated the effect of the addition of fiber on the strength of UHPC [
71,
79]. A study conducted by Graybeal, in 2007, proved that UHPC gained over 70 MPa of strength within two days of curing time [
72,
79]. Another study by Wille et al., in 2011, even produced UHPC with a compressive strength of greater than 210 MPa without steam curing or pressure [
73,
79]. Khalil et al. studied the effect of using UHP-SHCC in retrofitting some existing structures subjected to repeated loading [
74,
75,
76,
77].
Naeimi and Moustafa [
78] studied the overall behavior, failing mechanism, and reinforcement values of UHPC, as well as the detailing of its design using finite element modeling and using pushover analysis on DIANA FEA. A bridge with two columns was investigated with the use of UHPC instead of traditional concrete. A parametric analysis was investigated in order to study the effect of different steel ratios and other parameters on the overall behavior of the two piers. To evaluate the difference, a similar model was prepared and analyzed using conventional concrete. The results obtained showed that high-strength steel is essential to utilize the mechanical properties of UHPC to the fullest. The results also showed that the UHPC columns carried more flexure when compared to conventional columns, and this may prove to be economical since smaller cross-sections can be utilized for the columns.
Azmee and Shafiq [
80] discussed the importance of using UHPC in the structural field and how it is regarded as the future of concrete. The different design procedures and applications of UHPC are discussed in the paper. UHPC is regarded as the future because it exhibits remarkable properties including very high compressive strength values and self-placing and densifying properties. At the end of the paper, the authors manage to confirm that UHPC possesses better properties than conventional concrete and that it can help to improve the sustainability of buildings. However, it is not very commonly used because of its perceived high cost and challenging design procedure.
Oesch et al. [
81] examined the properties and behavior of cementitious material in conventional concrete and UHPC at the micro-scale. This was done in order to better understand the role of these materials and further advance concrete design in the future. The authors identified relationships between design parameters of different materials used in concrete mixing, such as the stiffness parameter and information regarding cracking parameters and the crack volume. A negative gradient relationship was obtained between the stiffness parameters and crack volume for both concrete types.
Valikhani et al. [
26] discussed the fact that bridges are always exposed to harsh environmental conditions around them. These harsh conditions can lead to the deterioration of concrete and reduce the service life of the bridge. This paper used UHPC as a repair material to fix issues with bridges in service. The UHPC was used as a sealing layer or crack filler on top of the bridge made of conventional concrete. The issue with this repair strategy is that there is very limited knowledge regarding the bond strength between UHPC and conventional concrete. This paper tested the bond strength using 30 specimens tested under the bi-surface shear test with different surface preparation and roughness degrees. The results concluded that the presence of some roughness between the two surfaces (UHPC and conventional concrete) allows for the growth of a strong bond between the two surfaces involved.
Since the flexural capacity of reinforced columns deteriorates with underground motion due to the crushing of the concrete and the buckling of longitudinal steel bars, research has focused on lessening the damage at the plastic hinge regions of bridge columns in order to develop bridge columns that are seismic-resistant. In their efforts to mitigate this issue, Ichikawa et al. [
79] studied the use of locating UHPC segments in plastic hinge regions. Three small-scale columns were tested. Two columns were tested by bilateral cyclic loading and had different plastic hinge detailing. One column had a reinforced concrete core covered in a UHPC jacket. Another column was post-tensioned and had a UHPC hollow-core plastic hinge. Despite the differences in detailing, both columns were designed to have the same strength. The point behind the bilateral cyclic loading was to inflict deformations due to flexure and test the possibility of having torsional modes within the columns. The results proved that the concrete core column successfully carried the applied axial loads to a drift of 6%, while the post-tensioned column carried the applied axial load to a drift of 3.5%. The results showed that the UHPC column proved to have a good performance against seismic loads.
Wille et al. [
82] tested the validity of most of the flexural stress and strain models available for calculating the flexure strength of UHPC. The authors considered nine models and tested them by comparing the theoretical results obtained from the models to real experimental data. The reason behind this comparison was the common confusion regarding which model accurately represents the performance of UHPC in flexure. For instance, design guidelines suggest that the way to design ultra-high-performance concrete is by using stress–strain relationships and stress block assumptions. However, assumptions are not enough, and the paper in question demonstrated that ultra-high-performance concrete requires more accurate information. Therefore, the given paper evaluated different combinations of compression and tension stress blocks with experimental data and previous research results in order to find the most precise model for UHPC.
Arora et al. [
14] explored the different ways of mixing aggregates and binder to produce sustainable and economical UHPC. Both the aggregates and the binder were selected using the maximum packing density suggested by a compressible packing model to produce UHPC with compressive strengths higher than 150 MPa. In order to achieve UHPC, a cement mass replacement between 30% to 50% was studied. Three aggregate sizes of 6.25 mm, 4.75 mm, and 2.36 mm were selected. The preliminary cost analysis indicated that the use of filler materials and cement replacements reduces material costs. This could be attributed to the reduction in ordinary cement paste content.
Many researchers have studied the performance of UHPC, focusing on its development and behavior from a materials-related perspective. Dong [
83] examined the findings obtained from material-related journal papers in a structural design context. To do that, Dong modeled prestressed bridge girders made of UHPC instead of traditional concrete. Dong studied their durability, structural integrity, environmental impact, and cost. After comparing the life-cycle cost of the same bridge modeled using UHPC and normal concrete, the results proved that UHPC is sufficient to prolong the service life of a structure while improving the environmental impact in the long run and reducing the annual cost.
Numerous computational methods exist for describing the behavior of UHPC structures under seismic loading. Wang et al. [
84] studied a more straightforward method to model the seismic performance of UHPC bridge columns based on the identified fap-shaped hysteretic model. In this model, the elastic stiffness, yield lateral force, post-yield stiffness ratio, and energy dissipation coefficient are calculated. This method focuses on the yield and ultimate states of the columns. The results of a nonlinear dynamic response state that the identified fap-shaped hysteretic model is suitable for estimating the dynamic responses and maximum drift ratio for UHPC bridge columns under earthquake loading. This simplified method can predict the hysteretic characteristics of UHPC.
Experimental and numerical investigations were carried out by Ren et al. [
85] to describe the performance of UHPC box piers subjected to seismic loading. The failure mode, hysteretic characteristics, energy dissipation, and stiffness degradation were investigated. In order to better understand the performance of UHPC box piers, parametric analysis was analyzed, and the effect of the axial load ratio and the longitudinal reinforcement ratio on the ductility of UHPC box piers was studied. The results showed that the ductility of UHPC box piers decreases with an increase in the longitudinal reinforcement ratio. The test angle plays a significant role in ductility. The ductility is reduced to the minimum when the direction of the lateral loads becomes normal to the diagonal of the cross-section.
This study investigates the feasibility of using UHPC in tall buildings located in moderate seismicity regions in terms of seismic performance and cost-effectiveness. UHPC demonstrates enhanced properties such as a higher compressive strength, a higher ductility and durability, and a better tensile strength [
14,
15,
16,
17,
18]. Moreover, seismic hazard maps for the United Arab Emirates (UAE) have not been published in international standards yet since it was considered a zero seismic region as per UBC’ 97 code [
86]. However, several research studies have performed a seismic hazard assessment of the UAE and reported that most parts of the UAE have moderate to low seismic hazard activities [
87]. The coastal areas of the UAE have relatively higher seismic levels as the closest potential earthquake fault is only 100 km away from them [
88]. ETABS software was used to model, analyze, and design most of the structural elements, as discussed in more detail in the coming sections. ETABS software has been produced by Computers and Structures, Inc. (CSI), California [
89]. Four different buildings were modeled using different compressive strengths of concrete. The first model was for high-strength concrete with a compressive strength of 60 MPa. In comparison, the other three models were for UHPC with compressive strengths of 150, 185, and 220 MPa.
3. Materials and Methods
Since many high-rise buildings worldwide, especially in the UAE, have a typical number of stories ranging from 40 to 80, the average number of 60 floors was chosen in this research to study the effect of the concrete type. Typically, a 60-story building is considered a tall building, and, particularly in the UAE, plenty of buildings with 60 stories or more exist [
91]. However, the authors think that design and ETABS analysis of buildings with greater than 60 stories would be time-consuming and tedious, as numerous iterations were conducted to obtain the most optimum solution for each concrete-type model. Meanwhile, investigating the seismic response of buildings with fewer than 60 stories may not be sufficient to properly understand the seismic effect on tall buildings, especially in the study region. Hence, 60 stories seemed to be the best and most optimum choice for the seismic analysis of tall buildings. In addition, the common concrete grade used in the design of tall buildings is C60 (60 MPa), especially in the UAE. Therefore, the reference model was chosen with a concrete grade of C60. Moreover, UHPC has a compressive strength of 150 to 220 MPa, which can be practically implemented in the construction industry. Therefore, the authors selected three different values, a lowest (150 MPa), a median (185 MPa), and the highest (220 MPa), to be studied in this research.
The purpose of this research paper was to assess and compare difference in terms of structural performance as well as cost when using high-strength concrete or ultra-high-performance concrete in a 60-story building. First, four buildings with different concrete grades (60, 150, 185, and 220 MPa) were modeled and analyzed using ETABS software [
89] to meet the seismic serviceability limits (drift-design) according to the ASCE 7-16 standard [
92]. For each model, several runs were performed to optimize the structure in terms of the size of elements. Further, design processes for all structural elements were conducted using ETABS [
89] and Quick Concrete Wall (QCW) software [
93], following IBC-18 [
94] and ACI 318-19 [
95]. QCW has been produced by Integrated Engineering Software, Inc., Bozeman. In particular, QCW was used in this study, as it provides more detailed design aspects. The comparison between the four models was discussed and summarized in terms of seismic response and cost-effectiveness to provide valuable recommendations at the end of this study.
According to the formula mentioned in ACI 318-19 [
71], section 19.2.2, the concrete elastic modulus was determined and defined in the models considering a concrete unit weight of 2500 kg/m
3. The high-strength concrete had a compressive strength equal to 60 MPa and the UHPC had three different compressive strength values; 150 MPa; 185 MPa; and 220 MPa. Practically, some engineers consider 60 MPa as a conventional reinforced concrete compressive strength for tall buildings, while others consider it the strength of high-strength concrete. Meanwhile, concrete is considered UHPC if it has a compressive strength greater than or equal to 150 MPa [
96]. Other properties for UHPC and HSC were assumed according to Akhnoukh et al. [
97,
98,
99]. Along with the building’s self-weight, it was also subjected to a superimposed dead load of 42.5 psf [2.036 kN/m
2], a live load of 50 psf [2.394 kN/m
2], a roof live load of 20 psf [0.958 kN/m
2] and wall perimeter (cladding) load of 7.5 psf [0.359 kN/m
2]. Other parameters such as the S
s and S
1 were set equal to 0.935 and 0.365, respectively. These values were obtained based on the S
s and S
1 values for moderate seismic regions. The long period was set equal to 8 s. The building was designed to meet the seismic requirements of ASCE 7-16 and IBC-18. The layout of each floor of the building being modeled is shown in
Figure 1a. The 60-story building was modeled on ETABS with a lateral force resisting system (LFRS) of a special concrete shear wall system coupled with ductile coupling beams. The reason for choosing this LFRS is that it is the most common system used in the United Arab Emirates for tall buildings. The gravity system consisted of flat-plate slabs and square columns.
Figure 1b shows the typical plan modeled in ETABS and the labels of structural elements that will be discussed in the coming sections. The structural elements in the model (columns, walls, and coupling beams) were designed and analyzed once with the 60 MPa concrete material and once with UHPC using the ETABS and Quick Concrete Wall software programs. The design process of the columns, piers, and spandrels were carried out every five floors for practical purposes. All structural elements were designed using the load combinations shown in
Table 1 according to ACI 318-19 [
95]. For vertical (flexural) reinforcement and horizontal (shear) reinforcement, structural steel grades of 460 MPa and 420 MPa, respectively, were used.
Gravity loads such as self-weight, dead, live, and roof live loads were evaluated using the equivalent lateral force (ELF) method. A linear static load case type was used for these loads. For the seismic load, four load cases were produced, namely, seismic load (E), seismic drift (E-Drift), response spectrum without eccentricity (E_RS_X), and response spectrum with eccentricity (E_RS_X-ecc). Seismic load (E) and seismic drift (E-Drift) load cases were analyzed using the equivalent lateral force (ELF) method, while response spectrum without eccentricity (E_RS_X) and response spectrum with eccentricity (E_RS_X-ecc) load cases were evaluated for response spectrum analysis. The main difference between the two response spectrum load cases is that the response spectrum without eccentricity (E_RS_X) has no diaphragm eccentricity, but the response spectrum with eccentricity (E_RS_X-ecc) has a diaphragm eccentricity of 0.05.
3.1. Gravity System
The design of the gravity system included both a slab and columns design. The thickness of slabs was determined based on the applied gravity loads as well as the span between vertical elements. In other words, a serviceability check (deflection and vibration) and strength design were performed to calculate the minimum slab thickness to minimize the building’s weight. The building’s weight significantly affects the seismic analysis, as will be discussed in the coming sections. Thus, a flat plate with a thickness of 0.2 m was selected for all stories. The building was supposed to move laterally under seismic events. Therefore, the gravity system had to be designed to resist this movement without losing its functionality. ASCE 7-16 [
92] mentions the importance of achieving the deformation compatibility between all structural elements in the building. Hence, all columns were designed on an additional moment, which was calculated based on the maximum design drift at every five floors of the building. Stiffness modifiers of 0.7 and 0.25 were applied to the columns and slabs of the models as per ACI 318-19 [
95] specifications, respectively.
3.2. Lateral Force Resisting System (LFRS)
Special coupled shear walls were used to resist the seismic forces. In ETABS models, all columns were released laterally to consider only the special coupled shear wall in the lateral design. The total dead mass, which includes the self-weight of the entire building and the superimposed loads for finishes, cladding, and brick walls, was used to determine the base shear in ETABS according to ASCE 7-16 [
92]. A stiffness modifier of 0.7 was used for the shear walls as per ACI 318-19 [
95] specifications. In addition, response spectrum analysis (RSA) was used in the design of the lateral system. The accidental eccentricity of 0.05 was selected according to ASCE 7-16. The scaling factor between the static equivalent force and the response spectrum (RS) force was used to scale the last one up according to ASCE 7-16. Moreover, this standard recommends multiplying the determined scaling factor by (I.g/R), where I is the importance factor, g is the gravity acceleration, and R is the response modification coefficient. Typically, the R value varies between 4.0 and 8.0 [
100]. For the moderate seismic region in the UAE, an R-value of 8.0 was considered. According to Ghosh [
101], for ductile coupled shear walls, the value of R should be considered as 8.0. For comparison purposes, the value of R was kept constant for both high-strength concrete and UHPC. However, further studies might be conducted to evaluate the validity of this assumption using, for example, nonlinear pushover analysis. Constant seismic parameters were used for high-strength concrete and UHPC models to study the effects of UHPC in detail without the influence of other parameters.
3.2.1. Drift Design
For the drift design, the ETABS software has been used to design the lengths and thicknesses of the shear walls. Several iterations were performed for each model in order to obtain the optimum model in terms of drift-design limits. During these iterations, certain limitations were considered, such as the structure’s natural period being kept in the range of 0.1–0.15 N to prevent the structure from being excessively rigid or flexible. Furthermore, the aspect ratio of the spandrels was kept between 3 to 5. According to Ghosh [
101], maintaining an aspect ratio of 3–5 results in flexural dominant behavior and plastic hinging, which is desirable for coupling beams. The cross-sectional aspect ratio of the walls was kept in the range of 6–12 to prevent unnecessary slenderness complications. In addition, the wall lengths were fixed throughout the height of the building, whereas the thicknesses were varied every five floors. The building was designed to ensure the maximum drift in the stories to be less than 2%, according to ASCE 7-16 [
92]. The drift values determined from ETABS are elastic and should be converted to the design story drift by multiplying the elastic values by C
d/I, where C
d is the deflection amplification factor taken as per ASCE 7-16 [
92].
3.2.2. Strength Design
Both the ETABS and Quick Concrete Wall (QCW) programs were utilized for the strength design. ETABS was used in the design of the columns, shear walls, and coupling beams. Moreover, QCW was used to review the design of shear walls in addition to calculating the boundary elements requirements according to ACI 318-19 [
95]. Due to the symmetrical shape of the building, only three piers (piers 1, 2, and 3), two spandrels (spandrels 1 and 2), and three columns (columns 1, 2, and 3) were designed every five floors. The notations of the piers, spandrels, and columns are depicted in
Figure 1b.