Choosing the Right Construction Method: A Comparative Study of Cost and Timeline for Top-Down and Bottom-Up Approaches

Abstract

The selection of an appropriate construction method stands as a pivotal decision in ensuring the success of any building project. This paper undertakes a comprehensive comparative analysis of cost and timeline implications between the top-down and bottom-up construction methodologies. The research focuses on ten distinct underground structure models, each characterized by varying depths and base areas. Through rigorous design and analysis, the cost and projected duration for each model are meticulously evaluated under the lens of both top-down and bottom-up construction techniques. The findings reveal that while the bottom-up approach emerges as the most cost-effective alternative at a depth of 8 m, the top-down method exhibits superior cost efficiency with increasing depth. Interestingly, alterations in the structure’s base area exert a minimal influence on the cost differentials between the two methodologies. Moreover, the top-down construction method consistently outshines its counterpart in terms of project duration across all ten models examined, with the percentage increase in base area yielding insignificant impacts on project timeline discrepancies. This study furnishes construction project managers with invaluable insights to navigate the complexities of method selection. By judiciously considering parameters such as project scale, depth, and base area, managers can strategically optimize both cost and timeline outcomes, thereby facilitating the seamless execution of construction projects.

1. Introduction

In the process of planning and executing building projects, one of the most critical decisions involves the selection of the most suitable construction methodology. Two of the most prevalent strategies utilized in the industry are the top-down and bottom-up methods, each harboring unique advantages as well as challenges [1]. The top or up/down method, as the name implies, involves building up and down at the same time, or, in other words, constructing a building’s substructures and superstructures simultaneously [2]. The method is also referred to as the top-down construction method when there is no superstructure to a project [3]. The top-down method is characterized by initiating construction from the top of the building and working downwards [4,5,6]. This strategy enables construction teams to work on upper floors concurrently with the lower levels, making it particularly suitable for large-scale projects with deep foundations, such as skyscrapers or multi-level parking structures [7,8]. Long [9] assessed these approaches and concluded that top-down construction methods yielded the most favorable results. Conversely, the bottom-up method involves building the structure from the ground up, beginning with site excavation and foundation installation [1,5]. This approach proceeds with the construction of lower floors, gradually progressing upwards [1,10]. It is generally more appropriate for smaller-scale projects, like single-family homes or low-rise apartments [8].

Cost is a critical consideration when selecting a construction method. Numerous factors, including site conditions, project complexity, and size, can influence construction costs [7]. The top-down method is often more expensive than the bottom-up approach, especially for smaller-scale projects, due to the need for extensive excavation and foundation work. Additionally, the top-down method may not be compatible with all soil types, and controlling the quality of lower construction levels can prove challenging [7,11]. Moreover, this method necessitates meticulous planning and coordination, potentially increasing overall project costs. In contrast, the bottom-up method is a more straightforward approach, which may result in lower expenses [5,8]. Construction teams can work on each level sequentially, simplifying cost management. However, site excavation and foundation installation can incur significant costs [5].

Timeline is another crucial factor in determining the optimal construction method. Project managers must strive to complete projects within the allotted timeline to avoid expensive delays. Generally, the top-down method can accelerate project completion compared to the bottom-up approach [8,12]. The simultaneous work on multiple levels enabled by the top-down method can expedite the overall construction process [1]. The structural support system also allows work on upper floors to commence even while lower floors are under construction, ultimately saving time [3,13]. On the other hand, the bottom-up method can be more time-consuming, particularly when constructing tall structures [8]. Site excavation, foundation installation, and the requirement to wait for the completion of each level before progressing upwards can lead to delays and extended timelines.

In conclusion, both top-down and bottom-up construction methods present their unique advantages and disadvantages. When choosing a construction method, project managers must weigh multiple factors, including project size, budget, timeline, and the type of structure being built [1]. Additional considerations involve the project’s complexity and the availability of resources, such as equipment and skilled labor. Project managers should also evaluate the project’s budget and timeline constraints to determine the most suitable method. The choice of construction method can significantly impact the project’s overall success, necessitating informed decision-making. Furthermore, project managers must consider the potential risks and challenges associated with each construction method to ensure a sound decision.

1.1. Literature Review

The construction industry has witnessed numerous innovations and advancements in techniques and methodologies. Among these, the top-down and bottom-up approaches have emerged as two primary construction methods. This literature review investigates the cost and timeline implications of these methods, drawing upon a wide range of studies from various locations and projects.

The top-down construction method has been extensively researched in various contexts, including China [7,14,15], Japan [16], Koren [17], United Kingdom [18,19,20], Hong Kong [21,22], Bangkok [23,24], Iran [25], Russia [26], and Qatar [27]. These studies have examined the assembly methods, steel columns, installation processes, environmental impacts, and structural design aspects of top-down construction. These studies provide valuable insights into the assembly processes and material choices, which are essential for successful top-down construction projects. Furthermore, specific applications of top-down construction, such as in metro stations [11,28,29,30], high-rise buildings [7,31], and residential projects [31,32], have been analyzed.

Comparing the cost and timeline of top-down and bottom-up construction methods has been the primary focus of numerous studies. The concept of top-down construction was initially introduced in Japan with the aim of reducing construction schedules [16]. Li et al. [11], Rhim et al. [17], Lui et al. [21], Basarkar et al. [29], and Nikiforova et al. [31] explored the cost and time efficiency of top-down construction, providing evidence that supports its advantages over the traditional bottom-up method.

The environmental impact and geotechnical factors of top-down construction have been examined in various studies. Jamsawang et al. [24], Chang [33], Kung [34], Jia et al. [35], and Tan et al. [36] analyzed the soil and groundwater conditions, as well as the potential environmental consequences of top-down construction. These studies contribute valuable information on how to design and construct top-down projects in a more sustainable and environmentally friendly manner.

The structural analysis and performance of top-down construction have been the focus of several studies [7,13,17,28,37,38,39,40,41,42]. These investigations offer insights into the structural behavior and design considerations for top-down construction projects, such as load transfer mechanisms, column connections, and soil–structure interactions.

Many researchers have conducted case studies on top-down construction projects to evaluate their performance and applicability in real-world settings. Examples of such studies include Ren et al. [12], Ghorbani et al. [25], Finno et al. [31], Sakharkar [32], and Chiu et al. [43]. These case studies provide practical knowledge and lessons learned from real-life projects, which can be valuable for future top-down construction endeavors.

The literature review reveals that the choice between top-down and bottom-up construction methods depends on factors such as soil conditions, structural design, cost, and construction timeline. Top-down construction methods have been shown to offer advantages in terms of cost and time savings, particularly in challenging soil conditions and densely populated areas. Nevertheless, it is essential to evaluate each project individually, taking into consideration its unique requirements and constraints.

The studies reviewed in this section have significantly contributed to the understanding of top-down and bottom-up construction methods. However, it is crucial to acknowledge the limitations and applicability of the findings in the context of different construction projects. Future research could explore innovative construction techniques and materials that further enhance the benefits of top-down and bottom-up approaches. However, it is crucial for project managers and construction professionals to carefully consider the unique requirements and challenges of each project to determine the most appropriate construction method.

1.2. Objectives and Scope

The primary aim of this study is to investigate the time and cost differences that arise when constructing buildings using the top-down method as opposed to employing traditional bottom-up techniques. To enable a fair and accurate comparison, the study has limited the selection of the construction depth and ground class of the construction area by considering the application boundaries of the respective technologies. To this end, the cost and project durations for constructing underground structures with varying depths and areas using both top-down and bottom-up methods have been calculated and compared based on depth. Within this scope, models, and their specifications for both top-down and bottom-up construction methods have been identified, static analyses have been performed, work items have been established, and unit prices and man–hour values have been determined. Utilizing the obtained data, cost tables and project schedules have been created based on the measurements of the statically approved models, and project durations have been calculated. The results have been compared and represented graphically to illustrate the distinctions between the top-down and bottom-up construction methods in terms of cost and project duration.

2. Materials and Methods

In this study, a detailed analysis and examination of underground structures constructed using top-down and bottom-up (conventional) construction methods were carried out, comparing their costs and time (project duration). The study was conducted in seven steps: Firstly, ten separate underground structure models were designed with two different sizes of construction areas and five different structure depths (or the number of basement levels). Then, the static analysis of these models was performed to ensure structural integrity. The details of the structure models were determined, and basic construction works were created based on the construction method. Unit prices were established to calculate the costs of the items, and man–hour values were determined for scheduling purposes. Quantities for each construction item were estimated, enabling cost calculations both at the item level and project level. Work schedules for each project were developed using the man–hour and quantity data, allowing for the calculation of project durations. Finally, the obtained data was compared using graphs, providing visual and quantitative insights. The flowchart illustrating the seven steps conducted in this study is provided in Figure 1.

Within the scope of the study, 10 different underground structure models have been determined. These designated structure models have two different sizes of construction areas and five different structure depths (or number of basement floors). These structure models consist of two main types based on the construction area. The type 1 group represents models with a construction area of 40 m × 40 m, while the Type 2 group represents models with a construction area of 80 m × 80 m. These main types also consist of five different subtypes based on the structure depth (or number of basement floors). For example, Type 1.1 represents a model with a construction area of 40 m × 40 m and a structure depth of 8 m (or two basement floors). Detailed information about all types of the 10 structure models determined within the scope of the study is provided below:

• Type 1: Represents structure models with a construction area of 40 m × 40 m;
• Type 1.1: Represents a model with a construction area of 40 m × 40 m and a depth of 8 m;
• Type 1.2: Represents a model with a construction area of 40 m × 40 m and a depth of 16 m;
• Type 1.3: Represents a model with a construction area of 40 m × 40 m and a depth of 24 m;
• Type 1.4: Represents a model with a construction area of 40 m × 40 m and a depth of 32 m;
• Type 1.5: Represents a model with a construction area of 40 m × 40 m and a depth of 40 m;
• Type 2: Represents structure models with a construction area of 80 m × 80 m;
• Type 2.1: Represents a model with a construction area of 80 m × 80 m and a depth of 8 m;
• Type 2.2: Represents a model with a construction area of 80 m × 80 m and a depth of 16 m;
• Type 2.3: Represents a model with a construction area of 80 m × 80 m and a depth of 24 m;
• Type 2.4: Represents a model with a construction area of 80 m × 80 m and a depth of 32 m;
• Type 2.5: Represents a model with a construction area of 80 m × 80 m and a depth of 40 m.

The ten different underground structure models identified within the scope of this study have been meticulously selected and determined according to various criteria. The selection of these models aims to provide a broad perspective when comparing the effects of construction methods. The primary criteria for selecting underground structure models are as follows:

Diverse Dimensions: The chosen models have different construction area sizes and structure depths. This diversity enables us to obtain results applicable to projects of varying scales.

Types of Structures: Each model is classified under two main categories with different construction areas. This classification allows for the examination of structural complexities and costs at different levels.

Structure Depths: Models are designed to have different numbers of underground floors. This facilitates the comparison of both construction methods and structural characteristics.

The determination of each model has been carefully conducted to ensure our comparative analysis yields robust results. The selection of these models is aimed at contributing to the academic literature and providing practical guidance to practitioners in the construction industry.

Additionally, an elevation of Type 1.1 is provided in Figure 2 to serve as an example of the sections, dimensions, and other relevant information determined based on the construction method and load-bearing system characteristics of the identified Type 1 and Type 2 building group models.

In order to compare the top-down and bottom-up construction techniques examined within the scope of the study, although the project sizes were selected to be the same, there are some project differences due to the significantly different construction techniques. In order to avoid the influence of these differences on the comparison, the commonly used load-bearing systems and details were used in the modelling of the structures. The assumptions and preferences made within this scope are provided below:

• Şişli, Istanbul has been chosen as the project construction site for the structure models;
• It is assumed that all structure models will be constructed on soft clayey and sandy soils;
• The structure models have completely symmetrical plan geometries;
• For the structures modelled using the top-down construction method, the preferred load-bearing system consists of “bored steel column + reinforced concrete slab”;
• For the structures modelled using the bottom-up construction method, two different load-bearing systems have been preferred. One of them has a “reinforced concrete column + reinforced concrete slab,” while the other has a “steel column + reinforced concrete slab”;
• The foundation systems for the structures to be built using the top-down and bottom-up construction methods consist of secant piles and raft foundation, respectively;
• All models of the structures to be built using the top-down and bottom-up construction methods have excavation support systems, and it is assumed that the same system is used in all of them. The excavation support system applied in the structure models consists of steel diaphragm walls and steel bracing elements;
• In the construction of all models according to the top-down and bottom-up construction methods, only rough construction cost and duration have been taken into account. Since the construction work and quantities within the structures to be used in both techniques will be the same, the data obtained will not affect the cost and duration comparison. Therefore, detailed labor, mechanical, and electrical work items have not been considered in the calculations;
• It is assumed that the steel bars and profiles used in the structure models are supplied from İçdaş factory in Bağcılar, Istanbul, and the bored and steel columns are supplied from Rainham Steel in the United Kingdom;
• It is assumed that the same number of machinery and workers are employed during the construction stages of Type 1 and Type 2 structure groups;
• For the top-down construction method with a “bored steel column + reinforced concrete slab” load-bearing system, a total of 116 workers are employed, while for the bottom-up construction method with a “steel column + reinforced concrete slab” load-bearing system, a total of 228 workers are employed. For the bottom-up construction method with a “reinforced concrete column + reinforced concrete slab” load-bearing system, a total of 220 workers are employed;
• When developing the work plan and estimating project durations, the date of 1 January 2021 is assumed as the start date of each of the 10 underground models.

The dimensions of the load-bearing structural elements of the examined building models were determined based on the static analyses conducted within the scope of the study. The analyses were performed on 10 building models with completely symmetrical plan geometry and stiffness distribution using the Sap2000 [44] and IdeCAD Static [45] software packages. As an example, some views of the analyses made using Sap2000 and IdeCAD Static software packages for the building model of Type 1.1 are given in Figure 3.

Within the scope of the study, only the rough construction items were taken into account in terms of cost and duration for the construction of the identified building models using the top-down and bottom-up construction methods. Detailed craftsmanship, mechanical, and electrical work items were not considered. Accordingly, the construction items determined for each construction method and its corresponding load-bearing system are provided in

Table 1. Construction items determined for top-down and bottom-up construction methods.

Construction Method (Load-Bearing System)	Item No	Item Description
Top-Down (Steel plunge column + Reinforced concrete slab)	1	Steel diaphragm wall construction
	2	Excavation for plunge column placement
	3	Fore pile works
	4	Steel plunge column assembly
	5	Slab reinforcement work
	6	Slab concrete pouring
	7	Excavation
	8	Mobile crane operation
	9	Steel transportation
	10	Profile transportation
Bottom-Up (Steel column + Reinforced concrete slab)	1	Steel diaphragm wall construction
	2	Excavation
	3	Strapping
	4	Foundation reinforcement work
	5	Foundation concrete pouring
	6	Steel column installation
	7	Formwork scaffolding construction
	8	Slab formwork construction
	9	Slab reinforcement work
	10	Slab concrete pouring
	11	Steel transportation
	12	Profile transportation
Bottom-Up (Reinforced concrete column + Reinforced concrete slab)	1	Steel diaphragm wall construction
	2	Excavation
	3	Strapping
	4	Foundation reinforcement work
	5	Foundation concrete pouring
	6	Column formwork construction
	7	Column reinforcement work
	8	Column concrete pouring
	9	Formwork scaffolding construction
	10	Slab formwork construction
	11	Slab reinforcement work
	12	Slab concrete pouring
	13	Steel transportation
	14	Profile transportation

.

In order to calculate the construction cost of the determined structure models, it is necessary to have quantity and unit price information for the construction items.

Quantities for certain construction items related to the construction methods of the determined structure models were obtained from the computer programs used for the static analysis of these models, while the remaining quantities were manually calculated. In this context, the quantities of the following construction items were calculated for the construction measurement:

• Concrete, formwork, lightweight reinforcement, and heavy reinforcement quantities for rough construction;
• Diaphragm wall quantities for shoring systems;
• Steel strapping quantities to be used in shoring systems;
• Quantities of plunge steel columns.

For the cost calculations in the construction of the determined structure models according to the top-down and bottom-up construction methods, the unit price list of the Ministry of Environment, Urbanization, and Climate Change for the year 2021 was used. The prices of materials to be procured from abroad were converted to Turkish lira based on the exchange rate of the Central Bank on the day of the study. Unit prices for special items were determined based on market data.

The total costs of the determined structure models according to the top-down and bottom-up construction methods were obtained by multiplying the quantities and unit prices. As an example, the details of the total cost of Model 1.1, including the quantities and unit prices of the construction items determined based on the construction method and load-bearing system, are provided in

Table 2. Cost table for Model 1.1 using the top-down construction method (plunge steel column + reinforced concrete slab).

No	Item No	Description	Material	Quantity	Unit	Unit Price (Turkish Liras, TL)	Total Cost (TL)
1	Special item 011	Steel diaphragm wall construction	Larssen 25 steel profile	1408.00	m²	1228.84	1,730,206.72
2	19.100.1058	Excavation for plunge steel column	Foundation pile drilling machine (440 HP)	60.16	hour	776.38	46,707.02
3	15.140.1209	Pile construction	120 cm diameter C30/37 class concrete	240.00	m	1221.79	293,229.60
4	Special item 001	Plunge steel column installation	UC 305 × 305 × 240 steel column	43.70	ton	20,629.46	901,507.40
5	15.160.1004	Slab reinforcement installation	Ø16/15 ribbed bars	197.89	ton	4362.90	863,356.83
6	15.150.1006	Slab concrete casting	d = 30 cm C30/37 class concrete	1440.00	m³	262.38	377,827.20
7	15.120.1101	Excavation	Soft and hard soil	13,280.00	m³	6.29	83,531.20
8	KGM/03.587/3	Mobile crane	80-ton lifting capacity	221.33	hour	750.56	166,123.95
9	N.YF.07	Transportation of steel	Ø8-Ø28 ribbed bars	222.96	ton	7.79	1736.83
10	N.YF.26	Transportation of profiles	UC 305 × 305 × 240 steel column	43.70	ton	810.31	35,410.55
Total							4,499,637.29

.

To create construction schedules for the determined building models based on the top-down and bottom-up construction methods, the man–hour values from the labor durations in the unit price analyses of the Ministry of Environment, Urbanization and Climate Change were utilized. The man–hour values for special items were calculated using data obtained from the market.

The number of machines and workers employed in the construction of the building models are provided in

Table 3. Numbers of construction machinery and teams.

Construction Method (Load-Bearing System)	Machine Type and Workforce	Number of Machines	Number of Workers
Top-down (Plunge Steel Column + Reinforced Concrete Slab)	Larssen steel profile driving machine	4
	Fore pile drilling machine (440 HP)	2
	Fore pile construction team		10
	Plunge steel column assembly team		6
	Reinforcement installation team for slab		80
	Concrete pouring team for slab		20
	Excavator for subgrade excavation (0.3 m3 bucket)	8
	80-ton mobile crane	1
	Total	15	116
Bottom-up (Steel Column + Reinforced Concrete Slab)	Larsen steel profile driving machine	4
	Excavator for excavation (0.5 m³ bucket)	8
	Banding steel assembly team		15
	Foundation and slab reinforcement installation team		80
	Concrete pouring team		20
	Steel column assembly team		8
	Formwork and scaffolding construction team		45
	Slab formwork team		60
	Total	12	228
Bottom-up (Reinforced Concrete Column + Reinforced Concrete Slab)	Larsen steel profile driving machine	4
	Excavator for excavation (0.5 m3 bucket)	8
	Banding steel assembly team		15
	Foundation and slab reinforcement installation team		80
	Concrete pouring team		20
	Column formwork team		60
	Column reinforcement team		80
	Formwork and scaffolding construction team		45
	Slab formwork team		60
	Total	12	220

. It is assumed that the same number of machines and workers are involved in the construction methods and load-bearing systems of Type 1 and Type 2 building groups. For the top-down construction method using the “plunge steel column + reinforced concrete slab” load-bearing system, a total of 116 workers are employed. For the bottom-up construction method using the “steel column + reinforced concrete slab” load-bearing system, a total of 228 workers are employed. For the bottom-up construction method using the “reinforced concrete column + reinforced concrete slab” load-bearing system, a total of 220 workers are employed.

Within the scope of the study, the reinforcement teams for foundation and slab work also participate in the column reinforcement fabrication of the completed floor of the analyzed building models. The slab formwork teams also participate in the column formwork fabrication of the completed floor. Accordingly, the activity relationships of the work items have been determined for all building models based on the construction method and load-bearing system. The duration of the work items for each model is calculated using the formula provided in Equation (1). It is assumed that 8 h are worked per day. In addition, the duration of the work items performed with construction machinery is calculated, taking into account the number of machines and the work they can accomplish in a unit of time.

Number of Workdays = (Work Item Quantity × Man hours)/(Number of Workers × 8)

(1)

As an example, the activity relationships and durations of the work items determined based on the construction method and load-bearing system for the Type 1.1 model are presented in

Table 4. Activity relationships of the identified work items for Type 1.1 building model using the top-down construction method (plunge steel column + reinforced concrete slab).

Activity No	Work Item	Start Date	Predecessor	Relationship	Duration (Days)
1	Steel diaphragm wall construction	1 January 2021	(-)		22
2	Excavation for fore piles where plunge steel columns will be placed	23 January 2021	1	FS	4
3	Fore pile construction	27 January 2021	2	FS	3
4	Plunge steel column installation	29 January 2021	3	FF	1
5	Reinforcement installation for elevation 0 slab	30 January 2021	4	FS	5
6	Concreting of elevation 0 slab	4 February 2021	5	FS	1
7	Excavation for (elevation 0 to elevation 4)	5 February 2021	6	FS	12
8	Reinforcement installation for (−4) slab	17 February 2021	7	FS	5
9	Concreting of (−4) slab	22 February 2021	8	FS	1
10	Excavation for (elevation −4 to elevation −8)	23 February 2021	9	FS	13
11	Reinforcement installation for (−8) slab	8 March 2021	10	FS	5
12	Concreting of (−8) slab	13 March 2021	11	FS	1

.

In order to calculate the project duration for the identified models within the scope of the study, the work program for each model was first drawn based on the determined activity relationships and durations. By performing the necessary calculations based on the work program, the project duration was obtained. As an example, the views of the work programs obtained by considering the activity relationships of the tasks determined according to the top-down and down-up construction methods and the structural systems for Type 1.1 building models are presented in Figure 4, Figure 5 and Figure 6.

The start and end dates of the tasks for all Type 1 and Type 2 building models, along with the calculated project durations, are provided in

Table 5. Completion dates and project durations for all building models of type 1 and type 2.

Type	Structure Depth (m)	Start Date	Top-Down Construction Method (Plunge Steel Column + Reinforced Concrete Slab)		Bottom-Up Construction Method (Steel Column + Reinforced Concrete Slab)		Bottom-Up Construction Method (Reinforced Concrete Column + Reinforced Concrete Slab)
			End Date	Project Duration (Days)	End Date	Project Duration (Days)	End Date	Project Duration (Days)
Type1.1	8	1 January 2021	14 March 2021	72	29 March 2021	87	28 March 2021	86
Type 1.2	16	1 January 2021	15 May 2021	134	10 June 2021	160	18 June 2021	168
Type 1.3	24	1 January 2021	16 July 2021	196	28 August 2021	239	6 Sep. 2021	248
Type 1.4	32	1 January 2021	16 Sep. 2021	258	12 Nov. 2021	315	26 Nov. 2021	329
Type 1.5	40	1 January 2021	17 Nov. 2021	320	6 Feb. 2022	401	25 Feb. 2022	420
Type 2.1	8	1 January 2021	21 August 2021	232	11 Oct. 2021	283	23 Sep. 2021	265
Type 2.2	16	1 January 2021	6 March 2022	429	31 May 2022	515	3 June 2022	518
Type 2.3	24	1 January 2021	25 Sep. 2022	632	30 Jan. 2023	759	2 March 2023	790
Type 2.4	32	1 January 2021	10 April 2023	829	25 Oct. 2023	1027	15 Nov. 2023	1048
Type 2.5	40	1 January 2021	24 Oct. 2023	1026	24 July 2024	1300	11 August 2024	1318

.

3. Results

This study aims to compare the cost and time (project duration) of structures built using top-down and bottom-up (conventional) construction methods. The comparative analysis and interpretations of the time (project duration) and cost data obtained through calculations are presented under the subheadings below.

3.1. Costs of Structure Models

Within the scope of this study, 10 different underground structure models were examined. These structure models have two different sizes of construction area and five different depths (or basement floor numbers). The structure models consist of two main types based on the construction area. The Type 1 group represents models with a construction area of 40 m × 40 m, while the Type 2 group represents models with a construction area of 80 m × 80 m. Both Type 1 and Type 2 structure groups consist of five different subtypes depending on the depth (or basement floor numbers).

3.1.1. Costs of Type 1 Structure Group Models

The construction cost values for the models belonging to the Type 1 structure group, obtained through calculations, are presented in

Table 6. Construction cost values for Type 1 structure group models based on the top-down and bottom-up construction methods and structural system shape.

Type 1 40 m × 40 m	Structure Depth (m)	Top-Down Construction Method (Plunge Steel Column + Reinforced Concrete Slab) (TL)	Bottom-Up Construction Method (Steel Column + Reinforced Concrete Slab) (TL)	Bottom-Up Construction Method (Reinforced Concrete Column + Reinforced Concrete Slab) (TL)
Type 1.1	8	4,499,637.29	5,384,483.02	208,304.69
Type 1.2	16	7,898,206.49	10,822,064.99	9,505,706.24
Type 1.3	24	12,195,519.22	17,750,047.59	15,670,620.40
Type 1.4	32	17,766,145.08	27,965,323.84	23,597,430.94
Type 1.5	40	24,010,518.69	40,029,750.60	2,973,964.40

, based on the top-down and bottom-up construction methods and the structural system shape.

According to the data in Table 6, it appears that the most cost-effective method among structures with an 8 m depth is the “bottom-up (reinforced concrete column + reinforced concrete slab)” construction method. For all other depths greater than 8 m (16 m, 24 m, 32 m, 40 m), the most cost-effective method is found to be the “top-down (immersed steel column + reinforced concrete slab)” construction method. It was determined that the construction method with the highest cost is the “bottom-up (steel column + reinforced concrete slab)” construction method at every depth.

The depth and cost graphs for the Type 1 structure group models based on the construction method and structural system shape are provided in Figure 7.

According to Figure 7, as the depth increases, the depth–cost graphs of the top-down (conventional) construction methods diverge from the depth–cost graph of the “top-down (plunge steel column + reinforced concrete slab)” construction method. The construction costs of the “bottom-up (reinforced concrete column + reinforced concrete slab)” method is found to be 0.93 times for Type 1.1, 1.20 times for Type 1.2, 1.28 times for Type 1.3, 1.32 times for Type 1.4, and 1.37 times for Type 1.5, compared to the construction costs of the “top-down (immersed steel column + reinforced concrete slab)” method. The construction costs of the “bottom-up (steel column + reinforced concrete slab)” method are determined to be 1.19 times for Type 1.1, 1.37 times for Type 1.2, 1.45 times for Type 1.3, 1.57 times for Type 1.4, and 1.66 times for Type 1.5, compared to the construction costs of the “top-down (plunge steel column + reinforced concrete slab)” method.

3.1.2. Costs of Type 2 Structure Group Models

The construction cost values for the models belonging to the Type 2 structure group, obtained through calculations, are presented in

Table 7. Construction cost values for Type 2 structure group models based on the top-down and bottom-up construction methods and structural system shape.

Type 1 80 m × 80 m	Structure Depth (m)	Top-Down Construction Method (Plunge Steel Column + Reinforced Concrete Slab) (TL)	Bottom-Up Construction Method (Steel Column + Reinforced Concrete Slab) (TL)	Bottom-Up Construction Method (Reinforced Concrete Column + Reinforced Concrete Slab) (TL)
Type 2.1	8	15,894,655.22	18,183,952.81	12,918,897.01
Type 2.2	16	26,665,439.65	35,164,930.18	28,963,152.09
Type 2.3	24	41,985,859.39	58,125,960.18	47,925,676.12
Type 2.4	32	63,752,323.52	94,062,502.50	72,330,234.13
Type 2.5	40	88,929,574.67	136,826,816.87	101,317,586.56

based on the top-down and bottom-up construction methods and the structural system shape.

According to the data in Table 7, it appears that the most cost-effective method among structures with an 8 m depth is the “bottom-up (reinforced concrete column + reinforced concrete slab)” construction method. For all other depths greater than 8 m (16 m, 24 m, 32 m, 40 m), the most cost-effective method is found to be the “top-down (plunge steel column + reinforced concrete slab)” construction method. It was determined that the construction method with the highest cost is the “bottom-up (steel column + reinforced concrete slab)” construction method at every depth.

The depth and cost graphs for the Type 2 structure group models based on the construction method and structural system shape are provided in Figure 8.

According to Figure 8, as the depth increases, the depth–cost graphs of the top-down (conventional) construction methods diverge from the depth–cost graph of the “top-down (plunge steel column + reinforced concrete slab)” construction method. The construction costs of the “bottom-up (reinforced concrete column + reinforced concrete slab)” method is found to be 0.81 times for Type 2.1, 1.08 times for Type 2.2, 1.14 times for Type 2.3, 1.134 times for Type 2.4, and 1.139 times for Type 2.5, compared to the construction costs of the “top-down (plunge steel column + reinforced concrete slab)” method. The construction costs of the “bottom-up (steel column + reinforced concrete slab)” method are determined to be 1.14 times for Type 2.1, 1.31 times for Type 2.2, 1.38 times for Type 2.3, 1.47 times for Type 2.4, and 1.53 times for Type 2.5, compared to the construction costs of the “top-down (plunge steel column + reinforced concrete slab)” method.

3.2. Project Durations of Structure Models

3.2.1. Project Durations of Type 1 Structure Group Models

The project durations of the models belonging to the Type 1 structure group, based on the top-down and bottom-up construction methods and the structural system shape, obtained through calculations, are presented in Figure 9. According to the data, it is apparent that the most suitable duration for each structure depth is achieved with the “top-down (plunge steel column + reinforced concrete slab)” construction method. For models with an 8 m depth, the longest project duration is observed in the “bottom-up (steel column + reinforced concrete slab)” construction method, while for models with 16 m, 24 m, 32 m, and 40 m depths, the “bottom-up (reinforced concrete slab + reinforced concrete column)” construction method has the longest duration.

3.2.2. Project Durations of Type 2 Structure Group Models

The project durations of the models belonging to the Type 2 structure group, based on the top-down and bottom-up construction methods and the structural system shape, obtained through calculations, are presented in Figure 10. According to the data, it is determined that the most suitable duration for each structure depth is achieved using the “top-down (plunge steel column + reinforced concrete slab)” construction method. For models with an 8 m depth, the longest project duration is observed in the “bottom-up (steel column + reinforced concrete slab)” construction method, while for models with 16 m, 24 m, 32 m, and 40 m depths, the “bottom-up (reinforced concrete slab + reinforced concrete column)” construction method has the longest duration.

4. Discussion

In this study, a comparison was made between the top-down and bottom-up construction methods in terms of cost and project duration. For this purpose, 10 different models with varying base areas and heights, equipped with the top-down construction method involving plunge steel column + reinforced concrete slab, and the bottom-up construction method with steel column + reinforced concrete slab and reinforced concrete column + reinforced concrete slab as structural systems, were designed and analyzed.

According to the results obtained, the bottom-up construction method is more economical for shallow depths, while the top-down construction method becomes more economical as the depth increases. The increase in the base area of the structure did not significantly affect the percentage difference between the two methods in terms of cost. Regarding project duration, the top-down construction method provided a time advantage compared to the conventional method for each depth. Similarly, the increase in the base area did not alter the percentage difference between the two methods in terms of project duration. While models with an 8 m depth showed a project duration change of approximately 16% (Type 1.1) to 12% (Type 2.1), models with a 40 m depth exhibited a project duration change of approximately 24% (Type 1.5) to 22% (Type 2.1).

In the literature, the comparison between the two systems was conducted based on project duration. In these studies, a reference construction model was chosen, and the comparison was made based on that specific construction. Sakharkar [32] stated that the top-down construction method reduced the project duration by 30% compared to the conventional method, while Prience et al. [2] found this difference to be around 15%. The results obtained in this study parallel the literature. According to these results, when favorable conditions are met, the use of the top-down construction method will facilitate an early return on investment and increase financial profitability.

However, it is imperative to acknowledge that the comparison of construction methods should not be limited solely to cost and project duration. Factors such as environmental impact, safety, and feasibility in specific geological conditions should also be considered. Further research and analysis could delve into these aspects to provide a comprehensive understanding of the suitability of each construction method in various contexts.

When selecting a construction method, there are numerous constraints and variables that need to be considered. The careful evaluation and examination of all factors associated with a project are necessary when choosing a construction method.

While this study primarily focused on cost and project duration, future research could explore additional dimensions of comparison, such as environmental impact and safety considerations. Integrating insights from the existing literature on these aspects could enrich the understanding of the advantages and limitations of different construction methods.

The top-down method allows simultaneous construction in both downward and upward directions. The development of civil and industrial construction in densely populated urban areas is directly related to the rational use of underground spaces and the construction of high-rise structures. In addition to the time and cost advantages of the top-down construction method, it is particularly ideal for projects where pile bracing construction is not feasible and soil movement needs to be minimized. The structural slab serves as support for excavation. It requires less space in the construction area. It generates less environmental pollution related to pre-construction earthworks. It eliminates the need for supporting adjacent structures. It does not require formwork, formwork scaffolding, and formwork installation for the slab; instead, it utilizes the ground floor formwork.

5. Conclusions

In this comparative study of cost and timeline for top-down and bottom-up approaches in construction, we have designed and analyzed 10 underground structure models with varying sizes and depths. The evaluation was based on predetermined construction activities, considering potential time and cost disparities between the two methods. Detailed cost estimations and time calculations were conducted using unit prices and man–hour values.

Based on the results obtained from the comparison of the 10 underground structure models, considering both time and cost aspects, the following conclusions emerge:

• The bottom-up construction method proves to be the most cost-effective choice at a depth of 8 m, while the top-down method becomes increasingly advantageous as the depth increases;
• Variation in the structure’s base area has minimal impact on the cost disparity between the two methods;
• The top-down construction method consistently outperforms the other systems in terms of project duration for all 10 analyzed models;
• Similar to the cost results, the percentage increase in the base area does not significantly affect the difference in project duration.

While our study provides valuable insights into the comparison of top-down and bottom-up construction methods, it is essential to acknowledge certain limitations inherent in the research methodology and assumptions made:

• Site-Specific Assumptions: The study was conducted assuming Şişli, Istanbul as the project construction site, with soft clayey and sandy soils. These site-specific conditions may not fully represent the diverse geological and environmental factors encountered in other locations;
• Standardization of Structural Models: Although efforts were made to standardize the structural models for fair comparison, variations in construction techniques and site conditions may exist, potentially impacting the generalizability of the findings;
• Simplifications in Cost and Duration Estimations: Detailed labor, mechanical, and electrical work items were not considered in the cost and duration estimations. While this approach facilitated a broad comparison of the construction methods, it may overlook certain project-specific complexities and nuances;
• Assumptions Regarding Materials and Resources: The study assumed specific suppliers for materials and standardized labor resources, which may not accurately reflect real-world procurement processes and availability, particularly in different geographical regions;
• Scope of Analysis: Our analysis primarily focused on rough construction items and did not delve into detailed craftsmanship, mechanical, and electrical work items. Future studies may benefit from a more comprehensive examination of these aspects.

By acknowledging these limitations, we aim to provide a transparent assessment of the study’s scope and ensure the appropriate interpretation of the results.

Selecting the right construction method is paramount for project success. Project managers should meticulously consider various factors such as project size, complexity, budget, and timeline when making this decision. The findings of this study offer valuable insights for project managers and stakeholders in selecting the most suitable construction method for their specific projects.

Both top-down and bottom-up approaches have their own merits and drawbacks. While the top-down approach offers faster project completion, it comes with higher costs and requires meticulous planning and coordination. Conversely, the bottom-up approach is relatively simpler and less expensive, but it can be time-consuming, particularly for tall structures, due to excavation and foundation installation requirements.

In summary, the choice between top-down and bottom-up construction methods depends on project-specific factors. Large-scale projects with deep foundations may benefit from the top-down approach, prioritizing faster construction and reduced excavation. Conversely, smaller-scale projects or those in challenging site conditions may find the bottom-up approach more suitable due to its lower cost and better control over construction quality.