Factors Affecting Construction Duration Of High Rise Buildings

Arab Academy for Science, Technology & Maritime Transport

June, 2017

FACTORS AFFECTING CONSTRUCTION DURATION OF HIGH RISE BUILDINGS Ehab Yasser E-Tahlawy, Ahmed Darwish, Mohammed M. Hamed, Islam Magdy Department of Construction & Building Engineer, AAST – Smart Village, Egypt

ABSTRACT The study in this paper reviews the general factors that can affect the overall construction duration of high rise buildings. Also shows the difference between these variable choices and the advantage of each one. The main factors studied are 1) Role of formwork type choice in reducing the slab cycle time and choice parameters. 2) Vertical delivery challenges and the huge role of temporary hoists, concrete pumping system and tower cranes to reduce the wasted time during the construction phase. 3) The choice of the scheduling method either CPM, LOB or LBRS and how to choose the suitable method for the project to be executed. The purpose of the study to determine some effective factors to be considered in the planning and construction phases of high-rise building in order to reduce effort exerted to accomplish the project in the required time frame.

INTRODUCTION High rise building in the world is a phenomenon which has emerged since the late 19th century and in the early 20th century, when the first steps were taken towards producing skyscrapers about 1880-1900 in Chicago. At the turn of the nineteenth century, steel structure began to substitute for heavy masonry structures in construction of multi-storey buildings. Coincided with the Industrial Revolution (accompanied by urbanization and rapid population growth), mass building was common.

Positive and Negative Effects of High Rise Building Like other human-made artifacts, high rise buildings have both positive effects and advantages and negative effects and disadvantages obtained from their construction in urban environments [1]. Negative effects of high rise building can be studied in different environmental, traffic, social, aesthetical and … categories. In addition, positive effects of tall buildings can be known as such things as reduced cost, visual aspects and … [1]. Negative effects:-

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June, 2017

Table 1: High rise buildings negative effects Positive effects:

Table 2: High rise buildings Positive effects

1- Role of Formwork Form work significantly influences successful project completion in high-rise building construction with reinforced concrete structures. One of the effective ways for reducing the form work duration is to strip the forms without delay when concrete placed in the form is sufficiently cured to stand by itself. The selection of formwork systems in high-rise buildings is often governed by their competence in optimizing concrete activities in an isolated manner, without relating this choice to the entire construction workflow[2].

❖ Formwork Choice and Construction Workflows The logic of work execution followed to satisfy building safety and integrity shapes the scheduling of involved activities. Structural works are the skeleton of every construction project and they set the pace to other downstream architectural and MEP activities. Hence, ensuring a continuous and on-time execution of structural framing is essential to keep the project on schedule. Assuming adequate availability of labor and material, the choice of formwork systems (e.g., the climbing technique) directly affects the progress of concrete works, and greatly influences the interlocking workflows of walls, shafts, and slabs where several tasks from different trades are involved. For instance, crane-lifted formwork used for core wall erection can congest the crane schedule and consequently delay the delivery of materials to other site zones. The quality of formwork system and the tolerance range it can provide is essential to ensure a smooth progress of work from one phase to another, and from one trade to another. Nonetheless, smaller tolerance ranges allow the incorporation of prefabrication and remote assembly that can boost construction speed [3].

❖ Formwork Selection Parameters Construction of high-rise buildings requires innovative formwork system technologies to overcome the limitations of space, budget, and time. However, many parameters affect the choice of formwork systems and are mainly divided into internal and external parameters as shown in Table 1. While internal parameters fall under designers and contractors

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June, 2017

control, external ones are affected by owner requirements, project milestones, project location, and corresponding local rules and regulations

Table 3: Formwork selection parameters

❖ Schedule Optimization and Workflow Improvements Advanced formwork systems allow project managers to optimize the project schedule. The trailing platform plays a major role in opening downstream work early in the project and benefitting from the available core wall shaft areas which are usually wasted work spaces in regular formwork technologies as conveyed through Figure 1. The early engagement of elevator crews reduces corresponding material batches where required accessories can be delivered to site on demand without storing large inventories. On the other hand, the independent progress of columns and slabs helps streamline both activities together and reduce the risk of one process delaying the other as is the case of using regular formwork. It also boosts production of both activities due to learning curve effects. Higher availability of tower crane for none concrete activities when using advanced systems, along with making building service elevators functional at core mid-height, can increase material and labor delivery rates to work zones, thus increasing production rates of nonconcrete activities as presented Figure 2. Nonetheless, core wall labors could be relocated to other activities once the core wall is erected, therefore decreasing total labor costs and allowing for more flexibility in resource allocation[3].

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June, 2017

Figure 1:- Schedule Using Regular Formwork

Figure 2:- Schedule Using advanced Formwork

2- Vertical Delivery Challenges The vertical delivery of materials and labor on construction sites is mainly achieved using temporary hoists, tower cranes and concrete pumping systems. However, the increasing number of high-rise building projects worldwide has introduced new challenges in vertical delivery. As the height of buildings grows, the efficiency of vertical transportation drops exponentially, thus affecting the safety, cost and overall schedule of projects. In particular, those sites in spatiallyconstrained urban areas are limited in the number of equipment that can be installed on site [4]. First, construction temporary hoists are the main method of transporting labor. Unlike materials that can be lifted during the night, workers only can be transported at the time of executing their assigned tasks Therefore, a significant amount of unproductive time is spent being transported to their designated floors, particularly in high-rise buildings. Second, concrete is a common building material that is used throughout a project. Traditional methods of delivering concrete by crane and bucket are constrained by the limited number of cranes and weather conditions. The use of concrete pumps and an innovative method to reduce costs and guarantee constant flow will be highlighted. Finally, tower cranes are used to lift and/or move heavy materials and large pieces of building components, and can be a big challenge in terms of safety. Special challenges for the use of tower cranes in tall buildings will be discussed.

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❖ Temporary Hoist A construction temporary hoist, also known as a temporary elevator, construction elevator, or construction lift, is commonly used in building construction projects to lift materials and labor. For most building projects, hoists are leased or rented by the general contractor. Unlike a permanent elevator, temporary hoists are operated manually by an on-board operator, allowing direction changes or stops at any time according to the operator. Hoists are commonly have single or double cages (1 or 2 cages per tower respectively) with a capacity varying from 25 to 35 people per cage. Capacity is reduced when materials and tools are carried with workers. The growing height of a building, limited number of hoists, and reduced capacity can make workers’ movement between floors inefficient, resulting in a significant amount of time wasted waiting for hoists each day. Hoist operations are also affected by weather and local regulations, which may restrict hours of operation. Therefore, optimized planning and operation of construction hoists is needed to reduce worker waiting time and improve overall schedule performance. Furthermore, since workers typically begin work simultaneously every morning, the biggest challenge of hoist operation optimization is to solve peak-hour congestion Strategies used in the elevator industry, such as zoning, can be applied to minimize worker transit time in high-rise construction .Zoning is used to divide buildings into groups of continuous vertical floors. These groups form zones that can be reached only by specifically assigned hoists. In this way, the number of stops each hoist has to make is reduced, Another, less restrictive way to reduce lifting times for sites with multiple hosts is to coordinate the hoists by optimizing the stops for each hoist when calls come for a pick up. Another alternative for dealing with peak hour congestion is to apply staggered arrivals to the workers’ daily schedule. Staggered arrivals have been studied in transportation and elevator planning for office buildings to reduce the queue during morning peak hours [4].

Figure 3:- Temporary Hoist

❖ Concrete Pumping System The majority of tall buildings under construction generally are concrete. The significant increase in the use of concrete in tall building construction is attributed to improvements in concrete technology including strength, admixtures, pumping, construction techniques and structural systems. To accommodate tight construction schedules, hydraulic concrete pumping has developed as a fast and economical method of transporting concrete due to its reduction in labour requirements and the ability to deliver continuous concrete pours. The traditional crane and bucket method of delivering concrete is limited by the availability of cranes, the effects of heavy winds, and its inability to deliver a continuous flow of concrete. Pumping methods aim to address these limitations.

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June, 2017

Factors Affecting Concrete Pumping Concrete pumping for construction purposes is directly related to two major factors, concrete composition and the mechanical characteristics of the pumping equipment. Concrete mix composition affects the properties of fresh concrete such as bleeding, segregation, viscosity, cohesion and compactness. These properties have a significant impact on the pumpability of the concrete and the formation of a boundary layer (also referred to as lubrication layer), which forms at the interface between the concrete and the pipe. This lubrication layer is crucial to reducing the shear stress between the concrete flow and pipe wall to achieve the lower pressures necessary for pumping [4].

Figure 4: - Example Of Concrete Pumping system

Figure 5: - Example Of Tower Cranes

❖ Tower Cranes Tower cranes are considered as the centerpiece of construction equipment in building projects. They play a key role in transporting a variety of materials vertically and horizontally. The efficiency of tower cranes largely depends on their type, number, location, operator skill and many more. As the number of work tasks and the demand for tower cranes increase, planners may experience difficulties in making an appropriate decision about the optimum layout of tower cranes. A poor decision will have negative effects, which will lead to additional costs and possible delays. The use of tower cranes during tall building construction is even more challenging because the increasing height adds problems of visibility, wind load and safety issues for the operator.

3- Scheduling Method Due to an increasingly competitive environment, construction companies are forced to be more efficient and achieve competitive operational advantage this encourages them to become specialists in certain types of construction projects. This specialization requires more focused project planning and controlling techniques that prove to be better for certain type of projects. The benefits of effective planning, scheduling and control of construction projects are: reduced construction time, reduced cost overruns and the minimization of disputes [5].

❖ Current Scheduling Methods The most common scheduling method used in the construction industry is the Gantt chart (Bar Chart) and Critical Path Method (CPM). Gantt chart (Bar chart) has gained wide acceptance and popularity because of its simplicity and ease of preparation and understanding. No “theory” or complicated calculations are involved. CPM network can show logic dependencies of activities, and estimate and predict the completion date of the project based on mathematical calculations. But both Gantt chart and CPM are unable to accurately model the repetitive nature of linear construction.

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This includes the inability of CPM to provide work continuity for crews or resources, to plan the large number of activities necessary to represent a repetitive or linear project and the inability of Gantt chart (Bar chart) and CPM to indicate rates of progress, and to accurately reflect actual conditions. The consequence of this is that there have been many attempts to find an effective scheduling technique for linear construction. These include, but not limited to, the Line of Balance (LOB). This section discusses the popular scheduling methods in construction industry [5].

❖ Critical Path Method (CPM) One of the major network scheduling methods which have been used in the construction industry is CPM (critical path method). This method involves the use of a geometric representation of flow chart which depicts the precedence between activities. The critical path method (CPM) is a duration-driven technique in which the basic inputs are project activities, their durations, and dependence relationships. Activity durations are functions of the resources required (rather than available) to complete each activity. The CPM formulation assumes that resources are not restricted in any sense. The use of network techniques and CPM by construction companies has reached a steady level after the enthusiastic boom of the early 1960‘s. Computer programmes eliminate the need to prepare a network, but the network notation provides an easily understood output format for management personnel. Advantages of Critical Path Method When comparing bar charts with networks, three advantages over bar charts. • Network show logic, the relationships among the activities. Bar charts do not • Networks can better represent large and complicated projects. • Networks can estimate, or predict, the completion date of the project, or other dates, on the basis of mathematical calculations of the CPM Limitation of Network Scheduling Method The first problem is the sheer size of the network. In a repetitive project of n units, the network prepared for one unit has to be repeated n times and linked to the others; this results in a huge network that is difficult to manage. This may cause difficulties in communication among the members of the construction management team. The second problem is that the CPM algorithm is designed primarily for optimizing project duration rather than dealing adequately with the special resource constraints of repetitive projects. The CPM algorithm has no capability that would ensure a smooth procession of crews from unit to unit with no conflict and no idle time for workers and equipment. This leads to hiring and procurement problems in the flow of labor and material during construction [5].

Figure 6:- Critical Path Method

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❖ Line Of Balance The line of balance (LOB) method was originated by the Goodyear Company in the early 1940s and was developed by the US Navy during the Second World War for the programming and control of both repetitive and non-repetitive projects. A common characteristic of LOB techniques is the typical unit network. Representative construction projects that fit into this category are a repetitive housing project or a high-rise building. Typical process production or flow line curves are depicted in Figure 7. The Figures 7 depicts the balanced production flow line of high rise building and unbalanced production flow lines. For example, the sequence of processes for a high rise building construction project may include form erection, steel installation, concrete placement, form removal, curtain wall installation, and glazing. The production curves for activities are plotted as a function of time. The production rate for a process can be determined from its slope. The horizontal distance between the production curves for two consecutive activities at given location indicates the time buffer. The difference between the cumulative number of production quantities delivered and the LOB quantity at any given time is termed the “criticality”. The negative criticality indicates the actual progress is less than the production forecast. The LOB is a quantity-time diagram. It focuses on the required delivery of completed quantities. Linear construction projects often consist of repetitive processes which have different production rates. This phenomenon of production rate imbalance has the potential for negatively impacting project performance by causing work stoppages, inefficient utilization of allocated resources, and excessive costs. Production rate imbalance occurs when the production curves of leading processes intersect the curves of following process because of different production rates and insufficient lag between start times of processes. (Fig.8) [5]. Advantages of Line Of Balance The major benefit of the LOB methodology is that it provides production rate and duration information in the form of an easily interpreted graphics format. The LOB plot for a linear construction project can be easily constructed, can show at a glance what is wrong with the progress of project, and can detect potential future bottlenecks. Also LOB can determine at any time:• Shortage of delivered materials which may impact production. • Materials which are being delivered in excess which may cause additional material handling or require additional storage space. • The jobs or processes which are falling behind and the required rate of acceleration to satisfy the required LOB quantities. • The jobs or processes which are ahead of schedule which may b e placing heavier demands on operating capital than necessary. • A forecast of partially completed production units by job, work station, or process to support the delivery schedule of finished units. Barriers to Implementing LOB The application of the LOB methodology by the US construction industry had been very limited. Some barriers to implementation of the LOB methodology include the following: • There is a lack of awareness among practitioners in the US construction industry that the LOB methodology exists. • Owners and contractors began adopting network techniques as planning tools at about the same time that the LOB methodology was originated and developed. These entities are reluctant to adopt new planning tools which are not being used by their counterparts or competitors. • Computerized tools employing network techniques are widely available whereas computerized tools employing the LOB methodology are not currently commercially available. Due to the popularity of the relatively inexpensive computer in the US construction industry, there is a resistance to change to a planning method which is currently not supported by computer. However, researchers at several universities have attempted to computerize the LOB methodology and have working prototypes [5].

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Figure 7:- Balanced LOB production curves

Figure 8:- Unbalanced LOB production curves

❖ Location Based Repetitive Scheduling Method LBRS varies from LOB in that the amount of time allocated to each work location varies based on the volume of work required in that location. The objective remains same, synchronizing the progress of different work crews to optimize the overall project delivery. LBRP are the projects where certain activities are continuously repeating at each locations of the project. Such as commercial housing projects, multistoried sky scrapers. Activities that repeat from one location to other location creates a very important need for a construction schedule that facilitate the uninterrupted work flow including work crews from one location to next, because it is often this requirement that establishes activity starting times and determines the overall project duration. Hence uninterrupted work flow becomes an extremely important issue for the planning and scheduling of high-rise building, hence high-rise building construction planners need to carefully design a process that ensures a continuous and reliable flow of resources through different locations in a project. The conventional Critical Path Method and its resource oriented extension such as time cost trader off, limited resources allocations and resource leveling does 9

Arab Academy for Science, Technology & Maritime Transport

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not consider the waste time during the transformation of work flow from one location to the other. Location Based Repetitive Scheduling Method (LBRSM) explicitly take care those entire requirements for repetitive nature projects like high rise buildings. The application of such work flow continuity during LBRSM leads to maximizing the use of a learning curve and minimizing idle time of each crew [6].

CONCLUSION There are some factors that can affect the overall construction duration of high rise buildings. The main factors studied are 1) Role of formwork type choice in reducing the slab cycle time and choice parameters. 2) Vertical delivery challenges and the huge role of temporary hoists, concrete pumping system and tower cranes to reduce the wasted time during the construction phase. 3) The choice of the scheduling method either CPM, LOB or LBRS and how to choose the suitable method for the project to be executed. By choosing the most suitable choice in each factor at the early planning stage in the project that can reduce the construction time frame and produce achievable schedule.

RERERENCES 1-

Mohammad Rahim Rahnama, Mohammad Hossein, HeraviTorbati, “Study of Physical-Spatial Effects of High-Rise Building”, Geography and Urban Planning, Ferdowsi University of Mashhad, 2014, Iran.

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Taehoon Kim, Hunhee Cho*, and Kyung-In Kang, “FORM WORK MANAGEMENT BASED ON UBIQUITOUS COMPUTING FOR HIGH-RISE BUILDING CONSTRUCTION”, School of Civil, Environmental and Architectural Engineering, Korea University, Korea.

3-

Hisham A. Abou Ibrahim, and Farook R. Hamzeh, “ROLE OF FORMWORK SYSTEMS IN HIGH-RISE CONSTRUCTION”, Department of Civil and Environmental Engineering, American University of Beirut, 2015, Lebanon.

4-

Yufeng Wei1, Andre Pinheiro, David Pedraza1, Bowen Wu1 and Brenda McCabe, “VERTICAL DELIVERY CHALLENGES FOR HIGH-RISE BUILDING CONSTRUCTION”, Department of Civil Engineering, Fumec University, 2015, Brazil.

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T. Subramani, A. Sarkunam, J. Jayalakshmi, “PLANNING AND SCHEDULING OF HIGH RISE BUILDING USING PRIMAVERA”, Department of Civil Engineering, VMKV Engg. College, Vinayaka Missions University, 2014, India.

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Atul R. Kolhe, Dr. J. E. M. Macwan, Dr. K. A. Chauhan, “PLANNING FOR HIGH-RISE BUILDING CONSTRUCTION USING LOCATION BASED REPETITIVE SCHEDULING METHOD (LBRSM)”, Atul R. Kolhe1, Dr. J. E. M. Macwan2, Dr. K. A. Chauhan3, 2014, India.

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Maria Isabel, “REDUCING TIME IN THE CONSTRUCTION OF HIGH RISE BUILDINGS”, Department of Civil and Environmental Engineering MASSACHUSETTS INSTITUTE OF TECHNOLOGY, 2000.

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