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MINISTRY OF EDUCATION AND TRAINING HANOI UNIVERSITY OF CIVIL ENGINEERING Nguyen Duc Xuan RESEARCH THE OPTIMAL COMPENSATION METHOD OF AXIAL SHORTENING OF REINFORCED CONCRETE COLUMNS IN SUPER HIGH-RISE BUILDINGS IN VIETNAM Major: Civil Engineering Code: 9580201 SUMMARY OF DISSERTATION Ha Noi - 2022 Completed at Hanoi University of Civil Engineering Academic advisor: Assoc Prof Dr Ho Ngoc Khoa Assoc Prof Dr Pham Hoang Anh Reviewer 1: Assoc Prof Dr Tran Chung Reviewer 2: Dr Nguyen Dai Minh Reviewer 3: Assoc Prof Dr Nguyen Ngoc Phuong The dissertation will be defended in front of the University-level Dissertation Evaluation Council at HUCE On hour date month 2022 For further information, please visit National Library and Library of HUCE PREFACE Research rationale Column shortening is the phenomenon that which vertical force-bearing reinforced concrete members (columns, walls, cores) are deformed and contracted in the axial direction under the action of loads, shortening, and creeping of concrete With the increase in building height, the cumulative shortening value becomes significant In particular, the shortening difference between columns, walls, and rigid cores (called warping or differential shortening), causes the floor beams to be tilted excessively, causing internal forces arising from forced displacements; which can cause damage to non-structural parts such as partition walls, glass walls, and deflect technical pipelines and elevators Finding solutions to limit warping between components is inevitable In particular, the solution to compensate for column shortening in the construction phase is being widely applied The actual construction of columns (walls) in super high-rise buildings in Vietnam poses the following problems to be solved: 1) Determine the minimum number of shortening compensation groups and corresponding shortening compensation values in order to optimize the adjustment of column formwork length in the construction organization; 2) Set up a specific construction process to compensate for the shortening in order to flexibly handle situations arising when the actual shortening value at each time during the construction process fluctuates The two problems mentioned above are new and need to be solved by theory and calculation not only in Vietnam but also in the world Currently, there are no specific standards or guidelines on this issue Therefore, it requires in-depth studies, thereby setting out a process to establish an effective construction plan for column shortening in Vietnamese conditions On the basis of the above analysis, the topic "Research on optimal methods of compensation for shortening of reinforced concrete columns of super-high-rise buildings in Vietnam", is urgent Research goal and objectives 1) Develop a method to select an effective option for column shortening compensation, ie find the optimal number of groups and the optimal shortening value; 2) On the basis of the theoretical method, propose a construction process to compensate for the construction of super high-rise buildings in Vietnam Research subject and scope - Overview of column shortening and compensation for columns in the construction of super high-rise buildings - Research scientific basis and practical compensation for columns - Develop problems and sequences to determine the optimal compensation plan - Proposing compensation construction process in conditions in Vietnam Scientific basis of research 4.1 Research subjects - Shortening compensation for the reinforced concrete structure of super high-rise buildings - Process of construction of shortening compensation for columns in the construction of super high-rise buildings 4.2 Research scope: - Applied within the default range of shortening values provided by design consultants - Do not study calculation methods and factors that affect short shortening Do not study the method of measuring and monitoring short shortening - Confused deformation limits a common value to the frame (line) Research methods - Mathematical methodology method (using optimal mathematical theory) - Statistical quantitative method (using micro-evolution algorithm) - Methods of analyzing and synthesizing practical data Novelty of dissertation - Proposing an optimal number of lumped groups to save costs and time to adjust the height of formwork in organizing the construction of columns - For the first time, the proposed method has been set to include random factors, in reality, to solve cases of compensation according to reliability - Proposing procedures and instructions for the construction of column shortening in super-high-rise buildings in Vietnam, showing the sequence and logic of the work in the construction process, especially handling manage the problems that arise when the actual shortening results at different times differ from the forecast Scientific and practical significance 7.1 Scientific - Proposing a new method to determine the compensation plan: the moving-optimal compensation method (MOC) The proposed method is highly flexible and gives better results than has been published - Methods of construction of columns in the construction of super high-rise buildings are clarified by specific processes, which can be processed flexibly combined with short-term monitoring values 7.2 Practical significance - The moving-optimal compensation method can be applied to set up construction plans in the construction of super high-rise building in Vietnam - The compensation construction process can be applied to the organization of construction of columns in the process of building super high-rise building in Vietnam - Can be applied to verify other compensation methods in Vietnamese conditions Structure of dissertation CHAPTER 1: OVERVIEW OF COLUMN SHORTENING AND SHORTENING COMPENSATION IN SUPER-HIGH-SURVEY CONSTRUCTION 1.1 General introduction 1.1.1 Cast-in-place reinforced concrete (reinforced concrete) super high-rise buildings In many respects and views, in line with the regulations of many countries worldwide, super high-rise buildings (SCT) have more than 60 floors This is also the accepted view in determining the scope and research object of the thesis 1.1.2 The concept of column shortening and shortening compensation in the construction of super high-rise buildings Column shortening is the phenomenon in which reinforced concrete members bearing vertical forces (cores, columns, walls) are deformed axially under the action of loads, shortening, and concrete creep The shortening value accumulates gradually with the height of the building Some concepts and terms used: - Shortening of column members is a value explicitly specified for a single column - Shortening of columns (walls, cores) for the calculation of shortening compensation is the cumulative shortening value after concrete floor pouring, determined according to the design floor height - Differential shortening or called warping: It is the difference between the shortening of the column and the adjacent hardcore - Compensating for column shortening or shortening compensation: This is the use of solutions that provide columns with a shortage of height during column construction to balance with the corresponding column shortening value during the use of the project - The shortening compensation error at each floor level is the difference between the design warping value and the shortening compensation value at the corresponding floor level 1.1.3 Causes and forecasts of column shortening The cause of column shortening is elastic deformation, shortening, and creep of reinforced concrete Accurate prediction of warping value is very complicated, especially for super high-rise buildings with modern architecture Elastic deformation occurs when the column is subjected to compressive load and depends on: Concrete strength, loading time, and the value of the load Concrete shortening is a volume change phenomenon, and it depends on: Ambient humidity, component dimensions, concrete composition, and reinforcement content The creep of concrete is a deformation over time and depends on: Ambient humidity, component dimensions; concrete composition and reinforcement content; concrete strength; the value of the load; loading time 1.2 Effects of column shortening on the structure 1.2.1 Effects on load-bearing structures When the warping value is large enough, it will cause additional internal forces for horizontal structures such as beams and floors due to forced displacement of supports, generating secondary internal forces in the structure They can affect the horizontal structure's bearing capacity and the structural system's safety 1.2.2 Effects on non-structural parts Excessive warping of columns in a floor affects non-structural parts of the building, such as cracking of the covering structure, partitions, and deviation of the technical piping system 1.2.3 Solutions to minimize warping in the design phase Optimize the design to distribute compressive stress evenly in vertical members, such as: using rigid formations (outriggers), and boundary beams (belts), and also provide a solution to increase the axial stiffness for the components The member is predicted to have a large shortening by adding reinforcement 1.3 Overview of short-column studies 1.3.1 Research on the phenomenon of column shortening in the world Before the 1960s, column shortening studies were not interesting because most buildings were under 20 stories and used large-section columns From 1960 to 1990, when the trend of building high-rise buildings (from 20 to 100 floors) developed strongly, there was better awareness of the shortening phenomenon From the 1990s up to now, along with the development of new generation super high-rise buildings with modern and creative architectural shapes, column shortening has been given due attention Research results have been proposed to be applied to European, American, and Australian Standards The shortening strain of vertical members is usually larger than that of the lower floors; it is recommended to use experimental data on concrete material properties such as modulus of elasticity, shortening, creep, and coefficient of thermal expansion to analyze and predict column shortening; Short-term design and field measurements are approximately the same during the first 200 days of construction, it starts to have large deviations in the longer term This finding indicates that the presumptive concrete material properties parameters in the basic design analysis and the test results data of concrete samples during the construction drawing design analysis phase may not reflect fully and accurately results in shortening Therefore, it is necessary to perform shortening measurement during construction for more accurate shortening analysis results; column warping in buildings under 40 stories (about 120 meters) is negligible; The effect of shortening deformation is very sensitive to environmental factors, construction conditions and construction progress through analysis of field shortening monitoring data 1.3.2 Research on the shortening of columns in Vietnam In Vietnam, the concept of shortening columns is relatively new and only has received attention since 2008, when super high-rise buildings began to be built Research results of Cao Duy Khoi (2013) suggest that the types of high-rise buildings in Vietnam (high-rise blocks above combined with larger podiums below) have problems related to the shortening of columns, even if the building is not super high-rise Some other studies show that the column shortening affects construction parts A general process is proposed to apply compensation treatment for column shortening from the design stage to the construction process In this study, the authors also proposed finding an effective optimal compensation plan to manage the compensation work quality better 1.4 Overview of the column shortening compensation method 1.4.1 Theoretical methods to determine the value of shortening and how to compensate According to a study by Park H.S et al in 2003, there are four methods of determining the value of shortening and how to compensate for the shortening of the column below: Absolute Compensation: Compensate by predictive warping This method leads to the number of times of adjusting the column formwork length many times, which is costly and time-consuming Moreover, having to manage many different compensation values during the construction process can be easily confused, affecting the quality of compensating construction work However, this method does not compute, and the column is correctly compensation Uniform Compensation: All floors are compensated by the same average value as the total warping value divided by the total number of floors This method is simple to calculate and compensate only once, so adjusting the length of column formwork to facilitate construction is very simple However, the cumulative error (the difference between the design warping value and the actual shortening value) at some floor levels may be larger than the allowable limit The method of compensating for shortening of equal groups (average): Divide the building into several small groups with the same number of floors in each group to compensate for shortening according to the uniform average group Thus, the number of times of adjustment of column formwork length is equal to the number of compensation groups This method of calculation is simple and easy to implement However, this method is challenging to control the cumulative shortening compensation error at some floor levels because it may be larger than the specified design or the number of compensation groups This can be costly and time-consuming for column formwork construction Optimal Compensation approach (OC): This method determines the number of groups and the compensation value for each group according to the optimization problem The number of compensation groups and the number of floors in each group are optimally calculated using the SA (Simulated Annealing Algorithm) algorithm and the constraints on the magnitude of the shortening compensation error at each floor At the same time, this method optimizes the number of compensation groups to be the smallest, which means that the number of floors in each group is the largest This makes it convenient to adjust the column formwork length However, this method must use an exceptional optimization algorithm, complicated calculations, and a variable number of design variables, which is challenging to apply when short fluctuations occur All of the above methods are very difficult to apply to the actual conditions arising in construction and to control the compensation error 1.4.2 Research on construction of shortening compensation in actual construction There are no published documents or research on the construction methods of shortening compensation in construction practice in the world Documents on the detailed process of compensation are still technological secrets and are not widely disseminated Some typical case studies are as follows: Alberto Cargnino et al (2012) propose compensating for gross shortening at some floor levels according to the concrete floor construction progress applied to The New Piedmont Goverment Office Tower (Italy) The research results show that: Absolute shortening compensation is complicated to manage, leading to easy confusion about the value of shortening compensation, which can cause reverse displacements that distort the structure calculation Figure However, absolute shortening compensation will minimize post-compensation warping, so it may have to be applied to some works or building parts with special requirements for finishing exterior architectural panels At the same time, the author also recommends that it is necessary to establish a more optimal compensation method This study has not mentioned the process of technical instructions and dealing with actual events that may occur when the shortening is changed Korean company MIDAS prepares compensation plan for Keangnam Landmark Hanoi project According to the proposed analysis report, it is necessary to deal with shortening compensation This report also proposes actual grouping compensation at floor levels of the 5th floor; 9, 16, and 26 with a corresponding compensation of 10; 20; 30, and 40 mm However, this report does not state the principles and calculation methods for determining compensation groups and controlling compensation errors The report does not guide the actual treatment process Some other studies also suggested grouping compensation to minimize the work of adjusting the column height However, the studies did not publish a description of the calculation and instructions for the compensation procedure 1.4.3 Sequence and technique of compensating construction According to practical studies, shortening compensation work follows the following steps: 1) Evaluation of input data provided by the design consultant; 2) Determine the value and compensation method; 3) Field shortening monitoring to forecast and correct shortening values; 4) Construction of shortening compensation; 5) Continue the process of compensating construction Construction techniques to compensate for shortening include column top shortening, column foot shortening compensation, and also floor shortening compensation 1.5 Problem and research orientation of the thesis 1.5.1 Issues raised The results of an overview study on column shortening and shortening construction methods in the construction of super high-rise buildings in the world and Vietnam can be summarized as follows: Column shortening needs considerable attention in the construction of super high-rise buildings Shortening column compensation is one of the solutions to minimize excessive warping An effective compensation solution must consider convenient in the organization of compensating construction The construction shortening compensation process needs to handle the actual boundary conditions It is essential in the context that the world and Vietnam have not yet fully published instructions for compensating for short columns of super highrise buildings 1.5.2 Research orientation of the thesis Based on the analysis results, the research direction of the thesis is proposed with the following contents: The study aims to develop the optimal method and actual construction process The research content of the thesis includes four basic tasks: (1) Research overview; (2) Research on the scientific and practical basis of column contraction compensation in the construction of super high-rise buildings; (3) Develop a method to determine the optimal column shortening value and compensation method; (4) Proposing the construction process to compensate for shortening in the construction of super high-rise buildings in Vietnam The research object of the thesis is the construction of short columns of super high-rise buildings The scope of the thesis research is limited: not study the short shortening calculation method; default the actual shortened values provided by the design consultant; some data on shortening values used in the study are referenced in some real works such as Keangnam Hanoi project; did not study shortening monitoring method The applied research methods are the theoretical analysis and synthesis method, the mathematical method (using optimal theories), and the quantitative statistical method CHAPTER THE SCIENTIFIC AND PRACTICAL BASIS OF COMPENSATING CONSTRUCTION IN THE CONSTRUCTION OF SUPER HIGH-RISE BUILDINGS 2.1 Mathematical basis of the optimization problem of column shortening compensation 2.1.1 Design variables The design variables for the column shortening compensation optimization problem are the required shortening compensation at each floor and the number of floors in a shortening compensation group 2.1.2 Objective function The optimization problem is expressed through an objective function of the form (1): y=F(x)=F (x1, x2, …, xn) (1) The goal is to find the minimum number of compensated groups (min), also called minimizing the number of compensated groups To achieve this goal, it is necessary to maximize the number of stages in each group of compensation (2): Max(F(X))=min{-F(X))} (2) 2.1.3 Constrains system Constraints are conditions that need to be satisfied In the construction of shortening compensation, these are the control conditions on the error after shortening compensation (limit on floor warping deformation), the maximum number of floors in a group of shortening compensation, and the compensation value that meets the conditions of construction 2.1.4 Methods of solving optimization problems 2.1.4.1 Mathematical programming (using gradients) The general optimization problem is stated as follows: Minimize (or maximize) the function of the form (3): F(x)→Min (Max) (3) with constraint condition: g_j (X)(≤ = ≥) b_i; X∈D⊂R^n;j=1÷m 2.1.4.2 Random Search Methods An optimization problem is a problem that finds the best solution in the space of possible solutions To overcome the disadvantages of gradient-based methods, random search methods are applied Random search techniques belonging to the class of artificial intelligence are often used in optimization problems, such as genetic algorithms (GA), swarm intelligence (PSO), and evolution Differential Evolution (DE) In 1995, two authors, Storn and Price introduced an optimal algorithm called differential evolution DE The evolutionary process performs de-duplication in steps: mutation, hybridization, and selection Currently, there are many variations and improvements of DE to increase the algorithm's efficiency for different problems 2.2 Forecasting and monitoring column shortening 2.2.1 The periods of forecasting the column shortening value Usually, to establish the plan and plan for shortening compensation, the design consultant needs to perform the column shortening analysis in stages: Preliminary design, construction drawing design, and construction phase The preliminary stage uses input data as hypothetical parameters to preliminarily determine the shortening value; the construction drawing design phase uses input data as parameters of material test results to determine the prediction of components that need to be controlled for shortening; The construction phase is shortened, calculated and adjusted in accordance with reality based on the field shortening measurement data and the results of cast-in-place concrete samples to serve the calculation of shortening compensation in construction 2.2.2 Monitoring column shortening The current popular method is to measure the strain using sensor systems attached to the base of the column in combination with total station monitoring 2.3 Compensating construction techniques 2.3.1 Selection of compensating construction components Structures that are predicted to have significant warping beyond the allowable limit will be selected for the construction of shortening compensation Several standards recommend limited warping: BS 8110-2:1985, Eurocode selects L/250 and L/500 or 20mm (whichever is smaller); Standard ACI 318-08 selects L/240 and L/480 Vietnamese standard TCVN 5574-2018 sets L/250 and L/500, where L is the beam span (distance between two adjacent vertical members) However, the design may have stricter requirements on limited deformation to improve the safety and aesthetic requirements of the building 2.3.2 Select the predicted shortening value according to the time points to calculate the shortening compensation The “target date” compensation plan has a forecasted shortening value at the time of shortening that can reach its maximum value (about 3-5 years after construction, also known as the “target date”) ) This option aims to calculate the shortening compensation so that the floor structure will have the desired position during longterm use According to the construction schedule, the plan of compensation according to the "floor completion date" has the estimated shortening compensation value at the time of floor completion The purpose is to calculate the shortening compensation so that the position of the floor structure will be equal to the design position at the time of floor finishing so that the finishing work of the floor will be convenient 2.3.3 Method of compensating construction Usually, the construction technique of column shortening compensation is performed at the top of the column or at the foot of the column during the construction of column concrete by adjusting the column formwork, or it is possible to make the column formwork with the height equal to the design height plus the amount of 11 3.2.3.1 Constraint by accumulative compensation error has the form (9) 𝑑 =∑ 𝑒 +∑ 𝑥 −𝛿 , 𝑗 = 1, … , 𝑁 (9) where, 𝑑 is the cumulative compensation error at the jth floor elevation of the i-th group (10) 𝑑 ≤ 𝜃 , 𝑗 = 1, … , 𝑁 (10) where 𝜃 is a predefined tolerance for compensation error 3.2.3.2 Constraint according to independent compensation error at each floor level, has the form (11) xij   i   , for i  1, , N F (11) where 𝜃 is a predefined tolerance for compensation error 3.3 Moving compensation optimizing algorithm 3.3.1 Deterministic optimal compensation (DOC) algorithm Assuming the forecast differential column shortenings data and the moving compensation optimizing are deterministic quantities (deterministic compensation problem), the sequence of steps to perform MCO is as shown in Figure 12 Start differential shortening; ; Total floors N i = 1, T=0 FALSE T