1 Foreword With the enhancement of China's comprehensive national strength, the continuous development and expansion of cities, and the needs of people's life, industrial production and environmental protection, the construction of pool structures has increased year by year. The pool structure in water supply and drainage engineering belongs to the category of special structure, and its load is mainly water pressure, earth pressure and temperature stress. This paper discusses some problems involved in the design of water tank from the perspective of structural specialty, and briefly summarizes some problems that should be paid attention to in the design of water tank. 2 Reasonable determination of design groundwater level The structural design of the pool is closely related to the elevation of groundwater level. Engineering accidents such as failure to grasp the groundwater level, insufficient anti-floating ability, and wrong structural selection occur from time to time. According to the current national design code, the groundwater level should be adopted according to the data provided by the survey department and the hydrological department, considering the possible highest groundwater level. In the design reference period of 50 years, the variable function of groundwater in general hydraulic structures is determined according to the principle of Unified Standard for Reliability Design of Engineering Structures, and the accidental function of rare floods is not considered. However, it is worth noting that the groundwater level provided by some engineering geological survey reports can not be obtained from the analysis of local hydrological data, but only reflects the groundwater level during the survey. If detailed investigation is carried out in the local dry season, the provided groundwater level elevation will not be used for design, or the structural calculation will be wrong. Therefore, designers should know the local hydrological conditions in detail and supplement the geological survey report that does not meet the design requirements. It is required to consider whether there are local rainstorms and typhoons, and whether the groundwater level will rise due to the failure to remove surface water in time. Civil designers should combine their own understanding of underground water level and geological conditions, determine the basement elevation of the pool together with water process designers, and make a plan by considering many factors such as process requirements, civil construction cost, operation cost, production years and so on. For example, when the groundwater level is high or there is a quicksand layer in the geological section, the designer should consider whether the elevation of the basement can be raised appropriately to reduce the influence of buoyancy on the structure and avoid the quicksand layer. 3 the setting of expansion joints and post-poured belts cracks caused by temperature, deformation and uneven settlement are often encountered in engineering. Most engineering examples with cracks show that temperature and concrete shrinkage deformation are the main reasons for the problems. The maximum expansion joint spacing specified in Article 6.2. 1 of Code for Structural Design of Water Supply and Drainage Engineering supplements that the maximum expansion joint spacing is not limited by regulations when concrete is mixed with a proper amount of reliable admixture or experienced post-pouring belt. What needs to be clear is that the code first emphasizes that when the length and width of the structure exceed the maximum expansion joint spacing, expansion joints should be set first, and only when the structure is difficult to handle can it be considered to be mixed or set. However, it must be clear that setting expansion joints in super-large structures is one of the main measures to reduce the cracks in the pool. Expansion joints are mainly set to reduce the influence of seasonal average temperature (humidity) difference in the middle surface. This method is usually used for rectangular storage tanks to avoid possible temperature and shrinkage cracks due to the long tank wall. For rectangular pools on the ground or semi-underground, expansion joints should generally be set along the pool wall, and the length should not exceed 15~20m. The underground pool shall be provided with expansion joints not exceeding 25~30m. When there are difficulties in the design and structural treatment of the long pool, construction methods such as post-pouring belt or UEA reinforcement belt can be adopted to reduce the equivalent temperature difference and unfavorable temperature difference caused by concrete shrinkage. The setting of post-pouring belt can avoid some unfavorable temperature difference before construction and equivalent temperature difference caused by early shrinkage of concrete, thus increasing the allowable spacing of structural expansion joints. The spacing of post-cast strip should first consider effectively reducing the temperature shrinkage stress, and then consider the combination with construction joints. Under normal construction conditions, the spacing of post-cast strip should be 20 ~ 30m. Of course, the longer the post-cast strip is retained, the better, but the subsequent process must not be affected during construction, generally not less than 40 days, and 60 days is the most suitable (considering the possibility of construction). When UEA concrete reinforcement belt is used, the large expansion strain of UEA concrete with reinforcement belt is used to compensate the temperature difference strain of concrete on both sides. Under the constraint conditions, the steel bars are elongated by the bonding of cement stone and steel bars, and the elongated steel bars produce compressive stress (called chemical prestress or self-stress) on the concrete itself, and the self-stress value of 0.2 ~ 0.7 MPa is generated in the concrete, which can roughly offset the shrinkage and tensile stress generated in the hardening process of the concrete, thus preventing or reducing the shrinkage and cracking of the concrete, making the concrete compact and improving the crack resistance and impermeability of the concrete structure. 4 Coordination between disciplines, design and construction In the design of water tanks, civil designers should understand the requirements of hydraulic process design, such as whether beams and columns are added to larger water tanks due to structural needs, or whether the requirement of adding axils (octagons) between walls and between wallboard and bottom plate will affect the process. If the process requirements do not allow adding beams, columns or axils, civil designers should meet the process requirements first, and then take other measures to meet the structural requirements. Designers should use some structural measures flexibly according to different situations on the basis of design specifications, with the cooperation of various professions. Design is closely related to construction. Many construction factors, such as water cement ratio, water consumption, concrete curing days, interval days of post-pouring zone, etc. , has been considered in the design calculation. These design conditions must be explained in detail to the construction unit, and relevant technical disclosure shall be made, requiring the construction unit to implement them one by one. To do this well, designers need to know about construction, new materials, new technologies and new methods in construction, construction sequence and requirements for design, so as to make the design suitable for construction and convenient for construction. For example, in order to facilitate formwork support and concrete pouring in pool construction, construction joints are generally reserved 300~500mm away from the pool bottom and above the armpit. Designers should consider the construction requirements and avoid designing reserved holes, embedded pipes, cantilever beams and slabs within this range. 5. Several problems that should be paid attention to in pool design and summary of engineering experience 5. 1 The reinforcement design of pool section should adopt the principle of smaller diameter and denser spacing as far as possible; 5.2 Try not to leave vertical construction joints during pool construction. It is suggested to leave horizontal construction joints 500mm above the top surface of the bottom plate. At the same time, pay attention to the waterproof between joints, and require surface roughening during secondary pouring to ensure the bonding between concrete. 5.3 According to the environment in which the pool is located and its specific use requirements, appropriate pool structural materials should be selected, including concrete strength grade and impermeability grade, whether other materials such as corrosion resistance or crack resistance should be added, and the thickness of concrete protective layer of steel bars should not be ignored; 5.4 When it is an aboveground pool, don't ignore the effects of temperature and humidity; 5.5 Pay attention to the structural measures at the corner of the pool wall, the pool wall and the roof, the pool wall and the bottom plate, and strengthen the reinforcement; 5.6 According to the phenomenon that the vertical cracks of the pool wall often extend downward from the top of the pool wall, it is best to strengthen the horizontal reinforcement concentrated at the top of the pool wall, and its diameter should not be less than the stress reinforcement of the pool wall; 5.7 It is best to add four vertical steel bars with a diameter not less than that of the vertical steel bars on the pond wall at the corner, which can not only play the role of structural column, but also strengthen the overall rigidity of the pond and increase the seismic performance; 5.8 It is suggested that the bottom plate of the pool should be selected to the outside of the pool wall with an appropriate length, not less than the thickness of the pool wall. It is best to set an axillary angle on the compressed side of the pool wall; 5.9 For the convenience of construction, don't forget to set up Mazar-e reinforcement for construction on the bottom plate and brace for construction on the side wall. Conclusion After taking part in the work, due to the different division of labor and work arrangement, I was exposed to more pool designs. Over time, some problems were found in the design of the pool, and some immature experience was accumulated. According to some problems in previous engineering design, combined with the design code of water supply and drainage structure and the mature experience of existing projects, some opinions and suggestions are put forward for peer discussion, hoping to provide some reference for pool design.
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