Why is it that in civil engineering, each span should preferably be a multiple of 3?

It should be a multiple of 6; I’ve seen ones with 4 spans, but each span is 30 meters long.

Chapter 7 Study on the Layout of Double-Row Turbine Halls and Static Structural Analysis 7.1 Research and Comparison of Hall Layout Options The upper reaches of the Yellow River feature narrow valleys, steep terrain, and complex geological conditions. Due to these constraints, it is difficult to determine the optimal locations for key structures such as flood control dams, water diversion systems, power generation halls, and spillway structures when planning large-scale hydroelectric power stations. The issue of locating the turbine halls is particularly challenging. If it is placed directly behind the dam, the excavation work required is often substantial, and high slope issues become prominent ； If it is placed underground, the high and steep slopes as well as the complex engineering geological conditions increase the difficulty of construction and prolong the project duration, making it difficult to meet both the requirements of ensuring safety and convenient operation, as well as those of saving costs and shortening the construction time. Starting from the requirement to properly determine the locations of the various main structures in the layout of large-scale hydropower stations, the double-row turbine hall layout is undoubtedly a good solution. By adopting this type of plant layout scheme, it is possible to better adapt to the terrain of deep river valleys, reduce the height of slope excavation on both banks, avoid large and complex underground engineering systems, and transform underground plants with poor construction and operation conditions into surface plants with better such conditions. Additionally, spillway structures can be arranged on the roof of the plant and along both banks, taking into account the terrain and hydrological characteristics of the river. Satisfactory results can be achieved by properly arranging the locations of the power generation water intake pipes and the inlets of the spillway structures on the dam, as well as by making full use of the space behind the dam. The former Soviet Union was the first to adopt a double-row unit layout, which is slightly wider than a single-row layout; two units are installed, one on the downstream side of the plant and the other on the upstream side. Our country began researching this layout method in the 1980s, and it was successfully applied at the Lijiaxia Hydropower Station. 7.1.1 Layout scheme for the mixed-type cave-style power house The primary reason for adopting a linear, mixed-type cave-style power house layout behind the dam in the initial design of the Lijiaxia Hydropower Station was that the station’s total installed capacity as well as the capacity of each individual unit were quite high; there were many units, resulting in a large scale for the power house and a significant land area required. If a conventional design were used, with all the units arranged in a line, the horizontal length of the power house would be around 180 meters. However, the width of the natural riverbed is only 30–50 meters, making it impossible to have a space of nearly 200 meters wide for the layout of the power house. The mountains on the left bank are thin, with developed fault fractures, and the Left Dam Gully is also present, making it unsuitable for locating industrial buildings. Although the right bank has steep mountains, it also has numerous fault fractures, resulting in complex engineering geological conditions. Since the diversion tunnel is located on the right bank, it is difficult to choose a location for an entirely underground power house. Moreover, there are landslides in the area immediately in front of the dam, which further complicates the task of establishing an inlet for the underground power house. For these reasons, layout plans for a dual-row turbine hall were studied at that time; however, due to the lack of design and operational experience in China regarding this type of layout, especially for large hydroelectric stations like Lijiaxia on page 85, the dual-row turbine hall layout was not chosen out of caution. Instead, the so-called mixed cave-style layout was adopted, which involved placing 3 turbines on the riverbed behind the dam and 2 turbines underground on the right bank. 7.1.2 Layout plan for the double-row unit plant Geological surveys show that, especially after the excavation of the dam foundation, the geological conditions of the dam foundation become clearer. The fault F27 runs through the right bank dam foundation and extends downstream; there is also a gently inclined fault with a northeast orientation that runs upstream along the riverbed above the roof of the underground plant, extending into the area of the dam foundation. The stratigraphic faults F33 and F35 both pass through the underground plant. The underground plant and main transformer room, with excavation dimensions of 72m X 30m X 70m (length X width X height), are located behind the dam and intersect these faults; two steel pipe water intake tunnels, each with a diameter of nearly 10m, pass through the rock masses above and below F27. The placement of these large underground chambers within a limited area behind the dam can weaken the integrity of the rock mass at the abutment behind the dam. In particular, after the excavation of these chambers, it will inevitably sever faults such as F27, resulting in situations where the unfavorable rock layers behind the dam are exposed or prone to sliding, which is detrimental to the anti-sliding stability and deformation resistance of the rock mass at the dam shoulder. 　　 In 1987, hydroelectric experts from the former Soviet Union visited China and inspected the Lijiaxia construction site. Based on their experience, they recommended that the Lijiaxia Hydropower Station adopt a double-row turbine arrangement. In 1990, experts from the Moscow Hydropower Design Institute of the former Soviet Union once again raised with China’s expert team the issue of changing the layout of the Lijiaxia Hydropower Station to a double-row arrangement. Considering the specific topographical and geological conditions of the Lijiaxia dam site, as well as the construction status at that time and the existing practical problems, the leaders of the General Institute for Water Resources and Hydropower Planning and Design suggested that, if possible, the Northwest Survey and Design Institute conduct a study on the feasibility of implementing a double-row turbine hall layout scheme at the Lijiaxia Hydropower Station. After comprehensive comparison, it is concluded that even when the diversion tunnel has been largely completed and flow interception is in progress, as long as the basic dam design and the overall layout of the complex remain unchanged, adopting a double-row turbine arrangement for the Liujiaxia Hydropower Station’s turbine hall is feasible from technical, economic, and construction timeline perspectives, with the advantages outweighing the disadvantages. 7.1.3 Layout of the double-row unit plant　　 1. Layout of the water intake system　　 Five pressure water intake pipes pass through the central section of the arch dam’s riverbed; the elevation of the inlet bottom is 2130.00 m. After passing through the dam body, these pipes run along the downstream dam surface in the form of pipes behind the dam. At an elevation of 2041.50 m, near the dam foot, they turn at almost 90 degrees and are then embedded in the solid concrete foundation of the auxiliary plant structure, extending to the inlet of the volute for each turbine at that elevation. The pressure tube is designed with a head of 162 m, an inner diameter of 8 m, a PD value of 1300 m^2. The maximum thickness of the steel plates is 40 mm, while the minimum thickness is 20 mm. Due to the arrangement of the hyperbolic arch dam and the twin units, the lengths of the various steel tubes vary; the longest one is 202.77 m, and the shortest is 167 m. The total weight of the steel tubes is 5140 t. 　　 2. Layout of the main power house 　　 In accordance with the overall layout of the complex and the fact that 5 units are to be installed, the double-row unit power houses are arranged in the main riverbed behind the dam. 3 units are arranged in the front row (on the downstream side), and 2 units are arranged in the back row (on the upstream side)

Reinforcing bars, channel steel, H-beams, etc., are all available in 6-meter and 9-meter lengths. Multiples of 3 are beneficial for the use of materials.

Not only are steel bars, channel steels, H-beams, etc. available in 6-meter and 9-meter lengths, but I-beams and pipes as well are available in these same lengths; this allows for material savings in construction, the fabrication of supports and pipe racks, as well as in pipeline installation processes.

Has it been considered comprehensively from the aspects of construction, design, transportation, and delivery? Please correct me! !

Many raw materials used in civil engineering are based on a modulus of 3; therefore, the span lengths also follow this modulus, which facilitates construction, reduces material usage, and simplifies installation and transportation.

To ensure dimensional consistency in architectural design, component production, and construction, thereby improving the level of industrialization in building construction, reducing costs, and enhancing the quality and speed of building design and construction, architectural design should adopt a **standardized unified modular system**. The building module is a selected standard scale unit that serves as the basis for coordinating the dimensions of buildings, building components, building products, and related equipment. The basic module is specified as 100 millimeters, denoted as M0. The export modules are divided into sub-modules and expanded modules, with the following base values specified: 1. Sub-modules: 1/10M0, 1/5M0, 1/2M0, with corresponding sizes of 10, 20, and 50 millimeters respectively. II. Expanded module sizes: 3M0, 6M0, 15M0, 30M0, 60M0, with corresponding dimensions of 300, 600, 1,500, 3,000, and 6,000 millimeters respectively. The amplitude values for the modulus sequence are as follows: First, for the 1/10M0 modulus sequence, the increments are of 10 millimeters, with amplitudes ranging from 10 millimeters to 150 millimeters ； The 1/5M0 series progresses in steps of 20 millimeters, with ranges from 20 millimeters to 400 millimeters ； The 1/2M0 series increments by 50 millimeters, with ranges from 50 millimeters to 800 millimeters.