Answers to 15 Common Questions on Civil Engineering Construction

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Answers to 15 Common Questions in Civil Engineering Construction I. What is the difference between diaphragm beams and frame beams? A connecting beam refers to a beam whose ends are connected to shear walls and whose span-to-height ratio is less than 5 (for specific provisions, see Article 7.1.8 of the Code for High-Rise Buildings) ； A frame beam refers to a beam whose ends are connected to frame columns, or a beam whose ends are connected to shear walls and whose span-to-height ratio is 5 or greater. The similarity between the two lies in the fact that, from the perspective of conceptual design, during seismic events it is desired that plastic hinges appear first in the frame beams or connecting beams, rather than in the frame columns or shear walls – this is what is referred to as \"strong columns and weak beams\" or \"strong walls and weak connecting beams\"” ； On the other hand, from a structural perspective, both must meet the seismic design requirements. Specifically, the longitudinal reinforcement in frame beams and connecting beams (including the reinforcement at the bottom and top of the beams) must satisfy the requirements regarding seismic anchorage length when anchored to the supports. For the same seismic rating, the diameter of the stirrups in frame beams and connecting beams as well as the spacing between them are the same. The difference between the two is that, in seismic design, the stiffness of connecting beams can be reduced significantly; in some cases, they can even be taken out of service. However, the stiffness of frame beams can only be reduced to a limited extent, and they cannot be taken out of service. Therefore, codes stipulate that secondary beams should not be placed on connecting beams, but they can be placed on frame beams. Generally, the span-to-height ratio of connecting beams is relatively small (less than 5), and their primary function is to transfer shear forces. Therefore, the codes specify certain construction requirements for connecting beams that differ from those for frame beams. First, the stirrups in connecting beams are required to be densely arranged throughout their entire length, whereas in frame beams there are areas with dense stirrups and areas without them. Second, clear requirements are set for the vertical reinforcement in connecting beams: the horizontal distribution reinforcement in the wall should serve as the vertical reinforcement for the connecting beam, being arranged continuously across its entire length ； When the cross-sectional height of the diaphragm beam is greater than 700 mm, the diameter of the longitudinal structural reinforcement bars (web bars) provided along the height of the beam on both sides should not be less than 10 mm, and the spacing between them should not be greater than 200 mm ； For coupling beams with a span-to-depth ratio not exceeding 2.5, the area ratio of longitudinal structural reinforcement (stirrups) on both sides of the beam shall not be less than 0.3%. This requirement has been incorporated into the mandatory provisions. As for frame beams, it is sufficient to ensure that “when the web height hw is ≥ 450 mm, longitudinal structural reinforcement must be provided along the height on both sides of the beam; the cross-sectional area of such longitudinal structural reinforcement on each side (excluding the tensile and compressive reinforcement as well as stiffening bars at the top and bottom of the beam) shall not be less than 0.1% of the web’s cross-sectional area bw·hw, and the spacing between these bars should not exceed 200 mm.” ” And it is not a mandatory provision. During the review of construction drawings, common errors made by designers include: first, classifying beams that are connected to shear walls at both ends and have a span-to-depth ratio of less than 5 as frame beams, with stirrups present in both reinforced and unreinforced areas; or classifying beams with a span-to-depth ratio of 5 or more as tie beams ； Secondly, in the reinforcement schedule for tie beams, no distinction is made between the height of the tie beam and its span-to-height ratio; instead, a general statement is provided stating that \"the vertical reinforcement in the tie beam is the same as that in the shear wall’s horizontal reinforcement.\" In such cases, if the height of the tie beam exceeds 700 mm or its span-to-height ratio is 2.5 or less, while the reinforcement ratio of the shear wall itself is less than 0.3% or the diameter of the horizontal reinforcement bars is 8 mm or less, it is easy to violate Article 7.2.26 of the \"High-Rise Building Code.\" Moreover, this is a mandatory provision, and designers should pay attention to this. II. What is the difference between frame beams and secondary beams? Generally, a secondary beam refers to a beam whose ends rest on frame beams. Such beams have no seismic requirements; therefore, structurally they differ from frame beams in the following ways. Taking the national standard atlas “03g101-1” as an example: (1) The anchorage length of the reinforcement at the top of the secondary beam at the supports is the tensile anchorage length la, whereas for frame beams, the anchorage length of the reinforcement at the top of the beam at the supports is the seismic anchorage length laE. (2) The anchorage length of the reinforcement at the bottom of secondary beams at supports is generally 12d, whereas the anchorage length of the reinforcement at the bottom of frame beams at supports is the seismic anchorage length laE. (3) The stirrups of secondary beams have no requirement for a minimum diameter, nor are there any requirements regarding compacted and non-compacted zones; only the calculation requirements need to be met. Frame beams have different requirements for the diameter and spacing of stirrups depending on the seismic resistance rating; they must not only meet the computational requirements but also the structural requirements. (4) In the planar representation, frame beams are numbered as KL, while secondary beams are numbered as L. In actual construction drawings, the main errors that designers tend to make fall into two categories: first, in the flat representation of secondary beams, the stirrups are indicated as belonging to reinforced areas or non-reinforced areas, such as φ8#100/200, etc. Secondly, when the beam in question is a single-span simply supported beam, the number of negative reinforcement bars at the supports often fails to meet the requirements specified in Clause 10.2.6 of the ‘Concrete Code’. According to this clause, when the beam ends are actually partially restrained but are still treated as simply supported beams, longitudinal structural reinforcement bars should be placed above the support area; the cross-sectional area of these bars should be no less than one-fourth of the cross-sectional area required for the longitudinal load-bearing bars in the middle and lower parts of the beam, and there should be no fewer than two such bars. III. What are the differences between primary tie beams and secondary tie beams? A foundation tie beam is a beam whose ends are connected to cap beams or individual column footings. Similar to secondary beams, foundation tie beams also have no seismic requirements; the anchorage length of the steel bars at the top of these beams in the supports is also the tensile anchorage length la. Moreover, there are no requirements regarding reinforced zones or non-reinforced zones for the stirrups in foundation tie beams. Unlike secondary beams, the reinforcement at the bottom of foundation tie beams must also meet the requirements regarding the tensile anchorage length la; the width of foundation tie beams should not be less than 250 mm. In addition to being determined based on calculation requirements, the diameter of the vertical reinforcement bars inside such beams should not be less than 12 mm, and there should be no fewer than 2 such bars (see Article 8.5.20 of the “Foundation Code”). The diameter of the stirrups should be at least Φ6@200 (see Article 3.12.1-9 of the “National Technical Measures for Civil Building Design – Structural Section”). In actual construction drawings, a common mistake made by designers is to apply the representation methods used for frame beams to foundation tie beams, assigning them the code JKL, and representing the stirrups as belonging to reinforced areas or non-reinforced areas, using notation such as φ8#100/200. The existing national standard flat method drawing sets do not provide specific details regarding the construction of foundation tie beams. If designers wish to use these drawing sets, it is appropriate to designate such beams as JL. It should also be specified in the notes that the reinforcement configuration for JL should follow that outlined in \"03G101-1\" for secondary beams (non-frame beams), and that the length to which the rebar at the bottom of the beam is anchored into the support must meet the requirements for the tensile anchorage length la. IV. Difference between frame columns and stand-alone columns: In terms of the sequence of concrete pouring, columns that are poured before wall construction are frame columns, while those poured after wall construction are stand-alone columns (structural columns). Of course, they can also be distinguished by their function: frame columns are primarily used to bear vertical loads, whereas stand-alone columns serve mainly to connect walls and provide shear resistance (this is most evident during earthquakes). V. What is the difference between structural columns and ordinary columns? Construction columns are not included in structural calculations. The common practice is to build the walls first and then pour the columns. Its longitudinal and stirrups only need to be arranged structurally, without having to meet a minimum reinforcement ratio. For ordinary columns, the longitudinal and stirrups must be arranged according to calculations, and must not be less than the minimum reinforcement ratio. VI. Why are piles driven at construction sites? A building is a structure built above the ground surface; it must have a solid foundation. Without proper treatment, uneven settlement of the foundation can lead to cracks in the building at worst, or even its collapse. To ensure the safety of a building, its foundation must be inspected by professional surveyors, who then determine the appropriate type of foundation based on the loads exerted by the superstructure. For taller buildings, deep foundations are required—such as driving piles down to a stable bearing layer, or excavating to a level below the basement level before driving piles (cement piles or vibro-compacted stone columns) into that layer. For shorter buildings, large-scale excavation or the use of vibro-compacted stone columns (which effectively mitigate seismic risks by pre-liquefying the soil) can be employed to prevent uneven settlement and meet the load-bearing requirements of the structure. Simply put, when the soil quality in the shallow layers of the foundation is poor and the competent soil layer lies at a greater depth, deep foundations are required to meet the structure’s demands regarding foundation strength, deformation, and stability; in such cases, pile foundations are used, which involves driving piles into the ground. Conversely, an open-cut method is used to expand the foundation. VII. Why are steel bars overlapped? Generally speaking, lapping is less costly and faster to implement than welding or mechanical joining; therefore, rebar lapping is usually used wherever it is permitted. During the construction of reinforced concrete structures, this is done to ensure the continuity of stress on the rebar and the reliable transmission of forces. For ease of transportation, steel bar manufacturers supply all steel bars other than coil bars in lengths of 6m or 9m each. Coiled rebar can be supplied in very long lengths, but there are also length limits for secondary transportation at the construction site; it’s hard to imagine transporting 80-meter-long rebar from the ground up to the 20th floor. Therefore, in engineering, there are inevitably areas where overlapping or welding is required. VIII. How does a tower crane rise step by step? Above the top part of the tower crane (below the beam section), there is an iron frame that is slightly wider than the main structure; the upper part of the tower crane is connected to this wider iron frame. Slides are provided at the four corners of the outer wide iron frame and those of the main iron frame, allowing the outer frame to move relative to the main frame. When adding a new section to the tower crane, a jack is used to lift the upper part upward until it reaches the height of one section of the structure. Then, a pre-lifted section is moved from the outer wide frame to its position between the existing sections, and it is fixed to the lower main iron frame. At this point, the tower crane has risen by one section in height. During the next addition of a section, the outer wide frame can slide upward further on top of this new section, and in this way the tower crane gradually rises higher. The outer wide iron frame is a bit longer than a single iron frame. Only in this way can section addition be carried out. When dismantling, it’s the opposite. The addition of sections to tower cranes is usually carried out at night in order to avoid disrupting work, so it is rare for anyone to see it happen; one only notices that the crane is getting taller day by day, without knowing exactly how it does so. When the image below is enlarged, the wider structure at the top is used for adding sections; it’s the movable part. IX. Why are some construction sites using suspended scaffolds while others use ground-based scaffolds? 1. If there is an urgent need to carry out outdoor backfilling or the inspection of structures below ground level, then the height of the scaffold that reaches the ground should be equal to the sum of the height of the basement and the height of the first floor. Suspended scaffolding should be erected starting from the top of the first floor, that is, at the level of the second floor; the height of such scaffolding is 18 meters (this figure already takes into account the height of the parapet walls and the protective railings). The advantage of this approach is that, after the scaffold is removed and inspected, it is possible to proceed directly with tasks such as waterproofing the exterior walls of the basement and carrying out outdoor backfilling. However, the disadvantages are also evident: a large number of suspended beams (usually 16# I-beams or channel steels) are required, along with wire ropes and U-shaped clamps. Additionally, there is an increase in labor costs and a certain delay in the project timeline. 2. If there is no urgency to backfill, it is recommended to erect the scaffolding all the way to the top; with a total height of no more than 30 meters, a double-row scaffolding system using individual vertical poles can be used effectively. This approach helps to speed up the construction process and save labor costs. However, the downside is that a large amount of steel pipe fasteners are required, and the scaffolding has a longer service life. X. What kind of glass is safety glass? Where is it mainly used? A type of glass that does not break under severe vibration or impact, and even if it does break, it is not likely to cause injury. Including tempered glass, laminated glass, etc. Used for doors and windows in cars, airplanes, and special buildings. (1) Exterior windows in buildings of 7 floors and above. (II) Floor-to-ceiling windows with window glass of an area greater than 1.5 m2, or those in which the bottom edge of the glass (the boundary between the transparent part of the glass and the opaque part covered by the glass mounting materials) is less than 500 mm from the final finished surface. (III) Curtain walls (except all-glass curtain walls). (IV) Asymmetricly installed windows, various types of ceilings (including skylights and daylight roofs), and suspended ceilings. 　 (5) Sightseeing elevators and their outer enclosures. (VI) Interior partitions and screens. (7) Guardrails for stairs, balconies, platform corridors, and handrails in atriums. (8) Floor slabs used to support pedestrian traffic. (IX) Entrance and exit areas, lobbies, etc., of public buildings (including 1. door glass, 2. glass installed above the door, 3. glass installed on both sides of the door, where the vertical edge nearest to the doorway opening is less than 300 mm away from that doorway opening). (10) Other parts of the body that are susceptible to impacts or blows, which may cause bodily injury. 11. What does C30 signify for this concrete? Concrete with a standard compressive strength value of 30 MPa. The strength grade of concrete is classified according to the standard values of cube compressive strength. The strength grade of concrete is denoted by the designation C and the characteristic value of cube compressive strength fcu,k, with the unit of measurement being MPa. The characteristic value of the compressive strength of cubes refers to the compressive strength, with a 95% level of confidence, measured using standard test methods on cube specimens with a side length of 150 mm that have been prepared and cured in accordance with standard procedures, at an age of 28 days. The strength grades of ordinary concrete are divided into twelve grades: C7.5, C10, C15, C20, C25, C30, C35, C40, C45, C50, C55, and C60. 12. Is it possible for Grade I steel bars to be ribbed steel bars? Could the secondary rebar be plain round rebar? How are Grade 1 steel, Grade 2 steel, Grade 3 steel, and Grade 4 steel classified? There are many types of steel bars, which are generally classified according to their chemical composition, manufacturing process, rolled shape, form of supply, diameter, and application in structures: (1) Classification by rolled shape: (a) Plain bars: Grade I steel bars (Q235 steel bars) are all rolled into plain, round cross-sections. They are supplied in coil form, with a diameter not exceeding 10 mm and a length ranging from 6 m to 12 m. (2) Ribbed steel bars: There are three types—spiral, herringbone, and crescent-shaped. Generally, Grade II and III steel bars are rolled into herringbone shapes, while Grade IV steel bars are rolled into spiral and crescent shapes. (3) Steel wires (divided into low-carbon steel wires and carbon steel wires) and steel strands. (4) Cold-rolled twisted steel bars: Formed through cold rolling and cold twisting. (II) Classification by diameter: steel wires (diameter 3–5 mm), fine rebar (diameter 6–10 mm), and thick rebar (diameter greater than 22 mm). (III) Classified by mechanical properties: Grade I steel bars (235/370 grade) ； Grade II steel bar (335/510 grade) ； Grade III steel bars (370/570) and Grade IV steel bars (540/835). (IV) Classified by production process: hot-rolled, cold-rolled, and cold-drawn steel bars; there are also heat-treated steel bars obtained by heat-treating Grade IV steel bars, which have higher strength than the former. (5) Classified by their function in the structure: compressive steel bars, tensile steel bars, erection steel bars, distribution steel bars, stirrups, etc. The steel bars used in reinforced concrete structures can be classified into the following categories according to their functions: 1. Load-bearing steel bars – steel bars that bear tensile and compressive stresses. 2. Stirrups – bear part of the tensile stress and fix the position of the load-bearing bars, commonly used in beams and columns. 3. Stirrups – used to fix the position of the steel rebar within the beam, forming the rebar framework inside it. 4. Distribution bars – Used in roof panels and floor slabs; they are arranged perpendicular to the load-bearing bars of the slab, distributing the applied weight evenly across those bars, fixing their position, and resisting temperature-induced deformation due to thermal expansion and contraction. 5. Others – Structural reinforcement bars provided due to the requirements of the component’s design or for construction and installation purposes. Such as lumbar tendons, embedded anchor bars, rings, etc. They look the same on the outside – what is the difference between grade 2 steel and grade 3 steel? In the construction industry, Grade II steel bars and Grade III steel bars were the terms used under the old standards; under the new standards, Grade II steel bars are now referred to as HRB335 grade steel bars, while Grade III steel bars are called HRB400 grade steel bars. Simply put, the similarity between these two types of steel bars is that they both belong to ordinary low-alloy hot-rolled steel bars ； All belong to ribbed steel bars (i.e., the commonly referred to deformed steel bars) ； All can be used in ordinary reinforced concrete structural projects. The main differences are: 1. Different steel grades (different chemical compositions). HRB335 grade rebar is 20MnSi (20 manganese silicon) ； HRB400 grade steel bars are 20MnSiV or 20MnSiNb or 20MnTi, etc ； 2. The strengths vary; the design tensile and compressive strengths of HRB335 grade steel bars are 300 MPa, while those of HRB400 grade steel bars are 360 MPa. 3. Due to the differences in the chemical composition and ultimate strength of steel bars, their properties such as toughness, cold bending capacity, and fatigue resistance also vary. The theoretical weight of the two types of rebar is the same when their nominal diameter and length are equal. The requirements for the anchorage length of the two types of rebar in concrete are different. The anchorage length of rebar is related to the tensile strength of the rebar, the tensile strength of the concrete, and the shape of the rebar. Here’s a formula for you: the anchorage length L of the tensile rebar in concrete is equal to a×(f1/f2)×d. In the formula, f1 represents the tensile design strength of the reinforcement ； f2 is the tensile design strength of concrete ； a is the shape coefficient of the rebar; it is 0.16 for smooth rebar and 0.14 for ribbed rebar ； d is the nominal diameter of the steel bar. Furthermore, when the rebar is of HRB335 or HRB400 grade and its diameter is greater than 25 mm, the anchorage length should be multiplied by a correction factor of 1.1. In seismic areas, a coefficient greater than 1 should also be applied based on the seismic resistance rating. The anchorage length of compressive steel bars in concrete is 0.7 times that of tensile steel bars. Regarding the grade of steel bars, it is not easy for inexperienced professionals to distinguish them based on their appearance, especially when the grades are similar to each other, such as 14 and 16, 6 and 8, 20 and 22. This is because different manufacturers produce steel bars with varying rib sizes, which can lead to visual confusion. It is best to use the grade designation for identification, along with measurements taken with calipers. In some areas, the old standards are still in use: 235 denotes grade 2 steel, identified by a starting number of 2, while 335 denotes grade 3 steel, identified by a starting number of 3, and so on. 13. Why are scaffolds and toeboards painted yellow and black? Why not paint it red? (1) Safety colors include four colors: red, yellow, blue, and green.
1. Meaning and uses of safety colors: Red denotes prohibition and stop. Devices, equipment, or environments that require prohibition, stopping, or indicate danger are marked in red, such as prohibition signs, traffic prohibition signs, and fire-fighting equipment. Yellow indicates attention and warning. Devices, equipment, or environments that need to alert people to potential hazards are marked in yellow, such as warning signs, traffic warning signs, traffic prohibition signs, and fire-fighting equipment. Blue signifies instructions that must be followed. For example, signs indicating the necessity of wearing personal protective equipment or traffic-related instructions are marked in blue. Green represents permission to proceed, safety, and providing information. Areas where it is safe to proceed or that indicate safety are marked in green, such as signs indicating permission to proceed, machine controls, start buttons, and safety signals.
2. Complementary colors: The complementary colors are black and white. The complementary color of yellow is black, while the complementary colors of red, blue, and green are all white. Black and white serve as complementary colors to each other. Black is used for the text and graphic symbols on safety signs, as well as for the combined graphics on warning signs and public information signs. White is used as the background color for the red, blue, and green safety colors on safety signs; it can also be used for the text and graphic symbols on safety signs, as well as for safety markings on walkways, traffic lanes, and safety lines on railway platforms. Stripes of red and white are more noticeable than red alone; they are used to indicate no entry or no crossing, and are applied to barriers and partitions in road traffic and similar contexts. Stripes of yellow and black are more prominent than yellow alone, serving as a reminder to be especially careful; they are used in lifting hooks, press clamps on shearing machines, and punch press sliders. Stripes of blue and white are more visible than blue alone, and are used to indicate directions, often in traffic guidance signs. (II) Use of safety lines: In industrial and manufacturing facilities, these lines are used to demarcate safe areas from hazardous areas. Lines marking safe passages within factories and safety lines on railway platforms are common examples of such safety lines. According to relevant regulations, safety lines should be white in color and at least 60 mm wide. With the presence of these safety lines during production, it becomes easier to distinguish between safe and hazardous areas, which helps us understand and assess them better. (III) Safety signs: Safety signs consist of safety colors, geometric shapes, and graphic symbols, and are used to convey specific safety information. The purpose of using safety signs is to alert people to potential hazards and prevent accidents, thereby ensuring safety. Of course, safety signs alone cannot eliminate any risks nor replace the necessary facilities for accident prevention. 1. Types of safety signs: Safety signs are divided into four categories: prohibition signs, warning signs, instruction signs, and guidance signs. 2. Meanings of safety signs: Prohibition signs are designed to prevent unsafe behaviors; they typically take the form of a circular frame with slashes. The circle and slashes are red, while the graphic symbol is black, with a white background. Warning signs are meant to alert people to potential dangers in their surroundings; they usually have a triangular frame. The triangle and its contents are black, with a yellow background. Instruction signs are used to require people to carry out certain actions or take preventive measures; they have a circular frame, with a white graphic symbol on a blue background. Guidance signs provide information to people; they have a square frame, with a white graphic symbol on a green background. XIV. What are the methods for waterproofing floor membranes: external application method or internal application method? What is the “external prevention and external application method”? What is the “external prevention and internal application method”? The methods for laying membrane materials in underground projects can be divided into the \"external protection with external laying method\" and the \"external protection with internal laying method\", depending on the sequence of wall construction and the location where the membrane is laid; the appropriate method can be chosen based on the specific circumstances during construction. The external application method involves first laying the bottom layer of membrane on the cushion layer, leaving joints around the edges; once the concrete for the base slab and the vertical surfaces is poured, the membrane waterproofing layer is applied directly to the outer surface of the exterior wall of the waterproof structure. “The specific construction sequence for the \"external prevention and external application method\" is as follows: ① Pour the concrete cushion for the waterproof structure base, then apply a 1:3 cement mortar leveling layer on top of the cushion, smoothing it out and finishing it with a trowel. ②Then, a permanent protective wall is constructed on the base slab layer; the height of this wall is B + (200~500 mm), where B is the thickness of the base slab. A layer of asphalt felt is laid flat at the bottom of the wall. ③A temporary protective wall is constructed on top of the permanent protective wall, with a height of 150×(number of asphalt shingle layers + 1). Temporary protective walls should be constructed using lime mortar. ④Apply a 1:3 cement mortar leveling layer on the permanent retaining wall and base layer, with the corners shaped into arcs. Apply lime mortar as a leveling layer on the temporary protective wall, and then paint it with lime slurry. If a form is used to replace the temporary retaining wall, an isolation agent should be applied to it. ⑤Once the leveling layer of the protective wall is basically dry, apply a coat of prime coat; however, no prime coat is required for temporary protective walls. ⑥ When applying the membrane waterproofing layer over the cushion layer and permanent protective wall, an additional membrane layer should be installed at the corners. During application, the bottom surface should be covered first, followed by the vertical surfaces; the joints around the edges should overlap each other and be attached to the protective wall. At the point where the membrane transitions from the cushion layer to the vertical surface of the permanent protective wall, it should be secured tightly using adhesive material. As for the area where it contacts the temporary protective wall (or the formwork of the retaining structure), it should be temporarily fixed to that wall (or formwork) in layers. ⑦After the linoleum has been installed, hot asphalt or bitumen should be applied to the surface of the base layer and the permanent protective wall membrane, and clean hot sand should be scattered over it while it is still hot. Once it has cooled, 1:3 cement mortar should be applied to the base layer, the permanent protective wall, and the temporary protective wall, serving as a protective layer for the membrane waterproofing system. ⑧When pouring the concrete base slab and wall concrete of the waterproof structure, the retaining wall serves as the formwork on the outside of the wall. ⑨After the pouring of the concrete for the waterproof structure is completed and inspected, the temporary protective walls are removed, and the waterproof membranes at the joint areas are cleared. Any damage to these membranes must be repaired before the waterproof membranes covering the outer surface of the waterproof structure are laid down layer by layer. Here, the sheets can be joined with overlapping seams; the upper sheet should overlap the lower one by at least 150 mm, and a cover strip should be used at the seam. ⑩Once the membrane waterproofing layer has been installed, a leakage test should be conducted immediately. Any leaks must be repaired right away; if there are no leaks, a permanent protective wall should be constructed. The permanent protective wall should have gaps every 5–6 meters as well as at corners, with the gap width being no less than 20 mm. These gaps should be filled with oilcloth or asphalt hemp. The gap between the retaining wall and the membrane waterproofing layer should be filled with 1:3 cement mortar as bricklaying progresses. Once the retaining wall construction is completed, the soil is backfilled immediately. “The “external protection with internal bonding” method is one of the techniques for laying membranes in underground projects. “The \"external protection with internal application\" method is a type of membrane construction technique in which a concrete cushion is poured first; the permanent protective wall is built entirely on top of this cushion, a cement mortar leveling layer is applied, and the membrane waterproofing layer is then directly laid on top of the cushion and the permanent protective wall. “The construction sequence for the \"external protection with internal bonding\" method is as follows: ① Construct a concrete filler layer; if the protective wall is quite tall, it is possible to increase the thickness of the base layer beneath the permanent protective wall, and reinforcing bars can be used if necessary. ②A permanent protective wall is constructed on the concrete pad; the thickness of this wall is that of a brick wall, with one layer of asphalt shingle laid directly beneath it. ③After the retaining wall is built, apply a 1:3 cement mortar leveling layer on the base course and the surface of the retaining wall; the corners should be made into obtuse or rounded shapes. ④After the leveling layer has dried, apply 1–2 coats of priming paint. Once the priming paint is dry, lay the membrane waterproofing layer directly on the protective wall and the base layer; when laying the membrane, start with the vertical surfaces first and then the horizontal surfaces. When laying the facade, start with the corners first, then the larger areas. ⑤Once the membrane waterproofing layer has been installed, a protective layer should be applied promptly. On the horizontal surfaces, a layer of fine aggregate concrete 30–50 mm thick can be poured, or a layer of 1:3 cement mortar can be applied. On vertical surfaces, an asphalt-based adhesive can be applied on the surface of the membrane, followed by the sprinkling of hot sand while it is still warm. After it cools, another layer of 1:3 cement mortar is applied as a protective layer, and this layer is textured to facilitate adhesion to the concrete wall. ⑥Pour the concrete for the base slab and walls of the waterproof structure. ⑦Backfill soil. 15. What are the differences between short-leg shear walls, irregular columns, and wall-type frames? To understand the difference between the first and third, one must first understand the classification of shear walls. 1. The types of shear walls are distinguished based on the integrity coefficient. a>=10, In/I