CHENG

LI MADE GREAT ACHIEVEMENTS IN THE FUTURE, 

AND DEVOTED HIMSELF TO PRAGMATIC INNOVATION

LI
The essence of reinforced concrete
      Confused:
      Because of the accident of fate, almost 30 years ago from other majors to do structural design.When I first contact, I feel that I have mastered the structure design as long as I master the "calculation + structure".After several years of design, I felt naive and ignorant.Steel structure and foundation, more easy to understand and reinforced concrete can't melt is well versed in, the design of a single component, or structure, good understanding and together, as if again desultorily, fragmented, this is ductile rupture, and that is brittle failure, etc., seems to be is the result of the experiment.When the structure is arranged, it is more "hard" to avoid that, some good understanding, some hard to make sense, such as the "frame short column" brittle failure principle;The axial force can improve the bending ability while the column is under pressure, but the axial compression ratio is also limited.There are no specific indicators for the prevention or control of the second channel;TBSA and PKPM can reflect the shear lag of the cylinder when the cylinder is in the cylinder.What happens if the first layer is not nested?!
      Old comrades:
      It's a very good topic, but it's really hard to talk about.After looking for a few days, decide to give a brick to lead the jade, have improper place, ask everybody to be very important.
      I am not a civil engineer (engineering mechanics), and I have two post-graduate degrees with the concrete edge (earthquake and wind).I feel the same way as you when I just touched the concrete.About twenty years ago, I consulted the university of southeast university (nanjing institute of technology) in my mind.Mr. Cheng, Mr. Cheng, is very rigorous and good at summarizing. He helped me to sum up a few points to make me feel more open-minded.Steel structure is isotropic material, the constitutive relation is extremely simple, as long as master strength, stability and structure, the rest is the force analysis, which happens to be our specialty.However, the reinforced concrete is very different, mainly because of the geometric change of the mechanical index of materials: strength of reinforcement, compressive strength of concrete, and an order of magnitude of tensile strength.The elongation of the reinforcement and the compressive resistance of concrete are further.2. The dispersion of mechanical indicators of concrete is larger.3. Creep properties of concrete materials.
      First nature of the reinforced concrete member is to use concrete compressive strength and tensile strength of steel, when the tensile strength of steel to determine the bearing capacity of components, because of its elongation is very big, and show the characteristics of ductility, vice is brittle failure.For example, flexural beam and super beam, the size eccentricity compression.And shear components, in the truss stress model, there is no strength is proportional relationship (bending even if it is not strictly proportional relationship, but the basic close to proportional), but simply bilinear relation, so its optimum reinforcement are not equal to the bending ductility of steel beam, only is the origin of the concept design of strong shear weak bending
      The second large discreteness of nature in order to satisfy the same degree of safety, you need greater strength rich (the ratio of average strength and design strength), and this reaction in the vii vii iv standard in safety factor K (bending 1.4, compression, shear is 1.55), the new standard in the formula was missing, but can be found from the background of statistical regression
      The third feature is the creep of concrete performance is one of the conditions of the plastic internal force redistribution, as a scholar said, the reasonable design of concrete structures can regulate its internal force according to the intention of the designers.The plastic hinge of the member with the crack work is not a point but a region.
      In the conceptual design of structure, there is a very important, is in a severe earthquake, the structure of rich and there is no strength only resistance to deformation ability of good and bad points, namely the structure should be entered the stage of plastic deformation (or elastic-plastic phase).Where the plastic hinge appears in the design;How many components are destroyed to absorb the earthquake input energy, and then easy to repair after the earthquake;Those key components are the last line of defense and so on, which is the essence of earthquake-resistant design.According to this idea, it is not difficult to understand many of the requirements in the seismic specification.For example, the short column has typical shear failure characteristics, and the coupling ratio and axial pressure ratio directly affect the ductility of the column.The frame-supported shear wall structure has been affected to the seismic performance due to the over-concentration of deformation. The structural stiffness of the switchboard has the largest mutation, and it is used as little as possible in the high intensity area.
      Furthermore, TBSA can not reflect the shear lag effect because of its use of seven-degree-of-freedom thin-walled bar model to analyze shear wall.At the same time, due to a mandatory constraint, there will be great errors in the conversion layer and the upper and lower layers, but the overall calculation and general floor analysis are good.The first layer as a nesting is just a supposition, as long as the horizontal force is metasomatic, it can be embedded there (underground one or two underground).In fact, the first layer thickness 100 or 500, vertex displacement effect is less than 5% of the building, I personally think it is not necessary just to meet the requirements of the simplified calculation model and thickness, but must understand the model and the actual discrepancy what may bring adverse effect, and adopts the corresponding remedial measures in the design.
      The norm, I think, is "do it, and you don't usually have a problem."In the design, if you think about it from time to time, if you don't abide by this letter, then you will naturally deepen your understanding of the specification and broaden your design thinking.Of course, I am not advocating that everyone should break through the norms, the superspecification, still have to let the general work clappers
      Generally, when the steel bar leaves the factory, there are signs and signs on the label.When it is not, the screw steel is marked (name), specification, grade;
      Level: Ⅰ, Ⅱ, Ⅲ, Ⅳ
      Code: 
      Generally denoted as: level marking specification.
      Generally denoted as: level marking specification.
      The reinforcement used in the building is mainly divided according to tensile strength.
      The tensile strength of 2400kg/c m squared is called a level 1 steel bar, and the drawing is represented by phi;
      The tensile strength of 2800kg/cm square is also a grade 1 steel bar, which is represented by the two vertical phi in the circle.
      The 16Mn steel of tensile strength of 3400kg/cm square is a 2 - grade steel bar, with a horizontal bar on the drawing.
      The 25MnSi steel of the tensile strength of 3800kg/cm square is a 3 - grade steel bar, which is represented by the two vertical and the bottom of the drawing.
      Now the data is the old standard, the existing standard stress unit is MPa, 1MPa = 10Kg/cm squared, which needs to be converted.
      You may be used to level of steel (Ⅰ  namely ), the engineering level of steel using hybrid structures (Ⅰ  namely) the quantity is relatively large, such as the belt type foundation, constructional column, ring girder, cast-in-place floor or roof board, etc.The existing buildings, structures and Bridges are more than two or three levels of steel.
     But if a careful analysis, in level 3 steel to save steel than level, under the condition of plate is not big, general plate structure reinforcement, this is the minimum reinforcement ratio to control, the minimum reinforcement ratio is directly related to steel grades, you compare the will find that the concrete level, level of steel and tertiary steel vary significantly (large plate, force control level 3 steel superiority is more obvious when)
      For example: c25 concrete
      When the level of steel: rho min = Max {0.20%, 0.45 ft/fy} = {0.20%, 0.27%} = 0.20% Max
      Level 3: when steel rho min = Max {0.20%, 0.45 ft/fy} = {0.20%, 0.16%} = 0.20% Max
      If thickness:
      H = 100 mm
      Amount of reinforcement per meter: AS1 = 1000 * 100 * 0.27% = 270mm__AS3 = 1000 * 100 * 0.20% = 200mm__
      H = 120 mm
      Amount of reinforcement per meter: AS1 = 1000 * 120 * 0.27% = 324mm__AS3 = 1000 * 120 * 0.20% = 240m__
      The difference between the two comes out, and the price of the two is about the same as the three levels of steel, and the support is not afraid to step on it.The bottom band does not need a hook.
     Of course, due to the high intensity, even by calculation, the province is not only constructed."The minimum reinforcement ratio is directly related to the steel grade" should also be reasonable.