Questions and answers of the portal steel structure 2

Questions and answers of the portal steel structure 2

  • 28 Jul 2020
  • steel structure

Questions and answers of the portal steel structure 2

36. What are the characteristics and functions of bolt welding?

Answer: The steel structure usually adopts only one connection method during a certain connection location, namely welding or riveting, or bolt connection. However, in some cases, the two connections are also mixed, such as bolt welding or riveting welding. The deformability of the weld bead is not as good as that of the bolt connection. The ultimate deformation of the side fillet weld bead is approximately equivalent to the deformation at the end of the sliding of the high-strength bolt connection with pretension. Therefore, when welding and high-strength bolts are used together, the ultimate load that the connection can bear is approximately equivalent to the welding ultimate load plus the anti-sliding load of the bolt connection. When the weld bead and high-strength bolt are shared, there is a construction procedure problem. If the welding is carried firstly and then the bolt is tightened, there may be gaps between the layers due to welding deformation, and it is not easy to achieve the required pre-tension when tightening. If the bolt is tightened first and then welded, high temperature may reduce the pretension of the bolt. A reasonable method is to firstly wring the bolts to 60% of the design pre-tension, then carry on welding, and finally tighten the bolts after welding.

37. What does stiffness mean?

Answer: The amount of force applied by the structure or member in the direction of deformation under unit deformation conditions. Needed for structural static or dynamic analysis. If the displacement method is used to analyze the internal force of the structure, the stiffness matrix is used, and the stiffness parameter of the structure is required when calculating the effect of earthquake or wind vibration. There are also structural stiffness parameters that need to be used when designing the foundation of a dynamic machine. Stiffness is directly related to material properties and cross-sectional properties.

38. When the wind resistant column and the steel beam are connected by a spring plate, is it unnecessary to consider the aspect ratio when the strength and stability are no problems due to the small axial force?

Answer: The wind resistant column is connected with the steel beam by a spring plate. The ideal state is that the wind resistant column is a bending member and can only bear its own weight and wind load. At this time, the slenderness ratio can be appropriately relaxed (220~250). In fact, many spring plate connections are invalid in reality, and some are simply replaced with Z-shaped steel plates. In this case, the wind resistant column should be considered as a bending member. It has to bear part of the roof load, so the slenderness ratio is better to be conservative. (180~200).

39. The steel frame maintenance walls are all covered outside of columns. Why not put it on the beam in the middle of the column?

Answer: If it is just a maintenance, it should be excluded from the structural system when it is arranged, so that it is neither a load-bearing member nor a anti-side force member, and it should not restrict the deformation of structural system (vertical and horizontal) . If the brick wall is built between the columns of the steel frame, because of the lateral displacement stiffness of the brick wall is much greater than that of the steel frame, which will let the brick wall constrain the lateral displacement of the steel frame and crack before the force of the steel frame. Therefore, it makes sense to place the maintenance wall on the steel beam supported by the corbel. In this way, the brick wall will move sideways along with the steel frame and will not restrain the steel frame and will not crack.

40. What is the impact on the structure if the slenderness ratio of the swing column is not satisfied?

Answer: The swing column itself has a negative effect on the stiffness of the structure. In other words, it has negative stiffness. It needs the system to provide it with stiffness to ensure its stability. Try to make the swing column meet its own stability. If necessary, you can use other means such as wall support to improve the stability of the swing column. If the stability of the swing column is not satisfied, the burden on the system is heavier.

41. What is the distance between purlins?

Answer: The description of the technical regulations of door specifications 6.3.7 states that the maximum purlin distance is generally not greater than 1.5m.

42. What does coupling mean?

Answer: As the saying goes:”you have me, and I in you”, and influenced by each other. The easiest example to understand this concept is to solve the multi-degree-of-freedom dynamic balance equation, especially the decoupling step in the mode decomposition method.

43. For a project with floors, do you need to turn the column at the wind resistant column by 90 degrees when designing the side span?

Answer: The lower column does not need to be turned. The upper column is calculated according to the wind resistant column. If the selected section is sufficient to calculate the wind resistance according to the direction of the weak axis, then there is no need to turn. The cross section of the upper column can be relatively smaller, but the processing of the upper and lower column nodes is more troublesome. It depends on the specific situation.

44. Did the shearing pieces must be set on the column foot?

Answer: In portal steel structure house specification, after the column foot is calculated according to the friction system, if it does not meet the shear resistant requirements, the shearing pieces should be set, the friction coefficient is considered according to 0.4, the calculation formula is ≥0.4N (need to be set); Q≤0.4 N (no need to set), there is also an important condition, the secondary pouring should be dense, and the concrete should be added with expansion agent.

45. Is it necessary to control the horizontal displacement of the top of the concrete column of the steel roof workshop?

Answer: If the structural system is a frame structure, it must meet the displacement requirement of 1/550, which is very important.

46. It is understandable that the web plate will destabilize under compression. Why is it destabilized under the action of shear force?

Answer: Although the shear force is not the main stress in the steel member, the H-shaped steel is a thin-walled steel. Because the web plate is weak, it is also possible for the phenomenon of "destabilize" to occur in weak areas. To be precise, it is a local destabilize problem. Local yielding is achieved. Taking the unit body for research, if only the shear force stress is applied, the main tensile stress and the main compressive stress form an angle of 45° with the horizontal direction. It is this main compressive stress that makes the web plate "stuck" and unstable.

47. There is a provision in the steel structure specification for high-rise civil buildings regarding the lateral ductility ratio between layers under the action of a large earthquake. What is the lateral ductility ratio between structural layers?

Answer: The interlayer lateral ductility ratio refers to the ratio of the maximum lateral displacement and its elastic displacement between structural layers, and its value must not exceed the following limits:

①All steel structure: frame system 3.5, frame eccentric support 3.0, frame center support 2.5;

②Steel frame structure: section steel-concrete frame 2.5, steel-concrete mix 2.0.

48. What is the ductility of steel structure?

Answer: The ductility of a structure, member or section refers to the deformation ability from the beginning of yield to the maximum bearing capacity to the period that the bearing ability has not significantly decreased after it has been reached. That is to say, the ductility is to reflect the deformation ability of the structure, member or section at a later stage. A structure, member or section with poor ductility has a small deformation ability at a later stage, and brittle failure will suddenly occur after reaching its maximum bearing capacity, which is inevitable. Therefore, in engineering structure design, it not only to meet the bearing capacity requirements, but also to meet certain ductility requirements.

49. The length of the round steel used to make the anchor bolts is not enough. Can welding measures be taken?

Answer: Bevel plug welds are used between anchor bolts and embedded plates, which are often used in engineering. The general view is acceptable. However, the anchor bolts are not long enough, and it is not possible to lengthen by welding, because the weldability of the anchor bolt steel is generally poor, resulting in a large residual stress and weld defects after welding, and it is easy to cause brittleness at the weld bead when pulled. It’s dangerous.

50. Why do some places require a steel beam to be installed on the gable wall of the steel workshop instead of bearing by the gable directly?

Answer: It should be set in the same structural unit of the factory building, the different structural types should not be used; roof truss should be set at the end of the factory building, and gable wall should not be used for loading; The mixed load bearing of horizontal wall and bent frame should not be adopted in the factory building unit, because of the different structure has different vibration characteristics, different material strength, and different lateral stiffness. Under the action of earthquake, the structure is often destroyed due to the imbalance of load, displacement and strength. Single-layer concrete column factory buildings bearing by gable walls and intermediate horizontal walls and skylight frames bearing by end brick walls. For this reason, different structural types should not be adopted in a structural unit of the factory building.

51. What is the difference between the bearing capacity of a member and the bearing capacity of a member's section?

Answer: In the design of concrete structures, we generally choose the weakest section of the member as the control section. At this time, the relationship between the bearing capacity of the member and the bearing capacity of the members’ section is like the relationship between the barrel and the wooden board: the bearing capacity of the member depends on the bearing capacity of the weakest cross-section of member. In the design of steel structure, the control section should also be selected. However, a very important aspect of steel structure design is structural stability. Therefore, the bearing capacity of the member at this time does not entirely depend on the bearing capacity of the weakest section of members, but also depends on the stability conditions of the member. Similarly, in the steel-concrete composite structure, the connection between steel and concrete should also be considered. At this time, the bearing capacity of the member is not entirely determined by the bearing of the weak section of member.

52. Do you need to spray paint the column feet buried underground?

Answer: The column foot buried underground does not need to be painted. The main purpose of painting the steel column is to protect the steel column from rust. The protection effect of concrete on the steel column is far greater than that of paint; and the use of plug-in foundation connection is for rigid connection, the paint can not guarantee the adhesion between steel plate and concrete.

53. What is a strong column and a weak beam?

Answer: The strong column and weak beam are to make the plastic hinge appear in the beam first, but not in the column. If plastic hinges appear in the column, the structure does not become a geometrically variable system, but only loses the ability to continue to withstand horizontal loads.

(1)Strong columns and weak beams, strong shearing and weak bending, and strong nodes and strong anchoring are all to achieve ductile frames;

(2)The column is a bending member, and the axial force is very large, so the ductility of the column is very small, the ductility of the frame is mainly provided by the beam, and the plastic hinge of the beam generally appears at the end, so that the ductility of the beam is again comes down to the nominal compression zone height of the beam end section;

(3)Strong shear and weak bending are also to achieve ductile frame because shear failure is brittle failure and bending failure is ductile failure. In order to achieve strong shear and weak bending, the measure taken is that the design shear force of the beam end and the column end is determined according to the flexural bearing capacity of the end beam and end column to make sure that the bending failure is earlier than the shear failure.

54. What is cycle?

Answer: Some things appear repeatedly in the development process of movement changes. The time when two consecutive occurrences is called a cycle.

Natural vibration cycle: the time required for the structure to complete a free vibration according to a certain vibration mode.

Basic cycle: the time required for the structure to complete a free vibration according to the basic vibration mode.

55. What is linear stiffness?

Answer: Stiffness refers to the amount of force applied by a structure or member in the direction of deformation under unit deformation conditions. Needed for structural static or dynamic analysis. The so-called linear stiffness is the magnitude of the generalized force required for a unit length of a member to produce a unit deformation.

56. What is steel core?

Answer: Steel core means to the coordinate position when a lateral load is applied at a point on a certain floor of the structure, and where the entire floor only produces a coordinate position but without torsion. This concept is similar to the concept of the shear center of the section of a member.

57. What is the effect of horizontal loads to the structure?

Answer: A structure must undergo shear deformation and bending deformation under the action of horizontal load. Shear deformation is caused by shear force, which is the direct accumulation of horizontal load, which means that the shear force is small in the upper part and large in the lower part, so the reinforcement part of the shear wall structure is at the bottom, and the horizontal ribs are strengthened to improve its shear resistance to prevent shear damage. This means that a transmission path for horizontal loads is directly transmitted through shear forces.

58. What is the difference between the four concepts of seismic wall, shear wall, load-bearing wall, and self-supporting wall?

Answer: The seismic wall is the shear wall.

Elf-supporting wall: a wall that mainly bears its own weight. The self-supporting wall accounts for more than 75% of the total load. It serves as a partition, fire partition, and thermal insulation. It is designed without considering its load bearing on the upper load.

Load-bearing wall: In addition to the above effects, the load-bearing of the upper load is considered in the design. According to the force classification, the wall can be divided into load-bearing walls and non-bearing walls according to the force of the wall. Walls that directly bear loads from floors and roofs are called load-bearing walls; walls that do not bear these external loads are called non-bearing walls.

Self-supporting wall: In a non-load-bearing wall, a wall that does not bear external loads and only bears its own weight and transmits it to the foundation is called a self-supporting wall; it only serves as a space separation, and its own weight is borne by the floor or beam is a partition wall; in the frame structure, the wall filled between the columns is called a filler wall, and the inner filler wall is a type of partition wall; the light-weight wall hanging outside the building is called a curtain wall, which has a metal curtain and a glass curtain Wait. Although the curtain wall and the outer filler wall cannot bear the load of the floor slab and the top of the floor, they bear the wind load and transmit it to the skeleton structure.

59. What is negative stiffness?

Answer: Due to the axial force acting on a pressure bar, its actual lateral stiffness is somewhat reduced. The reduction of its stiffness is due to the axial force, so it can be considered that the axial force produces negative stiffness.

60. Why the shock failure of torsion is greater than translation?

Answer: The stress generated by translation is basically uniform, but the stress generated by torsion is not evenly distributed, and the stress at the corners is concentrated. Moreover, the load (mass) distribution is not uniform in actual use, which will aggravate the difference between the maximum torsional displacement and the maximum interlayer displacement.

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