Answers to 30 steel questions in steel structure engineering

Answers to 30 steel questions in steel structure engineering

        1.What are the characteristics of steel structures compared with those of other materials?


 Answer: building steel has high strength, good plasticity and toughness, light weight of steel structure, uniform material, short construction period, good airtightness, poor corrosion resistance, heat resistance but not fire resistance, and brittle fracture of steel structure. 

Steel used in 3D steel structure engineering

  2.What functions must steel structure design meet?


 Answer: (1) it should be able to bear all kinds of situations that may occur in normal construction and normal use, including load and temperature changes, uneven settlement of foundation and earthquake action, etc.; (2) the structure has good working performance under normal use; (3) the structure has enough durability under normal maintenance; (4) it can still keep necessary after the occurrence of accidental events Overall stability.


   3.The basic requirements of steel structure for steel are briefly described.


 Answer: (1) high strength (tensile strength Fu and yield point FY); (2) sufficient deformation capacity (plasticity and toughness); (3) good process performance (cold working, hot working and weldability); (4) according to the specific working conditions of the structure, sometimes the steel is required to have the ability to adapt to low temperature, high temperature and corrosive environment).


   4.What are the plastic failure and brittle failure of steel?


 Answer: plastic failure is caused by excessive deformation, which exceeds the possible strain capacity of the material or component, and only occurs after the stress of the component reaches the tensile strength Fu of the steel, and the component produces large plastic deformation before failure; brittle failure refers to that the plastic deformation is very small or even no plastic deformation, and the calculated stress may be less than the yield point FY of the steel, and the fracture depends on the stress setThe form of destruction that began in the middle.


   5.What are the effects of carbon, sulfur and phosphorus on the properties of steel?


 Answer: with the increase of carbon content, the strength increases, the plasticity, toughness and fatigue strength decrease, and the weldability and corrosion resistance are deteriorated.Sulfur makes steel hot and phosphorus cold brittle.But phosphorus can also improve the strength and rust resistance of steel.


   6.Under what conditions will stress concentration occur, and what is the effect of stress concentration on steel properties?


 Answer: the actual steel structure components sometimes have holes, notches, concave corners, sudden changes in cross-section and internal defects of steel.At this time, the stress distribution in the component will no longer keep uniform, but will produce local peak stress in some areas, and reduce the stress in other areas, that is, stress concentration image.Under the action of negative temperature or dynamic load, the adverse effect of stress concentration is very prominent, which is often the source of brittle failure.


   7.What is fatigue fracture?Factors affecting fatigue strength of steel?


 Answer: the fatigue fracture of steel is a brittle failure in which the micro crack propagates continuously until it breaks under continuous and repeated loading.


 The fatigue strength of steel depends on the structural condition (stress concentration and residual stress), the applied stress amplitude Δ σ, and the number of cycles of repeated loading.


   8.In order to ensure the safety and reliability of steel structure and economic and reasonable use of materials, what factors should be considered when selecting steel?


 Answer: the importance of the structure or component; the nature of the load; the connection method; the working conditions of the structure; the thickness of the steel,


   9.The types of steel structure connection methods are briefly described.


 Answer: the connection method of steel structure can be divided into welding connection, bolt connection and rivet connection.


   10.The types of steel structure connection methods are briefly described.What are the advantages and disadvantages of steel structure welding connection method?


 Answer: the connection method of steel structure can be divided into welding connection, bolt connection and rivet connection.


 Advantages of welding connection: welding can be directly connected, simple structure, convenient production; do not weaken the cross-section, save materials; good airtight connection, structure stiffness; can realize automatic operation, improve the quality of welding structure.


 Disadvantages of welding connection: in the heat affected zone near the weld, the metallographic structure of steel changes, resulting in local material embrittlement; welding residual stress and residual deformation reduce the bearing capacity of compression members, and the welded structure is very sensitive to cracks. Once local cracks occur, it is easy to spread to the whole section, so the problem of low temperature cold embrittlement is more prominent.


   11.What is the main difference between high strength bolt connection and ordinary bolt connection?


 A: the main difference between high-strength bolt connection and ordinary bolt connection is that ordinary bolt connection relies on bolt rod to bear pressure and shear resistance to transfer shear force when it is sheared. When tightening nut, the pre tension of bolt is very small, and its influence can be ignored.In addition to its high material strength, high-strength bolt also exerts a great pre tension to tighten the bolt, which makes the contact surface of the connected plate produce compression force, so there is a great friction force between the plates.


   12.What are the possible failure modes of shear bolts?How to prevent it?


 Answer: when the bolt shear connection reaches the ultimate bearing capacity, there are four possible failure modes: ① the bolt bar is sheared; ② the bolt is under pressure; ③ the net section of the plate is broken; ④ the end plate is damaged by punching shear.


 The third failure mode is guaranteed by the component strength checking calculation; the fourth failure mode is guaranteed by the bolt end distance ≥ 2d0.The first and second failure modes are guaranteed by bolt calculation.


   13.Why should large and small allowable distances be specified for bolt arrangement?


 Answer: in order to avoid the mutual influence of stress concentration around the bolt, excessive weakening of the steel plate section, punching and shearing damage of the steel plate at the end, drum bending phenomenon between the connected plates and meeting the requirements of construction space, the large and small allowable distance of bolt arrangement is specified.


   14.What are the main principles to be considered when selecting the section of solid web axial compression members?


 Answer: (1) the area distribution should be carried out as far as possible to increase the moment of inertia and radius of rotation of the cross-section, and improve the overall stability bearing capacity and stiffness of the column; (2) the direction of the two main axes should be as stable as possible to achieve economic effect; (3) it is convenient to connect with other components, and the structure is as simple as possible, the manufacturing is labor-saving and the materials are convenient.


   15.Why is the conversion slenderness ratio adopted when calculating the overall stability of lattice axial compression members around the virtual axis?


 Answer: when the lattice axial compression members lose stability around the virtual axis, because the members are not continuous plates, but are connected by battens or battens at a certain distance, the shear deformation of the members is large, and the additional influence caused by shear can not be ignored.Therefore, the equivalent slenderness ratio is adopted to consider the influence of shear deformation of battens on the stable bearing capacity of lattice structures under axial compression. 

Steel type of steel structure

  16.What is the overall instability of beams?


 Answer: the beam is mainly used to bear bending moment. In order to give full play to the strength of the material, its section is usually designed in the form of high and narrow.When the load is small, it will only bend in the plane of moment action. When the load increases to a certain value, the beam will bend and twist suddenly and lose the ability to continue bearing. This phenomenon is called bending torsional buckling or overall instability of the beam.


   17.What factors affect the overall stability of the beam?What are the measures to improve the overall stability of the beam?


 Answer: the factors that affect the overall stability of the beam include the form and position of the load, the cross-section form of the beam, the spacing of the lateral supports and the form of the support.The measures to improve the overall stability of the beam include strengthening the compression flange and setting the lateral support.


   18.What should be calculated when designing tension bending and compression bending members?


 Answer: tension bending members need to calculate strength and stiffness (limiting slenderness ratio); compression bending members need to calculate strength, overall stability (in-plane stability of moment action and out of plane stability of moment action), local stability and stiffness (limiting slenderness ratio)


   19.What are the specific steps to select the section of solid web compression and bending members?


 Answer: (1) calculate the internal force design value of the structure; (2) select the section form; (3) determine the steel and strength design value; (4) determine the calculation length in and out of the plane under the action of bending moment; (5) preliminarily select the section size according to experience or existing data; (6) calculate whether the strength, stiffness and stability of the primary section meet the requirements.


   20.Why to set up the support system in the roof structure composed of trusses? What are the specific functions of the support system?


 Answer: the roof truss in its own plane is a geometry invariant system with large stiffness, and can bear various loads in the roof truss plane.However, the lateral stiffness and stability of the plane roof truss itself perpendicular to the roof truss plane are very poor and can not bear the horizontal load.Therefore, it is necessary to set up a support system.The specific functions of the support system are as follows: to ensure the overall space function of the structure; to avoid the lateral instability of the compression bar, to prevent excessive vibration of the pull rod; to bear and transfer the horizontal load; to ensure the stability and convenience of the structure installation.


   21.What is the meaning of “heat resistance but not fire resistance” of steel?What are the provisions of the code?


 Answer: when the steel is heated, its main mechanical properties, such as yield point and elastic modulus, decrease little when the temperature is within 200 ℃.When the temperature exceeds 200 ℃, the material changes greatly, not only the strength gradually decreases, but also blue brittleness and creep occur.When the temperature reaches 600 ℃, the steel enters the plastic state and can not continue to bear load.Therefore, the code for design of steel structures stipulates that when the surface temperature of steel exceeds 150 ℃, it is necessary to take thermal insulation protection measures for the structure that needs fire protection according to relevant specifications.


   22.What do age hardening and artificial aging mean?


 A: age hardening: a small amount of carbon and nitrogen melted in iron at high temperature will gradually precipitate from pure iron with the growth of time to form free carbon compounds and nitrides, which can restrain the plastic deformation of pure iron, so as to improve the strength of steel and reduce the plasticity and toughness. This phenomenon is called age hardening, commonly known as aging.


 Artificial aging: the process of age hardening is generally very long. Heating after plastic deformation of materials can make age hardening develop rapidly. This method is called artificial aging.


   23.Fatigue failure process of steel


 Answer: Steel under repeated load, the resistance and performance of the structure will change significantly, even fatigue failure.According to the test, under the direct continuous repeated dynamic load, the strength of steel will be reduced, that is, lower than the ultimate strength Fu of tensile test under one static load. This phenomenon is called steel fatigue.The fatigue failure is characterized by sudden brittle fracture.


   24.What are the common metallurgical defects of steel and what are their specific meanings?


 A: common metallurgical defects of steel include segregation, non-metallic inclusions, porosity, cracks and delamination.


 Segregation: it refers to that the chemical composition of steel is not uniform and uneven, especially the serious segregation of sulfur and phosphorus causes the deterioration of steel performance;


 Nonmetallic inclusion: it refers to the impurities such as sulfide and oxide in steel;


 Blowhole: when casting ingot, the carbon monoxide gas generated by the interaction of iron oxide and carbon can not fully escape.


   25.What factors are taken into account in the calculation formula of pre tension design value p of high strength bolt?


 Answer: (1) when tightening the bolt, the bolt is subjected to both the tensile stress caused by pretension and the torsional shear stress caused by torque; (2) in order to make up for the relaxation loss of pretension stress of high-strength bolt during construction, the over tension factor is generally 5% ~ 10%, so an over tension coefficient of 0.9 is considered; (3) considering the heterogeneity of bolt material, a reduction factor of 0.9 is introduced; (4) due to the non-uniformity of bolt material, a reduction factor of 0.9 is introducedIn order to ensure the safety, a safety factor of 0.9 is introduced.


   26.What are the design measures to reduce welding stress and welding deformation?


 Answer: (1) make the weld symmetrical to the neutral axis of the member section as far as possible, so as to reduce the welding deformation; (2) adopt appropriate weld leg size and weld length;


 (3) The weld should not be too concentrated.(4) Try to avoid two or three weld vertical cross; (5) try to avoid the shrinkage stress in the direction of base metal thickness.


   27.Design procedure of solid web axial compression member?


 Answer: (1) assuming the slenderness ratio λ of the member, calculate the required section area A.(2) The section steel is preferred according to the calculated section area a and the two main rotating radii.(4) According to the required a, h, B, etc., as well as considering the structural requirements, local stability and steel specifications, the primary size of the section is determined.(5) Checking calculation of strength, stability and stiffness of members


   28.How to determine the calculated height of steel beam web?


 Answer: (1) rolled steel beam refers to the distance between the starting points of two inner arcs at the intersection of web plate and upper and lower flange; (2) welded composite beam is the height of web plate; (3) riveted composite beam is the close distance between rivet (or high strength bolt) lines connecting upper and lower flange and web.


   29.What assumptions are made when determining the effective length of frame columns according to elastic stability theory?


 Answer: (1) the frame only bears the vertical load acting on the joint, neglecting the influence of beam end bending moment caused by beam load and horizontal load; (2) all frame columns lose stability at the same time, that is, all frame columns reach the critical load at the same time; (3) the rotation angles at both ends of the beam are equal.


   30.What are the specific steps to select the section of solid web compression and bending members?


 Answer: (1) calculate the internal force design value of the member; namely, the design value of bending moment MX, the design value of axial pressure n and the design value of shear force V: (2) select the section form; (3) determine the steel and strength design value; (4) determine the effective length in and out of the plane of bending moment action; (5) preliminarily select the section size based on experience or existing data; (6) conduct strength check and stiffness calculation on the preliminary selected sectionIf the checking calculation fails to meet the requirements, the primary section shall be adjusted and recalculated until the requirements are met.


   31.The differences between ordinary bolt connection and high strength bolt friction type connection under bending moment are briefly described.


 Answer: under the action of bending moment, the neutral axis is assumed to be located at the bolt below the bending moment in the calculation of ordinary bolt connection, and the neutral axis is located at the centroid axis of the bolt in the calculation of high-strength bolt friction connection.


   32.Is plastic development considered in the cross section of lattice members?Why?


 Answer: the plastic development is not considered in the cross-section of lattice members, and it is calculated according to the edge yield criterion, because the middle part of the section is hollow.


   33.What factors affect the stability bearing capacity of axial compression members?


 Answer: the initial bending of the member, the initial eccentricity of the load, the distribution of the residual stress and the restraint of the member.


   34.What should be calculated when designing tension bending and compression bending members?


 Answer: tension bending members need to calculate strength and stiffness (limiting slenderness ratio); compression bending members need to calculate strength, overall stability (in-plane stability of moment action and out of plane stability of moment action), local stability and stiffness (limiting slenderness ratio) 

Post time: Aug-05-2020