Questions and answers of the portal steel structure 1

Questions and answers of the portal steel structure 1

  • 15 Jul 2020
  • steel structure

Questions and answers of the portal steel structure 1

1、When looking at the bending moment diagram, you can see the bending moment, but don't know what is the relationship between the bending moment and the section of the members?

Answer: The bending bearing capacity of flexural members Mx/(γ x*Wx)+My/(γ y*Wy)≤f , While W is the section resistance moment, general section can be calculated according to the section resistance moments by hand.

2、 What are the regulations for flat-joints of H-type steel?

Answer: Any connection is possible. The main consideration is the transmission of bending moments and shear forces. In addition, where there are many dynamic loads, special care of flat-joints should be taken when designing welded joints.

3、 What is the role of a single tie rod? Is the column bracing support set by a single tie rod only required to meet the slenderness ratio requirement? If round steel is used as the support for a single tie rod, is there a slenderness ratio requirement?

Answer: a single tie rod, as the name implies, refers to the tie rod that can only bear the tensile force during calculation. When it is compressed, it is considered to withdraw from work, so it is generally necessary to set two symmetrically. b. Single tie rods, like other rods, must meet strength and rigidity requirements. In fact, intensity often does not take the effect of control. Generally, the rigidity controlled by the slenderness ratio can not be too weak, which can ensure its stability during installation and so on. c. Round steel tie rod is an exception, because the installation device with fastening is adopted, so its length-to-length ratio will not be controlled.

4、 When designing a steel structure, what happens after the deflection exceeds the limit?

Answer: Deformation that affects normal use or appearance; local damage (including cracks) that affects normal use or durability; vibration that affects normal use; other specific states that affect normal use.

5、 What is the role of extruded board?

Answer: Extruded polystyrene (XPS) insulation board, which uses polystyrene resin as the main raw material, is continuously extruded and formed into a rigid board by special process. It has a unique and perfect closed-cell honeycomb structure, and it is the environmentally friendly materials with high performance such as high pressure resistance, moisture resistance, air permeability, water absorption, corrosion resistance, low thermal conductivity, light weight, and long service life.

6、 What is the slenderness ratio?

Answer: The slenderness ratio of the structure λ = μ l/i, i is the slenderness ratio of the radius of gyration. The slenderness ratio is the ratio of the calculated length of the member to its corresponding radius of gyration. It can be seen from the formula that the concept of slenderness ratio comprehensively considers the end constraints of the members, the length of the member itself and the cross-sectional characteristics of the member.

7、 Is the buckling of the compressed flange of the I-beam under flexion along the weak axis or the strong axis direction of the I-beam?

Answer: When the load is not large, the beam basically bends in its plane of maximum stiffness, but when the load is large to a certain value, the beam will produce large lateral bending and torsional deformation at the same time, and eventually lose the load ability quickly. At this time, the overall instability of the beam must be lateral bending and twisting.

There are roughly three solutions:

① Increase the lateral support points of the beam or reduce the spacing of the lateral support points

② Adjust the cross-section of the beam to increase the lateral moment of inertia Iy of the beam or simply increase the width of the compressed flange (such as the upper flange of the crane beam)

③ The constraint of the beam end bearing on the cross section. If the bearing can provide rotation constraint, the overall stability of the beam will be greatly improved.

8、Why is there no torsion calculation for steel beams in the steel structure design code?

Answer: Normally, the steel beams are all open sections (except box-shaped sections), and their torsional section modulus is about an order of magnitude smaller than the bending section modulus, which means that their torsional capacity is about 1/ 10 of bending. In this way, it is not economical to use steel beams to bear torque. Therefore, the structure is usually used to ensure that it is not torsion, so the torsion calculation of the steel beam is not included in the steel structure design code.

9、 Is the displacement limit of the column top h/100 or h/240 when the masonry wall is adopted without crane?

Answer: The light steel specification has indeed errata this limit, mainly because the 1/100 column top displacement cannot guarantee that the wall will not be pulled apart. At the same time, if the wall is built inside the rigid frame (such as the internal partition wall), we do not consider the wall's embedding effect on the rigid frame when calculating the displacement of the column top (a bit exaggerated as a frame-shear structure).

10、 What is the maximum rigidity plane?

Answer: The maximum stiffness plane is the plane of rotation around the strong axis. Generally, there are two axis in the cross section. Among them, the moment of inertia around one of them is large called the strong axis, and the other is the weak axis.

11、 Straight seam steel pipe is used instead of seamless pipe, I wonder if it can be used?

Answer: The structural steel pipe should be the same in theory, the difference is not very big, the straight seam welded pipe is not so regular as the seamless pipe, and the centroid of the welded pipe may not be in the center, so pay special attention when used as a compression member. The probability of defects is relatively high, and important parts cannot be replaced by seamless pipes. The thickness of seamless tubes wall is impossible to be thin due to the limitations of processing technology (the average wall thickness of seamless tubes of the same diameter is thicker than welded pipes), In many cases, the use efficiency of seamless pipe materials is not as good as that of welded pipes, especially large-diameter pipes. The biggest difference between seamless pipe and welded pipe is that when it is used in the transmission of pressure gas or liquid (DN).

12、 Is there any difference between shear lag and shear force lag? What are their respective focuses?

Answer: The shear force lag effect is a universal mechanical phenomenon in structural engineering. As small as a member, as large as a super high-rise building, there will be shear force lag. Shear force lag, sometimes called shear lag, From the mechanical essence, it is Saint-Venant's principle. The specific performance is that within a certain local range, the shear force can play a limited role, so the distribution of normal stress is not uniform. This phenomenon of uneven distribution of normal stress is called shear lag. The hollow tube formed by opening a hole in the wall is also called a frame tube. After the hole is opened, there is a lag phenomenon in the shear force transmission due to the deformation of the horizontal beam, and the normal stress distribution in the column is parabolic, which is called the shear force lag phenomenon.

13、 How does the lengthening of anchoring length of anchor bolts affect the columns’ stress?

Answer: The axial tensile stress distribution in the anchor bolt is uneven, and is distributed in an inverted triangle shape. The upper part has the largest axial tensile stress and the lower part has zero axial tensile stress. As the anchoring depth increases, the stress gradually decreases, and finally decreases to zero when it reaches 25 to 30 times the diameter. Therefore, it is not useful to increase the anchoring length. As long as the anchoring length meets the above requirements, and the end is provided with a hook or anchor plate, the foundation concrete will generally not be damaged.

14、 What are the similarities and differences between the stress amplitude criterion and the stress ratio criterion and their respective characteristics?

Answer: For a long time, the fatigue design of steel structure has been carried out according to the stress ratio criterion. For a certain number of load cycles, the fatigue strength of the member σ max and the stress cycle characteristic represented by the stress ratio R are closely related. The introduction of safety coefficient for σ max , you can get the design fatigue stress allowable value [σ max] = f (R) to limit the stress within [σ max], this is the stress ratio criterion. Since the welded structure is used to withstand fatigue loads, the engineering community has gradually realized from practice that the fatigue strength of this type of structure is not related to the stress ratio R, but the stress amplitude Δ σ. The calculation formula of the stress amplitude criterion is Δ σ ≤〔 Δ σ 〔〔 Δ σ 〕is the allowable stress amplitude, which varies with the details of the structure, and also changes with the number of cycles before failure.Fatigue calculation of welded structure should be based on the stress amplitude, because of the residual stress inside the structure. Non-welded members,for stress cycles with R >= 0, the stress amplitude criterion is fully applicable, because the fatigue strength of members with and without residual stress is not much different. For stress cycles with R <0, the stress amplitude criterion is more safe.

15、 What is hot rolling and what is cold rolling, what are the differences?

Answer: The hot rolling is the steel is pressed out with a roller when above 1000 degrees, usually the plate is as small as 2MM thick, and the deformation heat during high-speed processing of the steel cannot withstand the heat dissipation of the increased area of the steel, that is, it is difficult to make sure to process at a temperature above 1000 degrees. Therefore, it is necessary to sacrifice hot rolling, which is an efficient and cheap processing method, to roll steel at room temperature, that is, to cold-roll the material after hot-rolled to meet the market requirements for thinner thickness. Of course, cold rolling brings new benefits, such as work hardening, which improves the strength of the steel, but it is not suitable for welding. At least the work hardening of the weld is eliminated, and the high strength is gone. Back to the strength of its hot rolled material, cold-formed steel can use hot-rolled materials, such as steel pipes; and can also use cold-rolled materials. Whether it is cold-rolled materials or hot-rolled materials, 2MM thick is a criterion. the thinnest hot-rolled material is 2MM thick, and the cold-rolled material is 3MM thickest.

16、 Why should the beam be compressed and bent calculate the out-of-plane and in-plane stability , but when the slope is smaller, only the in-plane stability should be calculated?

Answer: The beam only has the form of out-of-plane instability. There has never been a claim that the beam is unstable in the plane. As for the column, when there is an axial force, the calculated lengths of the out-of-plane and in-plane are different, so there is the in-plane and out-of-plane instability check. For a rigid frame beam, although it is called a beam, some of its internal forces are always axial forces, so its calculation strictly should use the model of the column, that is, it must be stable of the pressed and bending members both in-plane and out-of-plane . But when the slope of the roof is small, the axial force is small and can be ignored, so the beam model can be used, that is, the stability of in- plane can’t be calculated. When the slope of the roof is small, the inclined beam member only needs to calculate the strength in-plane, but it still needs to be stable out-of-plane.

17、 Why is the secondary beam generally designed to articulate with the primary beam?

Answer: If the secondary beam is just connected to the primary beam, when there is secondary beam with the same load on both sides at the same position of the primary beam will be okay, if not, the bending moment of the secondary beam end is twisted out-of- plane compared to the primary beam, and the torsion must be calculated. It involves torsional stiffness, fan moment of inertia, etc. In addition, the construction work will be increased for just connection, and the work load on the welding spot will be greatly increased. It is generally not necessary to compensate for the loss, so generally the secondary beam don’t use just connection.

18、 How to calculate the length of high-strength bolts?

Answer: The length of the high-strength bolt screw = 2 connecting end plate thickness + 1 nut thickness + 2 washer thickness + 3 wire opening length.

19、 What is the physical concept of bearing capacity after buckling?

Answer: The bearing capacity after buckling mainly refers to the ability of the member to continue to bear after partial buckling. It mainly occurs in thin-walled members, such as cold-formed thin-walled type steel. The effective width method is used to calculate the bearing capacity after buckling. The bearing capacity after buckling mainly depends on the width-thickness ratio of the plate and the constraint conditions of the edge of the plate. The larger the width-thickness ratio, the better the constraint and the higher the bearing capacity after buckling. In terms of analysis methods, the current domestic and foreign standards mainly use the effective width method. However, the influencing factors considered by different countries in calculating the effective width are different.

20、 What is a plastic algorithm? What is the considering strength after buckling?

Answer: The plastic algorithm refers to the occurrence of plastic hinges when the expected location reach the yielding strength in the statically indeterminate structure, and then to achieve the purpose of redistribution of plastic internal forces, and must ensure that the structure does not form a variable or transient system. Considering strength after buckling refers to a member calculation method in which the web plate of a flexural member still has a certain bearing capacity after losing local stability, and makes full use of its post-buckling strength.

21、What is the difference between a soft hook crane and a hard hook crane?

Answer: The soft hook crane refers to lifting heavy objects through steel ropes and hooks. Hard hook crane refers to lifting heavy objects through rigid objects such as clamps and material rakes. Hard hook cranes work frequently. The running speed is high, and the rigid cantilever structure attached to the trolley prevents the lifting weight from swinging freely.

22、 What is a rigid tie rod and what is a flexible tie rod?

Answer: Rigid tie rods can be pressed and pulled. Generally, double angle steel and round tubes are used. While flexible tie rods can only be pulled, and single angle steel or round tubes are generally used.

23、 What is the relationship between slenderness ratio and deflection?

Answer: ① Deflection is the deformation amount of the member after loading, that is, its displacement value.

② "The slenderness ratio is used to indicate the rigidity of the axial stress member" The slenderness ratio should be the material property. The properties possessed by any member, the rigidity of the axial force member, can be measured by the slenderness ratio.

③ Deflection and slenderness ratio are completely different concepts. The slenderness ratio is the ratio of the calculated length of the rod to the radius of gyration of the section. Deflection is the displacement value at a certain point after the member is stressed.

24、 How are the four levels of earthquake levels divided?

Answer: Earthquake resistance level: Class I, II, III, IV.

Earthquake fortification intensity: 6, 7, 8, 9 degrees.

Seismic fortification categories: A, B, C, D.

Earthquake level: frequent earthquakes, occasional earthquakes, rare earthquakes, uncommon earthquakes.

25、Can the brace be used as a support? Waht is the difference from other supports?

Answer: ① The brace and support are two structural concepts. The brace is used to ensure the stability of the steel beam section, while the support is used to form the stability of the structural system together with the steel frame, and to ensure that its deformation and bearing capacity meet the requirements.

② The brace can be used as a fulcrum out-of--plane of the compressed flange of the steel beam. It is used to ensure the overall stability of the steel beam.

26、 What must be considered when designing steel structure axial tension members?

Answer: a. Under the static load without fatigue, the residual stress has no effect on the bearing capacity of the tie rod.

b. If there is a sudden change in the cross-section of the tie rod, the distribution of stress at the change point is no longer uniform.

C. The design tie rod should take yield as the limit state of bearing capacity.

d. The limit state of bearing capacity should be considered from two aspects of gross section and net section.

e. Consider the efficiency of the net section

27、How to calculate the spring stiffness of steel column? What is the calculation formula?

Answer: The spring stiffness is to consider the column as a cantilever member, and apply a unit force on the top of the column to calculate the lateral displacement caused. This displacement is the spring stiffness, and the unit is generally KN/mm. If there is a ring beam, at the direction of beam without ring beam constraint, the spring stiffness calculation is the same as the cantilever member. In the other direction, because there is a ring beam at the top of the column, the EI in the calculation formula is the sum of all the columns in that direction.

28、What is the skin effect?

Answer: Under the effect of vertical load, the movement trend of the sloping door frame is that the roof ridge is downward and the eaves are deformed outward. The roof panel will work with the supporting purlins to resist this deformation trend in the form of deep beams. At this time, the roof panel is subjected to bear the shear forces and acts as the web plate of the deep beam. While the edge purlins bear the axial force and act as the flange of the deep beam. Obviously, the shear resistance of the roof panel is far greater than its bending resistance. Therefore, the skin effect refers to the resistance effect of the skin panel to the deformation load caused by its shear stiffness against the in-plane of panel. The main performance indicators of the skin effect are strength and stiffness.

29、 For the transverse stiffening ribs of the crane beam, it is advisable to start and drop the arc at the lower end of the ribs.What does it mean?

Answer: It means that the end of the stiffener should be continuously welded, such as fillet welding and surrounding welding. Prevent fatigue cracks on the web plate.

30、 How is the welding of the last weld beams of the partition in the box column performed?

Answer: The quality is easily guaranteed by electroslag welding!

31、 Cantilever beam and cantilever column have different calculated length coefficients, how to explain?

Answer: The cantilever beam has a calculated length factor of 1.0 and the cantilever column has a calculated length factor of 2.0. The column is a pressed and bending member, or simply is under compression, and the stability factor should be considered, so 2 is taken. While the beam is bending, it should be this difference.

32、 The deflection does not meet the specifications when design. Can arching be used to ensure this?

Answer: a. The structure controls the deflection and is designed according to the normal use limit state. For steel structures, excessive deflection can easily affect roof drainage and cause fear. For concrete structures, excessive deflection can cause local damage to durability (including concrete cracks). The above damage caused by excessive deflection of the building structure can be solved by arching.

b. Some structures are easy to arch, such as double-slope portal rigid frame beams. If the absolute deflection exceeds the limit, it can be adjusted by increasing the roof slope during production. Some structures are not easy to arch.such as large-span beams, if the relative deflection exceeds the limit, each beam must be arched. Since the arched beams is polyline after spliced, the deflection deformation is a curve, and the two lines are difficult to coincide.which will cause uneven roofing. For frame flat beams, it is more difficult to arch, and the flat beams cannot always be curved.

c. If you plan to use arching to reduce the steel consumption of the structure controlled by deflection, the deflection control regulations should be reduced. At this time, the deflection under live load must be controlled. The deflection caused by constant load is guaranteed by arching.

33、 What is the center seat pad method of steel structure columns?

Answer: The center seat pad method of steel structure column installation saves labor and time, and the construction accuracy can be controlled within 2mm, and the overall benefit can be increased by more than 20%.

The construction steps are as follows:

(1)Carry out the construction of the steel column foundation according to the construction drawing (same as the usual construction method), and the installation height above the foundation is 30-50mm lower than the bottom surface of the steel column to prepare for placing the center seat pad

(2)Calculate the minimum bearing area Amin according to the steel column weight Q, bolt preload F, and foundation concrete bearing strength P

(3)A square or circular center seat pad is made of steel plates with a thickness of 10 and 12 mm, and its area should not be less than twice the minimum bearing area Amin.

(4)Place the slurry on the completed foundation and place the center slurry pad. During construction, tools such as spirit level and spirit level should be used for accurate measurement to ensure the level of the center pad, the center of the pad is consistent with the installation axis, and the elevation above the pad is consistent with the installation elevation on the bottom of the steel column.

(5)When the concrete strength of the slurry layer reaches more than 75% of the design strength, the steel column is hoisted. The lifting of the steel column can be carried out directly, and the leveling and straightening can be carried out only by adjusting the anchor bolts.

(6)For secondary grouting, use non-shrinkage concrete or slightly expanded concrete to perform secondary grouting.

34、What is the "Baoxinge effect" and does it have a great influence on the design of steel structures?

Answer: The Baoxinge effect is the irrecoverable deformation left after the uninstalling when material reaches plastic deformation , and this deformation is plastic deformation. It is of course conceivable whether this deformation has an effect on the structure!

35、Brittle fracture of steel material or steel structure refers to the crack failure that occurs when the stress is lower than the tensile strength or yield strength of the steel.

Answer: Steel structures, especially welded structures, often have defects similar to cracks due to quality and structural reasons such as steel material, manufacturing, and welding. Most of brittle fractures occur due to the development of these defects, which leads to the instability of the cracks. When the cracks slowly expand to a certain degree, the fractures will expand at a very high speed, and they will suddenly occur without any warning before brittle failure.

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