38 Questions on Necessary Knowledge of Steel Structures Part 1

01 When looking at the bending moment diagram of a portal frame, you can see the bending moment, but what is the relationship between the bending moment and the cross-section of the member? Answer: The bending bearing capacity of the bent member is Mx/(γxWx)+My/(γyWy)≤f where W is the section modulus. The approximate section can be calculated manually based on the section modulus.

02 How is the butt joint of H-section steel defined? Answer: The main consideration is the transfer of bending moment and/or shear force. In addition, in places with a lot of dynamic loads, the design of welded joints should be particularly careful about butt joints.

03 “Planing and tightening”, do you need to weld after planing and tightening? Answer: Grinding and tightening is a way of force transmission, often used in places subject to dynamic loads. It is a way of force transmission adopted to avoid fatigue cracks in welds. There are requirements for grinding and tightening without welding, and there are also requirements for welding. It depends on the specific drawing requirements. The contact surface requires a smoothness of not less than 12.5, and the contact area is checked with a feeler gauge. The purpose of planing and tightening is to increase the contact area of the contact surface, which is generally used in nodes with a certain horizontal displacement and simply supported, and such nodes should have other connection methods (such as flange tightening, the web may be bolted). Generally, the parts of this kind of node that require planing and tightening do not need to be welded. If welding is required, planing and tightening is not conducive to the deep penetration of the melt during welding, the weld quality will be very poor, and the welded parts will not require tightening even if the groove is not opened. Tightening and welding are contradictory, so it is not accurate to say that the tightened parts are welded again. However, there is also a situation where tightening and welding may occur, that is, the node that is tightened does not have enough constraints on other degrees of freedom, and there are no other parts to provide constraints. It is possible to weld at the tightened part to constrain other directions of freedom. This kind of weld is an installation weld, and it is impossible to fully weld, let alone use it as a main force-bearing weld.

04 When designing steel structures, what are the consequences of exceeding the deflection limit? Answer: Affect normal use or appearance deformation; affect normal use or durability of local damage (including cracks); affect normal use of vibration; affect other specific states of normal use.

05 What is the role of extruded board? Answer: Extruded polystyrene (XPS) insulation board, with polystyrene resin as the main raw material, is a hard board formed by continuous extrusion foaming with a special process. It has a unique and perfect closed-cell honeycomb structure, has high pressure resistance, moisture resistance, non-permeability, non-absorption, corrosion resistance, low thermal conductivity, light weight, long service life and other high-quality performance of environmentally friendly materials. Extruded polystyrene insulation board is widely used in wall insulation, low-temperature storage facilities, parking platforms, building concrete roof pole structure roof and other fields of decoration industry cost-effective moisture-proof materials. Extruded board has excellent durable characteristics: the performance of the extruded board is stable and not easy to age. It can be used for 30–50 years, and has extremely excellent moisture resistance. In an environment with high water vapor pressure, it can still maintain low thermal conductivity. Extruded board has unparalleled thermal insulation performance: because the extruded board has a closed-cell performance structure, and its closed-cell rate reaches 99%, so its thermal insulation performance is good. Although the foamed polyurethane is a closed-cell structure, its closed-cell rate is smaller than that of the extruded board, only about 80%. The extruded board is superior to other insulation materials in terms of thermal insulation performance, water absorption performance and compressive strength. Therefore, it is also unmatched by other insulation materials in terms of thermal insulation performance. Extruded board has an unexpected compressive strength: the compressive strength of the extruded board can reach 150–500 kPa or more according to its different model thickness, while the compressive strength of other materials is only 150–300 kPa or more, it can be clearly seen that the compressive strength of other materials is far lower than the compressive strength of the extruded board. Extruded board has a foolproof water absorption performance: it is used under the road surface and road base to effectively prevent water seepage. Especially in the north, it can reduce the occurrence of frost and soil freezing affected by frost, control the situation of ground frost heave, and effectively block the ground gas from being damaged by moisture.

06 What is the slenderness ratio? Turning radius = √(moment of inertia/area) Slenderness ratio = calculation length / turning radius Answer: The slenderness ratio of the structure is λ＝μl/i, where i is the turning radius slenderness ratio. The concept can be simply seen from the calculation formula: the slenderness ratio is the ratio of the calculation length of the member to its corresponding turning radius. From this formula, it can be seen that the concept of slenderness ratio comprehensively considers the end constraint of the member, the length of the member itself and the cross-sectional characteristics of the member. The concept of slenderness ratio has a very obvious impact on the stability calculation of compressed members, because the larger the slenderness ratio, the easier the member is to lose stability. You can look at the calculation formulas for axial compression and bending members, which all have parameters related to the slenderness ratio. For tension members, the specification also gives the limit requirements for the slenderness ratio, which is to ensure the rigidity of the member during transportation and installation. The higher the stability requirements of the member, the smaller the stability limit given by the specification.

07 Does the buckling of the compressed flange of the bent I-beam occur along the weak axis direction of the I-beam or the strong axis direction? Answer: When the load is not large, the beam basically bends within its maximum stiffness plane, but when the load is large to a certain value, the beam will simultaneously produce a large lateral bending and torsional deformation, and quickly lose the ability to continue bearing. At this time, the overall instability of the beam must be lateral torsional buckling. There are roughly three solutions:

1. Increase the lateral support points of the beam or reduce the distance between the lateral support points.
2. Adjust the cross-section of the beam, 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 support on the cross-section, if the support can provide rotational constraint, the overall stability performance of the beam will be greatly improved.

08 Why is there no torsion calculation of steel beams in the steel structure design specification? Answer: Under normal circumstances, steel beams are all open sections (except for box sections), and their torsional section modulus is about an order of magnitude smaller than the bending section modulus, that is, its torsional capacity is about 1/10 of the bending, so if it is used Steel beams to bear torque are not economical. Therefore, the structure is usually used to ensure that it is not twisted, so there is no torsion calculation of steel beams in the steel structure design specification.

09 When using masonry walls without cranes, is the displacement limit of the column top h/100 or h/240? Answer: The light steel specification has indeed corrected this limit value, mainly because the column top displacement of 1/100 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 inner partition wall), we do not consider the embedding effect of the wall on the rigid frame when calculating the column top displacement (exaggeratedly metaphor as a frame shear structure).

10 What is called the maximum stiffness plane? Answer: The maximum stiffness plane is the plane of rotation around the strong axis. There are generally two axes in a section, one of which has a large moment of inertia, which is called the strong axis, and the other is the weak axis.

11 Can straight seam steel pipes be used instead of seamless pipes, I don’t know if they can be used? Answer: In theory, it should be the same in structural steel pipes, the difference is not very big, straight seam welded pipe is not as regular as seamless pipe, the centroid of the welded pipe may not be in the center, so it should be especially noted when used as a compressed member, The probability of defects in the weld seam of the welded pipe is relatively high, important parts cannot replace seamless pipes, and the wall thickness of seamless pipes cannot be made very thin due to the limitations of processing technology (the average wall thickness of seamless pipes of the same diameter is thicker than that of 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 pipes and welded pipes is that they are used in pressure gas or liquid transmission (DN).

12 What is the difference between shear lag and shear lag? What are their respective focuses? Answer: The shear lag effect is a common mechanical phenomenon in structural engineering, from a small component to a super high-rise building, there will be shear lag phenomenon. Shear lag, sometimes also called shear lag, from the mechanical essence, is Saint-Venant’s principle, the specific performance is that in a certain local range, the role of shear force is limited, so the normal stress distribution is uneven, which is called shear lag. The hollow tube formed by opening holes in the wall is also called a frame tube. After opening the hole, due to the deformation of the beam, there is a lag in the transmission of shear force, which makes the normal stress distribution in the column parabolic, which is called the shear lag phenomenon.

13 What impact will the increase in the anchor length of the anchor bolt have on the force of the column? Answer: The axial tensile stress distribution in the anchor bolt is uneven, and it is distributed in an inverted triangle shape. The upper part has the maximum 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 25~30 times the diameter. Therefore, it is useless to increase the anchor length. As long as the anchor length meets the above requirements, and there are hooks or anchor plates at the end, the foundation concrete is generally not pulled apart.

14 What is the difference and similarity between the stress amplitude criterion and the stress ratio criterion, and their respective characteristics? Answer: The fatigue design of steel structures has been carried out according to the stress ratio criterion for a long time. For a certain number of load cycles, the fatigue strength σmax of the component is closely related to the stress cycle characteristics represented by the stress ratio R. Introduce a safety factor into σmax to get the allowable fatigue stress value used for design [σmax]=f®. Limit the stress within [σmax], this is the stress ratio criterion. Since the use of welded structures to bear fatigue loads, the engineering community has gradually recognized from practice that what is closely related to the fatigue strength of such structures is not the stress ratio R, but the stress amplitude Δσ. The calculation formula of the stress amplitude criterion is Δσ≤[Δσ]. [Δσ] is the allowable stress amplitude, which varies with the construction details and varies with the number of cycles before failure. Welded structure fatigue calculation should be based on the stress amplitude criterion, the reason is the internal residual stress of the structure. For non-welded members, for R >=0 stress cycles, the stress amplitude criterion is fully applicable, because the fatigue strength of members with residual stress and without residual stress is not much different. For R<0 stress cycles, the use of the stress amplitude criterion is much safer.

15 What is hot rolling, what is cold rolling, what is the difference? Answer: Hot rolling is the steel being pressed out by a rolling mill at over 1000 degrees, usually the plate is as small as 2MM thick, the deformation heat of the steel during high-speed processing cannot reach the heat dissipation of the steel’s area increase, that is, it is difficult to maintain the temperature above 1000 degrees for processing, so it has to sacrifice the efficient and cheap processing method of hot rolling, and roll the steel at room temperature, that is, re-cold roll the hot-rolled material to meet the market’s demand for thinner thickness. Of course, cold rolling brings new benefits, such as work hardening, which increases 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, returning to the strength of its hot-rolled material. Although the cold-rolled material is still a hot-rolled material, 2MM thickness is a judgment, the thinnest hot-rolled material is 2MM thick, and the thickest cold-rolled material is 3MM.

16 Why do beam compression members calculate plane stability outside and inside, but when the slope is small, they can only calculate plane stability inside? Answer: Beams only have the form of out-of-plane instability. There has never been a saying that beams are unstable in-plane. For columns, when there is axial force, the calculation lengths of out-of-plane and in-plane are different, so there are out-of-plane and in-plane stability checks. For rigid frame beams, although they are called beams, there is always a part of the internal force that is axial force, so its check should strictly use the column model, that is, according to the compression and bending members, both the in-plane and out-of-plane stability must be calculated. But when the roof slope is small, the axial force is small and can be ignored, so the beam model can be used, that is, there is no need to calculate the in-plane stability. The meaning in the door specification (P33, Article 6.1.6–1) is that when the roof slope is small, the inclined beam members only need to calculate the strength in-plane, but the stability still needs to be calculated out-of-plane.