Essential Knowledge Points for Steel Structures

1. When planning steel structures, what will happen if the deflection exceeds the limit value?

Deformation that affects normal use or appearance; Partial damage (including cracks) that affects normal use or durability functions; Vibration that affects normal operation; Other specific conditions that affect normal use.

2. Is it possible to use straight seam steel pipes instead of seamless pipes?

In theory, the structural steel pipes should be the same, but the differences are not significant. Straight seam welded pipes are not as regular as seamless pipes, and the centroid of welded pipes may not be in the center. Therefore, when used as compression components, it is particularly important to pay attention to the high probability of defects in welded pipe welds, and important parts cannot be replaced by seamless pipes. Seamless pipes cannot be made very thin due to the constraints of processing technology (seamless pipes with the same diameter have a uniform wall thickness that is thicker than welded pipes), In many cases, the use of seamless pipe materials is not as powerful as welded pipes, especially for large diameter pipes.

The biggest difference between seamless and welded pipes is when used for pressure gas or liquid transmission (DN).

3. What is slenderness ratio?

Slenderness ratio of structure λ＝μ L/i, where i is the radius of rotation. The concept can be roughly seen from the calculation formula: slenderness ratio refers to the ratio of the calculated length of a component to its corresponding turning radius. From this formula, it can be seen that the concept of slenderness ratio takes into account the end restraint of the component, the length of the component itself, and the cross-sectional characteristics of the component. The concept of slenderness ratio has a significant impact on the stability calculation of compression members, as components with higher slenderness ratios are more prone to instability. Can you take a look at the calculation formulas for axial compression and bending components, which all have parameters related to slenderness ratio. The standard for tensile components also provides requirements for slenderness ratio restraint, which is to ensure the stiffness of the components under transportation and installation conditions. The higher the safety requirements for components, the smaller the safety limit given by the standard.

What is the relationship between slenderness ratio and deflection?

1. Deflection is the deformation of a component after loading, which is its displacement value.

2. Slenderness ratio is used to represent the stiffness of axially loaded components. "Slenderness ratio should be a material property. Any component has a property, and the stiffness of axially loaded components can be measured by slenderness ratio.

3. Deflection and slenderness ratio are completely different concepts. Slenderness ratio is the ratio of the calculated length of a member to the radius of rotation of the cross-section. Deflection is the displacement value of a component at a certain point after being subjected to force.

5. Deflection does not meet the standard during planning, can it be ensured by arching?

1. The control of deflection by the structure is planned according to the normal operating limit state. For steel structures, excessive deflection can easily affect roof drainage and create a sense of fear, while for concrete structures, excessive deflection can cause partial damage to durability (including concrete cracks). I believe that the above damages caused by excessive deflection of building structures can be solved through arching.

2. Some structures have simple arches, such as double slope portal frame beams. If the absolute deflection exceeds the limit, it can be adjusted by increasing the roof slope during production. Some structures are not very simple in arches, such as for large-span beams. If the relative deflection exceeds the limit, each section of the beam needs to be arched because the arched beams are spliced into a broken line, while the deflection deformation is a curve. It is difficult for the two lines to overlap, resulting in uneven roofs. Regarding frame flat beams, it is even more difficult to arch them, and they cannot be made into curved beams.

3. Assuming that you are planning to use arching to reduce the amount of steel used in a structure controlled by deflection, the deflection control requirement must be reduced. At this point, the deflection under live load must be controlled, and the deflection generated by dead load must be ensured by arching.

6. Is the buckling of the compression flange of a bent I-beam along the weak axis direction or the strong axis direction?

When the load is not large, the beam basically twists and turns in its maximum stiffness plane. However, when the load reaches a certain value, the beam will simultaneously experience significant lateral twists and torsional deformation, and ultimately quickly lose its ability to continue bearing. At this point, the overall instability of the beam is inevitably due to lateral bending and twisting.

There are roughly three solutions:

1. Add lateral support points for beams or reduce the spacing between lateral support points;

2. Adjust the cross-section of the beam, add the lateral moment of inertia Iy of the beam, or simply add the width of the compression flange (such as the upper flange of the crane beam);

3. The restraint of the beam end support on the cross-section, if the support can provide rotational restraint, the overall stability function of the beam will be greatly improved.

What is the physical concept of post buckling bearing capacity?

The load-bearing capacity after buckling mainly refers to the ability of a component to continue to bear after partial buckling, mainly generated in thin-walled components, such as cold-formed thin-walled steel. The effective width method is used to consider the load-bearing capacity after buckling in accounting. The size of the load-bearing capacity after bending mainly depends on the width to thickness ratio of the plate and the binding conditions at the edge of the plate. The larger the width to thickness ratio, the better the binding, and the higher the load-bearing capacity after bending. In terms of analysis methods, the current domestic and international standards mainly use the effective width method. However, the influencing factors considered by national standards in calculating effective width vary.

Why is there no torsion calculation for steel beams in the steel structure planning standards?

Usually, steel beams are of open cross-section (excluding box sections), and their torsional section modulus is about one order of magnitude smaller than the flexural section modulus, which means that their torsional capacity is about 1/10 of that of bending. Therefore, it is not economical to use steel beams to receive torque. Therefore, construction is usually used to ensure that it is not subjected to torsion, so there is no torsion calculation for steel beams in the steel structure planning standards.

9. Is the displacement limit of the column top when using masonry walls without a crane h/100 or h/240?

The light steel regulations have indeed corrected this limit value, mainly because a displacement of 1/100 of the column top cannot ensure that the wall is not pulled apart. At the same time, if the wall is built inside the rigid frame (such as an internal partition wall), we did not consider the embedding effect of the wall on the rigid frame when calculating the displacement of the column top (which is exaggerated and compared to a frame shear structure).

10. What is the maximum stiffness plane?

The maximum stiffness plane is a plane that rotates around a strong axis. Generally, a cross-section has two axes, one of which has a large moment of inertia and is called the strong axis, while the other is called the weak axis.

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

The shear lag effect is a common mechanical phenomenon in structural engineering, ranging from a component to a super high-rise building. Shear lag, sometimes also known as shear lag, is essentially the Saint Venant principle in mechanics. It is manifested in detail that within a certain range, the effect of shear is limited, so the distribution of normal stress is uneven. This phenomenon of uneven distribution of normal stress is called shear lag.

The hollow tube formed by opening on the wall, also known as a frame tube, undergoes shear lag due to the deformation of the crossbeam after opening, resulting in a parabolic distribution of normal stress in the column, known as shear lag.

12. What impact will the lengthening of anchor bolt anchoring length have on the stress of the column?

The axial tensile stress distribution in the anchor bolt is uneven, forming an inverted triangular distribution. The upper axial tensile stress is the highest, and the lower axial tensile stress is 0. As the anchoring depth increases, the stress gradually decreases, and finally decreases to 0 when it reaches 25-30 times the diameter. Therefore, adding anchor length again is of no use. As long as the anchoring length meets the above requirements and there are hooks or anchor plates at the ends, the bottom concrete will generally not be damaged by pulling.

How is the length of high-strength bolts calculated?

The length of high-strength bolt screw=2 connecting end plate thicknesses+1 nut thickness+2 washer thicknesses+3 thread mouth lengths.

14. What are the similarities and differences between the stress amplitude principle and the stress ratio principle, and their respective characteristics?

For a long time, the fatigue planning of steel structures has been carried out according to the principle of stress ratio. Regarding a certain number of load cycles and the fatigue strength of components σ Max is closely related to the stress cycle characteristics represented by stress ratio R. right σ By introducing a safety factor of max, the allowable fatigue stress value for planning can be obtained σ Max]=f (R). Constrain stress to [ σ Within max, this is the principle of stress ratio.

Since welded structures have been used to withstand fatigue loads, the engineering community has gradually realized from practice that the fatigue strength of such structures is closely related not to the stress ratio R, but to the stress amplitude Δσ。 The calculation formula for the stress amplitude principle is Δσ≤〔Δσ〕。

［ Δσ〕 It 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 structures should follow the principle of stress amplitude, as the residual stress inside the structure is not a welded component. The stress amplitude principle is fully applicable for stress cycles with R>=0, as the fatigue strength of components with and without residual stress is not significantly different. Regarding the stress cycle with R<0, adopting the stress amplitude principle tends to be more safe.

15. Why should beams be subjected to in-plane stability calculation for compression bending components? When the slope is small, only in-plane stability can be calculated?

The beam only has an out of plane instability form. There has never been a theory of instability in the plane of a beam. For columns, when there is axial force, the calculated lengths outside the plane and inside the plane are different, which is the only way to check the instability inside and outside the plane. For rigid frame beams, although they are called beams, some of their internal forces are always axial forces. Therefore, strictly speaking, their calculation should be based on a column model, which means that both the plane inside and outside of the compression bending component must be considered stable. But when the roof slope is small, the axial force is small and can be ignored, so a beam model can be used, which does not need to calculate the stability in the plane. The meaning in the door regulations (P33, Article 6.1.6-1) refers to when the roof slope is small, the diagonal beam components only need to be calculated for strength in the plane, but still need to be calculated for stability outside the plane.

Why is the secondary beam generally planned to be hinged with the main beam?

If the secondary beam is rigidly connected to the main beam, and there are secondary beams with the same load on both sides of the main beam in the same direction, it is okay. If there is no secondary beam, the bending moment at the end of the secondary beam is out of plane torsion for the main beam, and the calculation of torsion resistance also involves torsional stiffness, sectorial moment of inertia, etc. In addition, the construction workload needs to be added for the rigid connection, and the on-site welding workload is greatly increased, which is not worth the loss. Generally, it is not necessary to not make the secondary beam into a rigid connection.

17. What is plastic algorithm? What is the consideration of post buckling strength?

The plastic algorithm refers to the occurrence of plastic hinges in a statically indeterminate structure that yield to the expected strength at a predetermined location, thereby achieving the redistribution of plastic internal forces, and must ensure that the structure does not form a variable or transient system. Considering the post buckling strength refers to a component accounting method in which the web of a flexural component loses some stability and still has a certain bearing capacity, and fully utilizes its post buckling strength.

18. What is a rigid tie rod and a flexible tie rod?

Rigid tie rods can be both compressed and tensioned, usually using double angle steel and circular tubes, while flexible tie rods can only be tensioned, usually using single angle steel or circular tubes.

Can corner braces serve as support? What are the differences with other supports?

1. Corner braces and braces are two structural concepts. Corner braces are used to ensure the stability of the steel beam cross-section, while braces are used to form a structural system with the steel frame for stability and ensure that its deformation and bearing capacity meet the requirements.

2. Corner braces can serve as support points outside the plane of the compression flange of steel beams. It is used to ensure the overall stability of steel beams.

What should be considered when planning axial tension components of steel structures?

1. Under the static load effect of not generating fatigue, residual stress has no effect on the bearing capacity of the tie rod.

2. 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.

3. The planning of tie rods should be based on yielding as the ultimate bearing capacity.

4. The ultimate bearing capacity should be considered from both gross and net sections.

5. Consider the power of the net cross-section.

How to calculate the stiffness of the tension spring of the steel column? What is the accounting formula? How to calculate the stiffness of the tension spring of the concrete column and the stiffness of the tension spring when there is a ring beam on the concrete column? What is the accounting formula?

The stiffness of the tension spring refers to the calculation of the lateral displacement caused by applying one unit force to the top of the column as a cantilever component. This displacement is called the stiffness of the tension spring, usually measured in KN/mm. If there is a ring beam, in the direction without ring beam constraints, the stiffness calculation of the tension spring is the same as that of the cantilever component. 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 columns in that direction.

22. What is skin effect?

Under the effect of vertical load, the movement trend of the roof portal frame is that the ridge is downward and the eaves are outward deformed. The roof panel will resist this deformation trend in the form of deep beams along with supporting purlins. At this point, the roof panel receives shear force and acts as the web of the deep beam. The edge purlins receive axial force to lift the deep beam flange. Obviously, the shear resistance of the roof panel is much greater than its bending resistance. So, the skin effect refers to the resistance effect of the skin plate due to its shear stiffness on the load that causes deformation in the plane of the plate. Regarding the roof portal frame, the skin effect of resisting vertical loads depends on the slope of the roof, and the skin effect becomes more significant as the slope increases; The skin effect that resists the horizontal load effect increases with the decrease of slope.

The skin elements constitute the entire structural skin effect. The skin unit consists of skin panels, edge components, connectors, and intermediate components between two rigid frames. Edge components refer to two adjacent rigid frame beams and edge purlins (ridge and eave purlins), while intermediate components refer to the purlins in the middle. The main functional indicators of skin effect are strength and stiffness.

23. The theory of small deflection and large deflection is used for the bending and buckling of axial compression components. I would like to know the difference between the theory of small deflection and small deformation?

The theory of small deformation states that changes in geometric dimensions after structural deformation can be disregarded, and internal forces are still calculated based on the dimensions before deformation! The deformation here includes all deformations: tension, compression, bending, shear, torsion, and their combinations. The small deflection theory assumes that displacement is very small and belongs to geometric linear problems. It can be approximated using a deflection curve equation, and then energy is established to derive the stability coefficient. The deformation curvature can be approximated by y "=1/ ρ Replace! Replacing curvature with 'y' is used to analyze the small deflection theory of elastic rods. In a rigid rod with a tension spring, that's not the case. Furthermore, using the theory of large deflection does not necessarily mean that after buckling, the load can still be added. For example, if a cylindrical shell is compressed, it can only maintain stability under lower loads after buckling. Simply put, the small deflection theory can only obtain the critical load and cannot determine the stability at critical load or after buckling. The theory of large deflection can solve for the post buckling function.

24. What is second-order bending moment and second-order elastic-plastic analysis?

For many structures, undeformed structures are often used as accounting graphics for analysis, and the results obtained are accurate enough. At this point, there is a linear relationship between the obtained deformation and load, and this analysis method is called geometric linear analysis, also known as first order analysis. For some structures, internal force analysis must be based on the deformed structure, otherwise the resulting error will be significant. At this point, the relationship between the obtained deformation and load presents a nonlinear analysis. This analysis method is called geometric nonlinear analysis, also known as second order analysis. Using the deformed structure as the accounting basis and considering the elastic-plastic (material nonlinearity) of the material for structural analysis is a second-order elastic-plastic analysis.