Steel column

Steel column

Steel column is industrial product, is made of steel column. Used for large and medium-sized industrial plants, large span public buildings, high-rise buildings, etc.

Large and medium-sized industrial plants, long-span public buildings, high-rise buildings, light mobile houses, working platforms, trestle and support columns, most of the use of steel columns.

Classification

Steel column can be divided into solid belly column and lattice column according to section form. Solid belly column has a whole section, the most commonly used is the work section; The section of lattice column is divided into two or more limbs, which are connected by stripes or plates. When the load is larger and the column body is wider, the amount of steel is less. Steel columns can be divided into axial compression columns and eccentric compression columns according to the stress conditions. The longitudinal pressure on the axial compression column overlapped with the section of the column. Eccentric compression column under axial pressure and bending moment at the same time, also known as bending member.

Design calculation

The section of steel column should meet the requirements of strength, stability and slenderness ratio limit, and the components of the section should also meet the requirements of local stability.

Strength

The maximum compressive or tensile normal stress of the column shall not exceed the design strength of the steel. For the axial compression column, the axial pressure causes uniform compression normal stress in the section. For eccentric compression column, due to the role of bending moment, cause uneven normal stress in the section, usually in the eccentric side of the outermost fiber stress is the maximum compressive stress, the other side of the outermost fiber stress is the minimum compressive stress, the maximum tensile stress may occur when the bending moment is larger. When the axial compression of solid abdominal column is increased to a certain size, the column will suddenly bend from the linear equilibrium state to the side with small stiffness, and sometimes sudden torsion or bending and torsion may occur at the same time. If the pressure increases slightly, the bending, torsional or flexoral-torsional deformation increases rapidly, so that the column loses the bearing capacity of the phenomenon known as the whole column loss of stability, and according to its instability deformation is called bending instability, torsion instability or flexoral-torsional instability. The minimum axial pressure that destabilizes a column is called the critical force. Critical force The stress resulting from the removal of the section area of the coat is called the critical stress. The critical stress is often below the yield point of the steel, that is, the column will lose stability before its strength reaches its ultimate state. The ratio of the critical stress to the yield point is called the stability factor of the axial compression column. Buckling buckling is the most common of the three situations in which axial compression columns lose stability. The main factor affecting the critical stress of buckling is the ratio of the column slenderness, that is, the ratio of the calculated length of the column to the rotation radius of the section. For a given steel, the longer or thinner the column is, that is, the larger the slenderness ratio is, the smaller the critical stress is and the easier it is to bend and become unstable. When the slenderness ratio of the two spines in the x and y directions is not equal, the bending instability of the column always occurs along the direction where the stiffness is weak, that is, the slenderness is larger. When the steel column has an open section and the wall thickness of the section is small, the torsional instability or flexural instability may occur under axial pressure due to the poor torsional rigidity of the section. When the section is biaxial symmetry (such as cross section) or point symmetry (such as Z-shaped section), the centroid where the axial pressure is located is reconnected with the shear center axis. When the length of the column is small, torsional instability may occur. When the section is uniaxial symmetry (such as groove or T-shaped section), the centroid where the axial pressure is located does not coincide with the shear central axis, the column may be bent and torsional instability; When the cross section has no axis of symmetry, the column instability under axial pressure is generally flexural and torsional instability. The critical stress of torsional and flexural instability is related to the section form and size, torsional and flexural stiffness, column length and support. The smaller the wall thickness of open thin wall section is, the smaller the torsional stiffness is, and the more easily torsion occurs. The steel column used in engineering often has defects, such as residual stress of section generated by uneven heating and cooling in the process of hot rolling and structural welding, manufacturing deviations such as initial bending of components, and installation deviations such as initial eccentricity of component connections. These defects will reduce the critical stress and the coefficient of stability, which is not the same for different section forms of steel column. The stability calculation formula of axial compression column is σ = N/A≤φf, where σ is compressive stress of gross section. N is the axial pressure; A is the gross cross-section area; φ is the stability coefficient; f is the design strength.

Stability of eccentric compression column with full belly

Eccentric compression column is subjected to both axial pressure and bending moment. Because of the bending moment, the column begins to bend in the plane of the bending moment. If the axial pressure and bending moment increase gradually at the same time, the bending deformation also increases correspondingly. But when the load increases to a certain amount of hours, even if the load remains the same or gradually decreases, and the deformation will continue to increase rapidly, then the column has lost the bearing capacity, this phenomenon is called eccentric compression column in the plane of bending moment loss of stability, a bending instability. If the lateral stiffness of the column is small and the lateral support is poor, when the load increases to a certain amount of hours, the column in addition to bending in the plane of bending moment, in the side may also suddenly bend outward from the original plane, at the same time torsion, then the bending and torsional deformation is rapidly increased, so that the column loses bearing capacity phenomenon known as eccentric compression column in the plane of bending moment loss of stability, is bending and torsional instability. The stability of the eccentric compression column in and out of the plane of bending moment is not only related to the slenderness ratio of the column, but also depends on the eccentricity. Eccentricity is usually measured by eccentricity, which is the ratio of the eccentricity to the core distance of the section. For a given steel and column section form, the larger the slenderness ratio and eccentricity of the column, the lower the critical average compressive stress when the column is unstable, that is, the more easily the column is unstable.

Allowable slenderness ratio

The slenderness ratio of a column is a measure of the stiffness of the column. Too large slenderness ratio is not only unfavorable to the stability of the column, but also makes the column easy to bend in the process of transportation and installation, and easy to vibrate under the dynamic load in the period of use. Therefore, the design to specify the column allowable slenderness ratio, usually used 150. When the thickness of the web, flange plate or other components of a solid abdominal column is relatively small, local buckling may occur under a small load before the column loses its overall stability, that is, the phenomenon that the web or flange plate under compression deviates from its original plane position and produces wavy buckling is called the column loses local stability. After the partial loss of stability of the column, the stress condition of the column deteriorates, which may lead to the early loss of the overall stability of the column. In order to make each component of steel column have enough local stability, the width/thickness ratio of steel column is usually limited to a certain value according to the stress and support of the plate. For lattice columns, strength, stability, slenderness limit and stability of individual limbs and components should also be calculated. The calculation method is similar to that of the solid abdominal column. However, the shear stiffness of lattice column infix or slab system is much worse than that of the web of solid column. When the column becomes unstable by bending around the virtual axis (the axis in the section intersecting the plane of the slab or slab), the column will not only undergo bending deformation, but also undergo a considerable degree of shear deformation. The function of the strips and plates is to connect the column limbs of the lattice column into a whole, to ensure that the common force of each column limb, and bear the shear force perpendicular to the imaginary axis of the column. The strips and column limbs constitute a plane truss system. Under the shear action of columns, the strips and column limbs will only bear axial forces, and generally have high stiffness. The slab and column limb form a multi-layer plane rigid frame system. The slab and column limb will bear bending moment and shear force under the action of shear force of column. In order to meet the force requirements and ensure the constant geometry of column section and increase the torsional stiffness of the column, transverse spacers should be set at the steel column subjected to a large horizontal force and at the end and middle of each transport unit. The transverse compartment can be made of steel plate or Angle steel. The foot of the column is the part of the column that transfers the load of the column body to the foundation. In addition to fixing the column to the foundation, the column foot also plays the role of transferring and distributing the load. The column foot is generally composed of a bottom plate and a boot beam; When the column body is small, also can not use the boot beam; When the column body is large and the bottom plate is wide, in order to strengthen the stiffness of the bottom plate, reduce its bending moment and thickness, it is also necessary to arrange the partition or rib plate appropriately. An anchor bolt is a connector that holds the column to the foundation. On the axial compression column and eccentric compression column bending moment is very small, anchor bolt installation fixed position role, generally according to the structure configuration, usually with two, diameter of 20 ~ 30 mm. For eccentric compression column with large bending moment, anchor bolt should resist the bending moment from column body; At this time, the anchor bolt under tension, its diameter and number should be calculated according to the maximum bending moment and minimum axial pressure at the bottom of the column.