[Steel Structure · Technology] How to consider the seismic performance of steel structures and what seismic measures are taken in design?

How to consider the seismic performance of steel structures and what seismic measures are taken during design?

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When the seismic fortification intensity of the steel structure is not higher than 8 degrees (0.20g), and the height of the structure is not higher than 100m, the seismic motion parameters and performance-based design principles for the seismic performance design of the components and nodes of the steel structure should comply with the current national standard "Code for Seismic Design of Buildings" GB 50011, The seismic fortification category should be adopted in accordance with the current national standard "Classification Standard for Seismic Protection of Building Engineering" GB50223.

The seismic performance-based design of steel structural components should be based on the seismic fortification category, fortification intensity, site conditions, structural type and irregularity of the building, the role of structural components in the entire structure, the requirements for usage functions and auxiliary facilities functions, investment size, post earthquake losses, and the difficulty of repair. After comprehensive analysis and comparison, their seismic performance goals should be selected.

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The seismic performance-based design of steel structural components can adopt the following basic steps and methods

(1) According to the current national standard "Code for Seismic Design of Buildings" GB 50011, the calculation of frequent seismic effects should be carried out. The structural bearing capacity and lateral displacement should meet its requirements. When calculating the bearing capacity of components located in the plastic energy dissipation zone, the stiffness of the component can be reduced to form an equivalent elastic model.

(2) For buildings with a seismic fortification category of standard fortification category (Class C), the bearing performance level of plastic energy consumption zone can be preliminarily selected according to the specifications.

(3) Carry out seismic verification of bearing capacity under seismic fortification according to the following relevant regulations

1) Establish appropriate structural calculation models for structural analysis;

2) Set the performance coefficient of the plastic energy dissipation zone, select the cross-section of the plastic energy dissipation zone, and make its actual bearing performance level as close as possible to the set performance coefficient;

3) The standard value of the bearing capacity of other components should be checked for the internal force combination effect included in the performance coefficient. When the bearing capacity of structural components meets the internal force combination effect check with ductility level V, the mechanism control check can be ignored;

4) If necessary, the cross-section can be adjusted or the performance coefficient of the plastic energy dissipation zone can be reset.

(4) The ductility level of components and nodes should be determined according to the fortification category and the minimum bearing capacity level of the plastic energy consumption zone according to the specifications, and seismic measures should be taken according to the corresponding requirements of different ductility levels in accordance with the provisions of this standard.

(5) When the minimum load-bearing performance level of the plastic energy dissipation zone is Performance 5, Performance 6, or Performance 7, the elastic-plastic analysis of the structure under rare earthquakes or the formation of a new equivalent elastic analysis model based on the working state of the components should be used to check the elastic-plastic interlayer displacement angle of vertical components, which should meet the limit value of the elastic-plastic interlayer displacement angle in the current national standard "Code for Seismic Design of Buildings" GB 50011; When all structural requirements meet the requirement of ductility level I for structural components, the limit value of elastic-plastic interlayer displacement angle can be increased by 25%.

2. The performance coefficient of steel structural components should comply with the following regulations

(1) Components in different parts of the entire structure, horizontal components in the same part, and vertical components can have different performance coefficients; The bearing capacity of the plastic energy dissipation zone and its connections should meet the requirements of strong nodes and weak members.

(2) For frame structures, the performance coefficient of frame columns on the same floor should be higher than that of frame beams.

(3) For support structures and frame center support structures, the performance coefficient of frame columns on the same floor should be higher than that of frame beams, and the performance coefficient of frame beams should be higher than that of supports.

(4) For the support system of frame eccentric support structure, the performance coefficient of the same floor frame column should be higher than that of the support, the performance coefficient of the support should be higher than that of the frame beam, and the performance coefficient of the frame beam should be higher than that of the energy dissipation beam section.

(5) The performance coefficient of key components should not be lower than that of general components.

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3. The materials of steel structural components designed with seismic performance-based design should comply with the following regulations

(1) The quality grade of steel should comply with the following regulations

1) When the working temperature is above 0 ℃, its quality level should not be lower than B level;

2) When the working temperature is not higher than 0 ℃ but higher than -20 ℃, Q235 and Q345 steel should not be lower than Grade B, and Q390, Q420, and Q460 steel should not be lower than Grade C;

3) When the working temperature is not higher than -20 ℃, Q235 and Q345 steel should not be lower than grade C, and Q390, Q420, and Q460 steel should not be lower than grade D.

(2) The steel used in the plastic energy consumption zone of components should also comply with the following regulations

1) The ratio of the measured yield strength to the measured tensile strength of the steel should not exceed 0.85;

2) The steel should have obvious yield steps, and the elongation should not be less than 20%;

3) The steel should meet the condition that the measured yield strength value is not higher than the specified yield strength value of the previous level of steel;

4) The Charpy impact toughness of steel at working temperature should not be lower than 27J.

(3) The filler metal for critical welds in steel structural components should be tested for the impact toughness of the V-shaped notch, and its Charpy impact toughness at operating temperature should not be lower than 27J.

4. The layout of steel structures should comply with the provisions of the current national standard "Code for Seismic Design of Buildings" GB 50011.

5. The structural analysis model and its parameters should comply with the following regulations

(1) The model should accurately reflect the working state of the components and their connections under different seismic levels.

(2) The elastic analysis of the entire structure can be carried out using linear methods, while elastic-plastic analysis can use equivalent linearization methods with increased damping and static or dynamic nonlinear design methods based on the expected working state of the components.

(3) In rare earthquakes, the second-order effect of gravity should be considered.

(4) The damping ratio for elastic analysis can be adopted in accordance with the current national standard "Code for Seismic Design of Buildings" GB 50011. The damping ratio for elastoplastic analysis can be appropriately increased, and should not exceed 5% when using the equivalent linearization method.

(5) When calculating the effect generated by the representative value of gravity load on the beams and columns that form the support system, the support effect should not be considered.

6. The bearing capacity calculation of the column base should comply with the following regulations

(1) The ultimate bearing capacity of the column base of the support system should not be less than the shear force and combined tension generated by 1.2 times of the yield tension of the connected slant support.

(2) When checking the shear bearing capacity of the column base, the shear performance coefficient should not be less than 1.0.

(3) For frame structures or double lateral force resistant structures that bear more than 50% of the total horizontal seismic shear force, when using exposed column footings, the anchor bolts should comply with the following regulations

1) The bending bearing capacity calculated based on the yield of the anchor bolt gross section should not be less than 50% of the plastic bending bearing capacity of the full section of the steel column when the solid web column is rigidly connected to the column base;

2) The standard value of the tensile bearing capacity of the anchor bolt gross section of the separated column base of the lattice column should not be less than 50% of the standard value of the tensile bearing capacity of the steel column's separated legs;

3) The standard value of the tensile bearing capacity of the anchor bolt gross section of the hinged column base of the solid web column should not be less than 50% of the standard value of the tensile bearing capacity of the weakest section of the steel column.

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7. The connection of steel structure nodes for seismic fortification should comply with the provisions of the "Code for Welding of Steel Structures" GB 50661-2011. For multi story and high-rise steel structures with a structural height greater than 50m or seismic intensity greater than 7 degrees, the width to thickness ratio grade of the section plate should not be S5; The width to thickness ratio grade of the section plate should be S5 grade components, and the plate should meet the S4 grade section requirements after correction.

8. The plastic energy consumption zone of the component should comply with the following regulations

(1) The connection between plates in the plastic energy dissipation zone should use fully welded butt welds.

(2) The beam or support located in the plastic energy consumption zone should be made of the entire material. When the hot-rolled steel exceeds the maximum length specification of the material, equal strength splicing can be carried out.

(3) The support located in the plastic energy dissipation zone should not be spliced on site.

9. When a rigidly connected steel beam directly connected to the support system components between the support systems yields before the buckling of the compression diagonal member, it should be designed according to the frame beam of the frame structure. The internal force adjustment coefficient in the non plastic energy dissipation zone can be taken as 1.0, and the width to thickness ratio of the section plate should meet the S1 level requirements of the bending component.

When the ductility level of the plastic energy dissipation zone in the frame structure is level I or II, the rigid nodes of beams and columns should comply with the following regulations

(1) When welding beam flanges and column flanges, full penetration welds should be used.

(2) Within the range of 600mm nodes above and below the beam flange, full penetration welds should be used for the connection welds between the column flange and the column web or between the box column wall panels. The thickness of horizontal stiffeners or partitions installed at the upper and lower flange levels of the beam should not be less than the thickness of the beam flange.

(3) The over welding holes of the beam web should be completely separated from the full penetration groove weld seam between its end and the beam flange and column flange, and it is recommended to use improved over welding holes or conventional over welding holes.

(4) The length of the welding lining plate under the welding holes of the beam flange and column flange should not be less than the flange width plus 50mm and the flange width plus twice the flange thickness; The welding structure with the column flange should comply with the following regulations

1) The welding lining plate of the upper flange can use fillet welds, and the arc striking part should use fillet welding;

2) The lower flange lining plate should be welded to the column flange using a weld seam that penetrates from the top to the bottom.

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When using bone shaped joints for beam column stiffness nodes, the following regulations should be met

(1) When the internal force analysis model is calculated based on the undamaged section, the lateral displacement limit of the unsupported frame structure should be multiplied by 0.95; The deflection limit of steel beams should be multiplied by 0.90.

(2) When checking the flexural bearing capacity of the weakened section, the bending moment of the weakened section can be calculated as 0.80 times the bending moment at the beam end.

(3) The linear stiffness of a beam can be calculated by multiplying the value calculated by an equal cross-section by 0.90 times.

(4) Strong columns and weak beams should meet the requirements of the specifications.

(5) The bone weakening section should be automatically cut, and the size can be calculated according to the specifications.

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When the beam end reinforcement method is used at the beam column node to ensure the plastic hinge displacement requirement, the following regulations should be met

(1) The change in plastic bending moment of the strengthening section should be similar to the bending moment diagram when plastic hinges are formed at the beam end.

(2) When using cover plates to strengthen nodes, the calculated length of the cover plate should start at a distance of 50mm from the surface of the column.

(3) When using the method of widening the flange, the slope of the flange edge should not exceed 1:2.5; The distance between the starting point of widening and the column flange should be (0.3-0.4) hb, where hb is the height of the beam cross-section; The width to thickness ratio of the widened flange should not exceed 13 ε K.

(4) When the column has a box section, it is advisable to increase the flange thickness.

When the frame beam is covered with a concrete floor slab, the floor reinforcement should be reliably anchored.

14. Frame - The frame part of the central support structure, that is, the beam column components that do not transmit the internal force of the support, should be designed according to the ductility level determined by the standard and adopted according to the frame structure.

15. The slenderness ratio of the support and the width to thickness ratio of the section plate should meet the requirements of the specification based on the ductility level of the structural components. The width to thickness ratio of the support section plate should be adopted according to the limit value of the corresponding component plate width to thickness ratio level.

16. The central support structure should comply with the following regulations

(1) The support should be set in pairs, and the difference in the horizontal projection area of the cross sections of bars with different inclined directions in the same horizontal seismic action direction of each layer should not exceed 10%.

(2) For the support system of cross supported structures and paired single diagonal rod support structures, when the slenderness ratio of the supporting diagonal rod is greater than 130, the internal force calculation can only be based on the tension diagonal rod without considering the compression effect. When the number of structural layers exceeds two, the slenderness ratio should not exceed 180.

17. Steel support connection nodes should comply with the following regulations

(1) When the support and frame are connected by a node plate, the distance from the support end to the nearest embedded point of the node plate along the axis of the support rod should not be less than twice that of the node plate.

(2) The connection node between the herringbone support and the crossbeam should be equipped with lateral support, and the axial force design value should not be less than 2% of the axial bearing capacity design value of the beam.

When the ductility level of structural components is Level I, the construction of energy dissipation beam segments should comply with the following regulations

(1) The web plate of the energy dissipation beam section shall not be welded or reinforced, nor shall it be perforated.

(2) The connection between the energy dissipation beam section and the support should be equipped with stiffeners on both sides of its web. The height of the stiffeners should be the height of the beam web, and the width of the stiffeners on one side should not be less than (bf/2-tw), and the thickness should not be less than the larger value of 0.75tw and 10mm.

(3) The energy dissipation beam section should be equipped with intermediate stiffeners on its web as required: the intermediate stiffeners should be at the same height as the web of the energy dissipation beam section; When the cross-sectional height of the energy dissipation beam section is not greater than 640mm, unidirectional stiffeners can be configured; When the cross-sectional height of the energy dissipation beam section is greater than 640mm, stiffeners should be installed on both sides. The width of one side of the stiffener should not be less than (bf/2-tw), and the thickness should not be less than the larger value of tw and 10mm.

(4) When the energy dissipation beam section is connected to the column, its length should not exceed 1.6Wp, lfy/Vl, and should meet the provisions of relevant standards.

(5) The upper and lower flanges at both ends of the energy dissipation beam section should be equipped with lateral support, and the axial force design value of the support should not be less than 6% of the axial bearing capacity design value of the energy dissipation beam section flange.