I. Core Seismic Advantages of Steel Structures
Primary components of steel structures adopt hot-rolled H-beams, welded box columns, circular tubes and steel beams. Benefiting from the inherent properties of steel including good ductility, light self-weight and high strength, steel structures exhibit prominent seismic advantages compared with concrete structures:
Light Self-weight
The total building load is low. Seismic action (seismic shear force) is proportional to self-weight. Under identical conditions, steel structures bear significantly lower seismic force than reinforced concrete structures.
Excellent Ductility of Steel
Steel is an ideal elastoplastic material. It can enter the plastic stage under strong earthquakes to absorb seismic energy, and brittle failure without warning seldom occurs.
Industrialized Assembly & Controllable Joints
Components are prefabricated in factories and assembled on site. Properly designed beam-column joints can realize the philosophy of "strong joints, weak members" and "strong columns, weak beams" to form an energy dissipation mechanism.
Strong Structural Deformation Capacity
A certain amount of inter-story deformation is permitted with sufficient deformation reserve, so instantaneous overall collapse is unlikely.
High Post-earthquake Restoration Potential
Most post-quake damages concentrate on preset energy-dissipating parts such as beam ends and bracings. Repair and replacement of components are less difficult than concrete structures.
Applicable Scenarios: Factories, poultry houses, warehouses, multi-story office buildings, large-span PEB portal frame steel structures.
II. Seismic Characteristics of Main Structural Systems (Common in Engineering)
1. Lightweight Portal Frame Steel Structure (Widely Used for Poultry Houses and Warehouses)
Structural form: Variable-section H-section rigid frame columns + tapered beams; hinged or rigid column bases; roof and wall purlins + thermal insulation sandwich panels ✅ Advantages: Extremely light self-weight resulting in low seismic effect; simple configuration and economical cost ⚠️ Key Seismic Weaknesses
Hinged column bases are adopted in most projects with weak capacity to resist horizontal seismic bending moment; pure hinged schemes shall be used cautiously in high seismic intensity zones.
Sudden stiffness change at variable-section beam ends tends to trigger local instability under earthquakes.
The bracing system (cross bracing) serves as the first seismic defense line. Missing bracings or excessive spacing easily lead to insufficient lateral stiffness.
Enclosure sandwich panels are mostly non-load-bearing members and cannot act as seismic walls to resist horizontal forces.
Poor out-of-plane stability. Complete purlins and knee bracings must be installed to prevent local instability of beam flanges.
Applicable seismic intensity: Conventional Zone 6 and Zone 7. Special reinforcement on column bases and additional transverse rigid bracings are required for Zone 8 and higher seismic intensity zones; rigid column bases shall be adopted when necessary.
2. Steel Frame Structure (Multi-story Factories and Comprehensive Buildings)
Uniform-section H-section columns + H-section beams with rigid beam-column connections Seismic design principles to be followed: strong column weak beam, strong joint weak member, strong anchorage weak member Seismic force transmission path: Floor slab → Steel beam → Steel column → Foundation Plastic hinges at beam ends can dissipate seismic energy, making it one of the systems with optimal comprehensive seismic performance.
3. Concentrically Braced Frame / Eccentrically Braced Frame
Concentric cross bracing: High stiffness, suitable for moderate and minor earthquakes. However, bracings are prone to buckling under compression during earthquakes with limited energy dissipation capacity.
Eccentrically braced frame: Equipped with link beams. Seismic energy is absorbed by yielding of link beams, recommended for high seismic intensity zones.
III. Typical Seismic Failure Modes of Steel Structures
Buckling and Fracture of Bracings (Most Common in Lightweight Factories)
Excessive slenderness ratio of cross bracings causes compressive buckling under cyclic seismic loads, loss of lateral stiffness and sharp increase of building lateral displacement.
Beam-column Joint Failure
Brittle tearing of welds frequently occurs in early welded joints. Current codes require groove welding at beam-end flanges, run-off tabs, weld ultrasonic testing, or dogbone reduced beam sections to shift plastic hinges outward.
Column Base Failure
Insufficient uplift and shear resistance of hinged column bases; inadequate anchor bolts and overly thin base plates lead to lifting and sliding of column bases under earthquakes, directly triggering overturning of rigid frames.
Typical seismic damage sequence for portal frame poultry houses and warehouses: Column base failure > Bracing failure > Flexural instability of members.
Local Instability of Components
Excessive width-thickness ratio of beam and column flanges/web plates causes local buckling after entering plastic state and loss of bearing capacity.
Secondary Disasters of Enclosure Systems
Detachment of connectors for sandwich panels and wall panels, falling non-structural components. Although they do not directly cause main structure collapse, they bring safety risks.
IV. Key Control Principles for Seismic Design (In accordance with GB 50011 Code for Seismic Design of Buildings)
1. Conceptual Design (Higher Priority Than Numerical Calculation)
Adopt regular planar and elevation layouts as far as possible to avoid abrupt stiffness change and mass eccentricity. Concave-convex planes and staggered floors will induce torsional effects and aggravate seismic damage.
Arrange reliable and continuous horizontal and vertical lateral force-resisting systems (bracings / frames).
Clarify seismic defense lines: prioritize secondary members (bracings, beams) to dissipate energy and prevent frame columns from yielding first.
2. Key Detailing Limits
Slenderness ratio: Strictly control slenderness ratio of seismic bracings to avoid premature buckling.
Plate width-thickness ratio: Limit width-thickness ratio of flanges and webs of steel beams and columns to ensure plastic development without local instability.
Joint requirements: For high seismic grades, single-sided fillet welds are prohibited to bear major seismic forces.
Column bases: For seismic intensity Zone 7 and above, optimize column bases for portal frames; rigid column bases are recommended for important buildings.
3. Key Points of Calculation and Analysis
Seismic calculation methods: Equivalent base shear method / response spectrum method; time-history analysis shall be supplemented for irregular buildings. Controlled inter-story drift angle limits:
Lightweight portal frame buildings: Code limit 1/60
Multi-story steel frames: Approximately 1/250 depending on seismic grade
Excessive displacement indicates insufficient lateral stiffness, which requires densified bracings and enlarged member sections. Torsional coupling effect shall be considered when mass center does not coincide with stiffness center. Dead load consideration: Enclosure sandwich panels, equipment and suspended ceilings of steel structures shall all be included in mass calculation for seismic force evaluation.
V. Special Seismic Suggestions for Portal Frame Poultry Houses / Steel Chicken Houses (Matching Business Scenarios)
Key explanations combining engineering characteristics of prefabricated steel poultry houses:
Bracing System Cannot Be Simplified Arbitrarily
Roof transverse bracings and column bracings must be arranged as a complete set and shall not be omitted to save steel consumption. Bracings adopt Q235B / Q355B steel tubes or angle steels.
Column Bases Are Seismic Weak Zones
Hinged column bases satisfy requirements for Zone 6 and Zone 7 in ordinary projects. For Zone 8 seismic regions: increase the quantity of anchor bolts, thicken base plates, set shear keys, and adopt rigid column bases if conditions permit.
Independent Reinforcement for Ventilation Equipment, Cooling Pad Supports and Fan Bearings
Fans and cooling pads are additional masses that generate extra horizontal forces under earthquakes to prevent connectors from tearing wall panels.
Control Building Aspect Ratio and Avoid Overlong Structures
Seismic expansion joints are recommended for overlong buildings to reduce thermal stress and mitigate seismic torsional effects.
Material Selection
Q355B is recommended for main members with qualified impact toughness. Inferior thin-walled steel sections are strictly forbidden. Critical welds shall meet performance requirements under low temperature and dynamic loads.