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EGB376 (completed)
Introduction to Steel Structures and Limit States Design
| Question | Answer |
|---|---|
| Standard Steel Sections | • Hot-rolled Sections: UB (150-610mm deep), UC (100-310 deep), PFC/TFC, EA/UA • Standard Welded Sections in the form of 3-plate I-sections: WB (700-1200), WC (350-500) |
| Standard Steel Sections (2) | • Cold-formed structural steel hollow sections: CHS, SHS, RHS (not hot-formed); cold-formed tubes - direct-formed or continuous-formed or continuous-formed & stress-relieved |
| Standard Steel Sections (3) | • Plates alone or Plates and Standard sections can be combined to make other sections |
| Steels are alloys of iron with small quantities of... | carbon, manganese, chromium, vanadium, copper |
| Yield Stress - For all design actions: | tension, compression, bending and shear – yield stress fy is used |
| Steel Grades | G250, G300, G350, G450 etc |
| fy | varies with both grade and thickness whereas |
| fu | depends only on grade |
| fy | depends on chemical composition, method of manufacture and amount of working (so thinner plates have higher fy) |
| Choice of Steel Grade | Suitability of a grade depends on its strength properties, weldability, ductility at the specified service temperature, low cost to strength ratio, and availability in the required sizes and shapes |
| Steel structures code AS 4100 specifies the minimum requirements for... | • Design, Fabrication, Erection and Modification of Steelwork in structures (buildings, wharves and cranes); also to roadway, railway and pedestrian bridges together with our bridge codes • Based on Limit States Design Method |
| AS4100 excludes the following | • Steel elements less than 3 mm thick • Steel elements with design yield stress f_y exceeding 690 MPa. |
| AS4100 excludes the following (2) | • Cold-formed members except the tubular sections (RHS, SHS & CHS) complying with AS1163 – use AS/NZS4600: Cold-formed steel code (section thickness can be < 3mm) • Composite steel-concrete members – use AS2327, the composite structures code |
| Unidentified steels – | use fy = 170, fu = 300 MPa |
| Design Actions (Loads) | • Permanent Action G (Dead Load) AS1170.1 • Imposed Action Q (Live Load) AS1170.1 • Wind Action / Load (Wu) AS1170.2 • Snow and Ice Actions AS1170.3 • Earthquake Action/Load Feq AS1170.4 • Temperature Induced Loads T • Construction Loads C |
| Load Factors - To allow for “overload” possibility and possible inaccuracy of calculated design action effects, a Load Factor is... | used with each load. • For G: 1.2, 0.9 • For Q: 1.5, 0.0 • For Wu: 1.0 |
| Static structures - Dynamic loads (wind and earthquake actions) = | Use Static load equivalents for static structures |
| Design Action Effects S* - Factored loads are used to obtain design action effects S* from the analysis of structural systems, ie. beam, frame, etc | • Axial Forces N* • Bending Moments M* • Shear Forces V* • Deflections |
| Limit States Design Method - Structures must remain stable, safe and serviceable under all design actions and combinations during their design life | • Strength Limit State • Serviceability Limit State • Stability Limit State • Fatigue Limit State • Fire Limit State • Brittle Fracture Limit State This is NOT an allowable/permissible stress method (single factor used to allow for all unknowns) |
| Load Combinations for Strength Limit State | Nominal loads x Load factors |
| Capacity Reduction Factor Φ - Allows for “under-strength” due to... | • Variability of material strength • Deviation in section properties of manufactured sections • Deterioration due to corrosion etc • Quality of workmanship • Extent of damage and loss of life resulting from failure |
| Capacity Reduction Factor Φ Does not Allow for “under-strength” due to... | • Human Error Relies on competent structural engineers from QUT Quality assurance procedures in the design offices and construction sites |
| Capacity Reduction Factor Φ Allows for “under-strength” | • Φ = 0.9 for members • Φ = 0.8 for bolted connections • Φ = 0.6 for GP welds; = 0.8 for SP welds ΦR = Ultimate design capacity from AS 4100 |
| Serviceability Limit State | • Deflection • Vibration due to wind, EQ loads, machinery, traffic etc. flexible structures so use damping devices • Bolt slip • Corrosion • These are checked against acceptable limit values under appropriate load combinations |
| Serviceability Limit State (example) | For example, δ_calculated ≤ δ_acceptable, and typically for beams, deflection limit is span/250 |
| Corrosion | Metallic (galvanizing) or Non-metallic (paint systems) can be used |
| Serviceability Limit State - Appropriate combinations may include one or more of the following using Short-term and Long-term factors: | • G • φ_s Q • φ_l Q • W_s • E_s where the short term (φ_s) and long term (φ_l) factors are given |
| Stability Limit State | The structure as a whole must be stable against overturning and sway caused by horizontal and earthquake actions, and by structural frame imperfections: Use triangulated bracing, moment resisting joints, shear walls, staircases/lift cores |
| Limit States Design •This does not mean elimination of failure. | • It means the failure is unlikely to occur, ie. the risk of failure during the structure’s intended life time is very low. • This means probability is 2 x 10-7 (compare with car travel –3.6 x 10-4, Swimming 2 x 10-5) |
| Design for Robustness | • All steel structures including members and connection components, shall comply with robustness requirements in AS/NZS1170.0 • Avoid progressive collapse |
| Structures shall be detailed such that all parts of the structure shall be tied together both in the horizontal and the vertical planes so that... | the structure can withstand an event without being damaged to an extent disproportionate to that event |
| Fatigue Limit State | • Load fluctuations (ex. Wind) • Fatigue cracks and fracture • Structural failures at stresses well below yield levels • Fatigue damage is repairable • Welded joints • Minimise stress concentrations • Fatigue life versus Stress range curves |
| Fire Limit State | • Prevent premature collapse & fire spread • Passive approach versus Active approach (sprinklers, monitoring, evacuation, reduce fire spread etc) • Open car parks – no fire protection is needed |
| Fire Limit State (2) | • Fire load versus Fire performance should be assessed and then protection provided • Fire resistance level versus Period of structural adequacy method |
| Brittle Fracture Limit State | • Occurs suddenly in the tensile stress regions at low temperature • Can occur at low stress levels (25% of fy) • Use steel with adequate notch toughness |
| Brittle Fracture Limit State (2) | • Limit max steel thickness for a given steel grade depending on the lowest service temperature. This allows steel with adequate notch toughness to operate in notch-ductile temperature range and thus no brittle failure |
| Design Process | • Step 1: Decide on the structural layout for the building • Step 2: Determine the design actions/loads and their design action/load combinations with appropriate load factors |
| Design Process (2) | • Step 3: Structural analysis to determine the design action effects such as maximum bending moment M*, axial tension or compression force N* and shear force V* |
| Design Process (3) | • Step 4: For the chosen member, determine the design capacity = capacity reduction factor φ x nominal capacity R based on AS4100 rules |
| Design Process (4) | • Step 5: If Design Action Effect M* or N* or V * ≤ Design Capacity φR, then design is ok. Otherwise choose another member size and repeat the process |
| Design Process (5) | • Step 6: Produce design drawings: They should include the following: design data and details |
Created by:
Asher - S
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