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16.20 HANDOUT
Fall, 2002
Review of Design Considerations
OVERVIEW OF STRUCTURAL DESIGN PROCESS
Purpose: Assure “structural integrity” while minimizing cost
Structural integrity: “Capability of a structure to carry out the operation for which it was designed”
Aspects to consider:
- Loads
- Deformations
- Corrosion
- Fatigue/Long life
Factors in Determining cost:
- Material
- Waste amount
- Manufacturing
- Weight
- Subassembly/Assembly
- Durability and maintenance
- Useful life
- Repair
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STRUCTURAL DESIGN PROCESS
Determine capability of structure to carry loads (of box 2) subject to design restrictions (of box 1)
size components
environment to which structure is subjected
2 1
Manufacturing & Maintenance considered throughout
Determine internal stresses and deflections
Layout structural arrangement, select materials,
Determine applied loads and operating
Design restrictions/specifications
SOURCES OF APPLED LOADS (and resulting stresses and strains)
- Normal operative environment
- Environmental effects
- Isolated effects/special conditions
TERMINOLOGY
Limit load/stress/condition: maximum load/stress/condition where structure shows no permanent deformation (operationally defined as maximum condition the structure is expected to see under normal operation)
Ultimate load/stress/condition: maximum load/stress/condition where structure does not “fail” (operationally defined as limit condition times factor of safety).
Ultimate Factor of Safety = Ultimate Condition (design value) Limit Condition
Margin of Safety = Tested Value – Design Value (experimental reality) Design Value
Definition of “failure” depends on operational requirements.
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