Structural Behavior

• Strength, stiffness, stability, and ductility are key concepts in understanding structural behavior.

• Multiple levels of scale: Structural behavior can be viewed at multiple levels of scale:

  • Material

  • Element

  • System

• Levels are related, but independent. It is possible to make a low stiffness element with high stiffness materials, a low strength system from high strength elements, and so on.

Design Approaches

• Building codes for public health and safety: Modern building codes focus on public health and safety, not the rights of owners, nor the potential conflict between the owner's interest and the public interest.

• Limit States: Structural design can be viewed in terms of Limit States: bounds on acceptable behavior for given conditions.

• Two key load conditions:

  • Overload

  • Service Loads

• Checks for each condition:

  • Overload

    • Strength

    • Stability

    • Ductility

  • Service Loads

    • Stiffness

    • Serviceability

• Design approaches for strength: There are two basic approaches to designing structures with adequate strength:

  • Allowable stress design

    • Design the structure so that stresses at service loads do not exceed a specified failure stress (typically yield stress) divided by a factor of safety.

    • Common factors of safety are in the range of 1.5 to 2 for steel and concrete, and higher for timber.

  • Load Factor Design

    • Design the structure so that members are at failure for magnified loads. The magnification factors (called load factors) vary depending on the uncertainty associated with the load type.

• Load factor design is replacing allowable stress design: Allowable stress design has a long history in America and Europe, but is has been replaced in conrete design and is in the process of replacement in steel and timber design.