William M. Bulleit, Michigan Tech Dave K. Adams, Lane Engineers, Inc Philosophy of Structural Building Codes
Chair: Jon Schmidt SEI/ASCE Engineering Philosophy Committee
Define minimum standards Give legal requirements Assist engineers Enhance consistency across designs Assist code officials Enhance safety Protect property Reduce uncertainty Maintain heuristics Why Building Codes?
Resistance > Load Effect Format definition Loads Load application Load combinations Resistance calculations Detailing Goal of Structural Building Codes
Implicit or Explicit Explicit requirements generally reduce variability Implicit requirements generally increase variability Optimize the relative amount of each Evolution Cost Uncertainty reduction Avoid future failures How?
Allowable Stress/Strength Design Load and Resistance Factor Design (Partial Factor Code) Reliability Design Performance-Based Design – likely based around reliability design Format
Loads should be explicit to use a consistent basis (e.g., 50-year return period, 2% annual probability of exceedance) Load application should be a balance. (ASCE 7 seems to have become too explicit in this area.) Load combinations are usually explicit due to the need for a consistent basis for loads 1.2D + 1.6L + 0.5S Loads
Codes such as ACI-318 and the AISC specification are strongly weighted toward resistance calculations. Generally becoming more and more explicit. Increases design consistency Helps engineers – to a point Helps code officials Often material based (e.g., ACI-318), but sometimes structure based (e.g., AASHTO bridge code) Codes should help reduce model error. e.g., rectangular stress block versus designer’s choice Resistance Calculations
1983 : 111 pgs. spec., 155 pgs. comm; 266 pgs. total 1989: 353 pgs. total 2005: 430 pgs. total ACI 318
Anchorage of concrete wall to roof diaphragm must resist 200 plf. (Explicit) Structures must provide a complete load path to transmit loads from their point of origin to the load resisting element. (Implicit) Seismic Design Example
Generally very explicit Sometimes difficult to separate where detailing requirements differ from design requirements Example: Minimum ties for square concrete columns. No. 3 bars for No. 10 or smaller longitudinal bars, No.4 for larger long. bars Vertical spacing: 16 long. bar dia’s, 48 tie dia’s, or least dimension Other placement requirements Detailing
In the end, codes should lead to safer structures. Generally, more explicit codes would appear to lead to safer structures. But at some point the complexity of the code may lead to more errors by the designer Too implicit increases designers’ decisions and increases inconsistency among designs Too explicit makes it harder for designers to know what part of the code to use. Tends to CYA. What is wrong?
Specification: Includes legal aspects and necessary explicit information (e.g., load characterization) Commentary: Includes possible techniques for design (e.g., load application, resistance calculation) and possible details. Supplement: Design aids for designs done to the Commentary Designs done to the Commentary and Supplement would be ‘deemed-to-comply’ with the Specification Possible Solution
Codes need to find a balance between explicit requirements and implicit requirements Over emphasis on either can reduce safety and/or increase cost One possible solution is to have a relatively implicit specification with a commentary that makes design suggestions that are ‘deemed-to-comply’ with the specification. Conclusions