Philosophy of Structural Building Codes
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This document discusses structural building codes and their goals of establishing minimum safety standards, consistency, and reducing uncertainty. It notes that codes aim to strike a balance between explicit requirements, which increase consistency but complexity, and implicit requirements, which allow flexibility but also variability. The document examines different code formats and highlights trends toward more explicit standards in areas like load definitions, resistance calculations, and detailing over time. It concludes that codes need to balance explicit and implicit rules to optimize safety and cost.
Philosophy of Structural Building Codes
- 1. William M. Bulleit, Michigan TechDave K. Adams, Lane Engineers, IncPhilosophy of Structural Building Codes
- 2. Chair: Jon SchmidtSEI/ASCE Engineering Philosophy Committee
- 3. Define minimum standardsGive legal requirementsAssist engineersEnhance consistency across designsAssist code officialsEnhance safetyProtect propertyReduce uncertaintyMaintain heuristicsWhy Building Codes?
- 4. Resistance > Load EffectFormat definitionLoadsLoad applicationLoad combinations Resistance calculationsDetailingGoal of Structural Building Codes
- 5. Implicit or ExplicitExplicit requirements generally reduce variabilityImplicit requirements generally increase variabilityOptimize the relative amount of eachEvolutionCostUncertainty reductionAvoid future failuresHow?
- 6. Allowable Stress/Strength DesignLoad and Resistance Factor Design (Partial Factor Code)Reliability DesignPerformance-Based Design – likely based around reliability designFormat
- 7. 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 loads1.2D + 1.6L + 0.5SLoads
- 8. Overall: 7-88 94 pages 7-05 388 pages (smaller font)Live Loads:7-88: 2.5 pgs. spec., 5.5 pgs. comm.7-05: 5 pgs. spec., 6 pgs. comm.Snow Loads:7-88: 9 pgs. spec., 13 pgs. comm.7-05: 13 pgs. Spec., 14 pgs. comm.Wind Loads:7-88: 15 pgs. spec., 11 pgs. comm.7-05: 60 pgs. spec., 40 pgs. comm.ASCE 7
- 9. Codes such as ACI-318 and the AISC specification are strongly weighted toward resistance calculations.Generally becoming more and more explicit.Increases design consistencyHelps engineers – to a pointHelps code officialsOften 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 choiceResistance Calculations
- 10. 1983 : 111 pgs. spec., 155 pgs. comm; 266 pgs. total1989: 353 pgs. total2005: 430 pgs. totalACI 318
- 11. 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
- 12. Generally very explicitSometimes difficult to separate where detailing requirements differ from design requirementsExample: Minimum ties for square concrete columns. No. 3 bars for No. 10 or smaller longitudinal bars, No.4 for larger long. barsVertical spacing: 16 long. bar dia’s, 48 tie dia’s, or least dimensionOther placement requirementsDetailing
- 13. 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 designerToo implicit increases designers’ decisions and increases inconsistency among designsToo explicit makes it harder for designers to know what part of the code to use. Tends to CYA.What is wrong?
- 14. 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 CommentaryDesigns done to the Commentary and Supplement would be ‘deemed-to-comply’ with the SpecificationPossible Solution
- 15. Codes need to find a balance between explicit requirements and implicit requirementsOver emphasis on either can reduce safety and/or increase costOne 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
- 16. Questions?