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 standardsGive legal requirementsAssist engineersEnhance consistency across designsAssist code officialsEnhance safetyProtect propertyReduce uncertaintyMaintain heuristics

Why Building Codes?

Resistance > Load EffectFormat definitionLoadsLoad applicationLoad combinations Resistance calculationsDetailing

Goal of Structural Building Codes

Implicit or ExplicitExplicit requirements generally reduce

variabilityImplicit requirements generally increase

variabilityOptimize the relative amount of each

EvolutionCostUncertainty reductionAvoid future failures

How?

Allowable Stress/Strength DesignLoad and Resistance Factor Design (Partial

Factor Code)Reliability DesignPerformance-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 loads1.2D + 1.6L + 0.5S

Loads

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

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 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. total2005: 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 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 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

Questions?