Finding Errors in Structural Designs How is it Typically Done? James Hanson, Ph.D., P.E. Rose-Hulman Institute of Technology Voted #1 Eight Consecutive Years ASCE Illinois Section – Structural Group Meeting November 15, 2006

Overview Categories of Errors Categories of Errors Experience of a Generation Experience of a Generation Shared Experiences Shared Experiences Tools for Finding Errors Tools for Finding Errors Teaching Tools for Finding Errors Teaching Tools for Finding Errors

Categories of Errors Idealization of Reality Idealization of Reality Assumptions Inherent to Analysis or Design Method Assumptions Inherent to Analysis or Design Method Roundoff Error Roundoff Error Human Error Human Error

Idealization of Reality

Assumptions Inherent to Analysis or Design Method Concrete Beam Depth Shear Strength Code Design Eqtn

Roundoff Error

Human Error From AISC Seminar “Field Fixes – Common Problems in Design, Fabrication and Erection – Solutions and Prevention”

Overview Categories of Errors Categories of Errors Experience of a Generation Experience of a Generation Shared Experiences Shared Experiences Tools for Finding Errors Tools for Finding Errors Teaching Tools for Finding Errors Teaching Tools for Finding Errors

Precomputer engineers adept at finding errors Precomputer engineers adept at finding errors Often called “experience” Often called “experience” Will experience be lost? Will experience be lost? End of a Generation

Gathering Experience Ten Firms: engineers Ten Firms: engineers Interviewees: 35 structural engr Interviewees: 35 structural engr Experience: 1-55 years Experience: 1-55 years PE’s: 29 PE’s: 29 BS: 8, MS: 26, PhD: 1 BS: 8, MS: 26, PhD: 1

Interviews Critical Incident 1: “Think of the most recent time you discovered something unreasonable in the results of analysis or design.” Critical Incident 2: “Think of the most alarming time you discovered something unreasonable in the results of analysis or design.”

Overview Categories of Errors Categories of Errors Experience of a Generation Experience of a Generation Shared Experiences Shared Experiences Tools for Finding Errors Tools for Finding Errors Teaching Tools for Finding Errors Teaching Tools for Finding Errors

Post-tensioned slab-on-grade cracks. Followed standard practice. Standard practice assumptions were not valid in this case.

Beam through stairwell at chest height. Accidentally left on drawings. Designer failed to check drawings.

Detail not appropriate. Copied from previous design. Reviewer recognized.

CB A B C Beams meet without supporting column. Conditions changed requiring removal of planned column, but beams not redesigned. Found when checking load paths.

Using in-house spreadsheet. Changing member length had no effect on design strength. Spreadsheet had an error.

Beam flange not thick enough to weld studs. Beam web too shallow to bolt to girder. Designed by computer for strength. Found by experienced reviewer.

Beams seemed too deep. Checked reactions at ends of beam with hand calculations. Double counted self weight in computer design. L2L2 L2L2  Span (ft) Depth (in) Span (ft) 2 ≈ Depth (in)

Foundation underdesigned. Forgot self weight of structure. Remembered later.

Common Problem: Structural layout does not match architectural and/or mechanical layout. Changes in layout not communicated by architect & not noticed by structural engineer. Will BIM fix this?

Overview Categories of Errors Categories of Errors Experience of a Generation Experience of a Generation Shared Experiences Shared Experiences Tools for Finding Errors Tools for Finding Errors Teaching Tools for Finding Errors Teaching Tools for Finding Errors

1.Comparisons (23 of 87) Hand CalculationComputer Analysis L 24 Categories of Tools 2.Rules of Thumb (7 of 87)

3.Visualization (5 of 87) 4.Other (14 of 87) a) Procedures b) Reflection

5.Previous Experience (22 of 87) Oops, holes don’t line up! Similar Projects 6.Field (14 of 87)

10 Quick Checks 1.Is the deflected shape consistent with what was expected? 2.Are the moment diagrams consistent with what was expected? = Identifying Features

10 Quick Checks 3.Does the building weigh what you anticipate? 4.Does total base shear equal total applied lateral load? = Checking Equilibrium

10 Quick Checks 5.Do beams deflect more than permitted? 6.If most beams are the same size, why are others not? 7.Is the beam depth consistent with standard rules-of-thumb?

10 Quick Checks 8.Do the connections and bracing provide a continuous load path? 9.Do connection details match the assumptions used in the analysis? 10.Are the primary structural member sizes similar in similar projects?

Overview Categories of Errors Categories of Errors Experience of a Generation Experience of a Generation Shared Experiences Shared Experiences Tools for Finding Errors Tools for Finding Errors Teaching Tools for Finding Errors Teaching Tools for Finding Errors

Teaching the New Generation Many firms do so informally Many firms do so informally Not in textbooks Not in textbooks Integrating at undergrad level Integrating at undergrad level

Learning the Tools 1. Comparisons 2. Rules of Thumb 3. Visualization 4. Other 5. Previous Experience 6. Field Can be Taught Procedures Over Time Least Preferred

Integrating Into Classroom Emphasize approximations Emphasize approximations Equilibrium always satisfied Equilibrium always satisfied Features of behavior Features of behavior

Approximations A B Plan View AC Unit 10 ft5 ft10 ft5 ft 100 plf 50 plf 300 plf 100 plf AB Situation: The roof shown experiences snow load with drifting adjacent to the AC unit. The resulting distributed load on member AB is shown. Objective: Find, approximately, the peak moment and shear experienced by member AB.

Equilibrium

Features of Behavior Situation: A simply supported beam with a cantilevered end experiences uniform distributed load. Objective: Construct the moment diagram. Identify at least four features of the diagram that suggest you have a reasonable solution.

Impact In Classroom Structural Analysis I Structural Analysis II

Summary Some errors not tolerated Some errors not tolerated Tools for finding them Tools for finding them Tools passed on informally Tools passed on informally Tools can be taught Tools can be taught Center for Structural Engineering Education

Acknowledgements Participating Firms Sponsor Grant: DUE Questions or comments: