Finding Errors in Structural Designs

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Finding Errors in Structural Designs

• How is it Typically Done?

ASCE Illinois Section Structural Group
Meeting November 15, 2006
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Overview

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

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Categories of Errors

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

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Idealization of Reality
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Assumptions Inherent to Analysis or Design Method
Shear Strength
Code Design Eqtn
Concrete Beam Depth
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Roundoff Error
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Human Error
From AISC Seminar Field Fixes Common Problems
in Design, Fabrication and Erection Solutions
and Prevention
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Overview

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

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End of a Generation

• Precomputer engineers adept at finding errors
• Often called experience
• Will experience be lost?

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Gathering Experience

• Ten Firms 1 - 700 engineers
• Interviewees 35 structural engr
• Experience 1-55 years
• PEs 29
• BS 8, MS 26, PhD 1

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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.
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Overview

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

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Post-tensioned slab-on-grade cracks. Followed
standard practice. Standard practice assumptions
were not valid in this case.
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Beam through stairwell at chest
height. Accidentally left on drawings. Designer
failed to check drawings.
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Detail not appropriate. Copied from previous
design. Reviewer recognized.
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Beams meet without supporting column. Conditions
changed requiring removal of planned column, but
beams not redesigned. Found when checking load
paths.
C
B
CB
CB
A
1
3
4
2
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Using in-house spreadsheet. Changing member
length had no effect on design strength.
Spreadsheet
had an error.
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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.
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Beams seemed too deep.
Checked reactions at ends of beam with hand
calculations.
Double counted self weight in computer design.
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Foundation underdesigned. Forgot self weight of
structure. Remembered later.
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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?
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Overview

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

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Categories of Tools

• 1. Comparisons (23 of 87)

2. Rules of Thumb (7 of 87)
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• 3. Visualization (5 of 87)

• 4. Other (14 of 87)
• Procedures
• Reflection

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• 5. Previous Experience (22 of 87)

6. Field (14 of 87)
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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
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10 Quick Checks

1. Does the building weigh what you anticipate?
2. Does total base shear equal total applied lateral
   load?

Checking Equilibrium
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10 Quick Checks

1. Do beams deflect more than permitted?
2. If most beams are the same size, why are others
   not?
3. Is the beam depth consistent with standard
   rules-of-thumb?

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10 Quick Checks

1. Do the connections and bracing provide a
   continuous load path?
2. Do connection details match the assumptions used
   in the analysis?
3. Are the primary structural member sizes similar
   in similar projects?

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Overview

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

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Teaching the New Generation

• Many firms do so informally
• Not in textbooks
• Integrating at undergrad level

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Learning the Tools

1. Comparisons
2. Rules of Thumb
3. Visualization
4. Other
5. Previous Experience
6. Field

Procedures
Over Time
Least Preferred
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Integrating Into Classroom

• Emphasize approximations
• Equilibrium always satisfied
• Features of behavior

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Approximations
Situation The roof shown experiences snow load
with drifting adjacent to the AC unit. The
resulting distributed load on member AB is shown.

300 plf
100 plf
100 plf
A
B
50 plf
AC Unit
A
B
10 ft
5 ft
10 ft
5 ft
Plan View
Objective Find, approximately, the peak moment
and shear experienced by member AB.
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Equilibrium
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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.
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Impact In Classroom
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Summary

• Some errors not tolerated
• Tools for finding them
• Tools passed on informally
• Tools can be taught

Center for Structural Engineering
Education www.rose-hulman.edu/csee
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Acknowledgements
Sponsor Grant DUE-0341212
Participating Firms
Questions or comments james.hanson_at_rose-hulman.ed
u