Truss Factor of Safety and Characterization

1
Truss Factor of Safetyand Characterization

• Factor of Safety
• Member Cross-Sections
• Effecting Tensile and Compressive Strengths
• Characterizing Truss Members

2
Learning Objectives

• Calculate cross sectional area of a truss member
• Explain factors that affect the tensile and
  compressive strength of a truss member
• Explain the principle of the lever tool
• Experimentally determine the tensile and
  compressive strengths of truss members
• Graph and analyze the experimental data
• Calculate Factor of Safety for every truss member

3
Factor of Safety

• Design uncertainty
• Excessive loads vehicles exceeding
  specifications, wind, snow, earthquakes, etc
• Construction material variances, workmanship,
  etc
• Design Models accuracy of design simulations

4
Factor of Safety

• Designers make a bridge stronger than design
  target
• Factor of Safety
• Most codes require minimum Factor of Safety gt 1.6

5
Truss Characterization and Factor of Safety

• Factor of Safety
• Actual level is calculated by Method of Joints
• Need characterization data of the failure level
  for a truss member

6
Cross-Sections and Cross-Sectional Area
7
Tensile Strength Characteristics

• depends on the cross-sectional area
• depends on the material
• does not depend on the length
• does not depend on the cross-sectional shape

8
Compressive Strength Characteristics

• depends on the cross-sectional area
• depends on the material
• depends on the length
• depends on the cross-sectional shape

9
Summary of Material Effects on Tensile and
Compressive Strengths
Tensile Compressive
Cross-Sectional Shape No Yes
Cross-Sectional Area Yes Yes
Material Yes Yes
Length No Yes
10
Lever Concept

• Lever Relationship F1 L1 F2 L2

11
Testing Machine
This tester works on the lever principle
12
Tensile Strength Characterization

• Make test specimens, see Learning Activity 2
• Clamp specimen to T-Line
• Conduct tension tests

13
Tensile Strength

• Plot Tensile Strength (Newtons) vs. Member Width
  (mm)
• Generate linear regression through (0,0)

14
Compression Strength Characterization

• Make test specimens, see Learning Activity 2
• Support specimen on felt pads at C-Line
• Conduct compression tests

15
Compressive Strength

• Plot Compressive Strength (Newtons) vs. Member
  Length (cm)
• Generate best fit curve to data

16
Acknowledgements

• This presentation is based on Learning Activity
  2, Test the Strength of Structural Members from
  the book by Colonel Stephen J. Ressler, P.E.,
  Ph.D., Designing and Building File-Folder Bridges