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ENGINEERING DESIGN LAB IIENGR-102 WINTER 2015WEEK 4 LECTURE – BRIDGE MODULE

Pramod Abichandani, Ph.D.

Richard Primerano, Ph.D.

LAB WEEK 4 – CAD AND SIMULATION TOOLSUSING DATA FROM SOFTWARE TOOLS

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TEAMWORK EVALUATIONS

Due this week This is meant mainly to help us identify

potential teamwork issues and is not the only factor in deriving teamwork scores

If you are having any issues with team members, please let us know ASAP (don’t wait for evaluations).

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DUE NEXT WEEK

Design Proposal See proposal template

Signoff sheets Lab notebook pages from weeks 1-4

See journal guidelines

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BRIDGE DESIGN MODULE WEEK 4ITERATIVE K’NEX TRUSS BRIDGE DESIGN

Often, engineers rely on a combination of (pen and paper) calculation, simulation, and testing to arrive at final designs.

In this lab, we will simulate then build and test several K’NEX structures to compare simulation and experimental results.

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BRIDGE DESIGN WORKFLOW

CAD Design

Simulation

Build and Test

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INTERPRETING SOFTWARE TOOL OUTPUT

Software tools often handle the computations that would be tedious (or impossible) for us to do by hand.

They should never be relied upon without understanding the underlying theory or without qualitatively understanding the results. It is easy to set up a simulation problem

incorrectly Simulation models always contain a certain

amount of uncertainty Simulation tools never capture everything

Approximations are made to simplify analysis

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INTERPRETING SOFTWARE TOOL OUTPUT Within Visual Analysis you must specify the

constraints and applied load It will calculate the relevant equations and

determine the tension/compression in each member Blue = tension, Red = compression

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SCALING OF FORCES WITHIN A STRUCTURE

Under the assumption that the structure is operating in its linear elastic range, changes in applied load lead to proportional changes in internal and reaction forces.

100 lb

28.9 lb (t)

57.7 lb (c)57.7 lb (c)

50 lb50 lb

200 lb

57.8 lb (t)

115 lb (c)115 lb (c)

100 lb100 lb

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DETERMINE WHERE THE BRIDGE WILL FAIL

Different joint types fail at different loads Joints are stronger in compression than

tension

weak in tension

strongerin tension

strongestin tension

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LOAD AMPLIFICATION

Be aware that certain geometries can lead to amplification of internal loads

100 lb

28.9 lb (t)

57.7 lb (c)

50 lb50 lb

60˚

100 lb

86.6 lb

100 lb

50 lb50 lb

30˚

100 lb

186 lb

193 lb

50 lb50 lb

15˚

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ORIENTATION OF CROSS BRACING

The orientation of cross bracing will effect whether it is in tension or compression.

k’NEX joints are generally stronger in compression

tension

Applied load

com

pres

sion

Applied load

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DETERMINING WHEN THE BRIDGE WILL FAIL

If we assume the bridge is linear elastic, it is simple to determine approximately when it will fail.

The analysis is complicated by the fact that different joint types fail at different strengths

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DETERMINING WHEN THE BRIDGE WILL FAIL

The bridge does not necessarily fail at its weakest joint. e.g. a very weak joint that has zero

tension/compression The bridge does not necessarily fail at in the

member with the largest scaling factor e.g. the member’s joints could be much stronger

than all other joints

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DESIGN PROPOSAL

Should contain the following Summary of the design constraints under which

you must design your bridge Detailed description of your intended bridge

design Truss type, experimental and simulation results

Include any sketches, pictures, calculations relevant to your design

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DUE NEXT WEEK!

Preliminary Design Proposal

First notebook check You should have a minimum of 15 pages by the

end of the quarter Document all experimental data, mechanism

sketches, pseudo code, etc…