EGR 106 – Project Intro / While Loop

Project description Project schedule Background information / terminology A simple example Examples of similar programs While loop

Working in teams of approx. four students, each team is to develop a Matlab program that can be used to design a lightweight truss structure

The code shall allow the designer to evaluate alternative designs quickly and easily

The actual truss analysis will be performed by the Matlab function “truss_analysis.m” to be provided by the instructor.

Project Description

The program developed by each team shall provide a user-friendly interface for both data input and interpretation of results

Each team’s code will be used in a design competition in which each team will be given 1 hour to develop a truss design for a particular load scenario

The team will then construct their design from balsa wood (required construction materials will be provided)

Project Description (cont.)

3/27 – (T) Project Introduction3/29 – (R) Quiz 2 make up / Team assignment #14/3 – (T) Lecture: Analysis of Trusses / Graphical Interface4/5 – (R) Project work / Team assignment #24/10 – (T) Lecture: Analysis of Trusses / Graphical Interface4/12 – (R) Quiz 3 / Project work4/17 – (T) Project work4/19 – (R) In-class design competition4/24 – (T) Quiz 4 / Model testing4/26 – (R) Project Presentations / Demonstrations5/1 – (T) Written reports due

Project Schedule (tentative)

Definition: A planar truss is a structure composed of slender members joined together at their end points (called joints).

Background – Truss Structures

Photo: Engineering Mechanics: Statics, by R. C. Hibbeler

Terminology – Truss Structures

Forces

Joint

Member

Supports

Trusses are supported at certain joints at which motion is constrained either in the x- direction, y-direction or both the x- and y-directions. These supports are also called boundary conditions.

Terminology (cont.)

X & Y –displacement=0 X - displacement=0 Y -displacement=0 (bctype=1) (bctype=2) (bctype=3)

Truss Analysis – A simple example

Cx = 100

Cy = -50

300 mm

150 mm

A simple example – input data(details to be discussed next class)

Cx = 100

Cy = -50

300 mm

150 mm

joint_def=[0, 0, 1, 0, 0; 50, 50, 0, 0, 0; 150, 150, 0, 100, -50; 250, 50, 0, 0, 0; 300, 0, 3, 0, 0; 200, 0, 0, 0, 0; 100, 0, 0, 0, 0];

member_def=[1, 2, 10, 10; 2, 3, 10, 10; 3, 4, 10, 10; 4, 5, 10, 10; 5, 6, 10, 10; 6, 7, 10, 10; 7, 1, 10, 10; 2, 7, 10, 10; 7, 3, 10, 10; 3, 6, 10, 10; 6, 4, 10, 10];

Joint Displacments

disp = 0.0000 0.0000 0.2457 -0.1750 0.6493 -0.4371 0.2043 -0.4578 0.4500 -0.0000 0.3000 -0.5536 0.1500 -0.2707

Cx = 100

Cy = -50

300 mm

150 mm

Member Stresses

stress =

0.3536

0.3536

-1.0607

-1.0607

0.7500

0.7500

0.7500

-0.0000

0.0000

0.0000

-0.0000

A simple example - resultsMatlab command (call to function ‘analyze_truss’):

[disp,stress]=analyze_truss(joint_def,member_def)

Examples of truss design programs

Johns Hopkins University:http://www.jhu.edu/~virtlab/bridge/truss.htm

West Point Bridge Design Contesthttp://bridgecontest.usma.edu/

Notes on academic integrity

Truss design and analysis programs are common.

Students are welcome to look at other programs for features to incorporate.

Direct copying of code, however, is not permitted.

Do not share code with other teams.Be sure not to leave your codes on public

computers.

Longer Running Loops

for loops repeat a fixed number of times:

for variable = {array of length n}

{commands}

end

and break can be used to stop earlier.

Question: How about repeating “until done”? Run as long as is needed.

Answer: MATLAB’s “while” loop:

while expression

{commands to be repeated as long

as expression is true}

end

Prior example – compounded interest untilthe amount doubles:

value = 1000;

for year = 1:1000

value = value * 1.08;

disp([num2str(year),' years: $ ',num2str(value) ])

if value > 2000

break

end

end

for loop

terminated with break

Expected output:

while version

format bank

value = 1000;

while value < 2000

value = value * 1.08;

disp(value)

end

Example – Collecting and storing data until a zero is entered:

x = [ ];

new = 1;

while new ~= 0

new = input('enter value ');

x = [ x, new ];

end

x = x(1:end–1)

empty array

to drop the zero

initialize

Example – Getting valid keyboard input:

E.g. forcing the user’s input to be between 0 and 10:

x = –3;

while ( x < 0 ) | ( x > 10 )

x = input( 'type a value ' );

end

or:

x = input('enter value ');

while (x<0)|(x>10)

disp('invalid choice');

x = input('enter value ');

end

disp('finally!');

Example – computing pi:

...13

4

11

4

9

4

7

4

5

4

3

44

Example – “infinite” Hi-Lo:

numb = round (10*rand(1)); done = 0;while ~done

guess = input('guess');if guess = = numb disp( 'You got it !!!' ); done = 1;elseif guess > numb disp('too high')else disp('too low')end

end

initialization

single guess

loopcontrol

Nesting of while loops

while expression1 {outer loop commands} while expression2 {inner loop commands} end {more outer loop commands}end

these can also be more than 2 levels deep