Truss analysis marathon (2 Viewers)

[arman]

Ok, I though it would help if we all did some truss analysis questions

to start off with an easy one:

Determine reactions R1 and R2

[edit: btw NOT to scale, all angles are meant to be 60deg (all equilateral triangles)]

 

[ShowStopper]

Ok, I though it would help if we all did some truss analysis questions

to start off with an easy one:

Determine reactions R1 and R2

lol, cbf writing it all out

R1= 270 N
R2=180 N

 

[arman]

R1= 270 N
R2=180 N

ok, now for something harder

determine force in member BG for the same truss

btw, feel free to post any truss questions you have

just draw the truss in paint or similar and upload to 'imageshack dot us' - no signup or anything and press on 'insert image' when you reply on bos, copy and paste link and its done

 

[ShowStopper]

using method of sections

BG =242.48 N (C) ( may be an error in calculations, as i used ipod calculator lols )

 

[Kaos1]

ShowStopper's right... R1 = 270 & R2 = 180

he just assumed that all the members were 1 unit long, and did a simple members equation to figure out each reaction

eg.
R1 = 300 x .5 + 30 x 1 + 120 x 1.5 - R2 x2
therefore
R2 = 360 / 2
R2 = 180
R1 = 450 - 180
R1 = 270


Whats happening in member FG?

 

[arman]

using method of sections

BG =242.48 N (C) ( may be an error in calculations, as i used ipod calculator lols )

I got answer as BG = 138.67N

 

[arman]

ShowStopper's right... R1 = 270 & R2 = 180

yes you are both correct

Whats happening in member FG?

I found FG using method of joints and got 34.64N
Used this to find BG when sum the horizontals

 

[ShowStopper]

joints is easy, sections with this q is :S

 

[arman]

Actually, there are two ways to do this:

method of joints, requires you to find horizontal and vertical components of AF and FG, then sum to find BG

method of sections, cuts through BG, FG and FE, consider LHS of truss, take moments about external point between ED (at 30N down): takes into account R1, 270N up, 300N down and BG across

(note, although no distances are given, assume 1 member length is 1 (as equilateral, and then use relative distances, such as 0.5 for half a member)

 

[arman]

joints is easy, sections with this q is :S

actually, working for sections is easy

(270*1)-(300*0.5)+(BG*0.87)=0 (BG assumed in tension)
BG= -120/0.87
=-137.93N
therefore assumed direction incorrect

BG = 137.93N in compression

(note: I got 138.57N compressive, using method of joints

 

[ShowStopper]

omg i left the height as root 3 on 2! arghhh

 

[arman]

Now:

Question: determine force in member CH

 

[ShowStopper]

sections is easy can yu do joints? im confused as to why thats the answer, my direcitons are confusing the shit out of me!

 

[Kaos1]

WOA WOA WOA!! STOP!!

i get the 272*1-300*.5 part ... but where the hell did .87 come from?!

 

[ShowStopper]

cos 60 = 0.866..

i thought itd be root 3 on 2

 

[ShowStopper]

Ch = 207.8 n

 

[arman]

cos 60 = 0.866..

i thought itd be root 3 on 2

using pythagorus

distance = sq.root of 1*1-0.5*0.5
=0.8666

 

[arman]

Ch = 207.8 n

yes

now anyone have any truss questions, while i draw up another (i have answers to it, btw)

 

[arman]

ok, seeing as no-one else has one:

Determine reactions Ra and Rb, and length x (length will help later on)

 

[arman]

Answers for anybody interested:

Ra = 184.85kN at 55deg above horizontal and to the left

Rb = 131.19kN to the right (i did this one first)

Now Question (for the same truss)

Determine force in member AB


(answer: AB = 86.82kN in tension)