Truss analysis marathon (2 Viewers)

[Bdogz]

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)

i used method of joints around reaction A and i got AB to be 90kN tension, how did you get your answer?

 

[mitchell.j.bell]

CF = 207.8 in Compression yes?


Sin60 =180/CF

CF = 180/sin60

CF = 207.8

Draw free bodied diagram to find direction direction and sense.

CF = 207.8 (C)

 

[arman]

i used method of joints around reaction A and i got AB to be 90kN tension, how did you get your answer?

yes, i used method of joints with a force polygon .. but wait, sry, i made error, used angle for Ra as 54deg instead of 55

so using sine rule, AB = sin(25) * (184.85/sin(120))

so AB = 90 [edit: yes it is in tension]

 

[arman]

CF = 207.8 in Compression yes?


Sin60 =180/CF

CF = 180/sin60

CF = 207.8

Draw free bodied diagram to find direction direction and sense.

CF = 207.8 (C)

u mean CH = 207.8 is correct

 

[arman]

New one:

:jaw:

Determine reactions then determine force and nature of force in member X

 

[tommykins]

Assuming they're all equilateral triangles - taking the section from the LHS

you get
Fy = -Xsin60 - 15 = 0
Thus,
X = -15/sin60 = -17.32 kN

so the member X is 17.32kN in compression

EDIT ; reactions is easy enough but it not required to find X

 

[arman]

Assuming they're all equilateral triangles - taking the section from the LHS

sry i forgot to add angles

so the member X is 17.32kN in compression

correct

 

[tommykins]

btw guys if you plan on doing engineering in uni (which i assume you are) - this is the kind of stuff you'll be doing.

also will be heavily relying on SFD and BMD as well as moment of intertia.

 

[arman]

btw guys if you plan on doing engineering in uni (which i assume you are) - this is the kind of stuff you'll be doing.

also will be heavily relying on SFD and BMD as well as moment of intertia.

i hate shear force and bending moment diagrams - although they're not very hard
btw wot is moment of inertia, is it kinda like centripetal force, or is it just normal moments

 

[tommykins]

shear force and bending moment diagrams can be done via observation (ie. just knowing the reactions) and constructed within like, 10seconds. annoying if they want an algebraic equation for each section though.

moment of inertia is the tendency of an object to rotate on a specific axis. has nothing to do with normal moments thus far.

 

[arman]

shear force and bending moment diagrams can be done via observation (ie. just knowing the reactions) and constructed within like, 10seconds. annoying if they want an algebraic equation for each section though.

moment of inertia is the tendency of an object to rotate on a specific axis. has nothing to do with normal moments thus far.

so i guess its like dynamics, rather than having an object that is at equilibrium (static) , for eg: a truss that is supported and does not move [edit: is static]

 

[tommykins]

hrmmm... no no

eh...it's kinda hard to explain, best wait till you actually do it or you can just go to -
http://en.wikipedia.org/wiki/Second_moment_of_area

 

[arman]

hrmmm... no no

eh...it's kinda hard to explain, best wait till you actually do it or you can just go to -
Second moment of area - Wikipedia, the free encyclopedia

thanks, now i realise, i no it as second moment of area: bending stress = My/I
need to know that for tomoz