Force Resolving Structural Engineering Problem

[The Gradient Guru]

Can someone please help me start this question, I missed some vital notes in class and don't know how to do it. I just need to know the method so I can go on to do it myself Thanks :)

http://img408.imageshack.us/img408/3743/question70yu.th.jpg

 

[Astronuc]

Apply the Method of Joints, i.e. in static equilibrium, the sum of the forces in the x and y directions is zero.

Reference - http://mac6.ma.psu.edu/em11/p09a.html - look at section on Method of Joints.

http://physics.uwstout.edu/StatStr/Strength/StatII/stat22.htm

Some examples

http://web.pdx.edu/~cpch/c415ep7.htm


Please don't miss class!

 

[The Gradient Guru]

Thanks for the help but I'm still a little stuck. I can work out the magnitudes fine but I'm a bit confused as to where the negatives should be and what they actually represent. This is my working so far (the joints are labelled A-K from left to right):

Fab = Force along member AB

Consider Joint A:

Sum of Verticals at A=25+Fab*sin60=0
Fab*sin60=-25
Fab=-28.8N

Since this is negative it is a compressive force.

Sum of Horizontals at A=Fab*cos60+Fac=0
-28.88*cos60=-Fac
Fac=14.4N

Since this is positive it is a tensile force.

Consider Joint B (This is where I get confused:

Sum of Verticals at B=Fab*sin60+Fbc*sin60+Fbd*sin0=0
28.88*sin60+Fbc*sin60+0=0
Fbc*sin60=-28.88*sin60
Fbc=-28.88

Since this is negative it is a compressive force.

Sum of Horizontals at B= Fab*cos60-Fbc*cos60+Fbd*cos0=0
28.88*cos60-Fbc*cos60+Fbd=0
-Fbc*cos60+Fbd=-28.88*cos60
Fbd=0N

Have I got all of the negatives right because I really have no idea how they are determined and also, is it even possible for Fbd to have a force of 0N on this bridge given that there is a force of 50N on the center of the bridge?

Thanks for any help.