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Homework Help: Force Resolving Structural Engineering Problem

  1. Nov 9, 2005 #1
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Nov 9, 2005 #2

    Astronuc

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    Last edited by a moderator: May 2, 2017
  4. Nov 14, 2005 #3
    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.
     
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