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Simple truss question driving me crazy =/

  1. May 8, 2009 #1
    simple truss question....driving me crazy!! =/

    1. The problem statement, all variables and given/known data
    the question wants the forces in all the members of the truss. i have found that members bc and ef are zero force members...i cannot find the others. I am at a loss because ive tried all the methods i have been taught and the answers are not right. I NEED HELP!!!


    2. Relevant equations



    3. The attempt at a solution
    i have tried to take sum of forces in x prime (the AD axis), for members tbd and tcd....and also have tried doing tab and tda.
     

    Attached Files:

  2. jcsd
  3. May 8, 2009 #2
    Re: simple truss question....driving me crazy!! =/

    just in case you can't see the pic....the lengths from the pin to d is 16 ft, while the horizontal distance from the pin to d is 12 ft. the load at b is 2000 and the load at d is 3000. i have found a_y to be 3500 lbs
     
  4. May 8, 2009 #3
    Re: simple truss question....driving me crazy!! =/

    the angles between ab and ac is 30, while the angles between db and dc is 30
     
  5. May 8, 2009 #4
    Re: simple truss question....driving me crazy!! =/

    If you want to show the picture fast, you can upload it to picture-servers and post the link. Sometimes the approval can take a day.
     
  6. May 9, 2009 #5
    Re: simple truss question....driving me crazy!! =/

    > i have found that members bc and ef are zero force members
    Very good!

    > i have found a_y to be 3500 lbs
    Very good. (note: in engineering/mechanics terms, the reaction at A is usually shown as Ra) Ay is understandable, you are probably doing a physics course.

    > ive tried all the methods i have been taught and the answers are not right.
    What methods have you been taught? Does 'triangle of forces' mean anything to you? How about 'summing forces in x and y'?
    Can you post the calculations and the answer you've got using your method, and the answers given. Sometimes the answers are misprinted or mis-transcribed by the teacher or the student.

    Hope to hear from you soon.
     
  7. May 10, 2009 #6

    djeitnstine

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    Gold Member

    Re: simple truss question....driving me crazy!! =/

    Hachi_roku, I have an idea. If the question really wants ALL of the forces in each member, start out by finding the reaction forces -of course. Then start by resolving the unknown forces in the pins A, B C, D... in that order. Surely every force in each member will become apparent.
     
    Last edited: May 10, 2009
  8. May 10, 2009 #7
    Re: simple truss question....driving me crazy!! =/

    Hint:
    with perfect symmetry of loading and geometry, you only have to work out half of the structure, for example ABCD. The other half (DEFG) can be replaced by a single horizontal reaction. Note, however, that the reaction at A has a vertical and horizontal component.
    The horizontal reaction at D can easily be obtained by taking moments about A.
     
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