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Homework Help: Mechanics of Solids ( Final exam question) please help!

  1. Feb 9, 2012 #1
    hello all!
    I am new to this forum. I am doing Mechanic of Solid ( summer course). I have come across with this problem , but I don't know how to solve it. I have attached the question. Could anyone help me?
    I don't know how to convert from density to force. in this questions
    given are

    weight of concrete= 25 kN/m^3
    Height of the water contained by the dam= 3.3 m
    density of water= 1000 kg/m^3
    Stresses at A and B=?

    In order to find out stresss, I need to know forces. This is where I get stuck!
    Thank you very much!

    Attached Files:

  2. jcsd
  3. Feb 9, 2012 #2
    Re: Mechanics of Solids ( Final exam question) plz help!

    You have density of water (mass per cubic meter) and gravity, and so you can calculate the weight (F=ma) of a column of water all the way to the bottom of the dam.
  4. Feb 9, 2012 #3
    Re: Mechanics of Solids ( Final exam question) plz help!

    Here is my workout for this problem. But I need to find stresses at Point A and B. I guess I take a moment at either A or B and find the forces. But stress= Force/area. Area is not given in this problem. How do I go from here? Thanks!!!!!!!!!!!!

    1) weight of the dam W= mg= pvg= (pg)v= rV= 25 kN/m^3 ( 2.16 X 3.6 /2 X b) pg= weight density, b= width= 97.2 b kN

    2) Resultant hydrostatic force of water= density of water X gravity X height= 1000 kg/m^3 X 9.8 m/s^2 X 3.3 m- 32.373 kPa therefore weight b= 32.373kN Magnitude of the resultant force F= 32.373 b kN X 3.3 / 2 = 53.42 kN

    I am stuck here again. Could anyone help me? Thank you so much!!!!!!!1
  5. Feb 9, 2012 #4


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    Re: Mechanics of Solids ( Final exam question) plz help!

    It doesn't matter how long the dam is. The stress won't change along the length. So if you want to do the math "properly" take the length as L and it will cancel out eventually. Or if you just want to get to the answer, assume the dam is 1 meter long.

    I can't help you on how to find the stresses, because the answer depends on making some assumptions about how the dam is fixed to the ground, but that is not included in the question. Maybe civil engineers are supposed to "know" what those assumptions are, but I don't.
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