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Mu of a Beam

  1. Sep 25, 2012 #1
    http://www.personal.psu.edu/kar5230/blogs/kyles_professional_portfolio/How%20to%20Design%20a%20Singly%20Reinforced%20Concrete%20Beam.pdf [Broken]

    I'm stuck with a formula above. It says that to calculate for Moment of a beam... use the formula

    Mu= w L^2 / 8

    now given w = 1000 lb/ft, L=20 feet... then Mu= w L^2/8 = 1000(20)^2/8 = 50,000 lb-ft
    (although it's written as 500,000 lb-ft in the site.. i think it's wrong?)

    Anyway. my question is.. i have spent half a day doing conversion from lb-ft to Kilonewton-meter and I can't seem to get it right.

    I found out the following formula for converting between the two units:

    1 kN - m = 737.56 ft-lb
    1 kN/m = 68.52 lb/ft

    Now in the above example given w = 1000 lb/ft, L=20 feet

    1000 lb/ft = (14.59 kn/m x 6^2) / 2 = 525 kn-m/8 = 65.65 Kn-m

    Now converting Kn-m to ft-lb...

    65.65 KN-m x 737.56 ft-lbs / 1 KN-m = 48424 ft-lb or (lb-ft I assume this is the same).

    Now why can't it be equal to the 50,000 lb-ft in earlier calculation??
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Sep 25, 2012 #2


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    20 feet = 6.096 meters, not 6.
  4. Sep 25, 2012 #3
    Thanks. I have spent a day trying to find out even going to this particular Theorem in math which says some equations just can't be solved. So 0.1 is significant.. :)
  5. Sep 28, 2012 #4
    I'm trying to determine the deflection of the above beam. An engineer told me that deflections of beams from 6 meter to 7 meter is non-linear and high that is why many in constructions limit beam span to 6 meters for economy. What do you think of this. Anyway. Let's prove it by formula/calculations. On the following page is online calculator to solve for deflection.


    Supposed span of beam is 6 meters, w of beam is 4.23 Kn/m, distance of deflection measured is at middle, Modulus of Elasticity is 57000x sqrt (fc=21) or 261206, what is the formula or how do you determine the Moment of Inertia so I can input it to the online calculator above, thanks?
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