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Maximum tensile and compressive bending stresses in a beam 
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#1
Jan2111, 04:44 PM

P: 43

1. The problem statement, all variables and given/known data
Draw the shear force and bending moment diagrams for the beam shown in Fig. 1 below. Determine the maximum tensile and compressive bending stresses and the positions at which they occur. The beam’s crosssectional area is shown in Fig. 2. http://i1225.photobucket.com/albums/..._jon_19/q2.jpg 2. Relevant equations Bending Stress = Mc/I Where I is inertia, c is distance from neutral axis, M is the bending moment 3. The attempt at a solution I have drawn the shear and bending moment forces and worked out the moment of inertia in the Tsection. But I am unsure how I work out the maximum tensile and compressive bending stresses. How do I apply this. Thanks. 


#2
Jan2111, 05:23 PM

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#3
Jan2211, 06:08 AM

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#4
Jan2211, 08:57 AM

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Maximum tensile and compressive bending stresses in a beam
dvep: You do not need to compute Iy, because there is no bending moment about the y axis. Your Ix value currently appears incorrect. Try again. Yes, M is the maximum moment in the bending moment diagram.



#5
Jan2211, 11:00 AM

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dvep: Your Ix value appears incorrect. Try again. Regarding the units of M, I recommend converting all units to N, mm, and MPa. Using N and mm, stresses will be N/mm^2, which is called and written MPa. Also, the bending stress formula is sigma = M*y/Ix.



#6
Jan2211, 12:12 PM

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dvep: Nice work. You forgot to use y = 22 mm, and y = 38 mm. Try that again. Tensile stress is positive.
By the way, for long numbers having five or more digits, the international standard says you can write the digits in groups of three, separated by spaces. E.g., 15 000 000 N*mm, instead of 15000000 N*mm. See the international standard for writing units (ISO 310). 


#7
Jan2211, 12:23 PM

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No, you forgot the negative sign in the bending stress formula, this time. Try again. Also, typically use asterisk for the multiplication symbol, instead of "x," because "x" can easily be confused with the variable x.



#8
Jan2211, 12:40 PM

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dvep: Switch the words tensile and compressive, because tensile stress is positive. Also, in post 9, your units on M should be N*mm, not N*mm^2. Also, see the last paragraph of post 3087641, regarding rounding your final answer.



#9
Jan2211, 12:46 PM

P: 43

Thank you nvn, you were very helpful. 


#10
Jan2211, 01:16 PM

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dvep: By the way, it is not allowed to delete your posts, the way you did, above. They call this abuse of the Edit feature. We will hopefully let it slide this time, since you are doing such excellent work. But I just wanted to warn you, so you can stay out of trouble, next time.
You did excellent work on your homework. 


#11
Jan2211, 01:23 PM

P: 43

Thank you again for you help. 


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