Calculate Reaction Forces at B & D: Beam Bending Homework

[dvep]

Homework Statement

Calculate the reaction forces at B and D

See attached jpeg.

The Attempt at a Solution

sum of vertical forces

By + Dy = 7 + (9*5)

Do I then solve for the moment about D or A or B?

for moment about D i have: -Dy -7 -(9*5)+9By+(12*2)=0

is this correct, please explain to me the process of and how I should go about these problems

Any advice is appreciated, thankyou.

 

[SteamKing]

The moment equation you have written is incorrect. For example, what is the moment of the 7 kN force about D? Remember, the sign of the moment is important. The moment of the distributed load is also incorrect. I don't know why you are multiplying the applied moment of 12 kNm by 2.

 

[dvep]

The moment equation you have written is incorrect. For example, what is the moment of the 7 kN force about D? Remember, the sign of the moment is important. The moment of the distributed load is also incorrect. I don't know why you are multiplying the applied moment of 12 kNm by 2.

When the moment is clockwise to the left of the point it is positive?

moment about D:

9By +12 - (9*5*2.5) -(7*4) = 0

Is that correct now?

 

[SteamKing]

On the moment of the distributed load, remember the center of the load must be taken about point D. Check your moment arm.

 

[dvep]

On the moment of the distributed load, remember the center of the load must be taken about point D. Check your moment arm.

9By +12 - (9*5*6.5) -(7*4) = 0

By= 34.28

I'm am still a bit unclear on the moments when they are positive and negative, is what I said before correct, when they are to the left of the point and clockwise , they are positive?

 

[SteamKing]

The convention chosen doesn't matter, as long as it is consistently applied. I use the convention where the horizontal direction is positive to the right, vertical is positive upward, and the counterclockwise moment about the origin is positive. This is a right-hand system, and it is used later on in other areas of physics.