Calculate Reaction Forces at B & D: Beam Bending Homework

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Homework Help Overview

The discussion revolves around calculating the reaction forces at points B and D on a beam subjected to various loads. The problem involves understanding the principles of static equilibrium, particularly focusing on vertical forces and moments.

Discussion Character

  • Exploratory, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • The original poster attempts to set up equations based on the sum of vertical forces and moments about different points. They question the correctness of their moment equations and seek clarification on the process.
  • Some participants point out errors in the original poster's moment calculations and emphasize the importance of sign conventions for moments.
  • There are discussions about the correct application of moment arms and the treatment of distributed loads.
  • Questions arise regarding the sign conventions for moments and how they should be consistently applied.

Discussion Status

The discussion is ongoing, with participants providing feedback on the original poster's calculations and clarifying concepts related to moments and forces. There is a focus on ensuring that the conventions used are consistent, and some guidance has been offered regarding the setup of moment equations.

Contextual Notes

Participants are navigating through the complexities of moment calculations and the implications of sign conventions, indicating a need for clarity on these foundational concepts. The original poster expresses uncertainty about the application of these principles in their calculations.

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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.
 

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

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