Reaction force calculations and questions

[whlock]

Here is a simple one...

I am trying to calculate force at A & B.

The L shape is a simple beam.

so summing the force at the center of the beam
A-B=1700 -------(1)
summing the moment at the center of the beam, assuming anti-clockwise is positive
-3.699A - 5.676B = 9.498 (1700) ------(2)

solving those equation give me
A = -693#

& B = -2393#

so that means I will have a force of 693# at A pointing downward and 2393# at B pointing upward. ie..direction of force for A & B flipped to the drawing.

Assuming the method I do this is correct, wouldnt the force direction should be as the diagram shown. The 1700# will try to tilt line A-B. Why are the calulcated force shown different?

 

[Denial]

Your arithmetic is fine.[ ] It's your diagram that has confused you.[ ] This should show the forces the supports apply to the structure, not the forces the structure applies to the supports.[ ] As Prof Newton explained to us about 300 years ago, courtesy of his Third Law, the two set of forces are equal and opposite.

Alternatively, stick with the force-directions as shown on your diagram, and listen to your arithmetic.

 

[rb1957]

math fine ... pic fine too (if your consider Ra and Rb are loads ... the applied load will push down at B and lift up at A)

Quando Omni Flunkus Moritati

 

[desertfox]

Hi

Your maths workout just fine, I find it easier to take moments about either A or B and eliminate one unknown reaction.
You made an assumption about which direction the reactions were in at the start and if your assumption is incorrect then the force gets the opposite sign.
You can see easily which direction the reaction forces have to go to maintain equilibrium by assuming your standing a point B which is a pivot point and the 1700 acting clockwise, it should be clear the reaction at A needs to point down, conversely the reaction at B when standing at A needs to go up.

 

[chicopee]

I did not do the math, however, I can not agree with the accuracy of the numbers that the above responders have indicated to be correct since the drawing is incomplete. Had I presented a project illustrated by the OP's drawing to my professor Mr. Smith of Montana State College back in the early "60's, you can be assured that my grade would have been an "F".

 

[hydtools]

I would guess Mr. Smith would have also instructed you on the incompleteness of your answer as it has no corrective response included with the critique.

Ted

 

[rb1957]

guess i'm not seeing what's not there ? (and yes, I've read Lewis Carroll)

are you complaining about the detail of the part between A and B ?

as far as i can see all the necessary information to calculate the reactions at A and B is there ...

Quando Omni Flunkus Moritati

 

[chicopee]

My response being incomplete, I agree but I think that anyone who has taken ME as a curriculum, the deficiency in the drawing is apparent.

 

[rb1957]

fixity at A & B ?

Quando Omni Flunkus Moritati