How do I find the force in each member?

  • Thread starter Thread starter shreddinglicks
  • Start date Start date
  • Tags Tags
    Force Member
Click For Summary

Discussion Overview

The discussion revolves around determining the forces in each member of a truss structure, specifically identifying whether the members are in tension or compression. Participants are addressing a homework problem that involves applying principles of static equilibrium, trigonometry, and free body diagrams.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expresses difficulty in obtaining correct answers and seeks guidance on how to approach the problem.
  • Several participants suggest using trigonometry to calculate angles and summing forces in both the y and x directions to find reaction forces at specific points.
  • There is discussion about the torque calculations around point A and how to relate them to the forces in the truss members.
  • Participants mention the need to set up equations for joints B, D, and E, noting that there are multiple unknowns and equations required to solve for the forces.
  • Some participants clarify that just because members CD and DE are collinear, it does not necessarily mean they have the same force value, especially when considering the presence of other members at joint D.
  • There is a suggestion to draw free body diagrams (FBD) for better visualization of the forces acting on the joints.
  • One participant successfully finds the answer after engaging with others, indicating a collaborative effort in problem-solving.

Areas of Agreement / Disagreement

Participants generally agree on the need to use equilibrium equations and free body diagrams to analyze the truss. However, there is disagreement regarding the treatment of collinear members and whether they can be assumed to have equal forces. The discussion remains unresolved on some technical aspects, particularly concerning the calculations at joint D.

Contextual Notes

Some participants note that the calculations depend on correctly identifying all forces acting at each joint and that assumptions about collinearity may not hold in all cases. The discussion highlights the complexity of solving for multiple unknowns in a truss system.

Who May Find This Useful

This discussion may be useful for students studying statics, structural engineering, or anyone interested in understanding the analysis of truss structures and the application of equilibrium principles.

shreddinglicks
Messages
225
Reaction score
7

Homework Statement


Determine the force in each member of the truss. State if the members are in tension or compression.
P1 = 450 lb, P2 = 600 lb

Homework Equations

The Attempt at a Solution


No matter what I try I get wrong answers. I need someone to get me going in the right direction.
 

Attachments

  • Hibbler.ch6.f6_new.jpg
    Hibbler.ch6.f6_new.jpg
    8.5 KB · Views: 1,741
Physics news on Phys.org
First use trigonometry to calculate all the angles of the triangles, then sum the forces in the y direction and sum the moments about point a to solve for your reaction forces at a and c. then you can make cuts around different joints and do free body diagrams to solve for all the internal forces.

Edit: also sum the forces in the x-direction to solve for the reaction force in the x direction at a
 
Last edited:
CodyZim said:
First use trigonometry to calculate all the angles of the triangles, then sum the forces in the y direction and sum the moments about point a to solve for your reaction forces at a and c. then you can make cuts around different joints and do free body diagrams to solve for all the internal forces.

Edit: also sum the forces in the x-direction to solve for the reaction force in the x direction at a

How do I sum the forces in the y direction when I only know one of the forces, the 600 lbs?
 
I did the torque about A. M = -450(3.46)+Fc(6)

used:
tan(30) = 0pp./6

to get 3.46 ft.

M = 259.5 ft*lb
 
shreddinglicks said:
How do I sum the forces in the y direction when I only know one of the forces, the 600 lbs?
Sum the moment about point A and set equal to zero to get an equation in terms of only the y reaction at point C.
 
shreddinglicks said:
I did the torque about A. M = -450(3.46)+Fc(6)

used:
tan(30) = 0pp./6

to get 3.46 ft.

M = 259.5 ft*lb
259.5 is the value of the force at C in the y direction not the torque(Torque is the same as moment)

Knowing that, sum the forces in the y direction and solve for point a
 
This is the work done so far, thanks in advance you have been great help.

My last question, how do I find the force at BE?
 

Attachments

  • CCI11132014.jpg
    CCI11132014.jpg
    30.6 KB · Views: 830
shreddinglicks said:
This is the work done so far, thanks in advance you have been great help.

My last question, how do I find the force at BE?
Looks like you need to set up the equations for the joints B, D, and E. You'll have three unknowns: Fbd, Fbe, and Fde. You'll also have enough equations to plug in for and solve for them. It won't be as analytic as finding the other joints, but it will work. If you need help with that or any more further explaining let me know
 
CodyZim said:
Looks like you need to set up the equations for the joints B, D, and E. You'll have three unknowns: Fbd, Fbe, and Fde. You'll also have enough equations to plug in for and solve for them. It won't be as analytic as finding the other joints, but it will work. If you need help with that or any more further explaining let me know

I'm confused since CD and DE are collinear they both have the value of 519lb. DB is 0 because it is part of a collinear member. So isn't my only unknown BE?
 
  • #10
shreddinglicks said:
I'm confused since CD and DE are collinear they both have the value of 519lb. DB is 0 because it is part of a collinear member. So isn't my only unknown BE?
Give me a second I'll work the math out really quickly.
 
  • #11
CodyZim said:
Give me a second I'll work the math out really quickly.
Your joint D math is incorrect, you still have to take into account that there are three members, and you have to include DE in your x and y calculations

If you set up equations in the x and y for points B, D, and E, you should have three unknowns and three equations, and be able to solve for them. Each point has two trusses that are unknown. Just because CD and DE are colinear does not mean they are the same value. Only if there was no point in between then with a truss going down would they be equal to each other always
 
  • #12
CodyZim said:
Your joint D math is incorrect, you still have to take into account that there are three members, and you have to include DE in your x and y calculations

If you set up equations in the x and y for points B, D, and E, you should have three unknowns and three equations, and be able to solve for them. Each point has two trusses that are unknown. Just because CD and DE are colinear does not mean they are the same value. Only if there was no point in between then with a truss going down would they be equal to each other always

Could you elaborate? I want to make sure I understand this. The answer key says CD and DE are equal, and going by my notes it says collinear members are equal.

As far as setting up a systems, Basically you are saying to take my x and y of each point and combine them so I get 3 equations?
 
  • #13
What he is saying is that you need to take in account all three members attached to joint D because we have three internal forces compressing or in tension around joint D. Draw a FBD of joint D with all the internal forces around it. If only know one internal force you can make a system of equations to solve for the two unknowns. Sum of the forces in x and sum of the forces in y to make the two equations.
 
Last edited:
  • #14
ME_student said:
What he is saying is that you need to take in account all three members attached to joint D because we have three internal forces compressing or in tension around joint D. Draw a FBD of joint D with all the internal forces around it. If only know one internal force you can make a system of equations to solve for the two unknowns. Sum of the forces in x and sum of the forces in y to make the two equations.
^ This
Just sum the x and y direction for points B,D, and E. you won't be able to solve for the internal sections with just one point. you need to set up multiple equations and solve in terms of one force, and plug that into another equation
Example:

Lets say you solve for the x and y in point B and get
Fbe = Fbc - Fbd (these are just theoretical values, NOT the actual answer)

Solving point d, we'd get:
Fdb = Fde + 2*Fdc

Solving Point E gives:
Fed = Fea + Feb + 450

You know Fbc, Fdc, and Fea. Other than that, just plug one value in and solve for the three values with the three equations
 
  • #15
So you can find Fed and Feb with a system of equations, but you need to solve for bata and alpha
 

Attachments

  • #16
I did end up getting the answer. Thanks guys for putting in the time to help me.
 

Similar threads

Engineering Loads on a shaft with V-Belts
  • · Replies 2 ·
Replies
2
Views
3K
How Do You Calculate Distributed Load on a Roof Truss Due to Snow?
  • · Replies 3 ·
Replies
3
Views
5K
Engineering Resultant Force and Direction using Parallelogram Law
  • · Replies 6 ·
Replies
6
Views
4K
Calculate Force Member in Truss
  • · Replies 2 ·
Replies
2
Views
2K
Finding Forces in Member AE & AB: Exam Follow-up
  • · Replies 1 ·
Replies
1
Views
3K
Statics - Compression and tension forces in a truss
  • · Replies 2 ·
Replies
2
Views
7K
Checking My Work: Solving for Force in Member GH
  • · Replies 3 ·
Replies
3
Views
2K
Truss Method of Joints: Solving Unequal Member Forces
  • · Replies 2 ·
Replies
2
Views
2K
How Do You Calculate Forces in Truss Members?
  • · Replies 1 ·
Replies
1
Views
2K
How do I determine the force in each member of the truss?
  • · Replies 3 ·
Replies
3
Views
4K