Force Resolving Structural Engineering Problem

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SUMMARY

The discussion focuses on applying the Method of Joints in structural engineering to analyze forces at various joints in a truss system. The user demonstrates calculations for joints A and B, determining forces such as Fab, Fac, and Fbc, and identifies compressive and tensile forces based on the sign of the results. The user seeks clarification on the determination of negative values in their calculations and questions the feasibility of a zero force at joint B despite an external load. Key references include resources from PSU and UW-Stout for further understanding of the Method of Joints.

PREREQUISITES
  • Understanding of static equilibrium principles
  • Familiarity with the Method of Joints in structural analysis
  • Basic knowledge of truss structures and force analysis
  • Ability to perform vector calculations involving sine and cosine functions
NEXT STEPS
  • Study the Method of Joints in detail using resources from PSU and UW-Stout
  • Practice calculating forces in truss systems with various loading conditions
  • Learn about the significance of negative and positive force values in structural analysis
  • Explore advanced topics in structural mechanics, such as the Method of Sections
USEFUL FOR

Students in civil or structural engineering, professionals analyzing truss systems, and anyone seeking to understand the principles of static equilibrium in structural design.

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Thanks for the help but I'm still a little stuck. I can work out the magnitudes fine but I'm a bit confused as to where the negatives should be and what they actually represent. This is my working so far (the joints are labelled A-K from left to right):

Fab = Force along member AB

Consider Joint A:

Sum of Verticals at A=25+Fab*sin60=0
Fab*sin60=-25
Fab=-28.8N

Since this is negative it is a compressive force.

Sum of Horizontals at A=Fab*cos60+Fac=0
-28.88*cos60=-Fac
Fac=14.4N

Since this is positive it is a tensile force.

Consider Joint B (This is where I get confused:

Sum of Verticals at B=Fab*sin60+Fbc*sin60+Fbd*sin0=0
28.88*sin60+Fbc*sin60+0=0
Fbc*sin60=-28.88*sin60
Fbc=-28.88

Since this is negative it is a compressive force.

Sum of Horizontals at B= Fab*cos60-Fbc*cos60+Fbd*cos0=0
28.88*cos60-Fbc*cos60+Fbd=0
-Fbc*cos60+Fbd=-28.88*cos60
Fbd=0N

Have I got all of the negatives right because I really have no idea how they are determined and also, is it even possible for Fbd to have a force of 0N on this bridge given that there is a force of 50N on the center of the bridge?

Thanks for any help.
 

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