#1
Nov2806, 12:32 PM

P: n/a

If there is a cross member of a bridge with two forces (F and R) acting as contacts at the ends of the member, is there a way to use equilibrium conditions to write the relationships between the forces? The member makes an angle theta with the horizontal (upward and to the right) and has a center of mass at its geometrical center, with the origion (O) chosen at the point where R is acting (on the left end of the member). I'm just not sure how to approach this. All I know is that net force and net torque are both equal to zero, but I have no idea where to go from here. Thanks.




#2
Nov2806, 12:49 PM

P: 226

drawing a FBD would help us interpret the problem




#3
Mar2508, 09:38 PM

P: 1

Hey, I know that this post is from a while ago, but I have the same problem...
Attached is an FBD of the problem. 



#4
Mar2708, 03:48 PM

Sci Advisor
HW Helper
PF Gold
P: 5,966

Conditions for EquilibriumIf the weight of the member, acting thru its c.m., is considered, it is often customary to split its weight half and half, and apply that value to the adjacent joints. You still get the same relationships of the forces. If the weight of the member is significant and must be considered as it actually acts thru its c.m., you still get the same equilibrium relationships between F and R, but you no longer have the Fy/Fx = tan theta relationship, because you no longer have a pure truss, you have a frame with shear loads introduced as well as axial loads. 


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