Calculating End Reactions: How to Sum Moments of All Forces About a Point

[manich43]

Hi,I am trying to calculate the end reactions for the beam shown in the diagram attached.

I have got this far but know I am going wrong somewhere.

To check ,RA + RB should equal the total load,point and UDL so I am told.

Can someone point me in the right direction please.

Many Thanks...Mark

 

[PhanthomJay]

First check your units for moment; A 70 kN force with a 2 m perpendicular moment arm produces a moment of 140 kN-m.

Your calculation for the moment from the distributed load is wrong...you forgot to multiply the total force from that distributed load by the lever arm distance from its cg to the point in question. Always check your results for force equilibrium (sum of all forces in vertical direction = 0 ).

 

[manich43]

Hi,Can you be more specific,excuse my ignorance but this is the first time I have encountered this kind of problem.Can you show me an example?

Thanks...Mark

 

[PhanthomJay]

Hi,Can you be more specific,excuse my ignorance but this is the first time I have encountered this kind of problem.Can you show me an example?

Thanks...Mark

When, in deteremining end reactions, you sum moments of all forces about any point of an object in equilibrium, the moments must sum to zero, paying careful attention to cw and ccw moments (plus and minus signs).
A moment of a force is the force times the perpendicular distance from the line of action of the force to the point. When the force is uniformly distributed (w=kN/m), you must first get the total resultant force from that distributed load ( w times the length over which it is distributed, which you have done), and then apply that force at the center of gravity of the distributed load (its midpoint for a uniformly distributed load) and then determine the moment from that resultant of the distributed load.