# Truss Analysis

1. Apr 23, 2013

### MMCS

Attached is a picture of the truss. I am trying to calculate the member forces.

I know a moment is = force * perpedicular distance, but in this case, for example, if i am taking moments about A, Would the equation be

-(1*2KN)+(1*c) = 0

or using an alternative perpedicular distance

-(2.4*2KN)+(0.8*c) = 0

For my preliminary working, assuming i have used correct perp distance i have

ƩFx = 0
ƩFy = 0
ƩMoments about a point = 0

Ay + By - 2KN = 0

Momments about A = -(2KN * 2.4) + 1*ReactionC = 0
-4.8KN = reactionC

Sub into first eq

Ay +(-4.8) -2 = 0
Ay = 6.8

To find out the reaction in member AB

I have

6.8 - Tab Cos(tan-10.8/1) = 0
Tab = 6.8/Cos(tan-10.8/1) = 8.738

I have the answer to be 2.56

#### Attached Files:

• ###### truss.png
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Last edited: Apr 23, 2013
2. Apr 23, 2013

### SteamKing

Staff Emeritus
In taking moments, the 2kN force has the same direction as the vertical distance on 1m, so its moment will be 0. If you get confused, you can always calculate M = r x F, where x is the cross product operator.

3. Apr 23, 2013

### MMCS

Im dont quite understand sorry, i would understand if the force was acting through the point i am taking momoments about but how can the 2KN force x distance away not cause a moment

4. Apr 23, 2013

### SteamKing

Staff Emeritus
5. Apr 23, 2013

### MMCS

Using the value of of 4.8 but the same working i end up with a final equation of

2.8/cos(38.66) = 3.59

6. Apr 23, 2013

### MMCS

Ok i realise where my confusion was on this problem thanks for your help