Simple truss question driving me crazy =/

[hachi_roku]

simple truss question....driving me crazy!! =/

Homework Statement

the question wants the forces in all the members of the truss. i have found that members bc and ef are zero force members...i cannot find the others. I am at a loss because ive tried all the methods i have been taught and the answers are not right. I NEED HELP!!!

Homework Equations

The Attempt at a Solution

i have tried to take sum of forces in x prime (the AD axis), for members tbd and tcd....and also have tried doing tab and tda.

 

[hachi_roku]

just in case you can't see the pic....the lengths from the pin to d is 16 ft, while the horizontal distance from the pin to d is 12 ft. the load at b is 2000 and the load at d is 3000. i have found a_y to be 3500 lbs

 

[hachi_roku]

the angles between ab and ac is 30, while the angles between db and dc is 30

 

[mathmate]

If you want to show the picture fast, you can upload it to picture-servers and post the link. Sometimes the approval can take a day.

 

[mathmate]

> i have found that members bc and ef are zero force members
Very good!

> i have found a_y to be 3500 lbs
Very good. (note: in engineering/mechanics terms, the reaction at A is usually shown as Ra) Ay is understandable, you are probably doing a physics course.

> ive tried all the methods i have been taught and the answers are not right.
What methods have you been taught? Does 'triangle of forces' mean anything to you? How about 'summing forces in x and y'?
Can you post the calculations and the answer you've got using your method, and the answers given. Sometimes the answers are misprinted or mis-transcribed by the teacher or the student.

Hope to hear from you soon.

 

[djeitnstine]

Hachi_roku, I have an idea. If the question really wants ALL of the forces in each member, start out by finding the reaction forces -of course. Then start by resolving the unknown forces in the pins A, B C, D... in that order. Surely every force in each member will become apparent.

 

[mathmate]

Hint:
with perfect symmetry of loading and geometry, you only have to work out half of the structure, for example ABCD. The other half (DEFG) can be replaced by a single horizontal reaction. Note, however, that the reaction at A has a vertical and horizontal component.
The horizontal reaction at D can easily be obtained by taking moments about A.