7.4 THE METHOD OF SECTIONS

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Basically: cut out a part of a truss, and then treat it as if it is a rigid body. When done wisely, this allows simplified solutions. The alternative is using the method of joints to find all (or many) of the forces in the frame.

Keep in mind that while moments are very popular with the method of sections, it can also be used for forces as well.

Consider an example where we want to find forces in a structure, ([Hibbeler, 1992], prob 6-24, pg. )

7.4.1 Practice Problems

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1. Determine the tensions/compressions in members EF, JK and HJ for the bridge truss shown below. The method of sections is recommended.

2. Find the forces in FG and DF (indicate tension or compression). Assume all joints are pinned.

3. In the frame pictured below, find the forces in DE, EF, DF and CB.

4. Find the forces in members CD, CF, CG. (You will benefit most by using the method of sections to solve this problem)

(ans. CD= 11.25KN(C), CF= 3.21(T), CG= 6.8(C))

5. In the frame below members DL and EL can support up to 1kip of tension or compression, find the maximum load P that may be applied. (Hint: you could mix the method of joints and sections)

6. Find the forces in JM and KM.

7. Find the tension/compression in members CE, DE.

7.4.2 References

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Hibbeler, R.C., Engineering Mechanics: Statics and Dynamics, 6th edition, MacMillan Publishing Co., New York, USA, 1992.