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Hi, I'm having some problems with this truss question that I have in my homework. So far this is my solutions.

I worked out my solutions to the reaction forces but don't know if they're correct.

The next step is the find the internal forces ZB YB SU SV TV RT RS QS JL JK NP MP HO HM IO MO

Is there an easier way to find all these? e.g. are some of them the same?. Wouldn't using either method of joints or sections be too tedious here?. Thanks.

[![enter image description here][2]][2]

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  • $\begingroup$ Check your zero force members. A good way to visually see if they make sense is to remove them from the drawing. If you remove ZB and ZX, you only have a pin at W. This should visually show that these members can't be zero force. $\endgroup$ – hazzey Oct 10 '15 at 13:16
  • $\begingroup$ Lest I am mistaken, a truss where all the nodes are hinged will always be statically determinate. $\endgroup$ – Wasabi Oct 10 '15 at 14:38
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    $\begingroup$ @Wasabi, a pin-jointed truss can be indeterminate if there are 'redundant' elements (you can remove them without the truss collapsing). ae.msstate.edu/vlsm/truss/statically_det_indet_trusses/… $\endgroup$ – atom44 Oct 10 '15 at 18:58
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It may be worth reconsidering your deduced zero force members. But for this question it is not really necessary to determine which are zero force members in advance as this will come out in the analysis.

It is not important if the truss is determinate or not in order to get the member forces your are interested in (highlighted in red). Approach this question using the method of sections. I have marked sensible section cuts in green. You can get the member forces of JL and JK by considering the equilibrium of joint J. It is worth taking note that the member forces requested hints at the appropriate solution method. Figure1

For example, in order to get the member forces RT, RS, and QS make cut 3: Figure2

Notice that since you have the reaction forces at A already the member forces RT, RS, QS can be solved by equilibrium.

Getting member forces for NP, MP, and MO is a little bit more tricky, but once you have member force for MH (by making cut 4) you can make the cut marked in blue.

It is a bit tedious using method of sections, but there isn't a faster method that I am aware of. Even modelling this in a structural analysis software package would probably take longer ...

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  • $\begingroup$ It would be valuable for you to be able to check your own work. When you practice as an engineer how will you know what you have done is correct? Does your answer make sense? What checks could you do to make sure? In principal if you apply the correct method you should get the correct answer. Without intending to sound rude, I don't feel that it is appropriate to ask us to check your answers here. However, if you don't understand how to check if a truss is determinate or how to calculate reaction forces then please do ask. $\endgroup$ – atom44 Oct 10 '15 at 19:18
  • $\begingroup$ If you're looking for some verification to check your answers some easy-to-use and quick structural analysis software like SkyCiv Structural 3D comes in very handy. I quickly set up your truss system in about 10 minutes and found these results as the reactions. I'm quite familiar with the software though - so for a rookie it might take 20 minutes. Certainly quicker than by hand I'd imagine anyways. $\endgroup$ – pauloz1890 Oct 11 '15 at 7:00

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