I'm genuinley grateful, I've been dropping marks on these truss questions at every attempt. This and the rest of your videos are incredibly helpful, not to mention the website itself.
Hi Tim, I am not convinced with the second one, surely the whole bottom chord will be in tension, the way you have solved it the truss would be unstable with the zero force members you have identified as there is nothing stoping the roller support from sliding towards the right
As per the explanation at 7:31, the bottom chord cannot be in tension in the outer spans because of the roller support on the right. How else would you get the forces to resolve at the node? I agree that towards the centre of the truss the bottom chord carries axial tension, but not at the sides.
@@TheStructuralExam Thanks Tim, this is a concept im really struggling with. In the office I can easily identify zero force members by creating a model. Will I ever at any point be required to identify zero force members in this manner (in real life or during the exam) ?
@@RC-jx9nw What do you mean by "I can easily identify zero force members by creating a model"? If you put in a wrong model you get wrong answers. You can never use a computer to check your work, you must be the one checking the computer. It is important to understand WHY it is zero force to begin with, as this video attempts to explain. You should only ever use computers for quantitative work, but for qualitative work you need to be able to understand the behaviour of the structure without a computer. This is further tested at IStructE exams and CEng interviews, including the ones I examine. There is no hiding from it sadly.
@@TheStructuralExam For example, I can model a truss with pinned jointed connections and apply a generic load as per your diagram, after running the analysis I can see which members have 0 axial force and can identify the zero force members this way. I can also do this quantitively by hand using the method of joints resolving force joint by joint and you will find the zero force members this way. Also, for Q2 would it be correct to say that in practice this wouldn't actually work because as the truss bends, due to secondary order effects the roller support will start sliding towards the right so it is unstable. The non zero force members you have identified will very quickly start to go into tension as the load increases.
@@RC-jx9nw Q2 is a simply supported truss! It is a stable, determinate structure. There are no mechanisms involved! As mentioned, roller supports do not allow horizontal reactions therefore there is no axial force in the outer bottom chords under this load case.
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I'm genuinley grateful, I've been dropping marks on these truss questions at every attempt. This and the rest of your videos are incredibly helpful, not to mention the website itself.
Hi Sir... Would you please mention when will you post the next new video?
Thank you very much for your service!
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Hi Tim, I am not convinced with the second one, surely the whole bottom chord will be in tension, the way you have solved it the truss would be unstable with the zero force members you have identified as there is nothing stoping the roller support from sliding towards the right
As per the explanation at 7:31, the bottom chord cannot be in tension in the outer spans because of the roller support on the right. How else would you get the forces to resolve at the node? I agree that towards the centre of the truss the bottom chord carries axial tension, but not at the sides.
@@TheStructuralExam Thanks Tim, this is a concept im really struggling with. In the office I can easily identify zero force members by creating a model. Will I ever at any point be required to identify zero force members in this manner (in real life or during the exam) ?
@@RC-jx9nw What do you mean by "I can easily identify zero force members by creating a model"? If you put in a wrong model you get wrong answers. You can never use a computer to check your work, you must be the one checking the computer. It is important to understand WHY it is zero force to begin with, as this video attempts to explain. You should only ever use computers for quantitative work, but for qualitative work you need to be able to understand the behaviour of the structure without a computer. This is further tested at IStructE exams and CEng interviews, including the ones I examine. There is no hiding from it sadly.
@@TheStructuralExam For example, I can model a truss with pinned jointed connections and apply a generic load as per your diagram, after running the analysis I can see which members have 0 axial force and can identify the zero force members this way. I can also do this quantitively by hand using the method of joints resolving force joint by joint and you will find the zero force members this way.
Also, for Q2 would it be correct to say that in practice this wouldn't actually work because as the truss bends, due to secondary order effects the roller support will start sliding towards the right so it is unstable. The non zero force members you have identified will very quickly start to go into tension as the load increases.
@@RC-jx9nw Q2 is a simply supported truss! It is a stable, determinate structure. There are no mechanisms involved! As mentioned, roller supports do not allow horizontal reactions therefore there is no axial force in the outer bottom chords under this load case.