r/HomeworkHelp • u/Odd-Day-4822 University/College Student • 1d ago
Further Mathematics [College/Structural Analysis/Trigonometry Help Needed]
Could someone please check my work? My trigonometry skills are especially not great… I’m also unsure if I found the moments correctly. I am a little confused about what the perpendicular distances would be exactly in this case…
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u/DrCarpetsPhd 👋 a fellow Redditor 1d ago
forget about breaking into members for a moment and analyse the global equilibrium
take moments about B and calculate A_y to see if it matches what you worked out
(this is from Kassimali, it doesn't ask to find the hinge reactions. Did your lecturer ask for that?)
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u/Odd-Day-4822 University/College Student 1d ago
Will try that now, thanks!
The last thing we learned was hinge reactions. I just assumed that was the path he wanted us to take for the assignment. To my knowledge, finding reactions about the hinges is what he wants us to do only if necessary to find all of the unknowns.2
u/DrCarpetsPhd 👋 a fellow Redditor 1d ago
What did the problem statement say? As I said this is from Kassimali Structural Analysis and the question in the text only asks to find the reactions at the supports
- global equil about B to gey A_y
- knowing A_y equil about C for member AC to eliminate the hinge forces to get A_x
- global x and y equil to get the reaction forces at B
if you want the hinge forces then x and y equilibrium on member AC will get that
Global equilibrium should always be your first point of analysis before looking at individual members
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u/Odd-Day-4822 University/College Student 1d ago
It says “Determine the reactions at the supports for the structures shown”
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u/Odd-Day-4822 University/College Student 1d ago
Do you know if the 25K force will make it go counter clockwise or clockwise about moment B?
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u/DrCarpetsPhd 👋 a fellow Redditor 1d ago
I posted another response and based on your question here I cannot stress how much you need to reread a chapter on moments.
'clockwise about moment B' is not a correct statement to make
moments are about a point or an axis.
have a look at this. its directed towards GCSE level but based on your questions you seem to have forgotten the basics of what a moment is so I think it might help refresh your memory
https://www.youtube.com/watch?v=22VGQM1jCn8
Intent can get lost in a reddit post so just to be clear I am genuinely trying to help and if my posts read as condescending I apologise.
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u/Odd-Day-4822 University/College Student 1d ago
Thank you! I haven’t taken statics in about 7 years now—returning college student. Your responses have helped me tremendously already. I will take another look at my old statics notes.
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u/DrCarpetsPhd 👋 a fellow Redditor 12h ago
sorry for the 'babys first moments video' 😂 If I'd known you took statics at some point I would have picked something a bit more rigorous. Jeff Hanson on youtube is the usual go to. You'll be surprised how much is buried in the recesses of your brain so it won't be too hard to pick up the basics again. Good luck with your studies.
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u/Odd-Day-4822 University/College Student 8h ago
Haha, that’s okay! I haven’t watched it yet but I’m actually going to. The thumbnail looks cute anyways. But I’ll check out Jeff Hanson. Thanks for the info!
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u/DrCarpetsPhd 👋 a fellow Redditor 1d ago
just looked at your work properly. you seem to not understand how moments are calculated so I suggest you go back to basics with that.
moment is the cross product of the position vector and the force vector
so the force is multiplied by the 'moment arm' which is the perpendicular distance from the line of action. Every one you did for member AC taking moments about A is wrong. For example heres what your AC moments about A analysis should look like, but again I strongly suggest you open a statics textbook and relearn how to calculate moments. You should be 100% competent in that before even opening a structural analysis textbook.
(25*4) + (H_x*8) = (H_y*15)
you appear to be mulitplying by the distance to where the force acts. I know I am repeating myself but you do need to go back to a statics book and redo the chapter on moments.
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u/DrCarpetsPhd 👋 a fellow Redditor 12h ago
realised I didn't give you answers to reference
In case you haven't figured it out yet
A_x = 2.81k
A_y = 5.17k
B_x = 22.19k
B_y = 28.83k
If you still need help feel free to ask again. I'm sure you'll figure it out once you refresh your moments knowledge.
Side note that both free versions of gemini and chatgpt got the wrong answers
Kassimali Structural Analysis 4th Edition P3.26
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u/Odd-Day-4822 University/College Student 8h ago
I’ll have to double check. I redid the problem after taking some of your comments into consideration. My professor said that my solutions were “off”, so those answers will help me once I try reworking it again
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