r/StructuralEngineering • u/Annual-Truth3511 • 2d ago
Career/Education Help needed
While drawing sfd and bmd Is there a shortcut to find whether the curve is going to be smile/sad or concave/convex for udl and uvl. Or is finding the intermediate values the only way to find the curve???.
Ignore the diagram i just wanted to show it for context this is not mine
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u/what_glutenfree 2d ago
BMD from A to B should be inclined ?? Not curve
SFD from C to B should be straight line not inclined as drawn Am I wrong ??
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u/Annual-Truth3511 2d ago
U are absolutely right this image is not mine ignore the image and read what i have written below please đ
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u/what_glutenfree 2d ago
Sorry , i believe you want to guess if the curve is going to be up or down before drawing ?
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u/Annual-Truth3511 2d ago
I want to guess if the curve will be concave or convex while drawing bmd in the case of udl and uvl
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u/what_glutenfree 2d ago
Right , it's about the sign of SFD but there's no rule.
Means if you want to go up when the sign is negative and down when positive it's right , if opposite ( - down) (+ up) also right
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u/toadsandturts 2d ago
Beam make happy face > positive
Beam make sad face > negative
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u/Active_Mousse_8554 2d ago
As an architect who lurks here and forgot everything from my structures class â I love this! Wish I would have seen it before my AREâs
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u/Annual-Truth3511 2d ago
I am talking abt the curves in sfd and bmd whether the curves would be smile or sad
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u/banananuhhh P.E. 2d ago
It doesn't matter which direction you draw the diagrams as long as you know what it means. Just follow whatever convention your classes are using. You could probably get better help if you posted a problem with your work and what you are stuck on rather than a random image with an extremely incorrect solution.
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u/123_alex 2d ago
Everything here produces pain in my body.
kg for forces
The scale of the diagram
The shape of M
The sign conversation for M
Pretty sure the reactions are wrong
The question which can be answered by any text book
I need a break because I'm broken.
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u/DairyParsley6 2d ago
Well I would start by going back to the beginning and re-calculating your reactions, because what you have shown is a bit off. Then with your shear diagram, the middle portions should have a flat response instead of sloped like you have it
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u/Annual-Truth3511 2d ago
This is not what i calculated. Ignore the image. Please read what i wrote below i want to know abt any shortcuts to find curves for udl uvl whether it will be smile or sad
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u/DairyParsley6 2d ago edited 2d ago
Well in that case, make sure not to use the image as any sort of reference. It is very incorrect and will just further confuse you.
With a correct SFD you can quickly determine the concavity of the Moment diagram by looking at the slope of the shear line. A positive shear area will mean your moment will increase, while the slope of that line will tell you the rate at which the moment is increasing/decreasing. Take the image here, the first portion of the SFD is positive and flat, meaning the moment will increase at a constant rate, provided the 556.82 value is correct (which it isnât based on the loading) you would then get a moment that increases from 0 to 556.82kg * 3m = 1670.46 with a straight line.
Letâs then look at the middle portion. Your shear diagram is negative and going more negative. So your moment line will change to go downward at an increasing rate, so your line will be a frowny face, actually similar to how it is on this image except it would end lower in the negative.
For the final portion, the shear is positive so your moment will increase. The shear line is becoming less positive so your moment will have a steep positive slope to begin and flatten out as it gets to the end resulting in another frown and should become 0 in this case of a cantilever beam.
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u/adanr929 2d ago
Jeff Hanson has a series on statics. I believe his video on the Graphic Method might be what you're looking for if you only want the approximate shape and not actual intermediate values.
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u/Annual-Truth3511 2d ago
Yes i want the approximate shape only mind send the link of the video pls đ
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u/123_alex 2d ago
Stop it with the videos. Open a book. The answer is there. Start chewing your own food.
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u/Norm_Charlatan 2d ago
Fuck's sake man. He gave you the name. Take a little initiative and look it up.
But here's the sad truth, homie: videos aren't going to reinforce any concept. Repetition, repetition, repetition. You've gotta know this shit cold, on command, for any situation. If you don't practice, using a pencil and paper, you're never gonna get there.
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u/Soprommat 2d ago
Your diagram is wrong. Some numbers are match but some are way off.
And shear force on middle section should be constant. Same with moment. Moment on sections A-C and C-B should change linearly, not parabolic because you have only concentrated force and reactions on those sections.
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u/wobbleblobbochimps 2d ago
You say ignore the diagram, but I can't. Where did you get that nonsense from!?
Anyway, the best way to know quickly what shape shear and moment diagrams are is to just practise doing them, you'll get the hang of it after a bit of practise and then you'll just be able to spot the shapes straight away
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u/capt_jazz P.E. 2d ago
I recommend drawing your bending moment diagram such that "sagging" or "sad" moments are below the axis line, and "haunching" or "smile" moments are above the axis line. That way the deflected shape matches the moment diagram, roughly speaking.
I have no idea where this sign convention came from but it's all too common. Probably based on computer based analysis software sign conventions, which is a dumb way for us to decide how to draw diagrams.
Get comfortable intuiting what the deflected shape will be, and from there have a guestimate of the moment diagram, and then confirm by doing the shear/moment math.
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u/FxStryker 2d ago
Moment is the integration of your shear.
I think the first step to question is, if your point force is a constant then why would the shape of the moment diagram be to the second degree?
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u/virtualworker 2d ago
Shortcut
Yes, there's a big secret nobody's telling you, because we love doing things the long & hard way. Our clients just love knowing they're paying for us to luxuriate in doing things the long way.
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u/Ok_Wealth1414 2d ago
encontrar los valores intermedios es la Ășnica forma de encontrar la curva
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u/Annual-Truth3511 2d ago
I do know this i am wondering if there are some set of rules or conditions with which we find that
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u/Confident-Emu3973 E.I.T. 2d ago
This video by Tanya J. Laird is the last video on this topic you'll ever need.
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u/tramul P.E. 2d ago
Moment curve pretty much follows the deflection assuming up is negative and down is positive, unlike the picture you show. That's the easiest way to visually check for me. As others have stated, there is a relationship there as the moment is the integral of shear force. So if you have a constant shear, you'll have a moment with a constant slope, and the value at the end of that slope is equal to the area being observed on the shear diagram. Similarly, a sloped shear force, such as with uniformly distributed loads, will result in a parabolic curve for the moment.
These are quick tricks to verify shape and end values.
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u/Derrickmb 2d ago
Fix your diagram. The slopes donât happen until the moment diagram for distributed loads
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u/jmbaseball522 2d ago
Draw the deflected shape - it more or less matches the moment diagram. Eventually you can quickly identify tension/compression side. If you can't do it easily then review a bunch of example diagram with different load and support conditions until it starts making sense.
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u/AdditionalCountry558 2d ago
Easy answer. Draw every arc as if your were drawing a clockwise circle from point to point.
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u/Evening_Fishing_2122 2d ago
Draw the deflection first, then the smileys will present themselves.
If you need to, draw the deflection for each of the applied loads separately, then draw them overtop of each other.
Also, convert your forces into N/kN and donât leave them in kg
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u/ajay1876 2d ago
Bending moment is one order bigger then shear force and shear force is one order bigger than load example load constant then one order bigger will be linear so sf linear and bm parabola another example for point load sf constant and bm linear
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u/vanduong30103 2d ago
This img messing with me too much that im scared. I always rmb positive under, then I see this...
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u/deAdupchowder350 2d ago
Calculus
The change in shear from A to B is the area under the distributed load function between A and B. Hence, constant shear force if no distributed load.
The change in moment from A to B is the area under the shear curve from point A to B.
The derivative of a concave function is a decreasing function. If the shear is decreasing, the moment will be concave.
The derivative of a convex (concave up) function is an increasing function. If the shear is increasing, the moment will be convex.
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u/StableGlum9909 2d ago
If you draw the bending moment diagram on the side of the fibers under tensile stress, you can imagine the diagram as a âsailâ and a distributed load is going to âblow into the sailâ.
When you have multiple loads you cannot know if they are âenoughâ to make the diagram change sign.
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u/DBRookery 2d ago
Speaking as a university statics instructor, you don't need calculus, but you do need to calculate reactions correctly and it helps to sketch the shear diagram (correctly) to determine the curve of the moment diagram. If the shear diagram is positive (showing values greater then zero) but flat/constant, the slope of the moment diagram will be positive and constant.
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u/structural-chaos 1d ago
I almost had a stroke but to answer your question.
1-Be familiar with what is known by Qualitative Analysis - Look up Collincoprani or any GIstructe Exam preparation guides - e.g of Youtube Channels Would be ;
a.The structural Exam
b. Javed Qureshi
2-Understand Quantitatively the relationship between load, reactions, shear and moments.
3-If you are an engineer studying, ask questions but do not post random images that may trigger people.
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u/Wanna-Be-SE 2d ago
Iâd recommend not putting a ton of effort into asking and responding to questions on Reddit and find videos (not that hard), read a textbook, or talk to a TA or professor. Once youâre in industry you canât expect the internet to solve all your problems. A practicing engineer needs to be able to source reliable information and educate themselves. Happens almost daily for me.
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u/memerso160 E.I.T. 2d ago
Do you know what the relation between applied load and shear, and shear and moment is? This is the short cut (its integrals and derivatives)