Moment Diagram Sign Convention Poll 2

[271828]

I'm curious about how many people use tension side vs compression side M diagrams.

If you have a simply supported beam subject only to a gravity load, which side of the beam do you draw the diagram, top or bottom?

 

[steellion]

I draw them on the top, that's how we learned in school, though I acknowledge that bottom would be more logical and practical, especially for concrete.

 

[JAE]

I think it is NOT intuitive to match the moment diagram with the deflection diagram. By having them opposite, the eye avoids confusing the two.

I prefer the compression side - I also prefer the toilet paper going over the top rather than under the bottom.

They are both very important to me.....NOT.

 

[271828]

"...I also prefer the toilet paper going over the top rather than under the bottom."

The tension side vs compression side is an argument that I can live and let live, but not so for the toilet paper direction.

If the toilet paper goes over the top, then there is square footage of floor lost. Also, if a toddler or cat comes by and claws at it ferociously, it'll spool off a bunch of paper.

Come on man.

 

[Ron]

Top...it's the way I was taught and got stuck with it.

Paper on top. An absolute necessity. More important than the moment diagram. If the cat spools it off, you're close enough to the toilet to have a solution for the cat.

 

[JAE]

My cat died last year.

 

[JoshPlumSE]

I was taught structural analysis by a professor that specialized in concrete design. Hence, he always forced us to plot moment diagrams on the tension side of the member. That way, it would be very easy to know where we were going to have to place our flexural reinforcement.

Later on I had a boss who had specialized in steel design. He always encouraged us to plot the diagrams on the compression side of the members. That way, it would be very easy to see where we were going to have to add flange bracing.

In the end, both methods are completely valid. It's just a matter of personal preference (or perhaps personal bias?) based on what type of work you're doing and what you learned in school.

 

[msquared48]

My early boss had huge fits of tension if we did not plot it to the top. He had a patent on a bra design too.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[rb1957]

mike,

must've been a good boss to work for ...

good thing you didn't do a spoonerism on "fits of tension" particularly with the adverb "huge" ...

me, i'll have two pickets to tittsburg, please

 

[hokie66]

Tension side. Like everybody else, just because that's the way I learned it. Really, it makes no difference. I always did wonder why we called the ones on the bottom positive and the ones on the top negative.

 

[ToadJones]

I agree with everyone except BA Retired
He is retired and no longer has to draw or read moment diagrams.

 

[msquared48]

No. He is just of the age where everthing is done momentarily.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BAretired]

The CISC "Handbook of Steel Construction" under "Beam Diagrams and Formulae" show positive moments and shears above the line. What does the AISC show? Our Wood Design Manual shows it the same way.

I could be pedantic and say this is the correct way, but I don't want to offend all you "below the line" aficionados.

BA

 

[BAretired]

Your Wartiness,

How can you possibly not agree with me? I have upon occasion drawn my bending moment diagram on both sides of the line, uniform load above and point loads below and occasionally the other way around. It makes it easier to draw to scale without having to add numbers together as you plot.

BA

 

[gobsmacked]

I have had many discussions about this. Of course it doesn't matter, really, but you should avoid referring to moments as positive or negative. A better description is 'hogging' or 'sagging'. (Though, in columns, you may have to turn your head to one side or the other - I draw a 'physics eye' when I want to identify the point of view) Analysts favour a mathematical approach so that in complex frames the interpretation is unambiguous. Designers prefer the intuitive approach. Whatever convention you adopt will eventually lead to a contradiction which requires you to think about the meaning rather than the sign (like the meaning of a wurd as distinct from its speeling). For example, the bending moment in an arch is best drawn to match the line of thrust, which the other sign would fail to reveal. However this nice point is lost in other circumstances. Just state your convention for both moments and shear in the introduction your calcs. Beware of dogmatic people. They are ignoring one principle in favour of another.

 

[hokie66]

The problem with that is that hogs sag. I should have stayed out of this. A better answer would have been "I forget".

 

[IDS]

Surely the first question we need to settle is, is a tensile stress positive or negative?

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[paddingtongreen]

putting the BM above the beam was fine, it was positive, below the line was negative. Then I moved on to fixed ended beams and the direction of rotation got a sign, clockwise positive and anti clockwise negative, or vice versa. So the fixed end moments now have different sign, even if they are both above or both below the beam, so tension side can be positive or negative.

If you have performed as much Moment Distribution and Slope Deflection as I, you would see that consistency is the only thing that matters.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[rscassar]

Each to their own really. I was raised with concrete design and always showed the bending moment on the tension side to show where reinforcement was required.

Likewise I can understand why steel designers would prefer showing the bending moment diagram on the compression side to understand which flange requires attention/restraint.