Should new convertibles have taller than roof roll bars?

[Andres125sx]

What is this video from 2003 meant to show? All I see are cars which are evidently not terribly well designed for rollover safety causing bodily harm to crash test dummies. Are you actually trying to suggest that it is impossible to design a cantilevered beam to withstand extreme deflection during rollover? Because such a position is untenable.

You guys must pay more attention when reading...

The unfortunate result: Citroën C3 Pluriel, Mini Cooper and Peugeot 207 reveal - as well as in 2003 tested models of the middle class by Mazda, Opel and VW - a significant improvement.

Video is from april 2013, so they are current cars.... Do you think C3 pluriel or 207 are 10 years old cars?

What is the performance of current cars? If some of them are the same as the 2003 video then it would seem that the regulations would need to be better in terms of deflection, but we don't know that.

I guess we do, since they are current cars...

Do we agree then?

[Cold Fussion]

We´re discussing how to improve convertible´s safety. Some people said they´re not sure if it´s even needed, and I posted that video where, as you´ve stated yourself, convertible´s safety is not that good in case of a rollover. So yes, it´s needed

Then....

The 207 is not a current car, that mini is definitely some older variation. A very quick Google search will yield that exact same 20 rollover footage from a video uploaded in 2008. Given that the 207 went on sale in 2006, it is likely that the footage is from 2006.

At no point did I say that convertibles are safe, I said the those three tested cars did not have very good safety. In any case, this is purely anecdotal, as you've still yet to provide any real engineering analysis as to a convertible with a curved windshield cannot be made safe. Your position on cantilevers is also highly confusing, you are seemingly religiously against them, and then at the same time, point out why a straight rear roll hope (of some undisclosed geometry and comparison) is better in bending than a curved windshield. I cannot see how you can reconcile this point of view, as a curved windshield can be made just as strong and rigid as a straight roll hope

[Ciro Pabón]

A windshield is a simply supported beam

Ehem, sorry, Andrés, with all due respect, nope.

A simply supported beam is not affixed to anything, in the sense that is free to rotate at both ends. It is supported by its own weight and it's restricted in the vertical plane at both ends and in the horizontal plane at only one support (it's free to expand horizontally at the other support).

You're probably wanting to say a windshield is a cantilevered beam in a convertible and a fixed beam in a regular car.

Simply supported and cantilevered beams. A beam that can resist moments at both ends is called a fixed beam (not in the image)


[quote=""Andres125sx"]

That´s not a true cantilever, the suspended span makes cantilever arms work different way, they start working as compressed structures while a true cantilever works as a flexed one (compression at the lower side and traction at the upper one) with much higher moments at the support. [/quote]

I'm not sure where to start... [-o<

I'm the last person in this forum to invoke ipse dixit arguments (I hate when someone says "I know more than you, so you have to listen to me"), but, Andrés, con todo respeto y mucho cariño, I've been designing bridges (and beams) for over 30 years (yes, I was born "before dogs were invented", more or less), so you have to listen to me...

So, I'm pretty, pretty, pretty sure that this bridge has a reduced moment compared with a simply supported one. I'm also pretty sure that's the critical design consideration.

However, I am a teacher, so let me add, emphatically, that it's perfectly understandable (what appears to be, at least for me) your confusion.

Actually, when the first bridge was designed like that, in the world famous Firth of Forth, there was so much misunderstanding (not to mention that the previous bridge has collapsed) that the designer had to imagine a way to explain it in the simplest way possible.

If you're so kind, stay with me for a minute (yes, I know, my posts are loong and boring) and you will learn a couple of things, in case you have not learned them already.

If you have, then you perhaps will be enlightened about your knowledge.

There are no compressed structures on the beams of this bridge.

Both cantilevers have moment (what you call "compression at the lower side and traction (tension) at the upper one") , it's maximum at the ends of the beams closer to the center of the span and they are less than the moment you have in a simple beam.

At the intermediate supports you have shear, not compression.

The beams are cantilevered or affixed at the ends of the bridge, not at the columns.

Thus, at those pillars or columns, they need shear stirrups (vertical reinforcement: that is, the vertical squares at the right of the next image)...

Stirrups (estribos)


... instead of compression ties or hoops (horizontal reinforcement in the second following image).

Ties (aros en América Latina o presillas en España)


Now, the enlightenment:

Super-hyper-extra-majuscule-extraordinarily famous picture of the Firth of Forth bridge principle:
Ta daaaaaa!


Notice both guys at the extremes of the picture are working as true cantilevers: their arms are in tension in the upper part of the "beam" and the sticks that go from their hands to their hips are in compression in the lower part of the "beam". Quod erat demostratum...

The end result: it's well done, I say, even for Scottish engineers...


I also want to remark that I put forward this bridge as an example of a cantilevered design that is more efficient than a simply supported beam, not as an example of how you should analyze a windshield beam.

However, I insist, the critical factor in convertibles is the lack of rigidity of chassis, not windshield design.

Anyway, thanks for the opportunity to explain a couple of things about beams. I really love structural analysis, so, I think this thread is, as we say in Cali, "full HD".

[Andres125sx]

Warning: looong post, ignore if you´re not interested on structural analysis

You're probably wanting to say a windshield is a cantilevered beam in a convertible and a fixed beam in a regular car.

Exactly

I've been designing bridges (and beams) for over 30 years (yes, I was born "before dogs were invented", more or less), so you have to listen to me...

And I do, period. Don´t be fooled by my poor english, I usually strugle to explain my point of view talking about common things, imagine how hard it is to me trying to explain how a beam works in english... Try explaining this same post in spanish and you will probably understand me better

I perfectly know what you´re explaining about fixed beams, simple supported beams, etc. But I didn´t know what was refering each term. I was thinking a simple supported beam was refering to a cantilevered beam... simple support -> one support and the other end is free, that was my reasoning

So, I'm pretty, pretty, pretty sure that this bridge has a reduced moment compared with a simply supported one. I'm also pretty sure that's the critical design consideration.

And I agree, I was confused thinking a simple supported beam is a cantilevered beam, that´s all

There are no compressed structures on the beams of this bridge.

I´ll ask before discussing.... What are you refering?

Because the lower part of the beams are actually working by compression

They are the sticks on that nice picture of the three men

Both cantilevers have moment (what you call "compression at the lower side and traction (tension) at the upper one") , it's maximum at the ends of the beams closer to the center of the span and they are less than the moment you have in a simple beam.

At the intermediate supports you have shear, not compression.

The beams are cantilevered or affixed at the ends of the bridge, not at the columns.

Ok, I understand the bridge example was because I was talking about simple supported beams as if it was a cantilevered one, so I understand the example

And I obviously agree, but that is completely different to the subject we were discussing about. A windshield is a cantilevered beam, and there´s no cantilevered beams on that bridge, or any bridge

Bridges beams have no fixed supports as you know, because they must be able to expand and contract, so they are simple supported beams or cantilevered beams with an archor arm, but anycase their joints with columns must have free movement on, at least, one axis.

OTOH a winshield (as any cantilevered beam for obvious reasons) is completely fixed at one end, and completely free at the opposite

I also want to remark that I put forward this bridge as an example of a cantilevered design that is more efficient than a simply supported beam, not as an example of how you should analyze a windshield beam.

Obviously, as I said it was just a misunderstanding by my part, thanks for your great explanation

However, I insist, the critical factor in convertibles is the lack of rigidity of chassis, not windshield design.

Agree, but that´s a different debate, the thread is about convertible´s safety in case of a rollover, so windshield design actually is the critical factor

Anyway, thanks for the opportunity to explain a couple of things about beams. I really love structural analysis, so, I think this thread is, as we say in Cali, "full HD".

Thanks to you Ciro, I´ve learned a lot. I also love structural analysis, that´s the reason I´m so active on this topic. We probably are boring some F1T members tough

PS: upvoted, great explanation and I really appreciatte the effort =D>

[Andres125sx]

We´re discussing how to improve convertible´s safety. Some people said they´re not sure if it´s even needed, and I posted that video where, as you´ve stated yourself, convertible´s safety is not that good in case of a rollover. So yes, it´s needed

Then....

The 207 is not a current car, that mini is definitely some older variation. A very quick Google search will yield that exact same 20 rollover footage from a video uploaded in 2008. Given that the 207 went on sale in 2006, it is likely that the footage is from 2006.

At no point did I say that convertibles are safe, I said the those three tested cars did not have very good safety. In any case, this is purely anecdotal, as you've still yet to provide any real engineering analysis as to a convertible with a curved windshield cannot be made safe. Your position on cantilevers is also highly confusing, you are seemingly religiously against them, and then at the same time, point out why a straight rear roll hope (of some undisclosed geometry and comparison) is better in bending than a curved windshield. I cannot see how you can reconcile this point of view, as a curved windshield can be made just as strong and rigid as a straight roll hope

Any reason I should do that?. I´ve never said they cannot be made safe, I´ve only said after watching those rollover crash test and their conclusion, safety must improve. As you know because you deleted that part of my post, one of the options I was thinking about is reinforcing A pillar of curved windhields

I´m religiously against nothing. But I´m not sure if a cantilevered beam may be the best option to protect passengers. A straight roll hope is aroud a third of the windshield lenght, and that´s a huge factor when talking about cantilevered beams, span. A short cantilever can be very resistant easily, a long cantilever is not that easy to build it strong. That´s the reason cantilevered beams/structures usually need struts when span is not short. Direction of the load it must resist is also very important, a straight roll hope will receive the impact more or less axially, while the windshield will always receive the impact at a high angle what make it more prone to collapse

[Ciro Pabón]

Thanks for your kind words, Andrés. Just a couple of things:

...I´ll ask before discussing.... What are you refering?

Because the lower part of the beams are actually working by compression

They are the sticks on that nice picture of the three men

Well, when you load a beam you (as you correctly stated) create a compression force in the upper part of the beam and a tension force in the lower part of the beam, as the guys in the picture show. Here you can see the forces in a beam: there is a section (discontinuous line in the image) where forces are zero, it's called the neutral axis. The top of the beam (as shown by the arrows in the top center) is in compression, the bottom of the beam is in tensino (as shown by the arrows at bottom center).



Concrete is excellent at compression, while long, slender bars of steel are very good at tension: it's combination is called reinforced concrete.

A beam is designed to resist this bending moment, flexural stresses or, simply, momentum.

You can also adsorb those forces with a composite beam, in which some elements work at compression and some in tension, as in the Firth of Forth example.

However, is the whole beam what you design. You take in account what's called the third momentum of inertia, that is, the resistance of the whole section to this torque.

That's why the beam is so tall at the supports (the height is the maximum possible). Even if individual members of the composite beam are stressed to resist compression or tension, the resistance of the beam is given by the total bending momentum it can take.

The equation for the stress you create with the bending moment is:



This means that the stress increases with the distance to the neutral axis.

This combination of compression and tension at a distance of the neutral axis (where forces are zero) is what we call bending momentum is a torque. That's why taller beams are better.

You can see the beam to the left, the stresses to the right (the two triangles with stripes: the top triangle represents the compression forces, the bottom triangle represents the tension forces, you have to design the elements to resist the maximum force (represented by the length of the triangle at the top and at the bottom).

Diagram of forces in a beam


However, the resistance of the beam is given by its capacity to resist the torque generated by this triangles. That's why you try to put as much material as possible away from the neutral axis (where the two triangles touch and the force is zero). That's the origin of the I beams, as the one in the previous illustration.

It's the combination of tension in the lower part of the beam and compression on the upper part what is called momentum.

It's very different from compression: columns in compression are designed with a different method (Euler).

... Bridges beams have no fixed supports as you know, because they must be able to expand and contract, so they are simple supported beams or cantilevered beams with an archor arm, but anycase their joints with columns must have free movement on, at least, one axis.

Again, sorry, that's incorrect. A bridge with fixed supports is called an hyperstatic structure. This is the modern way to design a bridge. Its originators were Pier Luigi Nervi and Robert Maillart, our senseis...

Salginatobel Bridge by Maillart, 1929, no free movements at supports, all stresses created by thermal expansion are taken by steel reinforcement, three virtual hinges, 133 meters between supports

Orvieto Hangars, 1935, by Nervi, also with virtual hinges but no free movements at supports, shell structure, length 111 meters between supports, a masterpiece

[Andres125sx]

Thanks for your kind words, Andrés. Just a couple of things:

...I´ll ask before discussing.... What are you refering?

Because the lower part of the beams are actually working by compression

They are the sticks on that nice picture of the three men

Well, when you load a beam you (as you correctly stated) create a compression force in the upper part of the beam and a tension force in the lower part of the beam, as the guys in the picture show. Here you can see the forces in a beam: there is a section (discontinuous line in the image) where forces are zero, it's called

[...]

It's the combination of tension in the lower part of the beam and compression on the upper part what is called momentum.

It's very different from compression: columns in compression are designed with a different method

I´ve just realiced I´ve not said I´m contruction engineer. Anycase your explanation is great for me to know the english terms, thank you again

Obviously I´m not as experienced as you designing anything, I have the knowledge for designing but my job have usually been building, but I´ve also been checking buldings calculations for some time at a quality control company, if that´s the name in english

I didn´t know how to say when a beam must take a momentum, that´s the reason I talked about compression in the lower part. But I guess (never did it by myself) when designing a complex beam, you must consider what stress it is taking. If it´s an element of the lower part of the beam, it will be working by compression, so you must design that single element to resist compression

Actually momentum is just a combination of two stresses, theoretically speaking, there´s only 3 stresses, compression, tension, and shear (there are some more, but these are the basic ones, and I don´t know the english term of the fourth... rotational?).

Momentum is just a combination of compression and tension, so I understand what you say and agree, the beam is resisting a momentum, not compression, but I guess you can see what I was trying to say, the lower part of the beam is being compressed

... Bridges beams have no fixed supports as you know, because they must be able to expand and contract, so they are simple supported beams or cantilevered beams with an archor arm, but anycase their joints with columns must have free movement on, at least, one axis.

Again, sorry, that's incorrect. A bridge with fixed supports is called an hyperstatic structure. This is the modern way to design a bridge. Its originators were Pier Luigi Nervi and Robert Maillart, our senseis...

Salginatobel Bridge by Maillart, 1929, no free movements at supports, all stresses created by thermal expansion are taken by steel reinforcement, three virtual hinges, 133 meters between supports
http://upload.wikimedia.org/wikipedia/c ... g_4080.jpg[/url]

Orvieto Hangars, 1935, by Nervi, also with virtual hinges but no free movements at supports, shell structure, length 111 meters between supports, a masterpiece
http://1.bp.blogspot.com/_xJWa77A8OSw/S ... vieto1.jpg

Nice, looks like bridges are not my specialisation but again, I guess you can see what I was trying to say. Bridges usually don´t have rigid fixations between beams and columns, at least the example you put (as any other metallic bridge), and all this discussion started when talking about convertible windshields, wich is fixed to the car, so they are completely different structures that cannot be compared


Now I know most technical terms I didn´t know, what a fast technical english class Ciro =D> , I think now I could look for a job outside Spain

[g-force_addict]

That's the way KTM went with the X-Box


A tall rollbar for added protection even if it looks weird with the soft top on


I wonder why they didn't make the tall standing rollbars work as air intakes for the mid engine?
Pretty much like Bugatti did with the Veyron roadster but with a more smooth shape