Exactly. You will get tensile forces at the bottom edge, so that's where you want a lot of material. Then there's the question of lateral stability, and I don't have an answer to that.
Interesting about the forces in a T-beam! I also thought the T should stand as usual, regardless of the material, but wood/board is actually better in tension than in compression, so to speak? There you go!
Regarding what should be protected; I'm guessing it's a house construction, considering what TS wrote last!
But I still wonder why it should be protected from rain and frost but not from snow. :x Furthermore, I suspect that TS has another thread nearby that deals with this very project!
Edit: Now I know! But I cheated and went through TS's page...
Regarding what should be protected; I'm guessing it's a house construction, considering what TS wrote last!
But I still wonder why it should be protected from rain and frost but not from snow. :x Furthermore, I suspect that TS has another thread nearby that deals with this very project! Edit: Now I know! But I cheated and went through TS's page...
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Ok, then I understand, I think.
But now, I am more inclined to believe Protte. Because if the tensile strength is twice as high as the compressive strength, we must reinforce where the construction is subjected to compressive forces, as that is where the weak link is, since the forces become equally large. It is hardly meaningful to reinforce where it is already twice as strong.
Right?
But now, I am more inclined to believe Protte. Because if the tensile strength is twice as high as the compressive strength, we must reinforce where the construction is subjected to compressive forces, as that is where the weak link is, since the forces become equally large. It is hardly meaningful to reinforce where it is already twice as strong.
Right?
Not in my world. You want to use the material where it is most beneficial per amount of material used. Wood is weaker under compression forces, so I avoid using it that way, instead letting it handle tensile loads.
That sounded very convincing until I considered it a step further - perhaps a step too far(?) - and thought like this:Carl_Elvis said:
If you start with a plank, for simplicity's sake, a plank without joints and such as TS intended to have, at 120x21. If you place it on its edge and load it from above, isn't there a greater risk of the upper edge being compressed than the lower edge being pulled apart? If you then add another plank as reinforcement on the bottom edge, you have - as Snickerick is suggesting - reinforced the strong side but haven't done anything to the weak side. If anything, you might have subjected the top side to an even greater risk of compression since the bottom side can now take more load without breaking, and thus the top side experiences even more "pressure"? (Here I might be completely off...) Regardless, you at least have the same compressive forces on the top side, no matter how many planks you add to reinforce the bottom? (The self-weight should probably be considered too....)
Or, am I thinking entirely wrong?
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Considered a bit more:
What you (Carl-Elvis) write about wood being better in tension than in compression; is that really applicable here? That a plank you pull on will last longer than one you push together, I can accept, but only because the plank under compression will simply bend and break. In that case, it's the compression along the plank's length that causes it to buckle to one side and then break. (But if you prevented the plank from moving sideways, what would happen then? Would it fail due to tension or compress first?)
In this case, the force is applied from the side—or rather from above—and I would be very surprised if in this scenario, you get a concertina effect on the plank's top edge before the bottom edge simply breaks and splits. It's enough if you place a plank between two chairs and stand on it, it surely breaks from the underside, right!? It shouldn't matter if you set the plank on edge...?
With the above reasoning—given that I am correct, of course—Carl_Elvis's conclusion should be correct, the plank should be reinforced on the underside, but on completely opposite premises. Anyway, I vote for reinforcement on the underside!
Regards,
ToRy
Edit: It shows how much of a difference it can make when you think it through a bit...
What you (Carl-Elvis) write about wood being better in tension than in compression; is that really applicable here? That a plank you pull on will last longer than one you push together, I can accept, but only because the plank under compression will simply bend and break. In that case, it's the compression along the plank's length that causes it to buckle to one side and then break. (But if you prevented the plank from moving sideways, what would happen then? Would it fail due to tension or compress first?)
In this case, the force is applied from the side—or rather from above—and I would be very surprised if in this scenario, you get a concertina effect on the plank's top edge before the bottom edge simply breaks and splits. It's enough if you place a plank between two chairs and stand on it, it surely breaks from the underside, right!? It shouldn't matter if you set the plank on edge...?
With the above reasoning—given that I am correct, of course
—Carl_Elvis's conclusion should be correct, the plank should be reinforced on the underside, but on completely opposite premises. Anyway, I vote for reinforcement on the underside!Regards,
ToRy
Edit: It shows how much of a difference it can make when you think it through a bit...
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Sailing & sailing???ToRy said:
I think you can sail like this: With an upside-down T = ┴ the possibility of an accordion effect on the top side is reduced, i.e., more of the downward force is now absorbed by the lower "stick."
Thinks someone who has never asked.
Member
· Västerbottens län
· 18 060 posts
Jeez, I'll just say that.
In some posts, the reasoning is completely correct, but then when it comes to the conclusion, it's the opposite.
If you place a board on top, it reinforces against the fact that wood cannot withstand compression AND that the beam wants to overturn.
Then you have to distinguish between compressive strength and buckling. Compressive strength can be measured on a piece that is 5 cm. Buckling depends on the length relative to the cross-sectional area, and there you have to consider the slenderness ratio.
Protte with strength of materials 35 years ago + the school of hard knocks
In some posts, the reasoning is completely correct, but then when it comes to the conclusion, it's the opposite.
If you place a board on top, it reinforces against the fact that wood cannot withstand compression AND that the beam wants to overturn.
Then you have to distinguish between compressive strength and buckling. Compressive strength can be measured on a piece that is 5 cm. Buckling depends on the length relative to the cross-sectional area, and there you have to consider the slenderness ratio.
Protte with strength of materials 35 years ago + the school of hard knocks
However, it's not enough if the underside of the vertical board breaks due to tensile force. I think an H-beam is needed. A horizontal H, that is.
(My structural engineering knowledge is 38 years old.. :|)
(My structural engineering knowledge is 38 years old.. :|)
Wouldn't that be an I-beam?mycke_nu said:
I also considered that, but thought it was more fun to continue discussing the T-beam.
Hmmm.... I'm starting to doubt myself now.... I think I need to go to Byggmax and buy a plank. I have chairs at home. Does it really break from underneath? I based my second theory on that and it seems to be completely wrong - even if with the right (?) conclusion. The first theory seems to be completely right - even if with the wrong (?) conclusion.ToRy said:
I'm pondering whether I should add more but.... If now TS joins together a bunch of planks with overlaps to create an I-beam (which it is not called because it doesn't have any "edges" yet), 12 meters long; when he then attaches planks on the edge (regardless of if he attaches on the top or the bottom edge) he must also overlap these, otherwise there will be weak points where those planks meet. With double planks on the edge, it’s starting to get heavy.... Then a tarpaulin must be added too. Is there any chance that it will hold, even if we ignore possible side forces? (That's really part of the original question, but it feels like we've strayed from it a bit...?)
I want, I want, I WANT the plank to be attached on the underside but I can’t justify it anymore....
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Isn't an I-beam one where the "flanges" are narrower than the web, and an H-beam one where the "flanges" are as wide as the web, regardless of how you position it? The "flanges" probably have a fancy name, but I have no idea what it is.....Carl_Elvis said:Doesn't that become an I-beam?
I've also thought about it, but I found it more fun to continue discussing the T-beam.
Since it is the same boards in the web and the flanges, it becomes a wide flange beam andCarl_Elvis said:Doesn't that become an I-beam?
I also thought about it, but found it more fun to continue discussing T-beams.
then the I turns into H.
(According to Bodelind and Persson, Strength and Material Tables, Akademiförlaget 1971
)Hepp New problems.
How much does the tarp weigh, heavy or cheap and light.
If it's a light tarp, shouldn't it be possible to stretch a wire between the poles????
My perfor. instruction is 42 years old (1970) beat that if you can.
How much does the tarp weigh, heavy or cheap and light.
If it's a light tarp, shouldn't it be possible to stretch a wire between the poles????
My perfor. instruction is 42 years old (1970) beat that if you can.
Don't forget that if you have a short board that is braced at the top and a large load, it might be the shear strength that is critical - it won't break at the top or bottom but along the fibers in the middle..... ;-)
(Unfortunately, I don't have time for any strength of materials study right now, but I'm fascinated by some presented theories and conclusions....)
PS I also think that wire is a good idea at 12m, as long as you have something stable to attach it to!
(Unfortunately, I don't have time for any strength of materials study right now, but I'm fascinated by some presented theories and conclusions....)
PS I also think that wire is a good idea at 12m, as long as you have something stable to attach it to!