I was wondering what is the best material to use for a beam in the horizontal position, supported at both ends. Deflection wise is an i beam stronger than heavy wall square or rectangle tubing? I'm trying to find out pound for pound whats the best for the least amount of money, like everyone else :)
Thanks,
Doug

You can get what you need to calculate it here:

http://www.efunda.com/math/areas/Common_Cross_Section_Index.cfm

You are looking for the best moment of interia for the money.

You'll want to use a spreadsheet. :)

-Jeff

here the I-beam setup I used. Almost have my new playroom under house

As far as structural steel goes, the engineers calculate strength by the pounds/foot of the structural member. Having said that it is the vertical web that creates the load bearing strength. Ie using heavy flat bar the "flat" way is baaaad.
I very strongly recomend that you check with tlocal building codes. Your snow load factor is likely different than mine is.

Before I started my little digging project. I had an engineer over to look. Based on his math I would have had to use 3 19 1/4 LVL beams per selection to span 20' at a cost of $205/section. The cost of the steel at the time was $215/section.He suggested 12" I-beam 8' apart to replace carrier beams that where rotted. If I rebuild in future the middle beam comes out and I can span new building with 2 x 12 15'

There are several ways of looking at structural design. Usually us hobby type people look mainly at the cost of the project. This of-course translates into weight as we know steel is sold based usually on weight. If we sit down and look at the bare numbers typically for the same weight the beam will have more strength, however beam isn't w/out its little vice's either and the bottom line really depends on the application. One good rule of thumb is that if the beam can EVER be placed under some type of torsion or twisting load, use HSS [Hollow structural sections or tubing] If its purely a static application, beam can usually work. Beam are usually more work in making connections, usually it requires bolt up clips/plates/angles as has been shown in the pic's on this thread. Tubing can be just butted and welded. Both can be easily D & T'd, however the lighter walled tubing's might not lend themselves to this as well as the heavyer sizes.

I'd like to clarify a bit what DareBee said. If you are looking at two HSS sections of material, both are the same weight but one is rectangular [tall and skinny] and one is sqr, the tall/skinny piece will usually be stronger over the sqr pc of the same weight, but only when orientated so that the load is imparted on the skinny face, parrallel to the major length/side. ie; If the tube[when viewed from the end] is tall and skinny, and the load is imparted from above [or below] the member will take more load than if that same part was loaded from the side. The example of the flatbar is also a good illustration. A typical I-beam is another good example, you can see why the beam would be stronger when loaded on the flanges, across the depth of the beam, vs from the side where it would bow fairly easily..

Another option is to build a beam, from HSS members in a trussed style. This is actually how I'm building my system, its 20' long and has about 1/2 the weight of a standard beam of the same strength, it has taken alot more work to build however.

my 2 cents..

Jerry [hope I've helped and not confused the issue.. :)]

I was wondering what is the best material to use for a beam in the horizontal position, supported at both ends. Deflection wise is an i beam stronger than heavy wall square or rectangle tubing? I'm trying to find out pound for pound whats the best for the least amount of money, like everyone else :) Thanks, Doug

On a pound for pound basis the I beam is going to deflect the least, rectangular tube with the longest dimension vertical is next and square tube will deflect much more. As Inspiration Tool says you are looking for the best value of I the moment of inertia. An I-beam is designed to give the best I by having a larger proportion of the cross-sectional area further away from the centerline of the beam. Rectangular tube with the longest dimension vertical is next and square is worst because the I for regular sections depends on the third power of the vertical dimension.

On a dollar for dollar basis if you get some dimensions and prices for I beams and rectangular tube do the calculations and compare I per dollar you might find rectangular tube is lower cost.

Whatever you discover it is essential that if you are using the beam to support a structure or for lifting something you need to get a professional engineering opinion.

ignore everything else and the math is all about how much cross section in the direction of the load. Period!

I beams are designd for most of the load against the tall single part of the I. The more complex the shape the more complex the math. But it all boils down to cross section under the load.

ignore everything else and the math is all about how much cross section in the direction of the load. Period!

I beams are designd for most of the load against the tall single part of the I. The more complex the shape the more complex the math. But it all boils down to cross section under the load.

I do not like to disagree with such a vehement opinion :D but in my humble opinion and phrasing it as respectfully as possible it is only half right. The stiffness of a beam depends on the product of the cross section and the distance, in the direction of the load, of each element of the cross section from the neutral axis.

My thinking was that an I beam has 1 web and the rectangle tubing has 2-one on each side. So the tubing would be stronger. This application is for the rail/beam on a bridge saw. Approx. 14' long. supported on both ends. I don't want hardly any deflection in the z axis but more importantly none in the y axis. It has to cut in a straight line period. Again, it seems as if the rectangle tubing would be stronger, my thoughts.
Regards,
Doug

Considering the beam of a saw will see potential twisting load a tube is your best choice - heavy (it would have saved a lot of people a lot of typing if you had divulged this application up front - me I don't like to type and keep my answers short and over-simplistic in most cases anyway :-) )

My thinking was that an I beam has 1 web and the rectangle tubing has 2-one on each side. So the tubing would be stronger. This application is for the rail/beam on a bridge saw. Approx. 14' long. supported on both ends. I don't want hardly any deflection in the z axis but more importantly none in the y axis. It has to cut in a straight line period. Again, it seems as if the rectangle tubing would be stronger, my thoughts.
Regards,
Doug

I will second DareBee; if you give more detail you can get a more focussed answer. For this application you are probably better to go with a trussed structure as JerryFlyGuy suggests. Part of the reason I make this suggestion is that the amount of deflection in the Y direction depends on the spacing of your guide rails and the distance from the guide rails to the point where the load is applied. Closely spaced rails are not suitable and the only way you can get them well separated is with a tall beam, i.e., large rectangular tube; which is heavy. A trussed structure is more work but much less weight.

My apologies for the inconvenience. I started out thinking about this topic as a general one covering rails, bridges, and base for the rails. I think all of the answers apply but they get more detailed with the saw bridge and deflections under load. Hence my last reply.
Thanks,
Doug

Geof, thanks for jumping in here.. I always like to read your well written explanations :)

Miljnor, according to your explanation, a sqr solid bar of the same cross-sectional area as a beam, would have the same strength? If only this was true, imagine all the pain of dealing w/ complex shapes that could be elliminated! The cross sectional area has some bearing, esp in tension and compression loads, however in a beam calculation where your trying to elliminate as much flexture as possible [ keep it stiff] the cross sectional area in realation to the depth of section and/or it centroid is where the strength is, I can built a beam from 1/8" steel that is far stronger than a 4" I-beam if given the time and willingness to go the mile for complexity.. it would use a fair bit less mat'l, weigh less and be stiffer.. all w/ less x-sectional area than the beam. Take a pc of paper.. and you can flex it fairly easily.. take that same pc of paper and fold it into a long tube like box.. and it will be far stiffer.. and would in theory 'bare more weight' even if it's so small its hard to measure..

If one wants to learn more about how all this works.. go to your local hardware store and for a few dollars buy some yardsticks. Then using them.. make several shapes.. a I beam, a tube [sqr and a rect.] and maybe a channel.. make sure you compare apples to apples in that each pc must have the same x-sectional area [ number of yard stick's as viewed by the end profile] and see which is stronger, which twists easier than others.. if you really want to get scientific.. use a 10lb weight and measure deflections.. etc.. it would be a great little learning excercise.. You'll be amazed by what you think would make something stronger.. and what actually does make things stronger..

DEW, the vertical web's would seem to make things stronger [I also thought this when I first started into mechanical design] however if we added mat'l to the side of a tube and measured its stiffness and then took the same amount of mat'l and added it to the top and bottom, you'll find a significant increase in stiffness over the tube w/ the mat'l on the sides, I've actually done this to a beam which had too much deflection, we added a fb top and bottom to the beam and the deflection was less than 1/4 of what it had been previously. The web's actually only carry the shear load between the top and bottom flanges. Typically a beam's web is 1/2 of less of the thickness of the flange
on a beam. Where as, a tube has a uniform wall thickness and "wastes" some of this thickness where it doesn't need it.

As DareBee has already stated.. in your case the tube is better, not because it is stronger than the beam [in simple term's], but because it will resist the twisting moment added to the structure while the saw is in operation, your still going to have to spec a tube which will take the calculated loads of the saw and deliver the needed stiffness.

Another thought to concider is tolerances. The structure is going to deflect, period. If a fly lands on a 6ft cubed block of steel, the block deflects, infact.. if you really want to get down to the nitty gritty.. the changes in atmosphereic pressure deflect the block.. its not measureable.. but its there.. You have to decided what is the allowable tolerance for your system and then design accordingly.. if 0.0001" deflection at full load isn't acceptable.. then make changes to get your structure to the point where it has reached your tolerance. You have to sit down and quantify what those tolerance's are, before you ever start. Calling for a unit that cuts a straight line doesn't give you that tolerance. Saying that you want a machine which can cut a straight line w/ less than 0.001" deviation from center, in 10" is a tolerance.. Then the big hurdle is building to arrive at your decided tolerance.

Jerry [Sorry DareBee.. I tend to get windy... :D]

Miljnor, according to your explanation, a square solid bar of the same cross-sectional area as a beam, would have the same strength? If only this was true, imagine all the pain of dealing w/ complex shapes that could be eliminated! The cross sectional area has some bearing, especially in tension and compression loads, however in a beam calculation where your trying to eliminate as much flexure as possible [keep it stiff] the cross sectional area in relation to the depth of section and/or it centroid is where the strength is, I can built a beam from 1/8" steel that is far stronger than a 4" I-beam if given the time and willingness to go the mile for complexity.. it would use a fair bit less mat'l, weigh less and be stiffer.. all w/ less x-sectional area than the beam. Take a pc of paper.. and you can flex it fairly easily.. take that same pc of paper and fold it into a long tube like box.. and it will be far stiffer.. and would in theory 'bare more weight' even if it's so small its hard to measure..


Stiffness is not strength and no matter how you make it the Material properties won't change (unless of course you laminate/alloy them with something else, but that is a different argument.) I agree with your statement you can make something stiffer with a complex shape, but that gives you a trade off of fragility. if you make a honeycomb structure and one part of it fails the structure tends to fail very rapidly afterwards.

Were in a solid square piece of material, you have a toughness equal to its material composition and a stiffness of its cross-section. The more complex the shape (with the same weight) you will get more stiffness at the trade off of more easily damaged structure.



I do not like to disagree with such a vehement opinion but in my humble opinion and phrasing it as respectfully as possible it is only half right. The stiffness of a beam depends on the product of the cross section and the distance, in the direction of the load, of each element of the cross section from the neutral axis.

That’s pretty well what I meant by direction of the load but I am not so eloquent with words. :D

I made the mistake of over simplifying to the point of abstraction! :D

It's all good! :cheers:

Ps: I wasn't all the vehemet anyway! :p

....if you really want to get down to the nitty gritty.. the changes in atmosphereic pressure deflect the block.. its not measureable....]

I know it seems like biting the hand that compliments you, to mix a metaphor, but I find it necessary to insert a minor correction. Atmospheric pressure works on all sides so it does not exert a net load on the beam.

I do have to agree though you do get windy :D. I may have to hone my typing skills to keep up.

its not measureable.. but its there..


My wife tells me that about heaven and god all the time!

A professor that I new that was into practical application more than theory allways told me: "a differnce that makes no difference is no difference"

Atmospheric pressure works on all sides so it does not exert a net load on the beam..

True true... but!.. the atmoshpereic pressure is acting to compress the entire block, making it smaller in all dimensions.. :D [not to mention the effects of its own weight...Then you could take into account the effects of thermal expansion and differental heating.. oh.. we could make it complicated :p




I do have to agree though you do get windy :D. I may have to hone my typing skills to keep up.


Baaaa... I get it from my father.. I can honestly say.. I come by it honestly..:D my mother was a school teacher.. a stickler for details.. so this also has an impact.

Miljor, your correct in both accounts, however your talking in philosophy.. I'm talking in physic's ;) btw.. thanks for correcting me on stiffness and strength.. I often exchange the two [always in error] when trying to keep my fingers up to the speed at which my brain is spewing out junk...

Jerry [trying to be short and sweet.. at 6'3 and 220# :D]

Stiffness is not strength
Ps: I wasn't all the vehemet anyway! :p

Okay I concede the Ps and you are correct about stiffness is not strength although a very good principle to follow in designing almost anything is: If it is stiff enough to perform its function correctly it is almost certainly strong enough. One time when this does not work is what you referred to; a stiff but fragile structure if you accept that fragile implies it is suceptible to a buckling mode failure.

There is almost always a trade off between stiffness, weight and fragility.

from a practical point of veiw "big and strong you can't go wrong!" So the math becomes irrelevant. But if your designing an aircraft or rocket ect.... then "light and frail because the pilot can always bail" :D

I beams can usually be found cheaply if your not in a hurry you just have to start looking around at structural steel places for "take offs"

I didn't finish engineering school so I asked my partner (who has a masters in it and is older than me ) and he says that for load bearing the I beam is the master (of course he always prefaces it with the big and strong coment.) and if your using it for a crane it also has niffty place to put the wheels of your roller! :D

but what do I know I am after all a college drop out! I don't know nuttin! :D

but I do like a good tussle! (flame2)

I enjoy the rest of you guys getting "windy" it gives me something to read and saves my typing fingers.
As I said, most of my answers are very simplistic and abbreviated.

Interesting thread - sadly it brought back flashbacks to my junior collect mechanics class - we HAD to take a structural engineering course to "round out" our education. Yeah, round out this.....

The essential function of the rib of the beam is to move the sections of the beam that see the majority of the tensile and comperessive stresses farther away from the neutral axis of the beam. The neutral axis is the theoretical infinitely thin plane thru which there is neither compressive nor tensile stresses.

In a beam supported at each end, the top surface flanger is compressed and the bottom flange stretched in tension.

Now, the stiffness (resistance to deflection) is a funtion of the section modulus and where it is located.

In the case of the beam being "I" shaped and NOT seeing wind loads, this form is the stiffest for not deflecting vertically.

However if the beam has an "H" shape, it will have much more stiffness with respect to lateral bending but much more flexible in vertical bending than the I beam given same everything else (material, heat treat, thickness etc). All things equal, it isn't what you make the beam look like (I vs H) however it depends TREMENDOUSLY as to where you concentrate the material and how you apply the load.

The "big and strong you can't go wrong" philosophy is sadly quite WRONG. A bridge simply has to be able to hold itself up. Too heavy and it can't/won't. Oops!

Suspension bridges work because the wire rope that holds it up is loaded in the best possible way for that material in that form - pure TENSILE stress.

Concrete bridges work because the material is forced to be loaded in as pure a COMPRESSIVE stress. Interestingly, concrete and cast iron are some of the very few materials that want to be loaded in compression as opposed to tension.

An over simplification is easy and wrong to do when it comes to calculating beam sections for buildings - it simply ain't a SWAG science.

The "Galloping Gertie" bridge that at one time spanned the Puget Sound in Washington state is a perfect example of what happens when beams are improperly/incompletely analyzed. How so?

The bridge had plenty of beam strength. However, the designers failed to consider what a side wind load would do. Due to the fact that I beams instead of "clear span/bar joist" beams were initially used, the bridge 'saw' and reacted to tremendous side loading as applied by the wind.

In fact, on the fatefult day(s) at wind blew at a speed that side loaded the bridge in a fashion to excite it at its natural frequency - for hours upon hours on end.

This is the bridge that they show you in EVERY mechanical vibrations class in every univerity in the USA. It is the same bridge that is twisting an galloping up and down in tons of grade B horror/sci-fi flicks since the 50's.

The darn thing was twisting torsionally something like +/-6 feet from side to side and "galloping" up and down from end to end a like amount. It ultimately fell due to the torsional/bending overstressing.

The replacment was stiffer yet lighter due to "clear span" constrution. Instead of solid beams, they made the joists out of trusses similar to the 'open' roof trusses used in house roof construction. They made the top and bottom sections strong and far away form the neutral axis and the separating member made "open" so that the side wind loads blew "thru" the truss as opposed to against it.

Good thread that runs a huge risk of oversimplification. Calc'ing wind and beam loads on bridge trusses is a prime reason I did NOT get involved in civil engineering. Too hard for me to get thru/comprehend. Yet, those darn I versus H beam issues continued to plague me in automotive engineering.

How so?

Which would you rather have in your race engine? H beam Carrillo con rods or Crower I beams (same material and weight)??? AAARGH!!! Some of life's problems are simply put there to haunt you, forever and ever.

Well...just a simple point for the bridge of a bridge / gantry saw.....you should worry more about sag of the beam and how you'll compensate for that....

Viper.. its not that easy.. NC has it right.. [ as he usually does :D] you've also got to design things for the twisting force that is going to be imparted. If the z axis hangs down 2 ft below the yaxis.. thats a fairly huge lever.. w/ say an impact [ an oops] of 1500lb's force thats 3000ft-lbs of torque applied to that beam.. To put that into perspective.. thats me [ at 220 lb's] standing out on a board that is 13.6' long. [ Negating the weight of the board] you can imagine how much twisting that is putting on a 12" high beam..

Jerry... [ :tired:.. time for bed.. I think I'm coming down w/ strep throat... [sp] oh joy...]

... To put that into perspective.. thats me [ at 220 lb's] standing out on a board that is 13.6' long. [ Negating the weight of the board] you can imagine how much twisting that is putting on a 12" high beam..Jerry... [ :tired:.. time for bed.. I think I'm coming down w/ strep throat... [sp] oh joy...]

Actually it is worse than that. NC's example the Tacoma Narrows bridge fell down not because of a constant side load but because the solid sides of the bridge acted something like a flag in a strong wind. The wind passing over the edges creates vortexes that put an alternating force on each edge which happened at the natural torsional vibration frequency for the bridge. Even a small force applied in time with a natural vibration can build up to damaging torques. In a machine tool this is sometimes the origin of violent chatter and this can occur if your structure is very stiff but very light; it tends to vibrate at a high frequency that is excited by the high speed impact of the cutter striking the work. This is why some machines have hollow structures filled with something that both provides weight and naturally vibrates at a different frequency. This lowers the natural vibration frequency below what the tool will create and because there are now two or more natural frequencies they tend to cancel each other and stop damaging levels of vibration building up.

The "big and strong you can't go wrong" philosophy is sadly quite WRONG. A bridge simply has to be able to hold itself up. Too heavy and it can't/won't. Oops!


The web in an I-beam is to prevent shear loads in the upper and lower structure, thus making them act like a solid (without the solid ;) )

I like simplifications they make life easier (but I don't build bridges or rockets) here is another:

If you take 6 2x4s and stack them together, then bend them they will fail at some point, not much above (if any) a single 2x4 alone, but when you tie them together you get them all to act as a single unit (or nearly so) but you still have the ultimate compressive or enlongation limit of the material. The I-beam simulates this. But all in all a solid is still stronger but as you point out weight is sometimes an issue.

But by bringing up a unique situation (a singular situation) and a bridge no less you have now gone outside of the parameters of our discusion (a hoist I believe).

And I believe there are instances the one could bring up that would discount any theory. (I think its usually called "the exception to the rule")

So within the realm of a hoist "big and strong you can't go wrong" is WELL within this context! or as a recent song states "I believe I was right when I said you were wrong" at least on the big and strong! ;)

See I worked a rime into it too! so I must be on the right track! ;)

its all good. :cheers:

with lagre hoist that I've use it's not just the I-beam it is also the distance between trolley wheel (front to back) that displace the weight on the veritcal members and side rails

Wow, lots and lots of good stuff here. Thanks for the good reading material Guys. Keep it coming :cheers: :D

I enjoy the rest of you guys getting "windy" it gives me something to read and saves my typing fingers.
As I said, most of my answers are very simplistic and abbreviated.

Windy and some go one liners I'm with you on this one

Big and strong can be replaced by well and appropriately placed alternate and lighter construction methods.

Look at the boom on a construction crane. Open lattice contstruction with cable stiffeners. Yet it easily picks up MASSIVE loads with an essentially a spindley structure.

My cherry picker hoist for engines, has a big long clunky piece of tubing for the arm that does the lifting. Yet, to add stiffness w/o mass they put a joist like arch on top of it. SImple flat bar stock , mounted in an arch over top, and loaded in TENSION - the way steel likes to best be loaded. Much stiffer and yet ligher than another piece of tubing weled on top.

Suspension bridge holds up massive amounts of material - yet not via big n strong structure but many small cables loaded - again - in pure tension.

Finally, a related life's lesson that I'll cherish forever. My dad was a structural iron worker - "big and strong" philosophy with body and stubborness to match. One day we were building a frame for him to mount fire wood on. Long story but it got to be a big and strong vs engineered construction project execution (I was going thru college at the time - he'd said no kid of his was going to be an iron worker, they'd do better, God bless the man).

Anyway the episode was getting more and more tense and my mom, who loved us both and never intervened, sat by out of sight suffering pains of anguish as she listened to two like willed men arguing.

Finally, after heated exchanges, I blurted out "you're the one who's sending me to school to learn how to do things with the benefit of education, why don't you let me show you what I learned??? Besides, it's my welder that I bought so why don't we do it my way, after all it is your house and we played by the 'its mine, do it my way' rules up until now".

Silence but he acquiested and we did it the engineering schooled way. It went together and worked which didn't surprise me as the book said it would (it don't always work that way but this time it did). Not anymore was said as we finished things up and picked up afterwards.

I didn't think much of it.

Many years later, at my dad's wake, the usual story telling took place. My mom recounted the story - she didn't think I was within earshot. What my dad didn't/couldn't tell me was what he said to my mom afterwards:

"I don't know how that darn kid got it to work, I didn't think it would but it did. Still can't figure out how he did it ... but it did!?!?".

Too bad my dad's abusive family life prevented him from passing on and giving supportive recognition at time when it could really would have helped. Worse, yet, he didn't ask me to explain it to him. It was nice to learn however that he did respect what I did even if he couldn't understand or give accolades for it or coulndn't ask for guidance from his offspring for once.

Yes, big n strong has its place. But there is intrinsic beauty and efficiency in well engineered construction. Besides, any time you can get some spindly thing to work and generate a "how the heck did he do that?" comment, life simply doesn't get any better.

AAARGH, Deja vu all over again - more flash backs to structures class.

http://www.cnczone.com/forums/showthread.php?t=22121

Please make it stop!!!

Seriously, structural engineering is not to be taken lightly. I can make a huge difference in whether a thingie works or does or works well or poorly.

Structures is not a SWAG art, it is truly a science....

I still haven't decided on Crower vs Carrillo but some do work better in some instances that others and I think I'm on to why.... An engine with rod offset needs better bend resistance due the net offset loading -use H beam Carrillo's here. With non-eccentric loading, the I beam Crowers work nicely.

But I have friends who run Carrillo - the conflict never ends....

I'm with DareBee ; An appropriately sized tube section would be a better solution than an I Beam.

The beam has to support Two loads, vertical deformation due to load AND twist. I beam is optimised for the former, not the latter. For the same cross sectional area a Tube section has better torsion strength than an I beam as the material is as far from the neutral point as practical - that's why torsion suspension torsion members are always round.

In a combined load like this, for equivalent cross sectional areas a suitably sized rectangular tube, long side vertical would be the cheaper solution if material cost by the pound where the only issue. Add in the practicality of accurately attaching Z axis and saw to the beam, a tube section should be easier and cheaper too. What do the commercially available saws use?

The web in an I-beam doesn't 'prevent shear loads in the upper and lower structure, thus making them act like a solid' as there's no shear loads in the upper and lower structure. As NCCams showed the upper and lower carry stress and strain loads, compression and tension only. In deflction the upper and lower attempt to move in relation to each other along the length of the beam, the web keeps them in relation and that load is the shear load and is carried entirely by the web. Thats why its called the shear web in some applications. Same principle applies to torsion boxes, the material between the outer faces is a two dimensional shear web. Ugh - no, not torsion boxes again!

The shear web doesn't usually carry any significant load due to beam load until defelction occured and even then its only a small amount as the math would show, small X section close to the neutral line.

Whatever, do a google on Strength of materials.. its a useful read and usually 'strength' isn't what you thought it was...

Andrew

edit: I didn't read your post thoughly so I will get back at you... too damn early!

:cool:

The web in an I-beam doesn't 'prevent shear loads in the upper and lower structure, thus making them act like a solid' as there's no shear loads in the upper and lower structure.


As NCCams showed the upper and lower carry stress and strain loads, compression and tension only. In deflction the upper and lower attempt to move in relation to each other along the length of the beam, the web keeps them in relation and that load is the shear load and is carried entirely by the web. Thats why its called the shear web in some applications. Same principle applies to torsion boxes, the material between the outer faces is a two dimensional shear web. Ugh - no, not torsion boxes again!

is there something wrong with this or maybe it was too early or late for you?

the first part says does prevent shear loads and the second part say it does???

are you trying to confuse me! :stickpoke

its working! (chair)

Michael , not intending to confuse.. The statements are correct, note the ' quoted text' in the first line. Text quoted from an earlier post in the thread .

Again, there's no shear load on the upper and lower section. The shear is carried by the web alone.


Well, Strictly speaking thats true, not until you get to large elastic deformation or a torsion deflection. there.... thats really confused it now.

....Again, there's no shear load on the upper and lower section. The shear is carried by the web alone.
Well, Strictly speaking thats true, not until you get to large elastic deformation or a torsion deflection. there.... thats really confused it now.

Well if you want to get really nit-picky for a symmetric cross-section the shear is at a maximum on the neutral axis and goes to zero at each flange surface, the tension is maximum at the bottom face of the bottom flange and goes to zero at the neutral axis and the compression is maximum at the top face and goes to zero at the neutral axis. So shear occurs everywhere, tension occurs in the bottom section and compression in the top but the magnitude varies.

Well if you want to get really nit-picky for a symmetric cross-section the shear is at a maximum on the neutral axis and goes to zero at each flange surface, the tension is maximum at the bottom face of the bottom flange and goes to zero at the neutral axis and the compression is maximum at the top face and goes to zero at the neutral axis. So shear occurs everywhere, tension occurs in the bottom section and compression in the top but the magnitude varies.

It's getting better and better. Lot of good points/views and stuff to learn. How long do you guys thread will be when it is done? Really good reading material :D

How about lets take an I beam and plate each open side and weld solid.
3 vertical members - ot to be strong enough - we can also add a truss structure overhead.

he he he

We could also make one out of titanium

I didn't mean to start such a big thread but it is very interesting. The I beam with plates welded on both sides would be pretty strong, but that is a lot of welding and also a good chance of severe warpage. A good example of web strenth in thin but tall material is a simple wood floor joist. Something like a 2x3 on top and bottom with osb for the web. It allows huge floor spans and also allows pretty big areas cut out of the web without sacrificing strenth. The easiest way for my beam is the rectangle tubing. No cutting ,welding, warping. Just drill and tap holes, mount linear rails and its done. I haven't decided on where to mount the rails. Top and bottom on 1 vertical side, or top and bottom horizontally.
Thanks for all the input.
Doug

Doug, I was wondering how to orientate the rails myself, just a few short months ago. [there's a thread about it someplace..] The final decision was to put both rails on one vertical face, spaced as far appart as is possible. The reason being that getting the rails aligned accurately in any other configuration is just to difficult. The added benifit of having them located top & bottom of the member just isn't worth it, once you count the cost of getting it all aligned.


At least.. thats my experiance..

Jerry