HI,
So I am engineering student at the university of maryland. I have a design project which requires me to build a truss to withstand a minimum load of 750 lbs. I figured I would post this here because I know alot of you guys think outside the box and have alot of experience and passion when it comes to building things, So I figured I would make a post and see if you guys could give me some ideas.

The basics of the project are as follows. It has to be made of wood, each piece has to be individually attached at all joints. As far as joints go we can use just about anything from glue, to steel plates, nails , etcc. The load will be applied at the very center of the truss were there must be a joint. The dimension of the truss must be 25" in length, 5" high.

Some genral questions that I am pondering about.
1. What wood would be best suited for this? I have rescearched somewhat and found a engineering composite wood called laminated veneer which seems promising.

2. Joint methods, I think this is gonna be where it really matters. I have been toying with ideas of possibly using steel or aluminum with dimple died holes throughout. I have also thought about using carbon fiber as a possible material.

Anyways, I am sure some of you guys will have some great ideas, and I will be posting pictures of the build, and all the details after it is tested. Any help or ideas are greatly appreciated.


Thank you,

 

can you give a general description of the truss? is it a single triangle or what?

and also, please clarify dimensions of useable material.
and when you say 25" by 5" high, do you mean 25'(feet) x 5'(feet)

and you say "individually attached". what does that mean?

 

this is funny i did this project last year for my mechanics class. I used thin wall stainless steel tube that i got from my work and i crushed the ends on a vice. To join them I used mini nuts and bolts to hold them together. Wood is an absolute mess to try to pin together so stay away from it. A good design woud also be flat and wide pieces of metal

 

it has to be wood

 

wow im blind my bad. well what do you have that you can get a hold off as far a machine shop at school? best way to do it, is to cap the ends of the pieces of wood so the stress is dissipated. take a tube to slide over the ends and crush the end of the tube that is being joined. drill two holes and the on the piece of wood and the new cap so you have 2 pins to dissipate the stress through, or you can run a single bigger pin.




Modified by AWOC at 8:23 AM 11/2/2008

 

thatd actually be a bad design since you're relying on the thin crushed part to support your load. and it could buckle being the weak link.

 

25"x5" or 25'x5'?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by dfoxengr &raquo;</TD></TR><TR><TD CLASS="quote">thatd actually be a bad design since you're relying on the thin crushed part to support your load. and it could buckle being the weak link.</TD></TR></TABLE>

depends on how thick the tube is you'll be surprised the strength steel has. plus to join the crushed end you'll need two flat pieces, one on each side. this way there wont be any buckling.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by AWOC &raquo;</TD></TR><TR><TD CLASS="quote">

depends on how thick the tube is you'll be surprised the strength steel has. plus to join the crushed end you'll need two flat pieces, one on each side. this way there wont be any buckling. </TD></TR></TABLE>

Regardless of the strength of steel, his project has to be made of wood so that is the goal of this thread. Try to keep it on topic.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by AWOC &raquo;</TD></TR><TR><TD CLASS="quote">

depends on how thick the tube is you'll be surprised the strength steel has. plus to join the crushed end you'll need two flat pieces, one on each side. this way there wont be any buckling. </TD></TR></TABLE>

yes there could be with a large load. its a flat plate. very weak in buckling.
trust me im an engineer i know how strong steel is.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by -RedneckDave- &raquo;</TD></TR><TR><TD CLASS="quote">

Regardless of the strength of steel, his project has to be made of wood so that is the goal of this thread. Try to keep it on topic.</TD></TR></TABLE>
and the ends can be made of something else apparently.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by -RedneckDave- &raquo;</TD></TR><TR><TD CLASS="quote">

Regardless of the strength of steel, his project has to be made of wood so that is the goal of this thread. Try to keep it on topic.</TD></TR></TABLE>


he said steel plates can be used!!!!!!!!!!!!!!!!!!

well Mr. engineer since I'm only a junior year engineer to be, and since my ideas aren't good enough but you seat there and argue with mine. whats your idea then????????????? I hope you build wooden trusses for a living!!!!
Maybe you should look at a truss and see how the forces in tension and compression dissipate in the truss. Many members will dissipate the load without ANY problem. He needs to figure that out on his design and calculations! Plus there will be more than one crushed tube on that same joint. Trusses have many bars that join together in one joint. Making the tubes put together even stronger.

If 750 lbs is spread through many members, it is NOT a problem for a crushed high tensile steel thick wall tube and remember the two plates that are going to be holding it still. That plus 2 or 3 or 4 other bars that will be joined together in that joint to dissipate the weight. 750 lbs is not close enough weight to buckle the 4 high strength steel crushed tubes sandwiched together plus the mounting plates. I'm talking about tube that will 2"+. In a truss not one particular joint will see the 750 lbs of force on it. The force will be spread through all the joints. it is up to the designer to figure out the best design.

I'm open and interested in other ideas

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by AWOC &raquo;</TD></TR><TR><TD CLASS="quote">


he said steel plates can be used!!!!!!!!!!!!!!!!!!

well Mr. engineer since I'm only a junior year engineer to be, and since my ideas aren't good enough but you seat there and argue with mine. whats your idea then????????????? I hope you build wooden trusses for a living!!!!
Maybe you should look at a truss and see how the forces in tension and compression dissipate in the truss. Many members will dissipate the load without ANY problem. He needs to figure that out on his design and calculations! Plus there will be more than one crushed tube on that same joint. Trusses have many bars that join together in one joint. Making the tubes put together even stronger.

If 750 lbs is spread through many members, it is NOT a problem for a crushed high tensile steel thick wall tube and remember the two plates that are going to be holding it still. That plus 2 or 3 or 4 other bars that will be joined together in that joint to dissipate the weight. 750 lbs is not close enough weight to buckle the 4 high strength steel crushed tubes sandwiched together plus the mounting plates. I'm talking about tube that will 2"+. In a truss not one particular joint will see the 750 lbs of force on it. The force will be spread through all the joints. it is up to the designer to figure out the best design.

I'm open and interested in other ideas </TD></TR></TABLE>

why don't you just make a mild steel box out of 1/4" hot rolled steel?
i made a box to hold rod. 6010 ends horrible ugly welds and it is cabable of holding that easly. probably find a way to make the wood look like it suports the weight.

carbon fiber does have a high tensile strength but you sacrifice all the other properties of steel to get the lighter weight and higher tensile strength.

aluminum pretty much the same thing, with the lowest tensile of all of them. and being one of the weaker metals, there really isnt need for aluminum here unless it needs to disapate heat or for some sort of heat sink.

rember that you're pretty much using sheer strength, also use the thickest plates you can.

eh im not much help, im not an engineer, or and engineering student. or great at math. just a welding student that likes cars. so my experience with materials is only 1st hand.

i could'nt tell you how to use solidworks and all that you kids are pretty smart

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by dfoxengr &raquo;</TD></TR><TR><TD CLASS="quote">can you give a general description of the truss? is it a single triangle or what?

and also, please clarify dimensions of useable material.
and when you say 25" by 5" high, do you mean 25'(feet) x 5'(feet)

and you say "individually attached". what does that mean?</TD></TR></TABLE>

It is inches I put the inches symbol after the measurements. When I say individually attached it means we cannot have say one long piece running across the bottom or top, each piece must be seperate. The truss can consist of any design we want with as many or as few triangles as we want. The goal is to make the truss as light as possible while still being able to withstand the 750lb load.

 

I can tell that the other guy is an engineering student now since he has bad grammar. lol.

Also carbon and other compoosites arent that great in compression so I would stay clear of them.

Anyways. the crushed tube idea is still bad because theres no way to connect them flatly with multiple members intersecting at one spot. this would make it bulky and put the connecting rod or bolt or whatever in bending which is even worse. so you have poor design because of bending in that bolt/rod, and buckling case in the crushed tubes.
poor design.

if you have no material limitations then just make a simple truss
<U>/|\</U> out of 2x4" then some rectangle 1/8 wall tubing onto the end few inches, and put some 3/4" holes in the center of the capped ends to connect the capped ends with a piece of 3/4xsay .065" wall tubing by a press fit. make sure to double shear say by sandwiching that vertical member by the outter ones, as well as the vertical one by the two lower ones.

Now thats the easy way out.

You should really replace a lot of those with members that are sized properly for the job.
So say that your vertical wood is 5000psi in compressive strength parallel to the grain ( a low number for soft woods). Use this member as the only load supporting one and oversize it for a pretty good safety factor of say 10 to get 1.5" square. Use the angled piece and lower ones to just support the thing and they can be strips of like 1/4" thick by 1" wide. use 2 on each side(here / and here \ ) but make them sandwich your vertical on like this L R V R L where you have 5 total, L being left side, and R right, V being vertical. use the same endcap idea for the vertical, and then washers between the others will be sufficient. For the lower horizontals you could do the same thing as the angled ones with a 1.5" spacer in between to take up the space. thats a done deal and you have a large safety factor and its super light. But dont forget one buckling calc for the vertical, but that should be fine since itll be so short.

should be light and simple.


Modified by dfoxengr at 11:22 PM 11/5/2008


Modified by dfoxengr at 11:23 PM 11/5/2008

 

I found two videos of last years designs being tested. Btw, thank you guys for the input.

http://www.youtube.com/watch?v=1bG13kvgsjc

http://www.youtube.com/watch?v=FrFLJBdRjI8

 

Ok, my idea wont work for that then. You didn't say you were making a bridge
but you can still use the ideas i gave you to make something that will work.
good luck

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Race Egr &raquo;</TD></TR><TR><TD CLASS="quote">

It is inches I put the inches symbol after the measurements. </TD></TR></TABLE>

A simple "it is inches" would have worked....

 

I am not an engineer but I can give you some advice from my wood working experience....

one of the strongest joints for wood is a lap joint ....
if you combined those joints with a strong glue...
and then ran a bolt through the center of the joint with washers on both sides of the joint, you could apply compression to the joint to give even more strength to it

there are several variations on the lap joint....just do a search on them and see which one works the best for you, some of them like the dovetail could be tedious if you have a bunch of odd angles and alot of them to do

overall the weight wouldnt be that much higher than just the wood by itself, because hardware would be kept to minimum

heres kind of a rough idea of the pining technigue with a half lap joint
keep us posted with pics if you can.....sounds like a fun project


edit....what if you ran 2 beams (like 2 20"beams) almost the entire legnth of the span and had them half lapped/glued/pinned them for 17 1/2" giving you an overall legnth of 25"...technically speaking it wouldnt be a solid beam

then you could just build all the braces off of the main support beams

Or just bring in a big old chunk of 4x4 fence post,lol...I Know it would hold 750lbs

Modified by srmofo at 10:38 PM 11/6/2008


Modified by srmofo at 10:39 PM 11/6/2008


Modified by srmofo at 10:41 PM 11/6/2008

 

Wow, what a mess this started as...

Race Egr -

I don't have any genius easy plans to help you, but I do have some observations. Firstly, the videos you found obviously illustrate the direction you need to be taking. Between the two, however, you heard the rather massive failure point difference.

Video 2's bridge used similar sized beams, which we can assume are similar strength (which, of course, might be totally incorrect, but for the sake of argument yada yada). They also used thicker joint plates. They have 2 very distinct differences though.

Firstly, they appear to have used glue, instead of pins (bolts). The difference between the two gives you 2 very distinct load cases. The glue, which is going to be weaker, does allow you to better distribute the load across a larger area (a mini stressed skin, almost), and a pin is going to concentrate the stress at the leading edge of the hole it is located in. However, the glue is also going to act like a glass in failure, meaning that any bending component is going to create an increased failure point over a pin (because pins have no bending component of stress, just displacement), and doesn't really handle shear stress as well as the bolts. I think this is best shown from their initial failure, which didn't occur under the main area of load, but rather at the horizontal locations one beam out, which were seeing a significant bending load. Basically, their structure was too rigid (I know, it doesn't sound right, but deflection does not mean failure), and couldn't handle normal load deflection as well as a pin jointed one.

The other major difference is that, under the main load point, Video #1 chose to use compression based diagonals with no vertical support, while video #2 used a compressed vertical, with tension based diagonals. While, of course, a member in tension is going to be much stronger than one in compression (especially flat plates or crushed joints, AWOC), Video 1's bridge has zero bending component, and has a highly stressed member (the horizontal lower, directly under the load) in tension. They spread the compression component between 2 diagonals (where video 2 had one beam in major compression, that always created a 4 beam bending component), and it worked for them. Note that their main failure was the shear crack in their joint plates, not a major loss of a beam section. Different thickness or material choice would increase the strength there.

Just some food for thought...

P.S. - You noticed I talked about their bridges in 2d. This doesn't mean that you don't need to worry about cross braces, but the forces on those are going to be tiny compared to the 'walls'. Better construction standards will decrease them even more (a squared structure has little racking and splay, where as a non-squared one will induce a side load multiplying the rack/splay component)


Modified by Goullish at 11:38 PM 11/6/2008

 

pin and glue 2 pieces of wood, then stick weld a fat piece of 1" mild steel to use as a bracket

 

Go to a house being built and check out some trusses before they're installed, go to a company that manufactures prefab'd engineered trusses and talk to some of the engineers. Game over.

As a fellow engineering student, I'm extremely [freak]ing shocked to see you on Honda Tech getting advice on designing trusses! Didn't they teach you anything in engineering yet? Statics and strengths of materials? Structural Design? Not trying to be a dick but that's [freak]ed up. I'm in municipal civil engineering so I don't really do anything with wood or trusses but my buddies in structural civil take courses in steel design, timber design, reinforced concrete... If your project is making trusses I would hope you've taken some of these courses...

When I need help on a project I go to instructors/professors/text books, etc... Honda Tech is the distraction for me not the answer.


Modified by Justin Jones at 12:33 AM 11/7/2008

 

Easy, Rodney...

I'm guessing this is for a base Statics class, which would be late freshman/early sophomore level. Mechanics of materials comes later...

 

wow, this topic has gone nuts. And justin Jones guy, you need to relax, cause if we took those classes yet we wouldnt make the comments you didnt like so much.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by AWOC &raquo;</TD></TR><TR><TD CLASS="quote">
he said steel plates can be used!!!!!!!!!!!!!!!!!!</TD></TR></TABLE>

No need to get upset. I was only pointing out the original post said that if could be joined with metal. The object is to build something out of wood that will withstand the weight.
Just for reference
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Race Egr &raquo;</TD></TR><TR><TD CLASS="quote">The basics of the project are as follows. It has to be made of wood, each piece has to be individually attached at all joints. As far as joints go we can use just about anything from glue, to steel plates, nails , etcc. The load will be applied at the very center of the truss were there must be a joint. The dimension of the truss must be 25" in length, 5" high.</TD></TR></TABLE>

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by AWOC &raquo;</TD></TR><TR><TD CLASS="quote">well Mr. engineer since I'm only a junior year engineer to be, and since my ideas aren't good enough but you seat there and argue with mine. whats your idea then????????????? I hope you build wooden trusses for a living!!!!</TD></TR></TABLE>
Where did I say I was an engineer? Technically neither are you. I was not arguing with you.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by AWOC &raquo;</TD></TR><TR><TD CLASS="quote">If 750 lbs is spread through many members, it is NOT a problem for a crushed high tensile steel thick wall tube and remember the two plates that are going to be holding it still. That plus 2 or 3 or 4 other bars that will be joined together in that joint to dissipate the weight. 750 lbs is not close enough weight to buckle the 4 high strength steel crushed tubes sandwiched together plus the mounting plates. I'm talking about tube that will 2"+. In a truss not one particular joint will see the 750 lbs of force on it. The force will be spread through all the joints. it is up to the designer to figure out the best design.</TD></TR></TABLE>
From the original post.
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by race egr &raquo;</TD></TR><TR><TD CLASS="quote">The load will be applied at the very center of the truss were there must be a joint.</TD></TR></TABLE>

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by AWOC &raquo;</TD></TR><TR><TD CLASS="quote">I'm open and interested in other ideas </TD></TR></TABLE>
Well at least that is good.

 

Gorilla Glue

///TROPHY\\\