Truss Design
Common trusses used in engineering:
Warren
Pratt
Howe
K Truss
The following figures are shown under a load. The numbers represent percentage, where the total load = 100. The numbers are rounded to the nearest 5. For absolute numbers, visit the Bridge Designer.
Red represents tension, blue represents compression, and green is no load.
On this Warren truss, each of the down arrows represents 50% of the load. Notice how the two middle pieces have no load. Notice the load is quite a bit less on the ends of the top and bottom chord than in the middle. When you build for efficiency, keep in mind that for a bridge loaded in the center, the ends can be smaller than the middle.
Now examine the same Warren truss, but with added vertical members:
The added vertical members serve to break the top chord into smaller segments, making it stronger. However, the percentage of the load has increased both on top and bottom. Also notice where the green members are now.
Pratt Truss
I want you to notice the difference between the Pratt and Howe trusses under a load. The Pratt has bigger numbers on both the top and bottom chord, but its middle compression members are shorter, and hold less than those on the Howe. But the load is still concentrated in the middle, and gets less further to the ends on both.
Howe Truss
It is a trade off between the Pratt and Howe. For the Pratt, you’d have to use bigger top and bottom chords, while on the Howe you’d have to use bigger compression members.
K Truss
The K truss looks very good on paper. It shortens the lengths of the compression members compared to the other trusses. However, one must wonder if it adds additional weight simply because of the number of members. It is really interesting to note the two green members on the K truss, in theory those pieces could be taken off. However, I had to include them to make the truss design program work. This shows only one orientation of the K truss. If I reversed the direction of the K’s, I wonder how much it would change the forces.
The one thing I don’t like about this truss is the long vertical compression member in the middle of the bridge. If that one member could be shortened or even eliminated, I think the bridge would become more efficient.The K truss would be the hardest of these trusses to build. This is something worth considering. Making a strong joint that would make the most of the switch between compression and tension of the vertical members would be difficult.
If you are interested in learning more about trusses and truss design, check out Truss Fun, Second Edition
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so which one would hold the most weight??
that question can really not be answered. there are alot of determinig factors. 1)how the weight is applied. 2)what materials you use. 3)how much materials you have to work with. etc.
I want to build one project under civil department. its a competition..
I have to build a truss structure using Popsicle. But it will not be tested with vertical load but with horizontal force. Actually judges will attach a string on the top of my Popsicle truss structure which will go over a pulley and will carry a load at the end of the string..
Could anyone plz suggest what structure will be the Best for this very purpose! I need help as soon as possible.. Thnx.
I personally made a warren truss bridge out of popsicle sticks and wood glue. It held over 200 lbs. i cant say that this is the best design to use though. You just need to find which design distributes the wieght most evenly. But i didnt exactly understand how you said the weight would be applied.
i have to biuld a bridge for science fair do you think that popcile sticks would be the way to go , but i also need the bridge to break at some time with in 5-10 minutes. anyone can answer to this i just need some kind of answer.
/:( !!!!
i would also like it if you just posted here on the website!!
please someone respond soon!:D
thank you
p.s. i think that this website could use some more information on the different kinds of bridges and not just on your experiments you should have a viriety of appinions so that people will see two or more sides of the story!! OKAY? DO NOT take this as a insult i just think that it would be better that way , i thnk it would guve you more viewers than what you have now !! just my apinoin but i bet other people agree with me on this matter!! *****
sorry if that sounded wierd
Rose, part of the reason I have the option to leave comments is so people can share their opinions about my bridges or projects they have worked on. I also have an option for people to upload photos of their own bridges, so this site is not just my own work.
Any bridge can break within 5-10 minutes. It depends on how strong it is and how fast you load the bridge.
I am confused about what would be considered tension and what would be considered compression. I understand what the two are but I dont understand how to apply tension and how to apply compression. Heres an example. If you place a truck in the middle of a howe bridge, would that be applying compression tension or a combination of both. How about if you have a bucket dangling under the center of a Howe bridge, would that be applying tension compression or a combination of both.
Best regards,
Allan
For a science fair i have recently done, i had the warren, pratt and howe trusses all trusses.
the efficiencies in weight held over bridge weight were:
warren – 385.6 reoccurring
pratt – 425
howe – 340
These are not averages of several bridges, and they were only 1 foot long (cause thats what the stores had in stock.)
In this test, they were weighed by hooking a rope to the bottom beam, putting a bucket on the rope, and adding coins to the bucket (and a ten-pound weight).
I hope this answers some questions on which is best (the pratt)
Of course i had 30 pounds of coins.
p.s. thanks for the info, this is very helpful
Has anyone else noticed that the K Truss looks like it has arrows in it as well as Ks? Its like an optical illusion! Great Website, and thanks for the tips!
. It is the simplest design of the Truss Bridge, with two sides, a bottom, and the railing in simple triangle forms. The bottom of the side of the bridge is 11 inches in length and the top of the side is 10 inches in length. The little rails on the inside of the left and right side of the bridge are 2 inches in length. The bottom of the bridge…… Some of the things I learned in making this bridge were the different jobs involved in the process of actually building a real bridge. You need to have an engineer, an architect, a scientist, and a mathematic. You would need an engineer to help design the bridge. You would need an architect to oversee the construction of the bridge. Also, the architect would prepare information on the structure’s design and specifications, materials and equipment, estimated costs, safety, and construction time. They also make scale drawings and make sure that the bridge meets the building codes and laws. Also, to build a real life bridge, you would need a mathematic. They would make sure that all the little details in the bridge were exact and specific so the bridge was safe, and also look right.
what kind of glue works the best?
Thank you so much! My school is entering some groups into a Bridge building competition where all the materials you can use are 3 x 5 note cards and Elmer’s school glue! This website really helped us alot! thank you!
wow, this gave us alot of good ideas for school
YEAH THE BRIDGES ARE KINDA COOL…. IT WAS A BIG HELP THANKS
what would be the best truss design for a bridge in which the load is applied from the bottom,. my bridge is the same structure as the above ones , i’m just not sure about the truss design,.
how is the structure of trusse in which load has to hang from bottom my truss design is same as shown above
hi, im in yr 12 and im currently doing an assignment on bridges
ive built the warren truss bridge with vertical members,
ive had a look at your diagram with the load factors, but im quite confused, you say that the load factors add up to 100% but i dont see how the numbers come together to make 100%
i can only see that when 85% + 15% = 100% at the bottom line, but with the 60% lines i cant figure out how they would add up to give 100% please explain to me =]
Actually, he meant to say that the two black arrows above the bridge represent a single weight, distributed evenly between those two arrows (thus, 2 points at 50% each = 100%), the numbers on the bridge lines are the force amounts(not percentage) most likely, in Newtons. Hope that helps explain things better. There are many free online bridge building games for you to try your own designs and see how they work. I tried a K-truss design in one program from West Point, but the forces were completely reversed. It might have been the program’s physics, but I’m not sure. I had lots of compression at the top and tension at the bottom.
If you have a 5in section of your top chord in compression, which is stronger; a vertical member going from the bottom chord up and bisecting the top chord or save the weight and remove this vertical member and just add more lateral bracing?
I have an assignment atm and a cart is being moved across the top of the bridge, the bridge must weigh 150g max and we were thinking of using epoxy glue and doing a warren design with vertical supports in all triangles, and the vertical supports and slanted supports would be made of grouped spaghetti wheras the horizontal beams would be made of fettucine, is warren the best design to maximise its ability to hold weight in this instance?
I have a study project which uses ‘meranti’ timber instead of steel. The deck is designed to use the plywood. As you know, the more materials I involve in the bridge construction, the more the weight and truss will also involved. So how can I design the truss (the girders as well as piers) so that the bridge can support 4 men life load. The bridge is 2.5 m long, 500mm wide and height. The timber cross section is 12mm x 40mm.
hi garret i really like your website can you plese tell me how to glue the members togeather if you cant use nails. shuld i glue them dirrectly on top of each other or next to each other there are three members, i am using the warren truss
this is a good webstie for teens and it gives the motives for kids to learn how to build bridges.
is this Mrs. Goodfellow from CMS???
Why yes, I am from CMS.