Truss Design
Posted on August 16th, 2005
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.
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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.
well its not the best bridge i’ve seen i really like the idea of the warren bridge ppl should look into that
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???
I’m buliding a arch bridge out of flat toothpicks for a science project.
What truss do you think will be best for an arch bridge?
wow this site is soooooooooo great. I can’t even believe that miley cyrus and demi lovato use this site too! who knew?! well we have to build a bridge. We never have before and we’re SOOOOO excited. I never understand how to do it. but thats okay. Mrs. Konchesky is our inspiration. We love you. Woo. Holy Chum in a…mailbox…bucket…this site!
this is very helpful! i have a project where we need to build model bridges and i think this will help with drafting my design.
this site is very good for the studdy ofmechanics trusses
I love this site…it gave me a lot of help for our project. Thanks soo much. I just hope that the bridge that my group makes wins. Um, our bridge is meant to be 40 cm long and we have 150 paddle popsticks to complete it. Anyway, thanks a lot again.
thank you for the great info…it helped a lot on my project
Which truss can support the most weight if done correctly.
This helped my assignment a great deal,so I offer you my gratitude.
The school I got to St.Peters grade 6s had to build a bridge for Design+Technology and this website has really helped alot thank you!!!
thanks for the tips. i have to make a balsa wood bridge that will support 15 kilos in the center and the truss designs really helped.
hey, i dont really understand why is there no load in the middle part of the warren truss. Can anybody help me explain? thx
I am in need of willmot digram of forces.Would u please help in this regard
i made a truss bridge that was 40 cm long. i used the warren truss design. i added vertical suports also though. it was made from popsicle sticks and wood glue. the key is distrubuting the weight evenly. my bridge held over 200 lbs. depending on the materials and how much you have to work with, you should be able to do the same.
Also a determining factor would be exaxtly how the weight is going to be applied to your bridge. i also forgot to mention that my bridge was 10cm wide.