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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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 approve the Warren truss with added vertical member, because with a very smal cahged we can constat very big difference of load transmission. and also this can be very usefull.
Note: sory for the text error because i am not english speeking.
hi, i’m doing the Aureacon bridge building comp. , and was wondering if you could show some of those with the force IN THE MIDDLE, BUT PLACED AT THE BOTTOM OF THE TRUSS (as if cars were on it) so i can see the pressure distribution there.
e.g.
____________
- /\– /\—-/\– /\
/__\/__\./__\/__\
PRESSURE AT THE DOT
THX, and i hope it came out
trying to redo the bridge
–____________
, /\– /\— /\– /\
/__\/__\./__\/__\
PRESSURE AT THE DOT
THX, hope it came out
i’m so silly, this is a U.S. site, all the same, i am in AU, so if you can, is it possible to rely to me in metric units? i don’t really know how to convert it. But if you can’t either, I will use a google converter
please reply 2 me, i need help!!!
Nicholas, the force distribution is the same if the load is on the top or on the bottom of the bridge.
even though i agree with you, it is the problem of compression VS tension that i am up against.
nicolas is correct.
I am doing a science project on which bridge structure would hold the most weight/ be the most effiecint. How would you consider doing that?
I just don’t know which bridge designs are known as the best!
cool, this helped me a-lot in my research paper!
i think it is pretty cool. the resion i said that is because it show numbers and stuff
this is an awesome website for people who like building
bridges
hey I’m Jessica and I’m in high school and I take woodshop
We’re doing a project on building bridges and I decided to do the Town Trellis Truss on an Arch styled bridge. Our bridges have to hold over 100lbs (minimum). I was just wondering if you could PLEASE post some things on the Town Trellis Truss – that would help me out so much!!!! THANK YOU SO MUCH!
I am wondering if adding piece in the middle of the triangles in the K Truss would help any with the weight distribution.
i love the idiea of the toothpick bridges kids should be able to share their idieas and for this project.
Great website. it helped me a lot with my bridge structre