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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thanks………
i got all i needed ….
but i still think howe is better than the other two,but u said that all are just the same.
so can you please explain me that by dropping a mail to me.
Hi I have to make a bridge out of straws and masking tape that spans one meter in between two desks plus it has to hold two 5cm cubes full of sand which i calculated and can be no more than 20grams. The most efficient bridge able to hold this weight wins (least amount of straws). Any suggestions on what type of design i should use?
I have the same problem. Can SOMEONE please help us?
Where must the cubes be located in the span?
im making a warren for my elective with balsa wood.
what would be better a subdivided warren type or a warren with vertical supports?
I do not know what you mean by a subdivided Warren truss. What does this look like?
The vertical supports on the Warren truss allow the bridge to sustain more weight. =) <3
Reference the zero force members. In real life, the loads on bridges are applied at the panel points, so those members would carry a load, even if just a panel load.
You would make member identification much easier if you were to use Bowe’s Notation. With the external forces shown in place, number all the spaces between the forces. From left to right, label the interior spaces with alpha characters, starting with A. Each force can then be identified by thespace number before and after; each member can be identified either by a number-alpha or an alpha-alpha designation.
Thank you for your comment. Could you explain more about panel load?
I will look up Bowe’s notation. Do you know offhand where I might find an example (a picture) of it?
Thanks.
The panel points are the points where the load is applied, usually the joints on the top and bottom chords. For a bridge, beams, carrying the roadway or railroad, would span between two trusses and apply their loads to those points. For a roof truss, the purlins land at the top chord joints.
I found a site for you that shows Bow’s Notation, slightly different than I described because he needed to number the joints for the graphical solution that he describes. The graphical analysis is very accurate, before computers and calculators came on the scene, I used to check my calculated results that way.
http://wjesus.org/Frame_3.htm
Hello!!! im at school trying to win the bridge building competition !!! you guys really helped me!!!!
thx thx thx!!!
your Falaviena
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.
200 pds wow if u want us to believe u say something reasonable 40X10 cant hold 200 pds it would have to be 50 cm high considering u used normal wood glue.
hey josh im tiana and im in yr 7 and we are making bridges and how do you know if it is the write bridge and there is other choices what do you do ??????
When you stated that for a Howe Truss you would have to use larger compression members, were you referring to the diagonal members or vertical members? Thank you for your clarification.