Hello.  My name is Carrie and I am in a physics class that was assigned to make a bridge entirely out of toothpicks and glue.  It must be at least 30 cm long and the height does not matter.  My partner and I are using the Pratt truss, but we were wondering if it was necessary to put smaller triangles inside the larger ones. We are clamping our glue overnight and we will make the joints on another day.  Any advice that you could give us would be great.  Thank you!
-Carrie

Why are you using the Pratt truss? According to our experience, in the truss structure out of toothpicks, the strength of the adhesion part is very weak in comparison with the strength of the member. Most of my students choose Howe truss rather than Pratt truss. When Howe truss structure is adopted, it can be done small load to the tension members. How to make a contact point is important.

To put smaller triangles inside the larger ones may be effective if the member is in fear of buckling.
At first, Judge a compression member or a tension member. When that is a compression member and long enough, it has the possibility to break with buckling.

Send me a photograph of your bridge and your rules if you want more advice.

Good challenge!
Sincerely yours

From: Carrie Peters
Sent: Monday, December 16, 2002 8:43 AM
To: Tanimura
Subject: our bridge project

Dear Yasuyuki Tanimura,
Thank you so much for your advice. Unfortunately, we do not have a picture of our bridge because we are not done building it. Here are some more details about the assignment:

-only use standard sized toothpicks and glue
-bridge must span horizontal opening that is a minimum of 30 cm
-width of bridge must be at least 4 cm. There is no limit to the overall height of the structure, but it must provide a "roadbed" that is no more than 5 cm above the top surface of the test supports. The "roadbed" is placed where "cars" would drive if this bridge were a "real bridge." You cannot have toothpicks in the middle of the "road." The load used to test the strength of the bridge will be applied to the "roadbed"
-the bridge may touch the top surface of the supports for any length but it may NOT touch the inside vertical surfaces of the supports.
-the bridge must weigh less than 50 grams

STRENGTH TEST: a wooden block (50x50x12 mm) will be placed on the roadbed, approximately at the center of the structure. A bucket will be attached to the bottom of the rod and then mass will be slowly added to the bucket. Loading will continue until the bridge fails (can no longer hold weight).

We also have to write a report about our bridge. We chose to use the Pratt truss because we thought that it would be stronger than the Howe truss. We thought that the diagonal toothpicks in the Pratt truss would be able to handle more tension and the vertical components would be able to handle more compression. Is this true, or is the Howe truss better? The diagonals of the Howe truss go in the opposite direction.
So far, we have glued pairs of toothpicks together and clamped them while they are drying. We are not gluing them end to end, but overlapping them so the joints will be stronger (similar to how it says on your website). Our teacher told us to do this as well.


This is a picture of the Pratt truss, which my partner and I are using as of now.


When I asked if it was necessary to put smaller triangles within the larger triangles, I was referring to this design (Baltimore-Pratt truss). Would this be too difficult to build and would it be much stronger than the original Pratt truss (up above)?


You said that most of your students use the Howe truss. Do they use this simple Howe truss, or the one below with counter braces?

Thank you so much for your time. I hope that this makes it easier for you to understand what our project is and how we chose the Pratt truss. If you have any more advice, that would be great. Thank you!
-Carrie

>So far, we have glued pairs of toothpicks together and clamped them while they are drying. We are not gluing them end to end, but overlapping them so the joints will be stronger (similar to how it says on your website). Our teacher told us to do this as well.

That is a very good idea.

Here are some hints for you.

You will know that many toothpick bridges break from the joint part at the strength testing, as many actual bridges break from the welded section.Destruction begins from the weakest point, and does a chain.
The following figure shows the deformation of the section in the truss.

Which joint do you think breaks easily? I think that is the joint of the diagonal of the Pratt truss.

It may be OK if you devise a joint part like the following figure.

1. Make strong joint parts.
2. Make the members of the truss fat.
3. Make the height of the structure tall.

You can probably use about 300 toothpicks. You are very smart. If you take care of attention above, you might build a strong bridge. And your teacher might shout like this. “There is no weight! It's too dangerous!

I really want to read your report. If my advice is useful for you, that would be my great pleasure

Thank you so much for more advice. My partner and I will reconsider which truss we use and hopefully we will be able to build strong joints. Our report is not written, as we have not finished our bridge yet. If you would still like to read it when we are finished, I would be happy to send it to you. We are required to do research for our bridge and we are also required to use outside sources, so I plan on using our email correspondance and diagrams as one of our sources. One problem is that I did not save the second email that I sent to you with the truss diagrams and such. If you still have it and could send it back to me, that would be wonderful. If you already deleted it, it is not a problem...I think I remember what I wrote! I was wondering why making the height of the structure tall would increase the amount of weight the bridge would hold?

Thanks again for replying to our emails and for answering our questions.  I now understand tension and compression better. Thanks!

-Carrie

Thank you for your quick reply to our question on our paper. Your response was quite helpful.  We have not tested our bridge yet.  We will be doing so in about a week. My partner and I ended up combing some aspects of the Howe truss with our own design.  We hope that it will work well.  Both my partner and I are in high school and enjoy our physics class a lot. I will let you know how our bridge works. Thanks so much for your help.

Carrie