Science Buddies: "Ask an Expert"

we are looking for the best "strength to weight ratio"*...
i need the lightest and the strongest glue-i chose the weldbond glue..good choice?
can we use the stress formula to find the amount of load it can carry?
if so, how?
* strength to weight ratio is after we make the tower, we are going to weight it in and then check how much weight it can hold, the dead load divided by the load put on it is the strenght to weight ratio

Hi dhanya,

Welcome to the forum!

I found a few resources that you might find helpful, but to help you best we will need to know a bit more:

--Have you defined your project question? It sounds like this might be something like "How does tower weight affect tower strength?", but I can't quite tell from your question. You can find info about this part of the science-project process here: https://www.sciencebuddies.org/science- ... ndex.shtml (see "Your Question" and "Constructing a Hypothesis" in particular). By "strength", do you mean the amount of weight the tower can support, or something else? If, rather than for a traditional science fair, this project is for a competition to see how much weight your tower can support, you should have been provided with a description of how the test will be conducted, and that's probably relevant.

--What is the "stress formula" you mention? A possible project question could address whether this formula accurately describes your structure.

--What geometry will your tower have? Or are you thinking of comparing towers built in different shapes? You could have towers of the same weight built in many different ways, and they would be able to support different amounts of weight before collapsing. If you want to see how changing a tower's weight will affect the amount of weight it can support, you'll need to come up with a way of keeping the shape constant.

There are some recommendations for adhesives to use with balsa wood (a light wood popular in competitions with weight limitations) here:
http://answers.yahoo.com/question/index ... 031AAdXnU7

A few things that you might want to keep in mind as you're planning how to conduct the experiment: Ventilation can be important to avoid inhaling fumes from many glues. If you are going to build multiple identical structures (a good idea if you conduct a test that involves destroying a tower -- you would want average the weights that caused several identical towers to break), you probably want to think about how to make sure you apply to same amount of glue to the same places.

I don't think this is the same project you're discussing, but it has some related information: https://www.sciencebuddies.org/science- ... p013.shtml
If you're basing this on another Science Buddies project, could you post a link to it?

Amanda

Oh also: it looks like people have successfully used Weldbond for popsicle-stick structures before:
http://www.garrettsbridges.com/photos/p ... ick-bridge

And there are miscellaneous designs here:
http://www.garrettsbridges.com/category ... le-bridges

i need the lightest and the strongest glue-i chose the weldbond glue..good choice?

The weight of the amount of glue used compared to the weight of the popsickle sticks is probably very small so I would be more concerned about the ease of use and strength of the glue joints. "Weldbond" is definitely a readily available and easy to use glue for wood joints. In general, the strength of a wood glue joint is a function of the surface area and whether the joint was clamped/weighted with too much or too little pressure while it dried. Too much pressure will force out too much glue and it will be a "dry joint". Too little pressue will allow too much glue and it will be a "plastic joint" that can be pulled apart more easily as the glue will "flow" in a "plastic" state like taffy or play-doh. If the joint is allowed to move during the curing time, it will be accumulate fractures in the glue which will pre-weaken the bond.

can we use the stress formula to find the amount of load it can carry?
if so, how?

Yes and No and with great care and a deep understanding of the subject. There are many formulas that engineers use to "predict" the expected safe load and expected deflection of "beams" that are specific to specific applications. There are engineering tables produced by the wood industry for estimating certain strength properties for lumber by wood species and dimensions. The problem is that these are only "estimates" of "expected" values and any given piece of wood may behave better or worse than "expected" in a specific application. When applied by the most experienced engineers, these tables and formulas only provide a best guess at the expected failure point or to a meets or exceeds some commonly accepted "safety factor" for some type of application.

The "stability" of a structure such as a tower is often more important that its expected crush strength. If there is any "lean" in the tower, any downward force becomes a tipping force rendering the crush formulas mute.

The strength of a complex structure is only as strong as its weakest link. For example, if your tower consists of 4 corner posts and one is shorter than the others, it will likely have more weight transfered onto it. This means that it is very important to matching the lengths of all the parallel pieces.

If a vertical structural member bows or bends under the load, it will cause torque on the glue joints and it will become shorter and have even more weight transfered onto it. With popsickle sticks, the thinest dimension is one that is most likely to deflect so that needs to be considered in the geometry of your bracing design.