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Pratt vs Howe Truss

by DefyPhysics
Tags: howe, pratt, truss
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DefyPhysics
#1
Dec5-07, 04:20 PM
P: 3
1. The problem statement, all variables and given/known data
Hi, I have to build a Popsicle Stick bridge for Physics class. I really don't have much knowledge on bridges as most of this was to be researched. I did a lot of research but I'm having trouble deciding between these two trusses.

Restrictions/Requirements:
Length: 55 cm
Height: 4 cm
Width: 8 cm

So my main problem is the type of truss to use, Howe or Pratt. The load will be focused in the center.

2. Relevant equations



3. The attempt at a solution

I used a Bridge Designer program to determine the force/weight distribution for each truss. Pratt is on the top, Howe is on the bottom:



The main differences are that the Pratt truss has no force on the ends and the compression members are vertical. The Howe truss has no force on the center and the tension members are vertical instead.

Here's why I can't decide between the two:

- The Pratt truss uses vertical compression members. Since the height restriction is 4cm, wouldn't this allow the bridge using a Pratt truss to be stronger against compression? (Compression is inversely proportional to length, according to the research I've done)

- The Howe truss has no net force in the center, where the load will be placed. Does this automatically make it better than the Pratt?
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PhanthomJay
#2
Dec5-07, 10:34 PM
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It appears that you have inadvertently applied your load at the top of the truss instead of at the bottom, because that's the only way you're going to get a zero force in the center vertical chord for the Howe, and as compressive force of 150 for the Pratt (the way you have shown the load, those chord forces would be reversed). Beyond that, if your numbers are correct, I'm not sure which is better, it depends a lot on your l/r ratios for the compressive members. Off hand, you've got to weigh the 300 max tensile,225 max horiz chord compression, and 106 max compression of the diagonals, of the Howe, with the 187 max tensile and 250 max compression of the horiz chords, of the Pratt. And even for the zero force members, you're still going to use the same size popsicle stick in those members, so the zero's don't gain you anything. I'd vote for the Pratt, but it could be a wash either way. Just a thought, so don't take it for gospel. I'll let you be the engineer.
DefyPhysics
#3
Dec5-07, 10:49 PM
P: 3
Thanks for your help.

The weight is going to be loaded on the top of the bridge in the center. Does that change anything?

PhanthomJay
#4
Dec6-07, 08:28 AM
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Pratt vs Howe Truss

If the loading is as the top , your internal force diagrams as shown are correct.
vector3
#5
Dec10-07, 02:56 PM
P: 33
How about the deflection (vertical movement) at mid span. Does one indicate less deflection than the other? Also noticed that the maximum forces in Howe truss are less than the maximum in the Pratt. On a much larger scale, would the truss with the lower maximum forces result in in a lighter truss members, thereby reducing fabrication and erection costs?...


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