# Pratt vs Howe Truss Bridge Building: Deciding for Physics Class

• DefyPhysics
In summary: Based on the information provided, I would recommend the use of the Pratt truss. The Howe truss does not have a force in the center, which could cause it to fail under compression. Additionally, the Pratt truss has stronger components overall.
DefyPhysics

## Homework Statement

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.

## 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?

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.

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

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?...

## 1. What is the difference between a Pratt and Howe truss bridge?

The main difference between a Pratt and Howe truss bridge lies in the placement of the diagonal members. In a Pratt truss, the diagonals slant downwards towards the center of the bridge, while in a Howe truss, the diagonals slant upwards towards the center of the bridge.

## 2. Which type of truss bridge is stronger?

Both the Pratt and Howe truss bridges have their own strengths and weaknesses. The Pratt truss is better at handling compressive forces, while the Howe truss is better at handling tensile forces. Ultimately, the strength of a truss bridge depends on its design and construction.

## 3. How do you decide which truss bridge to build for a physics class project?

The decision of which truss bridge to build for a physics class project should be based on the specific learning objectives of the project. If the focus is on understanding compressive forces, a Pratt truss may be a better choice. If the focus is on understanding tensile forces, a Howe truss may be a better choice.

## 4. What are some real-world applications of Pratt and Howe truss bridges?

Pratt and Howe truss bridges are commonly used in the construction of railroad bridges and highway overpasses. They are also used in the construction of roof trusses for large buildings and in the design of cranes and other heavy machinery.

## 5. Is one type of truss bridge more cost-effective than the other?

The cost-effectiveness of a truss bridge depends on various factors such as the materials used, the length of the bridge, and the terrain it needs to cross. As a general rule, Howe truss bridges tend to be more cost-effective for longer spans, while Pratt truss bridges may be more cost-effective for shorter spans.

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