What is a Pratt truss structure?

A Pratt Truss is a style of truss which is styled as internal (web) truss members travelling in a diagonal top-down direction. This formation is quite simple and extremely effective in long spans such as bridges.

What is a Pratt truss design?

The Pratt truss was first developed in 1844 under patent of Thomas and Caleb Pratt. Prevalent from the 1840s through the early twentieth century, the Pratt has diagonals in tension, verticals in compression, except for the hip verticals immediately adjacent to the inclined end posts of the bridge.

How do you calculate strength to weight ratio of a bridge?

Strength to weight ratio is determined by dividing the maximum load carried by the weight of bridge.

What math is used to design a truss bridge?

Use trigonometric ratios to find force magnitudes at a truss’ nodes. Use the method of joints to set up a system of linear equations to calculate the tensions and compressions on the truss elements. Use the substitution method to solve systems of linear equations larger than 3 x 3.

How is a Pratt truss constructed?

The basic form of Pratt truss includes triangular truss design whose diagonal members slope toward the center of the bridge. When under load, this design makes diagonal members feel tension (the force that expands the object apart), while vertical members feel suspension (the force that pushes objects into one self).

Why is a Pratt truss bridge good?

Pratt Truss This simplifies and produces a more efficient design since the steel in the diagonal members (in tension) can be reduced. This has a few effects – it reduces the cost of the structure due to more efficient members, reduces the self-weight, and eases the constructability of the structure.

What makes a Pratt bridge strong?

A Pratt Truss has been used over the past two centuries as an effective truss method. The vertical members are in compression, whilst the diagonal members are in tension. This simplifies and produces a more efficient design since the steel in the diagonal members (in tension) can be reduced.

What is the strength to weight ratio of a truss?

After all the trusses were tested the breaking weight for each truss was divided by its weight. This gives the strength to weight ratio. Results: The average strength to weight ratio for the Pratt was 1128 ounces to 1 ounce. The average ratio for the Warren was 1013 ounces to 1 ounce.