Why are triangles the fundamental shape in truss design?
ATriangles use the least material
BTriangles are the only polygon that is rigid without bending its members
CTriangles look the best in bridges
DTriangles are easier to manufacture
A triangle cannot change shape without changing the length of at least one side. A square, pentagon, or any other polygon can deform into a different shape (parallelogram, etc.) without its sides changing length. This inherent rigidity makes triangles the basis for all truss structures.
Question 2 True / False
In a properly designed truss, individual members experience bending forces.
TTrue
FFalse
Answer: False
In an ideal truss (loads applied at joints, frictionless pin connections), members carry only axial forces -- pure tension or pure compression, not bending. This is what makes trusses efficient: each member resists force along its length, which is the strongest direction for a straight member.
Question 3 Short Answer
A Pratt truss and a Howe truss both span the same distance. What is the key difference between them?
Think about your answer, then reveal below.
Model answer: In a Pratt truss, the diagonal members are oriented so they carry tension under typical loading. In a Howe truss, the diagonals carry compression. This matters because tension members can be lighter (cables or thin rods) while compression members must be thicker to resist buckling.
The Pratt truss is often more efficient for steel structures because steel is equally strong in tension and compression, but tension members can be thinner since they do not buckle. The Howe truss was historically preferred for timber construction where compression members (thick wooden beams) were readily available.