The design process for prismatic beams subjected to bending begins by determining the allowable stress for the chosen material.
The allowable stress is generally obtained from a table of material properties or design specifications. Alternatively, it can be computed by dividing the material's ultimate strength by a suitable safety factor.
Then, the shear force and the bending moment diagrams corresponding to the specified loading conditions are drawn.
Next, the maximum absolute value of the bending moment is calculated using the shear force diagram.
The minimum allowable section modulus is then calculated using the beam's absolute bending moment value and allowable stress.
When a timber beam is designed, the ratio characterizing the shape of its cross-section is usually specified. The unknown dimensions are identified using the relationship between the dimensions of the beam and the section modulus.
While designing a rolled-steel beam, a table containing properties of the rolled-steel beam is referred to, and only those beam sections with a section modulus greater than or equal to the minimum required section modulus are considered.
