Midspansag

Midspansag is a term used in structural engineering to describe the vertical displacement of the midspan of a beam or girder under service loads. It is commonly called midspan sag and serves as a key serviceability criterion in beam design. The concept applies to materials such as steel, reinforced concrete, and timber.

Midspan sag occurs when bending moments cause downward bowing at midspan while the ends remain supported. It is influenced by span length, loading pattern (uniform, point, or variable), support conditions, material properties, and time-dependent effects such as creep and shrinkage in concrete. Temperature changes can also affect deflection.

Calculations use classical beam theory. For a simply supported beam with uniform load q, the maximum deflection is δmax = 5 q L^4 /(384 E I). For a central point load P, δmax = P L^3 /(48 E I). Continuous spans require more advanced analysis. Codes specify allowable deflections, often as a fraction of span length (for example L/360 or L/240).

Mitigation includes increasing stiffness (higher E or I), increasing depth, using prestressed concrete or steel, adding stiffeners or camber, or designing the beam as a continuous member with better moment distribution.

The term is used mainly in engineering literature and standards that govern deflection limits and serviceability.