The safety and structural integrity evaluation of buildings, bridges, or other structures involve a process primarily comprised of two key steps: demand and capacity calculations. The concept of “demand” refers to the internal forces, moments, or stresses within structural elements induced by applied forces, representing the actual load the structure will bear. To enhance safety, these applied forces undergo a scaling-up process to accommodate uncertainties in force magnitude, caused by inappropriate usage. The determination of “demand” involves a comprehensive structural analysis, often conducted through the finite element method, utilizing reliable commercial finite element analysis software.
On the other hand, “capacity” pertains to the structural member’s ability to withstand internal forces, moments, or stresses, derived from the geometric and mechanical properties, or strength, of the cross-sections. The strength of materials is subjected to a reduction factor to account for uncertainties in material strength during actual construction. Safety evaluation culminates in the comparison of “demand” and “capacity” through the Demand-to-Capacity Ratio (DCR). A DCR less than 1.0 indicates a safe condition where the capacity exceeds the demand. Conversely, a DCR greater than 1.0 signals a precarious scenario where the demand surpasses the capacity, foretelling potential structural failure.
If any structural member is deemed unsafe, a practical strengthening solution should be carried out to maintain the DCR lower than 1.0. Structural strengthening ranges from the surface application of carbon fiber onto reinforced concrete structures, concrete jacketing, steel jacketing, external prestressing, foundation underpinning, and so forth. In many cases, active strengthening is encouraged to ensure the effectiveness of the approach.