Design standards define safety margins that should cover resistance and load forces variabilities in a construction yet to be built. They cover construction tolerances, especially on geometry and materials that influence both load and resistance. They also cover the variability of actions that are quite large in Brazil, caused by a lack of Railways and Highway Traffic Control.
On the other hand they do not cover future variations that could occur, mainly because of release of new trucks, usually heavier, once there is a strong pressure, especially from transportation organizations to increase loads.
Well-built structures are able to withstand these increases since they compensate the loads growth with a reduction of design deviations, whether in geometry or in resistance.
When analyzing Existing Structures, the design criteria can only be revised since they meet the following prerequisites:
be quite old, about 30 years of good service, including the conditions of current loads and do not present structural impairment with respect to any ultimate limit state;
have good structural geometry, with acceptable deviations and some below the geometric tolerances;
have acceptable materials strength and some better performance than expected.
This occurs with some frequency in road construction and justifies alternative analysis to strengthen the structure, usually very expensive and with suspicious extension of the structure lifetime with respect to safety and durability.
The reduction of safety margins, according to European Practice 1 and 2, can be used since relevant recovery services are made, giving priority to their inspection and periodic maintenance to ensure an acceptable expected remaining lifetime, which can be less than 50 years.
This allows reduce the extreme values of loads, strength design and durability requirements by reducing the weighting coefficients without increasing failure probability (based on Reliability Theory) in the remaining lifetime. This solution without reinforcing structure was applied on the current DER-SP (São Paulo Transportation Dept) Program, allowing the use of TB45-1984 design traffic load instead of TB24-1950.