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If a normal linear calculation is performed and limit stress is achieved in any part of the structure, the dimension of critical elements must be increased. If however, plastic hinges are taken into account, the achievement of limit stress causes that plastic hinges are inserted into appropriate joints and the calculation can continue with another iteration step. The stress is redistributed to other parts of the structure and better utilisation of overall load bearing capacity of the structure is obtained.
On the other hand, there is a risk in this approach. If a hinge is added to the structure, its statical indeterminateness is reduced. If other hinges are added, it may happen that the structure becomes a mechanism. This would lead to a collapse of the structure and the calculation is stopped.
Plastic hinges can be thus used to calculate the plastic reliability margin of the structure. The applied load can be increased little by little (e.g. by increasing the load case coefficients in load case combination) until the structure collapses. This approach can be used to determine the maximum load multiple that the structure can sustain.
Plastic hinges are considered only at ends of individual 1D members. No selection of 1D members is made for the calculation with plastic hinges. If this type of calculation is selected, all 1D members in the structure are tested .
The calculation is similar to the calculation of beams with gaps. All 1D members in the structure are tested and if the limit stress is reached, the plastic hinge is inserted. If however the stress lowers in the next iteration step, the plastic hinge may be removed.
The Solver setup offers an item where a particular national code can be selected for correction of the limit moments. If option No Code is selected , the modification of the limit moments is performed as for option EC (Eurocode).
Approaches described in " Plastic hinges to EC3", ENV, DIN 18800 and NEN codes are implemented in SCIA Engineer.