This paper proposes an improved spread plasticity model that correctly identifies the initiation of yieldingrnanywhere in the beam, takes into account the gradual spread of plasticity, the shift of the points of contraflexure,rnthe variable location and actual length of the yield zones. The model assumes that columns andrnbeam column joints remain elastic. Beams are made up of elastic and spread plasticity sub elementsrnconnected in series. When a beam yields, its stiffness reduces and flexibility increases. Before yieldingrnspread plasticity sub element has a null matrix, as the beam yields, the magnitude of its coefficientsrnincrease. At each time step, the model updates the flexibility matrices of the spread plasticity subrnelements. Unlike existing spread and concentrated plasticity models, moments within the span arernmonitored and the effect of their yielding or "unyielding" taken into account.rnA number of examples are presented that demonstrate the limitations of the existing spread plasticityrnmodels. The paper concludes that spread plasticity models that only consider plasticity at the beamrncolumn connections are only accurate for lower stories and structures where the applied/design gravityrnload < 0.8. The examples also show that compared to existing spread plasticity models, the proposedrnmodel improves the accuracy in calculation of global displacements, joint rotations and inter story driftrnratios by up to 25%, 69% and 55% depending on the ratios of applied/design gravity load and bottom/toprnreinforcement.
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