Punching shear resistance usually constitutes the decisive design criterion for reinforced concrete flat slabs supported by columns. Modeling approaches based on Kinnunen and Nylander's rotation-symmetric sector model, such as the Quadrilinear Sector Model (QSM), allow the prediction of the slab's load rotation behavior. To take into account an orthogonal reinforcement layout, the steel Young's Modulus is usually reduced. A mechanically better justified flexural stiffness reduction factor is derived here which depends on the longitudinal reinforcement and modular ratio. The QSM also neglects shear deformations, which may reduce the flexural capacity. A Modified Sector Model (MSM) is derived which considers the influence of shear on the flexural behavior. A strength reduction factor for the shear crack-crossing longitudinal reinforcement is introduced which depends on the mechanical longitudinal reinforcement ratio. The factor takes into account that the longitudinal reinforcement also has to transmit forces resulting from shear. The MSM can also be applied to predict the load rotation responses and punching shear resistances of flat slabs strengthened with prestressed carbon fiber-reinforced polymer (CFRP) straps. (C) 2014 Elsevier Ltd. All rights reserved.