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Inner lever arm in EN 1992-1-1, clause 6.2.3 (3) is defined as distance of forces in tensile and compression chord, it follows that it is distance of position resultant of tensile force (tensile reinforcement) and position of resultant of compressive force (compressive reinforcement and compressive concrete). In EN code it is not defined how to calculate inner lever arm if difference between direction of resultant of bending moment and resultant of shear force is significant. Therefore in SCIA Engineer is used the same principle as for calculation of effective depth, it means that inner lever arm is calculated as projection to direction of shear force resultant. There are some exceptional cases, when inner lever arm is not calculated from plane of equilibrium:
In this cases, the inner lever arm is calculated according to formula
z = Coeffz ∙ Coeffd ∙ hl ∙ cos(|αV - αM|)2
The inner lever arm z calculated from plane of equilibrium or from formula above is compared with the minimal allowed inner lever arm. It can occur when cos(|αV - αM|)2 is lower than 0,45 (the distance between angle αM and αV is greater than 48 °). In that case is the inner lever arm calculated according formula
z = Coeffmin ∙ Coeffz ∙ Coeffd ∙ hl
where
| hl |
The height of cross-section perpendicular to neutral axis |
| Coeffd | The coefficient for calculation effective depth of cross-section, see "Coefficient for calculation of effective depth of cross-section" |
| Coeffz | The coefficient for calculation effective depth of cross-section, see "Coefficient for calculation of lever arm" |
| Coeffmin | The coefficient for minimal effective depth of cross-section, set to 0,45 |
| αV | The angle between direction of shear resultant and y-axis of cross-section |
| αM | The angle of slope line plane of equilibrium |