Crack width Calculation

[hannis]

Hi,

Does anyone know of a suitable method of calculating combined flexural and direct tension crack widths for reinforced concrete tanks. There are established method of calculating them separately but not combined.

Regards

Hannis

 

[Ginger]

Hannis

This requires some judgement to combine the two crack width effects. The bending solutions are well documented as the position of the neutral axis (NA) of the section can be calculated. Adding direct tensile forces to the section causes the NA to shift such that a smaller proportion of the section will be in compression. As the tensile load increases, eventually the NA will move outside the section & the entire section will be in tension. Trying to use modular ratio method to solve this gives cubic equations and you cannot solve directly.

See "Design of Liquid Retaining Structures" by RD Anchor. In here he demonstrates a solution method by looking at Moment M and tensile force T. An M/T ratio gives the value of the necessary eccentricity of a tensile force to produve the bending moment. The larger the ratio, the more predominant the bending crack effects and vice versa. If either bending or moment predominates to a large extent, then the best solution is to design for the predominant force and then adjust by approximation.

Anchor goes on to demonstrate a calculation for the cases where the ratio is "in the middle" but I can't replicate it here due to complexity of the equation.