Concrete Wall Serviceability Interaction Diagram

[cpb213]

Hi all,

I am working on an underground structure with strict crack control requirements. The perimeter walls are subjected to high out of plane flexural forces along with the axial compressive forces.

Does anyone know of a good program or reference for developing serviceability interaction diagrams? There are plenty of resources for designing beams for limiting crack widths but nothing that I can find for axially loaded members.

I would ignore the axial load but that would result in very high reinforcement ratios. I would like to utilize the axial loads to reduce the reinforcement ratios.

My other thought was to calculate the maximum allowable rebar stress for the max crack width/assumed rebar spacing and then use that stress as the yield stress in a regular concrete strength interaction diagram. This isn't correct either as it assumes a equivalent rectangular stress block instead of the triangular stress distribution.

Any help would be appreciated. Thanks

 

[Settingsun]

Never done it, but I would go with your "other thought". Set the bar stress to the limit (I'm guessing around 22 ksi = 150 MPa for strict crack control). For compression up to about f'c/2, concrete's stress strain is fairly linear so use Young's modulus instead of rectangular stress block. Plot a few points based on arbitrary neutral axis deptg to get the maximum eccentricity of the compression load for various load levels.

 

[JoelTXCive]

Have you looked at ACI 350-06? That is the manual for concrete water containing structures. I'm do not think there are equations in there for combined loading, but there is a lot of info about crack control for pure flexure.

That document recommends an elastic analysis and also limits bar stress as Steveh49 mentions above.