Stress determination of a RC slab for fatigue design

[kww2008]

Wonder if reasonable to include steel in the compression region of an RC slab (one with both top and bottom rebars) in the section analysis when calculating serviceability stress for fatigue design (to AS 5100.5 Bridge Code). Expect to give a more economical design if I can do so as including the steel in the compression region will reduce tensile stress under fatigue loading. With the tensile steel below permissible SLS-stress-limit levels, compression bars are unlikely going to buckle. What are your thoughts on this?

I am aware that I should not include rebars in the compression region for ULS bending strength calculation unless they are adequately restrained (as required by AS).

 

[rowingengineer]

Sorry, I am not following your logic here. But adding bars in the compression zone, how does this reduce tensile stress? I understand it isnwith a fatigue loading but i must have missed something in as5100. I can only see this happening on a thick slab with a larger compression stress depth by reducing the leaver arm.

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[IDS]

A few points:

In AS 5100 (and AS 3600) the confinement requirement for columns only applies where N* > 0.5Phi.Nu. There is no specified requirement for confinement in slabs, and since the axial load will be close to zero, I don't see any reason not to include compression steel for either ULS, SLS, or fatigue calculations (except for over-reinforced sections).

In a typical bridge deck slab the top reinforcement is likely to be below the centroid of the concrete compression zone, and will make very little difference to the stress in the tension steel if it is in compression.

For a lightly reinforced slab with large cover the top steel may be in the tension zone, in which case it will reduce the stress in the tension steel by a small but not negligible amount.

In summary, there is no reason to exclude the top steel except for over-reinforced sections, but it may make very little difference.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[kww2008]

Doug,
You are correct. The bars are both in tension since the neutral axis is above both the top layer of bars. I noticed that when I did the section analysis after I submitted the post. There is
some reduction in tensile stress of the lower bars but not much.