I recall a side comment by RAPT on a thread (that I haven't been able to find) regarding the appropriate use of compressive reinforcing for reducing long term creep and shrinkage deflections. As AS3600, CL 8.5.3.2 now clearly states that compression steel needs to be located in the compression zone (who would have thought! Amministrazione E Finanza Pdf Printer here. ) – I wanted to clarify the application of Asc in relation to slabs.
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I had a quick look at Ku values for typical slabs that I have designed (deflection controlled) and Ku seems to generally be in the order of 0.1 (treating as singly reinforced). Adopting this value of Ku =0.1 and assuming 25mm cover and d = D – 25 – 10, the minimum slab thickness to just scrape any reinforcing into the compressive zone is D = 300mm (kud = 26.5mm). As this is based on a singly reinforced section – accounting for this 'compressive reinforcing' would actually lower Ku further (or does it – if it is not in compression?!). I have read that under sustained loading (ie appropriate to the consideration of long term deflections) the neutral axis lowers further as internal stresses are redistributed into the compression reinforcing – I have not found anything to quantify this however.
How do you determine the long term neutral axis depth and is this appropriate for the assessment of 'compressive reinforcing'. How far into the compression zone does reinforcing have to be to be effective - Should the reinforcing be located within the 'compressive stress block'? Is there a limit to how much compression reinforcing can be effective (right up to 100%)?
Should the Kcs reference be removed from the slab chapter (is it really on appropriate to beams)? What is industry practice?