Australian Standard As2870 Pdf Files
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!) – I wanted to clarify the application of Asc in relation to slabs. 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. Calvin Richardson Country Boy Rapidshare Download. 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?!).
Mar 10, 2015. AS 2870—2011 Australian Standard® Residential slabs and footings AS 2870—2011 This is a fre1249669. This file you can free download and review.
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? Every engineer I have encountered (including myself) has used 'compressive' reinforcing to reduce deflections in slabs much thinner than 300mm. Since the industry dictates predominantly slabs with complete top and bottom mats of reinforcing (for ease of construction), I am loathe to end up specifying thicker slabs than everyone else. I understand that the formulas are significant simplifications of very complicated theory and are 'deemed to comply' – I am not disputing their appropriateness, I just want to know to apply them correctly.
Thanks in advance! RE: Long Term Deflection & Compression Reinforcing (Structural) 24 Feb 10 04:50. Cannot comment on your first three questions, however, I would use a triangular stress distribution when considering service loads and hence calculating deflections. I believe Prof. Ian Gilbert who chairs the sub-committee is against kcs factors for calculating deflections.
I find it to be a very helpful multiple when calculating additional deflections due to creep and shrinkage. When I say I find it a 'helpful multiple', I mean using a multiple of 2 gives my spreadsheet deflections very similar answers to what RAPT (software) does. I would never use a kcs of less than 2.0 for slab design because it is not industry practice to reinforce slabs in compression regions hence no help will be provided by the reinforcement to reduce the deflections. I have attached my spreadsheet. The process I go through basically follows: 1. Calculate service moments midspan and calculate Ieff based on these service moments.
Calculate instantaneous deflections based on short-term loading and Ieff (Normally G+0.7*Q). Calculate additional deflections from creep and shrinkage based on Ieff and kcs=2.0 (2.0*G+2.0*0.4*Q). Sum the two to give a total long-term deflection.
I have only ever used this approach to verify the outputs I have obtained from RAPT (software). • RE: Long Term Deflection & Compression Reinforcing (Structural) 24 Feb 10 17:31. I believe RAPT is in the UK at the moment on holidays I imagine, having fun sun banking on their beaches on doubt. So I think you may have to wait a while for him to chime in. Ian Gilbert says 'The use of the deflection multiplier kcs to calculate time-dependent deflections is simple and convenient and, provided the section is initially cracked under short term loads, it sometimes provides a 'ball-park' estimate of final deflection.
However, to calculate the shrinkage induced deflection by multiplying the load induced short-term deflection by a long-term deflection multiplier is fundamentally wrong. Shrinkage can cause significant deflection even in unloaded members (where the short-term deflection is zero). The approach ignores the creep and shrinkage characteristics of the concrete, the environment, the age at first loading and so on. At best, it provides a very approximate estimate. At worst, it is not worth the time involved in making the calculation.' (I doubt I can say it any better.) From the paper: I have also heard Ian say you should never use a KCS less than two if you must use the kcs method.