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When designing reinforced concrete slabs it is common to divide the slab into 'design strips', which are then used to rationalise the reinforcement design along (typically orthogonal) lengths of the slab (also known as the Hilleborg Strip Method).

I have mostly seen this approach used in design software (for example, SAFE), however they offer little to no advice as to how and where these design strips should be located/sized.

What rules and considerations do engineers apply when locating/sizing design strips?

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  • $\begingroup$ Are you referring to distribution of actual reinforcement, or to analyzing a representative portion of the slab? $\endgroup$ – Ethan48 Jan 26 '15 at 16:36
  • $\begingroup$ As far as I understand it- both: the method (I have later found out) is called the Hilleborg Strip Method. The reinforcement quantity is specific to the strip selected (as analysed) - although I appreciate there will be later rationalisation at detailing stage. $\endgroup$ – thomasmichaelwallace Jan 27 '15 at 13:30
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Per ACI 318 13.2, two-way slabs are designed based on "column strips" and "middle strips".

To paraphrase the code:

A Column strip is a design strip with a width on each side of a column centerline equal to 0.25L1 or 0.25L2, whichever is less. Column strip includes beams if any.

  • L1 and L1 are the span lengths in the two slab directions.

A Middle strip is a design strip bounded by two column strips.

This is a simplified method in the code and has some criteria to meet. This method then allows you to accommodate holes in the slab by placing the missing reinforcement on the edges of the hole.

I don't know how this compares to the Hilleborg method.

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Usually, this is done through trial and error, and you optimise for the economic cost (weight of concrete and steel).

You do this by using a calculation sheet (in excel, or MathCad) where the end result is the weight or cost of both the concrete and the steel. And you try and find the best compromise between the distance between the strips, their size, and the resulting slab size.

I'm not sure I answered you question perfectly though...

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  • $\begingroup$ Thanks @gromain, although I was more looking for a few rules of thumb / guidance about how to locate the strips initially- I've recently come across a good diagram about the method that I'll share when I get back to work; for example small strips around openings to provide extra local thickness, splitting strips at points of zero shear, etc. $\endgroup$ – thomasmichaelwallace Jan 31 '15 at 17:22
  • $\begingroup$ Oh yes, sorry then, I didn't fully understood the question (was a bit tired!). It's not my current field of work (though this is what I studied) so I can't offer any help on rules of thumb! $\endgroup$ – gromain Jan 31 '15 at 17:35
  • $\begingroup$ No worries- to be honest, like a lot of engineering questions, it took me a few edits to really understand what I was asking (i.e. discovering that it's the Hilleborg method!). $\endgroup$ – thomasmichaelwallace Jan 31 '15 at 17:57
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This question has no clear answer in its current form. The strip method is a lower bound approach (meaning that you approach the optimum solution from the safe side) based on beam analogy. So in theory (given endless ductility, etc.) you could arrange your strips in any way you want and you would always end up with a safe (but possibly horrible in all other aspects) solution. The recommendations given for the application of the strip method are to keep redistribution needs and service state-behavior in check as well as finding economical solutions. There's a lot of different recommendations in the strip method that vary between cases, corner angles for load dividers, moment ratios, widths of column strips vs mid-strips, widths of edge strips, reinforcement distributions within the strips, etc.

If you want to learn more about the strip method, and if you're going to apply it you really should, Hillerborg's book "Strip Method Design Handbook" is a good place to start. I personally find both the strip method and the strut and tie method to be very simple and elegant manifestations of applied engineering judgment.

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