Basic reinforcement mesh and additional bars 1

[Aakalim103]

I am designing a flat slab and I want to give a basic top and bottom reinforcement and then provide additional bars where required. I have seen that being done once or twice but I am not sure if there are some additional requirement that need to fulfilled. I mean should we consider them as bundled bars where we have additional reinforcement and will it effect the lap lengths and the development lengths?

 

[r13]

The bars in the base mat should be given generous spacing to accommodate the additional bars. You should try to avoid bundle bars, unless absolute necessity. I don't recall it (bundle) affects splice length, but the location of splices need to be staggered. You should check the code to confirm these statements.

 

[Lomarandil]

Yes, bundled bars need to be developed as if an equivalent single bar sharing the same cross sectional area. So it does increase development and lap length.

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just call me Lo.

 

[spieng89]

Typically for flat slabs, I'll dictate temp steel #4 at 16" c/c or similar and add in steel where needed at intervals that are evenly spaced within the inital temp steel spacing.

Ex:
temp/shrinkage steel - #4 @ 16" c/c
supplementary steel - #5 @ 16" c/c (centered within the temp steel)

I choose spacing and bar sizes that are easily verifiable during inspection.

 

[r13]

I choose spacing and bar sizes that are easily verifiable during inspection.

That's a good practice. I wasn't thrilled when seeing spacing designated with decimal point (8.5", 11-3/4"...). I prefer 6"/12"/18" arrangement for ease of measuring.

 

[KootK]

I do this for slab bottom mats. I find that basing it on a midspan moment of wL^2/20 works well for me (more than T&S). I instruct the contractor to place the additional bars such that they are spaced far enough from the basic mat bars that they need not be treated as bundled for development. That said, I suppose that it would be nice to give them the option to place bars side by side if they're willing to pay the small penalty of some additional development length.

I've yet to do a basic mat for the top steel but I know of at least one local contractor who swears by it.

 

[Tomfh]

We often do basic top and bottom mat plus extra bars as required. Extra bars typically go between the regular bars. They’re not specifically bundled. I will often tell them to space bars out a bit, as sometimes they do bundle them too much on site.

As for bar spacing we tend to do 300, 200, 150, 100 cts. This equates to 1 bar per boot, 3 bars per two boots, 2 bars per boot, three bars per boot, etc.

 

[Sujitha Menon]

At the outset fix the Slab mesh based with a spacing of 150mm to 250mm so that you can place the additional bars in between
No issues in providing bundled bars but do take care of the covers provided .

 

[Agent666]

I wasn't thrilled when seeing spacing designated with decimal point (8.5", 11-3/4"...)

On site I multiply spacing by 10, and measure 10 spaces in one go, if the average is ok it's near enough for me, then review for really out of tolerance once off spacings.

Works will if you have weird 'decimal' spacing like r13 noted. 85" or 117.5" is easier to measure than 8.5" or 11.75" without getting some creep on every spacing.