Expert: Arshad Khan - 6/25/2011

Question
QUESTION: Sir
At my site at first floor one of slab area is 3200mm x 3600mm
Dwg shows to fix the Dia8@150 c/c spacing as alternate bent up.
as per our BBS total wt in slab is 97kg, and slab thickness is 150mm
My question is at site Dia 8 finished and now i want to fix by higher dia let say 10mm then what will be the spacing ? How to calculate ? can i fix by 12mm or 16mm ?

Since it is sunken the bars are not xing the beams or going 30% inside the next slab.
Also in case the slab steel xing the beam & going in next slab by 30% of span then to change it to higher dia shall i consider the wt of steel which is going in next slab ?
Thanks & regards

ANSWER: Hi Abdul,

Y8 at 150 c/c equals 335 mm2 per m width of slab. The equivalent Y10 would be at about 225 c/c, which gives 353 mm2 per m width of slab.

I would not recommend fixing Y12 or Y16, as the spacing becomes too wide and will result in cracking (also will not meet code requirements).

If the bars cannot go through to next slab span, just bend them up and curtail at far face of the supporting beams. For curtailment, you should just consider the minimum rebar required for continuity- this can be in the larger diameter or smaller, so long as the rebar area is OK.

---------- FOLLOW-UP ----------

QUESTION: Thanks for your reply. I want to know how u reaced 225 ie method of calculation.
Also it is bad practice at site that slab steel at ends left straight while dwgs says that it should bend by 50mm. What are the consequences ?

Answer
Hi Abdul,

If you work out the cross-sectional area of the bars and multiply by the spacing, you get a similar area per running meter- i.e.  335 or 353mm2/m, as mentioned above.

So long as the bottom bars extend past the support by at least same distance as the slab depth, there is no need to have the bends. Of course top rebar shoul be properly anchored with either a bend down at an edge support or extend well past any internal support so as to provide full tension anchorage.