Expert: Bruce E. Johnson - 10/17/2008

Question
QUESTION: Dear Mr Johson

First of all let me tell you that you are doing a really great job by helping people in solving their construction problems. Well sir i am a 1st yr student of Quantity  Surveying at the University. My uncle is about to built up a new house which will be of ground level only and with 6 pieces of room (including dinning, kitchen, bedroom and toilet etc). Well sir my question here is, firstly how can i reduce the amount of steel bars for the the house and where (e.g columns, beam, slab...)? Secondly, if i have a roof slab of 12ft by 12ft(144sq/ft) how much reinforcement bars, at its minimum, should i place in both ways?

Along with this sir, i would like to draw your attention to  the mason way of working here in my country. Basically they put the reinforcement bars at a distance of 200m in between both ways.

according to my calculation:

> 12feets = 4608 mm
> 4608/200 = 23.04
> According to mason over here 24 bars of 12ft of steel in   
both ways will be nedded.

I want to know, if ever i can increase the distance btw the bars to 250mm - 300mm??
this will surly reduce the cost of steel bars and the roof slab will b lighter in wight!

Remember the roof slab is to be made of 150 mm depth and 8mm diameter of reinforcement bars is to be used for the roof slab?.

i hope i have provided you with the maximum info needed for you to explain me this situation.

Hoping to here from you soon>
Rgards
Noorany


ANSWER: Hi Noorany, I don't see why you can't increase the distance between the bars to 300mm.  This is less than 12" on center.  12" is a standard reinforcement grid for applications such as this.  The reinforcement size seems inadequate however.  13mm-16mm is preferable. 8mm is fine for temperature steel located within 26mm from the top of the slab.  The main slab reinforcement should be 13mm or 16mm steel located 37mm from the bottom of the slab.  If what your mason is doing is according to local standards and practices then I would tend to go by what they want to do.  But I feel that the steel size is inadequate. The last thing you want is the roof slab to collapse on top of your head due to the lack of a few pieces of rebar. I hope this information helps feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com

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

QUESTION: (a) Hi Johnson, thank for ur reply. But still i am a bit confused, can i go for it or not??? By the way, i had a lecture with an architect, where he mention that here, in my country, masons tend to "over-use" the amount of steel bars..????? u can take the e.g which i've given above, concerning the 144sq/ft room where 24 reinforcement bars of 12feet are used in both side??? i want to know, if really the mason "over-use" the bars here??? remember Mr Johson the architect is aware that the reinforcement bars is of 8mm, yet he said there is an over use????

(b) Generally contractors or even mason, in my country,  use the 8mm reinforcement bars for residential roof slab and also for columns so i think i will go by the 8mm itself but sir my question which arise here is...!!

From yrs they've been using 8mm they got no problem with it, so here if not 300mm,which was my 1st proposal, if i increase the distance to 250mm, which shows only 50mm difference, will it have a great impact on the roof??? and if so, then what are the qualities of the roof will be affected??? will it affect its lasting duration..???
OR, if i choose to use only 20 reinforcement bars instead of 24 bars, what opinion do you have on it.???
And what would you advise me to do, if u think the number of bars can be reduced??

Mr Johson please if you can, reply me in maximum detail that u can!!!??

Regards
Noorany


Answer
Hi again Noorany, I thought I was pretty clear about my answer.  I feel that there is not enough steel based on your description of the size of the steel etc. Any roof slabs I have done over the years, and a rough estimate is several hundred thousand square feet of concrete roof decks, the grid of reinforcing steel was #4 (13mm)rebar at 12"(305mm) on center each direction.  #4 rebar is 13mm in diameter which is half again the size of the 8mm steel you are using.  I don't like it, I'm sorry.  If the architect thinks that your slab is over built then by all means follow his directions and remove some of the reinforcement or extend the spacing. But I can't in all honesty tell you to do so.  I am not a structural engineer, I can only go by what I have personally built over the years and how that work was designed by structural engineers.  Sincerely bruce e johnson