Expert: Arshad Khan - 10/29/2010

Question
QUESTION: Great info / appreciate you taking the time here. I would like a rough idea of the column, beam & slab dimensions needed for my "Ground + 3 floors" RCC framed hotel. Not sure of the various dead & live load - data points : parking in gr floor ; banquet halls / restaurant / misc in 1st floor ; regular hotel rooms in the 2nd & 3rd floors ; 3 feet of soil on the terrace for a roof top garden - so what approx dead load would 3' of regular soil exert ?. I heard that wet soil can weigh more than eighty pounds per sq ft. I certainly will be hiring a structural engineer and I know that this is not the medium to design over - just trying to get a ballpark figure here. The columns need to be atleast 22' apart width wise(so continuous beam across 4 columns) and 24' apart length wise (to accomodate 2 rooms width of 12' each). My rough calculations tell me that I could accomodate this, with ~1.4' of beam thickness and ~0.6' of slab thickness - so I lose only 2 feet height in each floor due to the beams + slab - does this sound right ? I would prefer to have the columns further apart, but then looks like that it would eat into the floor height quiet a bit - so trying to guage the "sweet spot" here.

ANSWER: Hi Sudheer,

As you point out, this medium is not for designing, but just advising. RC Beams should nromally be about span/12 to span/15 deep depending on whether simply supported or continuous, though they can be shallower with the use of higher density of rebar and stronger grade of concrete.

RC Slabs can be span/20 to span/35 depending on loading, support conditions and whether they are designed/reinforced for one-way or two-way spanning.

3 feet of soil will exert a pressure of around 16 to 20 kN/m2 depending on its density/level of compaction and moisture content.

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

QUESTION: Thank you for re-iterating on this formula and details on the pressure exerted by the soil. If I throw few "small" fruit trees into the mix, could you please roughly guide me on what kind of increase in pressure should I be considering ?

Would you recomend going with the more expensive "Ready Mix Concrete" option - to ensure computer controlled / quality mix or the standard on-site mixing by local labour ? Could you point me to the right book on RCC structures that would help me communicate effectively with the structural engineer (not too technical / @ ground level jargon, more like a few feet high) ?

Answer
Hi Sudheer,

A few small fruit trees will only add around 0.5 to 1 kN/m2 loading, depending on spacing. You should also allow for live loading if people will have access to wander about. This can be from 1.5 kN/m2 for a few people, to 4 kN/m2 for crowd loading.

If the site supervision is good and the materials are from reliable sources, then there is no reason that site mixed concrete cannot be as good as RMC.

There are so many books available, I suggest you just Google and see what looks good to you. For design, The RC Designers manual by C E Reynolds and Steedman is very good.