Provision for Steel requirements in Estimates for items in construction of storied building works

Steel requirements for items in construction of storied building works - Provision to be made in the estimates-

A NOTE ON ESTIMATION OF R.C.C. SECTIONS AND REINFORCEMENT FOR BUILDINGS.

Layout

The first step in the procedure is to prepare a R.C.C. layout showing the locations of columns, beams and spanning of slabs. As far as possible the shorter span of the slab should be between 3 m. to 4.5 m. Depending on the ratio of longer span to shorter span slab may be two way if this ratio is less than 1.5  and one way if this ratio is greater than 1:5. The span of the main beams should not preferably exceed 8 metres depending upon obligatory column positions. Panel walls must be supported on beams unless supported by walls below.

Loadings

For estimating the load following total load per sq.m (including self load, live load and weight of floor finish) may be adopted.

A: Light Buildings. 

This category includes residential buildings, tenements, wards of hospitals, dormitories etc.

1) Floor slab - 600 Kg/m2

2) Terrace slab - 750 Kg/m2

3) Staircase slab - 1000 Kg/m2.

4) Sanitary Blocks . 750 Kg/m2.

B: Medium Heavy Buildings. 

This category includes all other buildings except in Class A above and except heavy buildings for factory, warehouse and garages.

1) Floor slab - 1000 Kg/m2.

2) Terrace slab - 750 kg/m2. 

3) Staircase slabs - 1000 Kg/m2.

4) Sanitary blocks - 750 Kg/m2.

Storeys. 

The buildings are divided into 2 types as below: 

Type - 1:  buildings have more than 5 Storeys or with ground + 4 or more floors above.

Type - 2: buildings with 5 Storeys and below or with ground + 4 or less floors above. 

The steel required in Kg/m2 for both these types may be assumed at the rates shown in Appendix 'A'

Slabs.

The thickness of the slab should be assumed to be.

100 mm. up to short span of 3.5 m. 

110 mm. up to short span of 3.85 m.

120 cm. up to short span of 4.2 m.

130 mm. up to short span of 4.5 m.

The steel shown in Appendix 'A' may be assumed. Beyond 4.5 m. shorter span detailed designs should be prepared even for estimating.

For staircase slabs, thickness should be assumed at 40 mm per metre of span. For cantilever slab thickness should be assumed as 80 mm per metre of span.

Beams.

For assuming the size of beam max. shear force should be calculated. The size should be worked out from the formula.

b x d = S.F./5

Where,

b = breadth in cm.

d = depth in cm.

S.F.= Shear Force in kg

A suitable value for 'b' (generally 200 mm.) should be assumed and 'd' computed from the value of b x d in the above formula. 

For buildings of type I, beams required for resisting horizontal forces should be marked out and steel assumed at the rate shown in Appendix 'A'.

Columns

The total load on the column at each floor level should be calculated. The area of column for building in type-2 should be computed by dividing the load in kg. by 40 MT/each floor level. For buildings in type 1 buildings should be divided in blocks of 5 Storeys starting from the top. The area of column and mix in each block should be kept the same. Area shall be worked out by dividing the load at bottom most floor level in each block by the permissible stress shown in the Appendix 'A'.

While determining dimensions of the column it should be ensured that the least dimension is not less than the value obtained by dividing the floor height in cm. by 15.

Footings

The total load on footings should be computed and the area of footing in sq.m can be worked out by dividing load in Tonnes by Safe Bearing Capacity of foundation strata in Tonnes/Sq.m. The depth of footings may be assumed in all cases as 600 mm. at column face and 300 mm at free edge.

Special Structures.

Quantities of special structural elements like shells, arches, footed plates, water tanks, etc. should be worked out only on the basis of the detailed designs.

 

Column		Concrete Mix	Permissible Stress in direct Compression	Building with more than 5 storeys Type-1		Building with less than 5 storeys Type-1
Column				Mild Steel N/mm2	HYSD N/mm2	Mild Steel N/mm2
	Top 5 Stories	M-15	4	270	200	260
	Next 5 storeys	M-20	5	340	270	-
	Next 5 storeys	M-25	6	-	320	-
	Next 5 storeys	M-30	8	-	370	-
	Next 5 storeys	M-35	9	-	420	-
Footing		M-15		67		67
Beams
A.	Resisting Horizontal forces	M-15	-	250	200	200
B.	Not resisting Horizontal forces	M-15	-	200	170	200
Slabs
	Slab upto 4.5 m span (short span)	M-15	-	67	-	67
	Staircase slab	M-15	-	135	-	135