Calculating Reinforcements or Iron Rods in Raft Foundation



Image 1, shows beams of section 1-1,2-2,,4-4 and 5-5 but for this purpose we are going to assume that section 2-2 is used for the entire raft.
Image 2, is the reinforcements details for the raft slab also showing its sections.
Lots of people are considering building their houses around water bodies and the large portions of the highbrow areas in Lagos state are waterlogged so the demand for raft foundation is high.
Starting with image 1 section 2-2 the dots represents the iron rods runners, these are usually the thicker of the rods and they lay them to run round the rings of the raft foundation in this case it is a 16mm rod in 9 places.
The ring which is usually the thinner one is represented by the thick line forming a rectangle of 1100mm x 225mm just as shown on the drawings.
Calculating the runners (16mm rods) we need to know the entire length of the foundation trench (excavated portion of the soil) which we can get from image 2, the trench will be along the dotted lines that crosses the thick square columns or pillars as shown in the drawings, the thick square dots are representing the pillars of the building so the trench are been represented by the dotted lines running and joining all of them, this you can easily calculate since the dimension are already given outside the perimeter “check dimensions A-B-C-D and 1-2-3-4-5-7-8
From our sums the total length of trench is 65.6m or 216 feets. Therefore the 16mm rods we need is (65.6m x 9 rods)/11
=54 lengths of 16mm rods.
11meters(m) is taken as the length of a rod so if the total length is (65.6 x 9) then just divide by 11 as was done above then you’ll know the number of rods you are to buy which is 54 lengths in this case,
If 16mm rods go for N2700/rod then
total for 16mm rods=N2700 x 54=N145,800
16mm on the drawing is represented as Y1601 and Y1602
Now to calculate the rings , remember the total length of the trench or foundation hole round site is 65.6m.
Calculate the length of each ring from the drawing , we take each ring to be approximately (175mm +1050mm+175mm+1050mm)=2450mm or 2.45m. Using section 2-2 for all, our rings will be taken as 12mm rods and they spaced at 300mm or 0.3m from each other or (center to center) (See BEAM drawings on image 1-300 c/c links)
So knowing that each ring is 2.45m, we got that by removing 25mm from the dimensions we have round the section of 225mm by 1100mm, so if you remove 50mm from 225 you’ll get 175mm same as removing 50mm (25mm top and bottom of drawings) for 1100m will give 1050mm , since they are spaced at 300mm to each other ignoring the overlap
that we are supposed to also add just for this purpose ,then to
Calculate the numbers of rings in total trench,
65.6m/0.3m or 65600mm/300mm=219 rings and each ring is 2.45m, therefore total lengths of rings
=2.45m x 219= 536.55
Rods are sold in 11m lengths ,so to know the numbers of rods you are going to buy , just divide 536.55 by 11=48 lengths of 12mm rods.
If 12mm rod is N1600 x 49=N78,400.

Calculating the reinforcements/iron rods in slab ( refer to image 2)
For easy calculation of the reinforcements just follow the specifications by the left of drawing then trace the line on each specification to the drawing then check where there is a dot, the line crossing that dot is what to be calculated. The dots are usually supported by arrows both sides, this indicates where that particular rod suppose to end.
So starting from the top
19Y1201-200C/C,
The total length of this is the broken line spanning the entire drawing so the total length is 3825+3950+1375+1250+4975+1250
=16.625(ignoring the return) multiply that by 19 because that is what is stated at 200c/c=315.875m
Divide by 11m=28 lengths of 12mm rods——-(1)
For second line,
3825+3950+1375=9.15m
divide by 11m=1 length of 12mm rods———(2)
Line 3,
This is a peculiar case the line is approximately 2m not really specified in the drawing but the arrows are in 4 places along that line and four of those rods are to be placed at each point where you have (4) written on the drawing.
so 16 x 2m=32, 32 divide by 11m=3 lengths of 10mm rods———–(3)
Line 4,
6Y1002-250C/CB
Make to be 6x2m =12m
divide by 11m =1 length of 10mm rods ——-(4)
Line 5
Trace line under and check the dot , then calculate the broken line crossing it.
12 x(3825+3950+1375+1250)=124.8, divide by 11
=11 1/2 lengths of 10mm rods—————–(5)
Line 6,
38 X 2=76, DIVIDE BY 11
=7 LENGTHS OF 12MM rods—————-(6)
Line 7,
In line 7, the line was not drawn directly to cross the dot, so trace where the dot is up and you’ll see another dot indicating how the iron rods will be placed ,the line is a broken line
83 x (3825)(ignoring the overlap and return)=317.475
divide by 11=29 lengths of 12mm rods———–(7)
Line 8,
8 x2m=16m, divide by 11m
=1 1/2 lengths of 10mm rods———–(8)
Line 9,
8 x2=16m
=1 1/2 lengths of 10mm rods ———-(9)
Note the short lines are immeasurable and are just estimated to be 2m each
Line 10,
77 x (2.910+3.825)=518.595 divide by 11
=47 lengths of 12mm rods——————(10)
Check the other dot at at the same line up and measure the broken line in our case the return and overlap was ignored.
Line 11
103 x 2 =206, divide by 11
=19 lengths of 12mm rods———(11)
Line 12,
19 x 2=38
=3 1/2 lengths of 12mm rods———(12)
Line 13,
19 x 2=38
=3 1/2 lengths of 12mm rods———(13)
Line 14,
34 x 2=68
=6 lengths of 10mm rods———(14)
Line 15
19×2=38
Divide by 11=3 1/2 lengths of 12mm rods ——–(15)
Line 16,
103 x 2 =206, divide by 11= 19 lengths of 12mm rods
Line 17
6 x 2=12m
divide by 11m=1 length of 10mm rods
Line 18
12 x 4975=59.7m
divide by 11m=5 1/2 length of 10mm rods
Line 19
10 x 2=20m
divide by 11m=2 length of 10mm rods
Line 20
8 x 2=16m
divide by 11m=1 1/2 length of 10mm rods
Line 21
19 x (1250+4975) ignoring overlap and return=130.725
divide by 11m=12 length of 12mm rods
Line 22
30 x 2=60m
divide by 11m=5 1/2 length of 10mm rods
Line 23
15 x 2=30m
divide by 11m=3 length of 12mm rods
Line 24
19 x(1250+ 4975+1250+1375)=168.15
divide by 11m=15 1/2 length of 12mm rods
Therefore total numbers of lengths of iron rods of 10mm=271 rods
If 10mm rods are sold for N1300, then 271 rods
=352,300
And the total for 12mm rods for the slab is =219 rods
If 12mm rods are sold for N1600, then 219 rods=N350,400
Total reinforcements for the raft foundation above is=
N145,800+N78,400+N352,300+N350,400
Total=N926,900

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