Learning that one way slab only have its (crank)/bent up bars running along short span (alternately placed with straight bottom bars) or we may say parallel to its shorter side, then when it comes to "two way slab" it will run to both directions.
You must also take note of the steel bar placements when dealing with two way slab.
where:
Main bars (bent up/cranked bars)
● L/2/spacing + 1 for short span area
● W/2/spacing + 1 for long span area
Main bars (bottom straight bars)
● L/spacing + 1 for short span area
● W/spacing + 1 for long span area
Main bars (cut/extra top bars)
● (L/2/spacing + 1)*2 sides for short span area
● (W/2/spacing + 1)*2 sides for long span area
Temp bars/Distribution bars (top bars)
● (L/4/spacing + 1)*2 sides for short span area
● (W/4/spacing + 1)*2 sides for long span area
Once you get the idea of proper placement of rebars you may now proceed for the estimate of number of pieces needed.
You may also watch an explanation from this video here.
TWO WAY SLAB REBAR ESTIMATE
Sample Computation:
➤ Say you have 3m x 3.75m slab
➤ 12mm Ø main bar spacing @ 0.30m o.c.
➤ 10mm Ø temp bars @ 0.30m o.c.
➤ 125mm slab thickness
a. Checking for type of slab (as general practice)
∴ 3.75m/3m = 1.25m less than 2 then ✔two way
b. Number of pcs per span
Bent up/Cranked bars
● others deduct concrete cover on both sides (optional)
● length/2/spacing + 1 = pcs
parallel to Short span
= 3.75m /2 /0.15
= 12.50 + 1
= 13.50 say 14pcs
= ✔ 14pcs bent up/crank bars
parallel to Long span
= 3m /2 /0.15
= 10 + 1
= 11pcs
= ✔ 11pcs bent up/crank bars
Bottom Straight Bars
● others deduct concrete cover on both sides (optional)
● total length/spacing + 1 = pcs
parallel to Short span
= 3.75m /0.15
= 25 + 1
= 26pcs
= ✔ 26pcs bottom straight bars
parallel to Long span
= 3m /0.15
= 20 + 1
= 21pcs
= ✔ 21pcs bottom straight bars
Cut/Extra Top bars
● total length/2/spacing + 1 = pcs (but less 1pc) x 2 sides
● or same pcs to bent bars just deduct 1 pc
● but take not these rebars will be on both sides
parallel to Short span
= 3.75m /2 /0.15
= 12.50 + 1
= (13.50 say 14pcs - 1pc) * 2 sides
= ✔ 26pcs cut/extra bars
parallel to Long span
● total length/2/spacing + 1 = pcs (but less 1pc) x 2 sides
= 3m /2 /0.15
= 10 + 1
= (11pcs - 1pc ) x 2 sides
= ✔ 20pcs cut/extra bars
Temp bars (Distribution bars)
● total length/4/spacing + 1 = pcs x 2 sides
parallel to Short span
= 3.75m /4 /0.15
= 6.25 + 1
= 7.25 say 7pcs x 2 sides
= ✔ 14pcs temp/distribution bars
parallel to Long span
● others deduct concrete cover on both sides (optional)
● total length/4/spacing + 1 = pcs x 2 sides
= 3m /4 /0.15
= 5+ 1
= 6pcs x 2 sides
= ✔ 12pcs temp/distribution bars
c. Cut size (length) per bar
Bent up/cranked bars
● others add 40d for Ld on both sides (optional since we are taking it at center to center of beams)
● length + 2(bend) + 2(hook)
where:
◆ hook=16D (d as diameter of bar)
◆ bend=0.42D (or we can get from table in previous post)
say 70mm or 0.07m (for 125mm slab thickness)
parallel to Short span
= 3m + 2(0.07) + 2(16x0.012)
= ✔ 3.524m cut length bent up/crank bars (14pcs)
parallel to Long span
= 3.75m + 2(0.07) + 2(16x0.012)
= ✔ 4.274m cut length bent up/crank bars (11pcs)
Bottom Straight bars
● length + 2(hook)
parallel to Short span
= 3m + 2(16x0.012)
= ✔ 3.384m cut length bottom straight bars (26pcs)
parallel to Long span
= 3.75m + 2(16x0.012)
= ✔ 4.134m cut length bent up/crank bars (21pcs)
Cut/Extra top bars
● length/4 + 1(hook)
● some use length/3 + 1(hook)
you may choose any of the two
parallel to Short span
= 3m/4 + 1(16x0.012) x 2 sides
= ✔ 0.942m say 0.95m cut length bottom straight bars (26pcs)
parallel to Long span
= 3.75m/4 + 1(16x0.012) x 2 sides
= ✔ 1.1295m say 1.15m cut length bent up/crank bars (20pcs)
Temp bars (Distribution bars)
● length + 2(hooks)
● same as your straight bottom bars except in quantity
parallel to Short span
= 3m + 2(16x0.010)
= ✔ 3.32m cut length bottom straight bars (14pcs)
parallel to Long span
= 3.75m + 2(16x0.010)
= ✔ 4.07m cut length bent up/crank bars (12pcs)
Final part after getting the possible cut size of rebars proceed to the number of pcs to be ordered from nearest hardware/suppliers.
d. Number of steel bar needed
Say we are limited with 6m and 7.5m length availability of steel in our local market, then choose which length gives the least wastage from the two.
Just a quick peek using my excel calcu to check which length is more economical to use
Bent/Cranked bars
= ✔ 3.524m cut length bent up/crank bars (14pcs) use 7.5m length
= ✔ 4.274m cut length bent up/crank bars (11pcs) use 6m length
● 7.5m/3.524m=2.12 say 2pcs usable
● 14/2
= ✔ 7pcs 12mm Ø x 7.5m
● 6m/4.274m=1.40 say 1pc usable
● 11/1
= ✔ 11pcs 12mm Ø x 6m
Do the same steps below if you could come up with the same results in highlight
Straight Bottom bars
= ✔ 3.384m cut length bottom straight bars (26pcs) use 7.5m length
= ✔ 4.134m cut length bent up/crank bars (21pcs) use 6m length
= ✔ 13pcs 12mm Ø x 7.5m
= ✔ 21pcs 12mm Ø x 6m
Cut/Extra top bars (disregard cut length in excel since L/3 was used)
= ✔ 0.95m cut length bottom straight bars (26pcs) use 6m length
= ✔ 1.15m cut length bent up/crank bars (20pcs) use 6m length
= ✔ 5pcs 12mm Ø x 7.5m
= ✔ 4pcs 12mm Ø x 6m
Temp/Distribution bars
= ✔ 3.32m cut length bottom straight bars (14pcs) use 7.5m length
= ✔ 4.07m cut length bent up/crank bars (12pcs) use 6m length
= ✔ 7pcs 10mm Ø x 7.5m
= ✔ 12pcs 10mm Ø x 6m
total:
= 25pcs 12mm Ø x 7.5m length (167kgs)
= 36pcs 12mm Ø x 6.0m length (192kgs)
= 7pcs 10mm Ø x 7.5m length (32kgs)
= 12pcs 10mm Ø x 6.0m length (44kgs)
e. Tie Wire Needed (in kgs)
Same process just like what we did in one way slab just compute all those joints where your rebars will cross with each other.
(x axis pcs) * (y axis pcs) * 0.30m
53m length
= ✔ 14pcs bent up/crank bars
= ✔ 11pcs bent up/crank bars
● (14 x 11 x 0.30) / 53 = 0.87kgs
= ✔ 26pcs bottom straight bars
= ✔ 21pcs bottom straight bars
● (26 x 21 x 0.30) / 53 = 3.09kgs
= ✔ 13pcs cut/extra bars
= ✔ 10pcs cut/extra bars
● (13 x 10 x 0.30) / 53 = 0.74kgs x 2 sides
= ✔ 7pcs temp/distribution bars
= ✔ 6pcs temp/distribution bars
● (7 x 6 x 0.30) / 53 = 0.24kgs x 2 sides
total = 5.92kgs say ✔ 6kgs #16 G.i. wire
Now try to get the kg per steel bar if you will come up with the same results above.
use formula:
(d)2/162) x steel length x no. of pcs = kgs
where:
d=diameter of bar
If you wish to try to compute for spacing of rebars, try to watch from this source quite a good start if you intend to understand how slabs are designed.
Various types of support conditions for two way slab
Share with friends if you find this helpful. Thanks!
CONCRETE SLAB (Two Way)
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Reviewed by b.o.m.
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