One Way Joist Floor System

ONE WAY JOIST FLOOR SYSTEM


Design Concept One way joist floor system consists of a series of

small, closely spaced, reinforced concrete “T” beams framing into monolithically cast concrete girders which in turn are carried by building columns.

The “T” beams called joist are formed by creating void spaces in what otherwise be a solid slab. The voids are formed by using special steel or fibre glass pans.

The concrete cast between the forms to create the joist is poured to a depth and over the top of the

POURING OF PAN JOIST



form so as to create a thin monolithic slab which

becomes flange of the joist.

Strength of the R.C.C “T” beam depends on the

amount of steel, therefore a considerable saving in

concrete dead load can be obtained without any loss

of strength.

Occasionally, joist floor are built using clay tile

fillers which serve as form for the concrete in the ribs

and are left in place to serve as ceiling. (ACI Code

8.13.5)


USES One way joist floor are economical for buildings,

apartments, houses, hospitals, hotels, and classrooms etc where live loads are relatively LIGHT and spans are comparatively LONG say up to 35 ft span.

DIMENSIONS Two modules of 2’-0”, with 20” wide pan and 3,-0”

with 30” pan are used. Slab thickness is governed by ACI Code 8.13.6.1 and is minimum 2” for 20” pan and 2.5” for 30” pan.

3 ft Module

30 “6 “ 6 “

16“

3.0“

TOP SLAB

RIB

X-SEC OF JOIST

6 “

16“

3.0“

RIB

BASIC ELEMENT FOR DESIGN

9 “

36 “


For one hour fire rating 0.75” clear concrete cover

and 3” - 3.5” slab thickness is required. For two hour

fire rating, 1” concrete cover and 4.5” thick slab is

required.

Depth of joist floor is governed by deflection

requirements of ACI Code table 9.5(a). Standard

depths of rib are 6”, 8”, 10”, 12”, 14”, 16”, or 20”.

To provide additional shear capacity, the end forms

are tapered in last 3 ft length. Forms are sloped,

minimum 1:12 to facilitate removal.

BENDING UP OF BAR AND TAPERING OF JOIST NEAR THE SUPPORT


DISTRIBUTION RIB CRSI handbook suggest no distribution rib in span

up to 20 ft, one at mid span for spans of 20’ to 30’ and two at third point for span exceeding 30’. It is generally reinforced by 1# 4 bar at top and bottom.

ACI CODE PROVISIONS. (ACI CODE 8.13) Ribs shall not be less than 4” width and depth not

greater than 3.5 times the rib width. Clear spacing of the ribs shall not exceed 30”. Slab thickness over top of pans must not be less

than 1/12 of clear span nor less than 2”.

ONE WAY JOIST FLOOR SYSTEM Reinforcement in slab shall be equal to temperature

and shrinkage reinforcement. Positive reinforcement shall consist of 2 bars with

one bar cut off or bent up where no longer required. Shear strength of the joist may be taken 10 % more

than normal beam i.e. Vc= 1.1x2x√f’cxbwxd

One way joist are normally proportioned with no shear reinforcement but shear reinforcement may be provided in exceptional cases.

Girders are cast monolithically with joist and slab. Joist construction not meeting the requirements of

ACI 8.13.1 to 3 are designed as slab and “T” beam.


DESIGN CONCEPT OF JOIST FLOOR The design of joist floor consist of:

● Design of slab

● Design of Joist

● Design of Girder Design of Slab. The slab is considered as if it were of

plain concrete and checked for modulus of rupture. The slab may be considered as fixed between joists and bending moment may be taken as wul2/12. It is generally reinforced with temperature and shrinkage reinforcement. (ACI code 8.13.6.2)


DESIGN OF ONEWAY JOIST

30 “6 “ 6 “

14“

3“

TOP SLAB

RIB

X-SEC OF JOIST

6 “

14“

3“

RIB

9 “

36 “


Joist.

● Load for design of each joist is taken equal to the floor load on the width equal to width of

module i.e. 3 ft or 2 ft

● It is designed as floor beam having rectangular section in the region of negative moment and as “T” section at mid span. Minimum depth of joist is taken as per ACI Code table 9.5 (a).

● ACI moment coefficient (ACI Code 8.3.3) or elastic analysis may be used to determine the bending moments.

A B C

-1/24 +1/14 -1/9

Spandrel beam

Spandrel beam

A B C

-1/16 +1/14 -1/9

Column Column

ACI MOMENT COEFICIENTS

1. TWO SPANS

Beam

Column

ACI MOMENT COEFICIENTS

1. MORE THAN TWO SPANS

-1/24

+1/14

-1/10

+1/16

-1/11 -1/11

+1/16

-1/11

-1/16

+1/14

-1/10

+1/16

-1/11 -1/11

+1/16

-1/11

0

+1/11

-1/10

+1/16

-1/11

WALL

-1/11


Girder. The girder is designed for the load transferred as uniformly distributed load from the joist – just like ordinary beam.

Transverse Distribution Rib. As per para above

ANY QUESTION ?

ThanksThanks


DESIGN EXAMPLE

It is proposed to add 6 x classrooms / labs to the existing academic block of MCE comprising of 3 classrooms along with a veranda at G.F and 3 x labs at F.F. The proposed LINE PLAN is shown in figure. Design the concrete one way joist floor using 3 ft module for the exterior classroom. Live load = 60 psf,

F.F = 2”, and ceiling plaster ½” thick. f’c = 4 ksi fy = 60 ksi . Assume exterior staircase and one hour fire rating.

DESIGN OF ONE WAY JOIST FLOOR SYSTEM

D W

CL ROOM

28’x 40’

GROUND FLOOR LINE PLAN2’ Projection

D W

13 ½ “ Thick Wall

VERANDAH 10 FT WIDE

B-1 B-2

D D W

CL ROOM

30’x 40’

CL ROOM

28’x 40’

B-3

DESIGN OF ONE WAY JOIST

30 “6 “ 6 “

16“

3.5“

TOP SLAB

RIB

X-SEC OF JOIST

6 “

16“

3.5“

RIB


9 “

36 “

2 # 4 2 # 6

16“

3.5“

Distr Rib

9’-4”

A

A

# 3 @ 12 “ c/c both ways1 # 4

Girder on wall2 # 5 Altn cut off Girder 3’-6”

13.5”13.5”28 ft

L- SEC OF JOIST

30 “6 “ 6 “

16“

3.5“

# 3 @ 12” c/c both ways

SECTION AA

2 # 5 2 # 5

2 # 62 # 6

0.75” cover

7’-0”

2 # 6 2 # 6

16“

3.5“

Distr Rib

10’A

A

# 3 @ 12 “ c/c both ways1 # 4

Girder on wall2 # 5 Altn cut off Girder

3’- 6”

13.5”13.5” 30 ft

L- SEC OF JOIST

30 “6” 6”

16“

3.5“


SECTION AA

2 # 5 2 # 5

2 # 62 # 6

0.75” cover

Load on Wall Footing

Calculate the load on interior wall assuming ceiling ht 14’, DPC 5’ above foundation level, roof live load = 20 psf, roof insulation weighting 30 psf, and ½” ceiling plaster. Design the wall footing if allowable soil pressure is 1.50 Ton/sq ft at 5’ below ground.

Solution

Wall footing is designed at service loads per foot length of wall. Dead load and live loads will be worked out separately. Dead loads are;

Wt of 3 ft long slab/ft= 3.5x12.5x3= 131 lbs/ft

Wt of rib= (6+9)/12x0.5x(16/12)x150=125 lbs/ft

Wt of slab+ rib= 131+125= 256 lbs/ft

Wt of insulation and C.P=30+6=36psf=36x3=108 plf

D = (256+108) x1.15 x 28 = 1.95 K D = (256+108)x30 /(2X3) =1.82 k2x3

L = 3x20x1.15x28 =0.322k L=(3x20)x30/(2x3) =0.30k 2x3

D = (256+90)1.15x28/(2x3)=1.86 K D = (256+90)x30/(2x3)=1.73 k

D =(13.5+1.5)/12x14x0.12 = 2.1 k

L = 3x60x1.15x28 =0.96 k L = (3x60)30/2x3 = 0.90 k G.F.ROOF

3x2 D =(13.5+1.5)/12x14x0.12 = 2.1 k

EXTERIOR CL ROOM INTERIOR CL ROOM

D.P.C

D =11.56 KL = 2.48 K

ROOF

13.5” WALL+C.P

DESIGN OF ONEWAY JOIST FLOOR SYSTEM

D D D DW W W W

CL ROOM

24’x 36’

CL ROOM

26’x 36’

LINE PLAN GROUND FLOOR 2’ Projection

13 ½ “ Thick Wall

VERANDAH 10 FT WIDE B-1 B-2


DESIGN OF ONEWAY JOIST

30 “6 “ 6 “

14“

3“

TOP SLAB

RIB

X-SEC OF JOIST

6 “

14“

3“

RIB

9 “

36 “

2 # 6 2 # 6

14“

3“

Distr Rib

8’-8”A

A

# 3 @ 12 “ c/c both ways1 # 4

Girder on wall 2 # 5 Altn cut off Girder 3’

13.5”13.5”26 ft

L- SEC OF JOIST

30 “6 “ 6 “

14“

3“


SECTION AA

2 # 5 2 # 52 # 52 # 5

2 # 62 # 6

0.75” cover

RELEVANT PROVISIONS OF ACI CODE FOR SHEAR IN BEAMS

Φ Vn ≥ Vu (ACI 11.1)

Vn = Vc + Vs (ACI 11.1)

Vc = 2 √fc’ bw.d (ACI 11.3.1.1)

Vs = (Av fyt.d)/S (ACI 11.5.6.2)

Sections located less than a distance ‘d’ from face of support can be designed for the same shear as that computed at a dist ‘d’. ACI 11.1.3.1

Design yield strength of shear reinforcement is not greater than 60 ksi. (ACI 11.4.2)


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One Way Joist Floor System