one way joist floor system
DESCRIPTION
SLAB SYSTEMSTRANSCRIPT
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
ONE WAY JOIST FLOOR SYSTEM
ONE WAY JOIST FLOOR SYSTEM
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)
ONE WAY JOIST FLOOR SYSTEM
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 “
ONE WAY JOIST FLOOR SYSTEM
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
ONE WAY JOIST FLOOR SYSTEM
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.
ONE WAY JOIST FLOOR SYSTEM
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)
BASIC ELEMENT FOR DESIGN
DESIGN OF ONEWAY JOIST
30 “6 “ 6 “
14“
3“
TOP SLAB
RIB
X-SEC OF JOIST
6 “
14“
3“
RIB
9 “
36 “
ONE WAY JOIST FLOOR SYSTEM
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
ONE WAY JOIST FLOOR SYSTEM
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 ?
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ONE WAY JOIST FLOOR SYSTEM
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
BASIC ELEMENT FOR DESIGN
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“
# 3 @ 12” c/c both ways
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
BASIC ELEMENT FOR DESIGN
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“
# 3 @ 12” c/c both ways
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)
ONE WAY JOIST FLOOR SYSTEM