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Lecture 23 - Two-Way Floor Slab System
November 29, 2001CVEN 444
Lecture Goals
Direct Method Reinforcement in Two-way Slabs Footing Classification Footing Design
Minimum extensions for reinforcement
The reinforcement is for slabs without beams.
Design Examples
Look at Example 13-7
Design of a flat plate floor without spandrel beams - Direct Design MethodLook at Example 13-9
Design of a Two-way slab with beams in both directions.
Shear Strength of Two-way Slabs
Shear failure occurs when inclined cracks form due to flexural and shearing stresses.
Two types of shear failure mechanism in 2-way slabs.
(a) One-way shear
(b) Two-way shear (Punching shear).
Shear Failure Mechanism
Two-way shearOne-way shear
Slabs failing in two-way shear
Inclined cracking Slab slides down column
Top (negative reinforcement pulls out of slab leaving no connection to the column
Inclined cracking
Design for two-way ShearCritical perimeter is located d / 2 from column face, where d = effective depth of slab
Design for two-way ShearCritical perimeter is located d / 2 from column face, where d = effective depth of slab
Slabs with Drop Panels
Design Equations
Two-way Shear with Negligible Moment Transfer
ACI Eqn. 11-1
Factored shear force (tributary area bounded by lines of zero shear)
Nominal shear resistance of slab
0.85
where, Vu =
Vn =
=f
nu VV
Design Equations
Two-way Shear with Negligible Moment Transfer
Shear resistance of concrete
Shear resistance of steel (in most slabs, Vs = 0)
where, Vc =
Vs =
scn VVV
Design Equations
(a) Two-way Shear with Negligible Moment Transfer
For two-way shear in slabs (& footings) Vc is smallest of
long side/short side of column concentrated load or reaction area
length of critical perimeter around the column
where, bc =
b0 =
ACI 11-35dbfV 0c
c
c 4
2
Design Equations
(b) Two-way Shear with Negligible Moment Transfer
40 for interior columns
30 for edge columns
20 for corner columns
where, as =
ACI 11-36dbfb
dV 0c
0
sc 2
Design Equations
(c) Two-way Shear with Negligible Moment Transfer
Take smaller of (a), (b) and (c)
ACI 11-37dbfV 0cc 4
Design Equations
Figure 13-41
MacGregor
Tributary area for Vu calculations
Slab Shear Reinforcement
If , Vn can be increased by:
Increase slab thickness
Use drop panel
Increase b0 by increasing column size or adding a fillet or capital
Add shear reinforcement
(1)
(2)
(3)
(4)
uc VV
Slab Shear Reinforcement
