Lecture 34 Design of TwoWay Floor Slab System

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Lecture 34 - Design of Two-Way Floor Slab System

• April 12, 2001
• CVEN 444

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Lecture Goals

• Thickness Example
• Shear Strength of Slabs
• Shear Example
• Direct Design Method

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Example
The floor system consists of solid slabs and
beams in two directions supported on 20 in.
square columns. Determine the minimum slab
thickness required for an interior panel. Use fc
4 ksi and fy 60 ksi
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Example
The cross-sections are
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Example
The resulting cross section
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Shear Strength of Slabs
In two-way floor systems, the slab must have
adequate thickness to resist both bending moments
and shear forces at critical section. There are
three cases to look at for shear.
Two-way Slabs supported on beams Two-Way Slabs
without beams Shear Reinforcement in two-way
slabs without beams.
1. 2. 3.
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Shear Strength of Slabs
Two-way slabs supported on beams
The critical location is found at d distance from
the column, where
The supporting beams are stiff and are capable of
transmitting floor loads to the columns.
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Shear Strength of Slabs
The shear force is calculated using the
triangular and trapezoidal areas. If no shear
reinforcement is provided, the shear force at a
distance d from the beam must equal
where,
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Shear Strength of Slabs
Two-Way Slabs without beams
There are two types of shear that need to be
addressed
One-way shear or beam shear at distance d from
the column Two-way or punch out shear which
occurs along a truncated cone.
1. 2.
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Shear Strength of Slabs
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Shear Strength of Slabs
One-way shear considers critical section a
distance d from the column and the slab is
considered as a wide beam spanning between
supports.
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Shear Strength of Slabs
Two-way shear fails along a a truncated cone or
pyramid around the column. The critical section
is located d/2 from the column face, column
capital, or drop panel.
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Shear Strength of Slabs
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Shear Strength of Slabs
If shear reinforcement is not provided, the shear
strength of concrete is the smaller of
as is 40 for interior columns, 30 for edge
columns, and 20 for corner columns.
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Shear Strength of Slabs
Shear Reinforcement in two-way slabs without
beams.
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Shear Strength of Slabs
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Shear Strength of Slabs
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Shear Strength of Slabs
Conventional stirrup cages
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Shear Strength of Slabs
Studded steel strips
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Shear Strength of Slabs
The reinforced slab follows section 11.12.4 in
the ACI Code, where Vn can not
The spacing, s, can not exceed d/2. If a
shearhead reinforcement is provided
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Example Problem
Determine the shear reinforcement required for an
interior flat panel considering the following
Vu 195k, slab thickness 9 in., d 7.5 in., fc
3 ksi, fy 60 ksi, and column is 20 x 20 in.
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Example Problem
The final result 13 stirrups at total distance
of 45.5 in