2 Way Slab
Lecture 7 – Two-Way Slabs
Two-way slabs have tension reinforcing spanning in BOTH directions, and may take the general form of one of the following:
Types of Two-Way Slab Systems
Lecture 7 – Page 1 of 13
The following Table may be used to determine minimum thickness of various twoway slabs based on deflection:
Minimum Suggested Thickness “h” for Two-Way Slabs
Two-Way Slab System: Minimum Thickness h: Flat plate Ln/30 Flat plate with spandrel beams Ln/33 Flat slab Ln/33 Flat slab with spandrel beams Ln/36 Two-way beam-supported slab Ln/33 Ln = clear distance in long direction Flat Plates Flat plates are the most common type of two-way slab system. It is commonly used in multi-story construction such as hotels, hospitals, offices and apartment buildings. It has several advantages: • • • Easy formwork Simple bar placement Low floor-to-floor heights
Direct Design Method of Flat Plates per ACI 318-02 Two-way slabs are inherently difficult to analyze by conventional methods of statics because of the two-way bending occurring. Accurately determining the moments on a two-way slab is typically accomplished by finite element computer analysis.
Computer analysis of two-way slab
Lecture 7 – Page 2 of 13
The ACI 318 code allows a direct design method that can be used in most typical situations. However, the following limitations apply: 1. Must have 3 or more continuous spans in each direction. 2. Slab panels must be rectangular with a ratio of the longer span to shorter span(measured as centerline-to-centerline of support) not greater than 2.0. 3. Successive span lengths in each direction must not differ by more than 1/3 of the longer span. 4. Columns must not be offset by more than 10% of the span (in direction of offset) from either axis between centerlines of successive columns. 5. Loads must be uniformly distributed, with the unfactored live load not more than 2 times the unfactored dead load (L/D < 2.0). Design Strips a) If L1 > L2: L2 L2
Column (typ.)
Interior
Two-way slabs have tension reinforcing spanning in BOTH directions, and may take the general form of one of the following:
Types of Two-Way Slab Systems
Lecture 7 – Page 1 of 13
The following Table may be used to determine minimum thickness of various twoway slabs based on deflection:
Minimum Suggested Thickness “h” for Two-Way Slabs
Two-Way Slab System: Minimum Thickness h: Flat plate Ln/30 Flat plate with spandrel beams Ln/33 Flat slab Ln/33 Flat slab with spandrel beams Ln/36 Two-way beam-supported slab Ln/33 Ln = clear distance in long direction Flat Plates Flat plates are the most common type of two-way slab system. It is commonly used in multi-story construction such as hotels, hospitals, offices and apartment buildings. It has several advantages: • • • Easy formwork Simple bar placement Low floor-to-floor heights
Direct Design Method of Flat Plates per ACI 318-02 Two-way slabs are inherently difficult to analyze by conventional methods of statics because of the two-way bending occurring. Accurately determining the moments on a two-way slab is typically accomplished by finite element computer analysis.
Computer analysis of two-way slab
Lecture 7 – Page 2 of 13
The ACI 318 code allows a direct design method that can be used in most typical situations. However, the following limitations apply: 1. Must have 3 or more continuous spans in each direction. 2. Slab panels must be rectangular with a ratio of the longer span to shorter span(measured as centerline-to-centerline of support) not greater than 2.0. 3. Successive span lengths in each direction must not differ by more than 1/3 of the longer span. 4. Columns must not be offset by more than 10% of the span (in direction of offset) from either axis between centerlines of successive columns. 5. Loads must be uniformly distributed, with the unfactored live load not more than 2 times the unfactored dead load (L/D < 2.0). Design Strips a) If L1 > L2: L2 L2
Column (typ.)
Interior