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F2009abn
twenty three
concrete construction:
T-beams & slabsConcrete Slabs 1
Lecture 23
Architectural Structures
ARCH 331
lecture
ARCHITECTURAL STRUCTURES:
FORM, BEHAVIOR, AND DESIGN
ARCH 331
DR. ANNE NICHOLS
FALL 2018
Concrete Slabs 2
Lecture 23
Foundations Structures
ARCH 331
F2008abn
Systems
• beams separate from slab
• beams integral with slab
– close spaced
• continuous beams
• no beams
Concrete Slabs 3
Lecture 23
Foundations Structures
ARCH 331
F2008abn
T sections
• two areas of compression in moment
possible
• one-way joists
• effective flange width
F2008abnConcrete Slabs 4
Lecture 23
Foundations Structures
ARCH 331
T sections
• negative bending: min As, larger of:
• effective width (interior)
– L/4
– bw + 16t
– center-to-
center of
beams
CfCw
T
)(6
dbf
fA w
y
c
s
)(
3db
f
fA f
y
c
s
2
F2008abnConcrete Slabs 5
Lecture 23
Foundations Structures
ARCH 331
T sections
• usual analysis steps
1. assume no compression in web
2. design like a
rectangular beam
3. needs reinforcement
in slab too
4. also analyze for negative
moment, if any
a/2
T
a=1c
0.85f’c
C
Concrete Slabs 6
Lecture 23
Foundations Structures
ARCH 331
F2008abn
One-Way
• Joists
– standard
stems
– 2.5” to 4.5”
slab
– ~30” widths
– reusable
forms
Concrete Slabs 7
Lecture 23
Foundations Structures
ARCH 331
F2008abn
One-Way
• Joists
– wide pans
– 5’, 6’ up
– light loads &
long spans
– one-leg
stirrups
Concrete Slabs 8
Lecture 23
Foundations Structures
ARCH 331
F2008abn
Compression Reinforcement
• doubly reinforced
• negative bending
• two compression forces
• bigger Mn
• control deflection
• increase ductility
• needs ties because
of buckling
3
F2008abnConcrete Slabs 9
Lecture 23
Foundations Structures
ARCH 331
Compression Reinforcement
• analysis
– As & As
– T = Cc + Cs
– T = Asfy
– Cs = As (f s - 0.85f c)
– Cc = 0.85f cba with
– fs not known, so solve for c (n.a.)
– fs < fy ?
– Mn = T(d-a/2)+Cs(d-d )
1a c
Concrete Slabs 10
Lecture 23
Foundations Structures
ARCH 331
F2008abn
Slabs
• one way behavior – like beams
• two way behavior – more complex
Concrete Slabs 11
Lecture 23
Foundations Structures
ARCH 331
F2008abn
Slab Design
• one unit wide “strip”
• with uniform loads
– like “wide” beams
– moment / unit width
– uniform curvature
• with point loads
– resisted by stiffness
of adjacent strips
– more curvature in middle
F2008abnConcrete Slabs 12
Lecture 23
Foundations Structures
ARCH 331
Slab Design
• min thickness by code
• reinforcement
– bars, welded wire mesh
– cover
– minimum by steel grade
• 40-50:
• 60:
0018.0bt
As
002.0bt
As
4
Concrete Slabs 13
Lecture 23
Foundations Structures
ARCH 331
F2008abn
One-Way Slabs
• As tables
• max spacing
– 3(t) and 18”
– 5(t) and 18” – temp & shrinkage steel
• no room for stirrups
Concrete Slabs 14
Lecture 23
Foundations Structures
ARCH 331
F2008abn
Precast
• prestressed
– PCI Design
Handbook
– double T’s
– hollow core
– L’s
• topping
• load tables
