no beams lecture 23 arch 331 concrete slabs 2 lecture 23...

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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

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Systems

• beams separate from slab

• beams integral with slab

– close spaced

• continuous beams

• no beams

Concrete Slabs 3

Lecture 23

Foundations Structures

ARCH 331

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T sections

• two areas of compression in moment

possible

• one-way joists

• effective flange width

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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

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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

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One-Way

• Joists

– standard

stems

– 2.5” to 4.5”

slab

– ~30” widths

– reusable

forms

Concrete Slabs 7

Lecture 23

Foundations Structures

ARCH 331

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One-Way

• Joists

– wide pans

– 5’, 6’ up

– light loads &

long spans

– one-leg

stirrups

Concrete Slabs 8

Lecture 23

Foundations Structures

ARCH 331

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Compression Reinforcement

• doubly reinforced

• negative bending

• two compression forces

• bigger Mn

• control deflection

• increase ductility

• needs ties because

of buckling

3

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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

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Slabs

• one way behavior – like beams

• two way behavior – more complex

Concrete Slabs 11

Lecture 23

Foundations Structures

ARCH 331

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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

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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

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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

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Precast

• prestressed

– PCI Design

Handbook

– double T’s

– hollow core

– L’s

• topping

• load tables