ENCE 710 Design of Steel

Structures

IV. COMPOSITE STEEL-CONCRET CONSTRUCTION

C. C. Fu, Ph.D., P.E.Civil and Environmental Engineering

DepartmentUniversity of Maryland

2

IntroductionFollowing subjects are covered: Composite Action Effective Width Nominal Moment Strength Shear Connectors, Strength and Fatigue Formed Steel DeckReading: Chapters 16 of Salmon & Johnson AISC LRFD Specification Chapters B (Design

Requirements) and I (Design of Composite Members)

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Composite Action

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Effective Width

AISC-I31. Interior

BE ≤ L/4BE ≤ b0 (for equal beam spacing)

2. ExteriorBE ≤ L/8 + (dist from beam center to edge of slabBE ≤ b0/2 + (dist from beam center to edge of slab)

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Nominal Moment Strength

Nominal Moment Strength of Fully Composite Section (AISC 14th Edition Art. I3.2a)

1.

Mn = based on plastic stress distribution on the Composite Section; Φb = 0.9

2.

Mn = based on superposition of elastic stresses, considering the effect of shoring;

Φb = 0.9

yf

pwc FEth /76.3/

yf

pwc FEth /76.3/

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Plastic Stress Distribution

Case 1 (if a ≤ ts): S & J Eq. (16.7.1 to 5)

Case 2 (if a > ts): S & J Eq. (16.7.6 to 10)

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Shear Connectors

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Shear Variation

V’ = Tmax =AsFy

V’ = Cmax = 0.85fc’bEts

N = Cmax/Qn or Tmax/Qn

Whichever is smaller

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Nominal Strength Qn

Qn = 1. Headed Steel Stud(AISC Eq. I8-1)

2. Channel Connectors (AISC Eq. I8-2)

uscpgccwn FARREfAQ '5.0

cccwfn EfLttQ ')5.0(3.0

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Nominal Strength Qn

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Connector Design – Fatigue Strength

(AASHTO LRFD Eq. 6.10.7.4.1b-1)

Zr = d 2 5.5 d 2/2; (AASHTO LRFD Eq. 6.10.7.4.2-1)

where = 34.5 – 4.28 log N (AASHTO LRFD Eq. 6.10.7.4.2-2)

QV

InZp

sr

r

Example:

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Composite Column Section (rolled steel shape encased in concrete)

AISC I2.1. Encased Composite Columns

AISC I2.2. Filled Composite Columns(Ref: Separate handout with examples.)

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Composite Column Section (rolled steel shape encased in concrete)

Using Effective Section Properties (I2-4, 5 & 6)ccyrsrys fAFAFAP '85.00

21

2

1LK

EIP effe

ccsessseff IECIEIEEI 15.0

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Filled Composite Column Example

AISC I2-2b(a)Compact(b)Noncompact(c)Slender

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Filled Composite Column Example

ccyrsrys fAFAFAP '85.00

ccsessseff IECIEIEEI 35.0

• For compact sections• Ac = bfhf+π(r-t)2+2bf(r-t)+2hf(r-t)

Ac = (8.5 in.)(4.5 in.) + π(0.375 in.)2 + (8.5 in.)(0.375 in.) + 2(4.5 in.)(0.375 in.) = 48.4 in.2

•

• • P0 = (10.4 in.2)(46ksi) + 0.85(48.4 in.2)(5 ksi) = 684kips•

• EIeff = (29,000 kis)(61.8 in.4) + (0.90)(3,900 ksi)(111 in.4) = 2,180,000 kip-in.2

42222

22211 .111)

3

)(4

2)(

2

)((2)

9

8

8)((2

12

))((2

12in

trhtrtr

hbhbIc

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Filled Composite Column Example

21

2

1LK

EIP effe

• Pe = π2(2,180,000 kip-in.2)/(1.0(14 ft)(12 in./ft))2 = 762 kips

• P0/Pe = 684 kips/762 kips = 0.898 ≤ 2.25

•

• φcPn = 0.75(470 kips) = 353 kips > 336 kips o.k.

kipskipsPP ePpn 470658.0)684(658.0 898.0/

00