ENCE 710
Design of Steel Structures
IV. COMPOSITE STEEL-CONCRET
CONSTRUCTION
C. C. Fu, Ph.D., P.E.
Civil and Environmental Engineering Department
University of Maryland
Introduction
Following subjects are covered:
 Composite Action
 Effective Width
 Nominal Moment Strength
 Shear Connectors, Strength and Fatigue
 Formed Steel Deck
Reading:
 Chapters 16 of Salmon & Johnson
 AISC LRFD Specification Chapters B (Design Requirements)
and I (Design of Composite Members)
2
Composite Action
3
Effective Width
AISC-I3
1. Interior
BE ≤ L/4
BE ≤ b0 (for equal beam spacing)
2. Exterior
BE ≤ L/8 + (dist from beam center to edge of slab
BE ≤ b0/2 + (dist from beam center to edge of slab)
4
Nominal Moment Strength
Nominal Moment Strength of Fully Composite Section
(AISC 14th Edition Art. I3.2a)
1.

h c / t w    p  3 . 76 /

E
F yf



Mn = based on plastic stress distribution
on the Composite Section;
Φb = 0.9
2.

h c / t w    p  3 . 76 /

E
F yf



Mn = based on superposition of elastic
stresses, considering the effect of shoring;
Φb = 0.9
5
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)
6
Shear Connectors
7
Shear Variation
V’ = Cmax = 0.85fc’bEts
V’ = Tmax =AsFy
N = Cmax/Qn or Tmax/Qn
Whichever is smaller
8
Nominal Strength Qn
Qn =
1.
Headed Steel Stud
(AISC Eq. I8-1)
Q n  0 .5 Aw
2.
f c ' E c  R g R p A sc Fu
Channel Connectors
(AISC Eq. I8-2)
Q n  0 . 3( t f  0 . 5 t w ) L c
fc ' Ec
9
Nominal Strength Qn
10
Connector Design – Fatigue Strength
p
nZ r I
(AASHTO LRFD Eq. 6.10.7.4.1b-1)
V sr Q
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)
Example:
11
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.)
12
Composite Column Section (rolled
steel shape encased in concrete)
Using Effective Section Properties (I2-4, 5 & 6)
P0  A s F y  A sr F yr  0 . 85 A c f ' c
 EI eff
2
Pe1 
K 1 L 2
EI eff  E s I s  0 . 5 E s I se  C 1 E c I c
13
Filled Composite Column Example
AISC I2-2b
(a) Compact
(b) Noncompact
(c) Slender
14
Filled Composite Column Example
• 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
2
•
Ic 
b1 h1
12
2

2 ( b 2 )( h 2 )
 2 ( r  t )(
12

8

8
9
)  2(
 (r  t )
2
2
)(
h2
2

4(r  t )
3
)  111 in .
2
• P0  A s F y  A sr F yr  0 . 85 A c f ' c
• P0 = (10.4 in.2)(46ksi) + 0.85(48.4 in.2)(5 ksi) = 684kips
•
EI eff  E s I s  0 . 5 E s I se  C 3 E c I c
• EIeff = (29,000 kis)(61.8 in.4) + (0.90)(3,900 ksi)(111 in.4)
= 2,180,000 kip-in.2
15
4
Filled Composite Column Example
 EI eff
2
Pe1 
K 1 L 2
• 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
•

Pn  P0 0 . 658
p 0 / Pe
  ( 684 kips ) 0 .658
0 . 898
  470 kips
• φcPn = 0.75(470 kips) = 353 kips > 336 kips o.k.
16