Page 254 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch05.qxd 9/29/05 5:12 PM Page 5.34

CRITERIA FOR BUILDING DESIGN

5.34 CHAPTER FIVE

To ensure that axial loads applied to encased composite columns are properly shared between the
steel and concrete, shear connectors are required [see Eq. (5.128) for nominal strength]. The fol-
lowing requirements must be met:
When the external force is applied directly to the steel section, shear connectors must be provid-
ed to transfer the required shear force V′, given by

 AF 
sy
V 1
V ′ =  −  (5.124)
 P o 
When the external force is applied directly to the concrete encasement, shear connectors must be
provided to transfer the required shear force V′, given by

 AF 
sy
V ′ =   (5.125)
V
 P o 
where V = shear force introduced to column, kips (N)
2
2
A s = area of steel section, in (mm )
F y = specified minimum yield stress of steel section, ksi (MPa)
P o = nominal axial compressive strength without length effects, kips (N)
Also, when load is applied to the encased composite concrete by direct bearing on the concrete,
the design bearing strength φ B P p and the allowable tensile strength P p /Ω B are determined from

(5.126)
P p = 1.7f c ′A B
φ B = 0.65 (LRFD) φ B = 2.31 (ASD) (5.127)
2
2
where A B is the loaded area, in (mm ).
Additionally, various detailing requirements for encased composite columns must be met. At
least four longitudinal bars must be used. Transverse reinforcement must be spaced at the lesser
of 16 longitudinal bar diameters, 48 tie bar diameters, or 0.5 times the least dimension of the
composite section. The encasement must provide at least 1.5 in (38 mm) of clear cover to the
steel.
Shear connectors, meeting the requirements of Eqs. (5.124) and (5.125), must be distributed
along the length of the member at least a distance of 2.5 times the depth of the encased composite
column above and below the load transfer region. The maximum connector spacing is 16 in (405 mm),
and connectors must be placed on at least two faces of the steel shape in a configuration symmetrical
about the steel shape axes.
If the composite cross section is built up from two or more encased steel shapes, the shapes must
be interconnected with lacing, tie plates, batten plates, or similar components to prevent buckling of
individual shapes as a result of loads applied prior to hardening of the concrete.
For encased composite columns, the nominal shear strength of one stud shear connector is

Q = 05 A sc f E ′ c c ≤ A F u (5.128)
.
n
sc
2
2
where A sc = cross-sectional area of stud, in (mm )
E c = modulus of elasticity of concrete, ksi (MPa)
F u = specified minimum tensile strength of stud shear connector, ksi (MPa)
The shear strength of an encased column should be based on the shear strength of the steel sec-
tion alone, plus the shear strength of any tie reinforcement, or, alternatively, on the shear strength of
the reinforced concrete (see ACI 318).

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