Page 255 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch05.qxd 9/29/05 5:12 PM Page 5.35

CRITERIA FOR BUILDING DESIGN

CRITERIA FOR BUILDING DESIGN 5.35

5.8.3 Filled Composite Columns
To qualify for design as a filled composite column, three conditions must be met: (1) the cross-
sectional area of the steel HSS shall comprise at least 1% of the total composite cross section; (2) for
rectangular HSS filled with concrete, the width–thickness ratio b/t must not exceed 2.26 EF/ y ; and
(3) for round HSS, the diameter–thickness ratio D/t must not exceed 0.15E/F y . However, higher b/t
and D/t ratios may be used when justified by testing and analysis.
The design compressive strength φ c P n and allowable compressive strength P n /Ω c for axially
loaded filled composite columns are determined for the limit state of flexural buckling using
Eqs. (5.118) to (5.123) with φ c = 0.75 (LRFD) and Ω c = 2.00 (ASD), but with the following modified
definitions:
P o = A s F y + A sr F yr + C 2 A c f c ′ (5.129)
(5.130)
EI eff = E s I s + E s I sr + C 3 E c I c
where C 2 = 0.85 for rectangular HSS, C 2 = 0.95 for circular sections, and
 A 
06 2 
C = . +  A + s A   ≤ 09 . (5.131)
3
s
c
For concrete-filled composite members subjected to tensile forces, the available tensile strength
is calculated as follows, ignoring any tensile strength of the concrete. The design tensile strength φ t P n
and the allowable tensile strength P n /Ω t for the limit state of yielding are determined using φ t = 0.90
(LRFD) and Ω t = 1.67 (ASD), where the nominal tensile strength P n , kips (kN), is
(5.132)
P n = P t = A s F y + A sr F yr
The available shear strength for filled composite columns is based on either the shear strength of
the steel section alone or the shear strength of the concrete (see ACI 318).
Loads applied to filled composite columns must be transferred between the steel and concrete.
When the external force is applied either to the steel section or to the concrete infill, transfer of force
from the steel section to the concrete core may be developed from direct bond interaction, shear con-
nection, or direct bearing. Use the force-transfer mechanism providing the largest nominal strength;
do not superpose mechanisms. When the load is applied to the concrete core by direct bearing, the
design bearing strength φ c P n and the allowable tensile strength P n /Ω c for the limit state of concrete
crushing are determined using φ c = 0.65 (LRFD) and Ω c = 2.31 (ASD). Calculate the nominal bearing
2
2
strength as P n = 1.7f c ′A B , where A B is the loaded area, in (mm ). When shear connectors are used to
transfer the required shear force, distribute them along the length of the member at least a distance
of 2.5 times the width of structural steel HSS or 2.5 times the diameter of round HSS, both above
and below the load transfer region. The maximum connector spacing should not exceed 16 in (405 mm).
5.8.4 Composite Beams with Shear Connectors
The most common application of composite construction is a flexural member with shear connec-
tors. See Arts. 5.8.5 and 5.8.6 for shear connector requirements. For such applications, the design
may be based on an effective concrete-steel T-beam, where the width of the concrete slab on either
side of the beam centerline is limited to the following:
1. One-eighth of the beam span, center-to-center of supports.
2. One-half the distance to the centerline of the adjacent beam.
3. The distance to the edge of the slab.
Temporary supports (shores) during construction are optional. However, when temporary shores are
not used during construction, the steel section alone must have adequate strength (see Art. 5.5) to
support all loads applied prior to the concrete attaining 75% of its specified strength f c ′.

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