Building Design and Construction Handbook - Merritt - Ventech!

CONCRETE CONSTRUCTION 9.139
crete slab is obtained by natural bond if the steel beam is fully encased with a
minimum of 2 in of concrete on the sides or soffit. If the beam is not encased, the
interaction may be accomplished with mechanical anchors (shear connectors). Requirements
for composite structural-steel-concrete members are given in the AISC
‘‘Specification for Structural Steel for Buildings—Allowable Stress Design and
Plastic Design,’’ and AISC ‘‘Load and Resistance Factor Design Specification for
Structural Steel Buildings,’’ American Institute of Steel Construction.
The design strength of composite flexural members is the same for both shored
and unshored construction. Shoring should not be removed, however, until the supported
elements have the design properties required to support all loads and limit
deflections and cracking. Individual elements should be designed to support all
loads prior to the full development of the design strength of the composite member.
Premature loading of individual precast elements can cause excessive deflections
as the result of creep and shrinkage.
According to the ACI 318 Building Code, the factored horizontal shear force
for a composite member may be transferred between individual concrete elements
by contact stresses or anchored ties, or both. The factored shear force V u at the
section considered must be equal to or less than the nominal horizontal shear
strength V nh multiplied by � � 0.85.
V � � V (9.116)
u nh
When Vu � �80bvd, where bv is the section width and d the distance from the
extreme compression surface to the centroid of tension reinforcement, the factored
shear force may be transferred by contact stresses without ties, if the contact surfaces
are clean, free of laitance and intentionally roughened. Otherwise, if the contact
surfaces are clean but not intentionally roughened, fully anchored minimum
ties [Eq. (9.81)], spaced not over 24 in or 4 times the least dimension of the
supported element are required when Vu � �80bvd. When fully anchored minimum ties are provided and the contact surfaces are
clean, free of laitance and intentionally roughened to a full amplitude of about 1 ⁄4
in, the Code permits transferring a factored shear force equal to �(260 � 0.6
�vƒy)�bvd but not more than �(500 bvd), where �v is the ratio of tie reinforcement
area to the area of the contact surface, ƒy is the yield strength of shear reinforcement,
and � is defined under Eq. (9.117).
When Vu exceeds �(500bvd), the factored shear force may be transferred by
shear-friction reinforcement placed perpendicular to assumed cracks. Shear force
Vu should not exceed 800Ac or 0.2ƒ�c Ac, where Ac is the area of the concrete section
resisting shear transfer, and ƒ�c is the specified concrete compressive strength. Re-
quired reinforcement area is
V u
Avƒ � (9.117)
�ƒ �
where ƒ y � yield strength of shear reinforcement
� � coefficient of friction
� 1.4� for monolithic concrete
� 1.0� for concrete cast against hardened concrete with surface intentionally
roughened to a full amplitude of about 0.25 in
� 0.7� for concrete anchored by headed studs or rebars to as-rolled structural
steel (clean and without paint)
� 0.6� for concrete cast against hardened concrete not intentionally
roughened
y