Lecture 36 – Concrete Design – Continuous Beams & Slabs, Columns Continuous Beams and Slabs


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Concrete structural members are typically poured integrally together.
Beams and slabs often span multiple supports and are not “simplysupported”
as steel and wood framed beams are. As discussed in Lecture
15, these concrete beams and slabs are continuous and have both
positive moments and negative moments.
The location of tension bars in the members is related to the location of


Rebar Placement:
At the transition between the Mpos and Mneg zones, a minimum overlap of
bars is required per ACI 318. These overlaps are required for developing
the full bar strength in tension. The friction developed between the
concrete and the ribs of the rebar must equal the tensile strength of the
bar. The necessary length of the bar embedment to achieve this friction
force is called the “Development Length”, Ld, and is specified as a
multiple of bar diameters. For example, the Ld for a Grade 60 rebar and
concrete f’c = 4000 PSI = 38 x bar diameter.



Concrete Columns:
As discussed in the previous lecture, concrete is good at resisting
compression but poor in resisting tension. So, it might make sense that
concrete would be the material of choice for columns. It is true that
concrete IS used for compression members such as columns, piers,
bearing walls and pedestals. Members under pure compression could
then (theoretically) be unreinforced. These members are often subject to
additional forces such as moment that would put some tensile forces into
the member and would thus necessitate the addition of tension
reinforcement.



Spiral Column:
A spiral column has a single rebar wrapped around the vertical bars in a
spiral and is stronger than a comparable tied column. It is more laborintensive
to build than a tied column. The ACI requires a minimum of 6
vertical bars, with the same minimum and maximum areas as a tied
column.


Column Load Capacity:
Columns are rarely under pure axial compression only. Typically they
experience moment in conjunction with axial loads and are under
combined compressive and bending stresses. For this reason, the
numeric calculations involved with determining the combined axial
capacity and bending capacity are daunting.