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Section 3
3-35
3.8.4 Design of I-shaped beams per ACI-318
I-shaped sections can be designed as beams per the ACI 318 code.
The property for these sections must be defined through a user
table, I-section, or using the tapered specification. Information on
assigning properties in this manner is available in sections 5.19 (I-
section type) and 5.20.3 (Tapered I shape) of the Technical
Reference manual.
From the standpoint of the analysis – determining member forces,
nodal displacements and support reactions – the same set of
facilities and rules which are applicable for any normal reinforced
concrete frames or other structures can be used when I-sections or
tapered concrete members are specified. In other words, there isn’t
anything unique or special to account for in the analysis model
simply because I-shaped concrete beams are part of it.
From the standpoint of design, the following rules are applicable:
1. The member can be designed as a beam using the general
principles explained in Chapter 3 of the Technical Reference
manual. It currently cannot be designed as a column. Design as a
beam is done for flexure (MZ), shear (FY) and torsion (MX) just
like that for rectangular, tee or trapezoidal beams. Axial forces
(FX) are used during the capacity computations in shear and
torsion. At each section along the length that the member is
designed at, the depth at that section location is used for effective
depth computation.
2. The program performs the following tests on the section
dimensions before starting the design:
• If the thickness of the web is the same as the width of the
top and bottom flanges, the member is designed as a
rectangular section.