= 6.3 — Modeling assumptions
== 6.3.1 General
=== 6.3.1.1 Relative stiffnesses of members within structural systems shall be selected based on a reasonable set of assumptions. The assumptions shall be consistent throughout each analysis.
=== 6.3.1.2 To calculate moments and shears caused by gravity loads in columns, beams, and slabs, it shall be permitted to use a model limited to the members in the level being considered and the columns above and below that level. It shall be permitted to assume far ends of columns built integrally with the structure to be fixed.
=== 6.3.1.3 The analysis model shall consider the effects of variation of member cross-sectional properties, such as that due to haunches.
= R6.3 — Modeling assumptions
== R6.3.1 General
=== R6.3.1.1 Separate analyses with different stiffness assumptions may be performed for different objectives such as to check serviceability and strength criteria or to bound the demands on elements where stiffness assumptions are critical.
Ideally, the member stiffnesses EcI and GJ should reflect the degree of cracking and inelastic action that has occurred along each member before yielding. However, the complexities involved in selecting different stiffnesses for all members of a frame would make frame analyses inefficient in the design process. Simpler assumptions are required to define flexural and torsional stiffnesses.
For braced frames, relative values of stiffness are important. A common assumption is to use 0.5Ig for beams and Ig for columns.
For sway frames, a realistic estimate of I is desirable and should be used if second-order analyses are performed. Guidance for the choice of
I for this case is given in 6.6.3.1.
Two conditions determine whether it is necessary to consider torsional stiffness in the analysis of a given structure: 1) the relative magnitude of the torsional and flexural stiffnesses; and 2) whether torsion is required for equilibrium of the structure (equilibrium torsion) or is due to members twisting to maintain deformation compatibility (compatibility torsion). In the case of equilibrium torsion, torsional stiffness should be included in the analysis. It is, for example, necessary to consider the torsional stiffnesses of edge beams. In the case of compatibility torsion, torsional stiffness usually is not included in the analysis. This is because the cracked torsional stiffness of a beam is a small fraction of the flexural stiffness of the members framing into it. Torsion should be considered in design as required
in Chapter 9.0 .
=== R6.3.1.3 Stiffness and fixed-end moment coefficients for haunched members may be obtained from the Portland Cement Association (1972).
== 6.3.2 T-beam geometry
=== 6.3.2.1 For nonprestressed T-beams supporting monolithic or composite slabs, the effective flange width bf shall include the beam web width bw plus an effective overhanging
flange width in accordance with Table 6.3.2.1 , where h is the slab thickness and sw is the clear distance to the adjacent web.
== R6.3.2 T-beam geometry
=== R6.3.2.1 In ACI 318M-11, the width of the slab effective as a T-beam flange was limited to one-fourth the span. The Code now allows one-eighth of the span on each side of the beam web. This
was done to simplify Table 6.3.2.1 and has negligible impact on designs.
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Table 6.3.2.1—Dimensional limits for effective overhanging flange width for T-beams
=== 6.3.2.2 Isolated nonprestressed T-beams in which the flange is used to provide additional compression area shall have a flange thickness greater than or equal to 0.5bw and an effective flange width less than or equal to 4bw.
=== 6.3.2.3 For prestressed T-beams, it shall be permitted to use the geometry provided by
[parts] 6.3.2.1 and 6.3.2.2.
=== R6.3.2.3 The empirical provisions of 6.3.2.1 and 6.3.2.2 were developed for nonprestressed T-beams. The flange widths in 6.3.2.1 and 6.3.2.2 should be used unless experience has proven that variations are safe and satisfactory. Although many standard prestressed products in use today do not satisfy the effective flange width requirements of 6.3.2.1 and 6.3.2.2, they demonstrate satisfactory performance. Therefore, determination of an effective flange width for prestressed T-beams is left to the experience and judgment of the licensed design professional. It is not always considered conservative in elastic analysis and design considerations to use the maximum flange width as permitted in 6.3.2.1.
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[ Lanjut Ke 6.4—Arrangement of live load
... ]

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