0125

where h is the clear distance between flanges, tw is the thickness of the web, and Fy is the yield strength.

In addition to the compactness of the section, the lateral unbraced length of beam members is also a limiting factor for the development of the full plastic moment capacity of members. The LRFD provisions provide the limit on spacing of braces for beam as:

• For nonseismic region where Lpd is the unbraced length, ry is the radius of gyration about y-axis, Fy is the yield strength, M1, M2 are smaller and larger end moments, and M1/M2 is the positive in double curvature bending.

The AISC-LRFD Specification explicitly specifies the limitations for beam members as described above, but not for beam-column members. More studies are necessary to determine the reasonable limits leading to adequate rotation capacity of beam-column members. Based on White's study [40], the limitations for beam members seem to be used for beam-column members until the specification provides the specific values for beam-column members.

5.4.13 Adjustment of Member Sizes

If one of the following three conditions — strength, serviceability, or ductility — is not satisfied, appropriate adjustments of the member sizes should be made. This can be done by referring to the sequence of plastic hinge formation shown in the P.OUT. For example, if the load-carrying capacity of a structural system is less than the factored load effect, the member with the first plastic hinge should be replaced with a stronger member. On the other hand, if the load-carrying capacity exceeds the factored load effect significantly, members without plastic hinges may be replaced with lighter members. If lateral drift exceeds drift requirements, columns or beams should be sized up, or a braced structural system should be considered instead to meet this serviceability limit.

In semirigid frames, behavior is influenced by the combined effects of members and connections. As an illustration, if an excessive lateral drift occurs in a structural system, the drift may be reduced by increasing member sizes or using more rigid connections. If the strength of a beam exceeds the required strength, it may be adjusted by reducing the beam size or using more flexible connections. Once the member and connection sizes are adjusted, the iteration leads to an optimum design. Figure 5.33 shows a flow chart of analysis and design procedure in the use of advanced analysis.

0 0

Post a comment