Redundancy Factors

RAM Frame calculates the so-called "reliability/redundancy factor", 'Rho' which is used in generating load combinations in the 1997 UBC and 2000/2003 IBC code. The Redundancy Factor, Rho, is a function of the shear taken by a given member and the ground floor area of the building.

In addition to evaluating the governing 'Rho' factor for the whole structure for each of the seismic load cases, the program also calculates and outputs the 'Rho" factor for each column, brace and wall element in the structure for all seismic load cases. The "Rho' factor will then be used in generating load combinations in the Steel post-processors. Note that rho is not applied to the load case results (member forces, reactions, etc.) in Analysis - Load Cases mode.

The following rules are enforced in calculation of redundancy factors for each member at stories:

If it is found that there is only one diaphragm at a story, the program uses total story shear and total story area for each member at the story without checking that the member is connected to the diaphragm.
If it is found that there are more than one diaphragm at a story, and
1. If a member is found to be connected to a diaphragm at the story, the diaphragm area and total shear at this diaphragm are used for the member
2. If a member is found to be not connected to any diaphragm at the story, the total story area and total story shear are used for the member
3. If a member is found to be connected more than one diaphragm, the sum of all diaphragm areas and the sum of shears at these diaphragms are used for the member

Note that the redundancy factor for IBC 2006/ASCE7-05 is different from previous provisions of IBC. It is given either as 1.0 or 1.3, and it is not calculated. The program does not do any determination of the Redundancy factor, and it will be input by the user as part of the Load Combination generators.

IBC 2000\IBC 2003

The calculation of the Redundancy Factor is the same as for UBC 97 with the following two exceptions: Every story must be investigated, not just those stories at or below the 2/3 height, and the floor area used in the equation is the area of the floor at the level being investigated, not the area of the base.

In equation 16-32, if the area of the current level is zero, the program uses the area of the nearest non-zero floor above. If there are no floors above with an area, ρi is set to 0.00.

For structures in Seismic Design Category A, B or C, the Redundancy Factor is permitted to be 1.0 per Section 1617.2.1. The program does not consider the Seismic Design Category so it does not automatically set the value to1.0. In the combination generation the user should specify to use a value of 1.0, rather than the calculated value, when appropriate.

There are limits on the permissible values of the Redundancy factors for Special Moment Frames in structures assigned to Seismic Design Category D, E and F. This provision is not implemented; no warning will be given if the permissible values are exceeded.

For structures with vertical combinations of frames (e.g., moment frames sitting on walls), there is a requirement for modifying the rho factor for the lower portion of the structure. This provision is not implemented.