The intent of the redundancy coefficient is to encourage the design of more redundant structures, with a greater number of elements provided to resist lateral forces. Introduction of the redundancy coefficient into the building code was a direct reaction of the observation of structures damaged by the Northridge earthquake and the resulting conclusion that economic pressures had led many engineers to design structures with very little redundancy. This was particularly observed to be a problem for certain classes of moment-resisting steel frame and concrete shear wall buildings.
The International Code Council’s 2003 International Building Code (IBC) Section 1617.2.1 references the American Society of Civil Engineers’ Minimum Design Loads for Buildings and Other Structures (ASCE 7-02) Section 188.8.131.52 for redundancy provisions and then makes some modifications. For structures analyzed under the simplified analysis procedure, the exception requires that IBC Section 1617.2.2 be followed. The modifications found in 2003 IBC Section 1617.2.1 to the ASCE 7-02 provisions make them virtually identical to those in Section 1617.2.2.
Answers to FAQs
Q: Does the redundancy factor (p) apply to the design of diaphragms, collectors, and foundations?
A: The redundancy factor does not apply to the design of diaphragms or collectors; see Section 1617.2.1, Item 3, after the statement, "The value p shall be permitted to be taken equal to 1.0 in the following circumstances …" Also, for the Simplified Method, see Item 3 after the same statement in Section 16184.108.40.206. (This will be more explicitly stated in ASCE 7-05.)
Foundations do, however, need to be designed using the calculated p. Situations under which p is permitted to be set equal to 1.0 are listed in Table 1.
(This question and answer was reprinted with permission from the 2003 IBC Structural Q and A Application Guide published by the ICC.)
Q: Will there be any changes to the redundancy coefficient in the 2006 IBC?
A: In the new seismic provisions of ASCE 7-05 (which is scheduled to be adopted into the 2006 IBC), p is equal to either 1.0 or 1.3, depending upon whether or not an individual element can be removed (deemed to have failed or lost its moment-resisting capabilities) from the lateral-force-resisting system without causing the remaining structure to suffer a reduction in story strength of more than 33 percent or creating an extreme torsional irregularity (plan irregularity Type 1b).
Braced frame, moment frame, and shear wall systems have to conform to redundancy requirements. Dual systems are also included, but in most cases are inherently redundant. Shear walls with a height-to-length ratio greater than 1.0 have been included in redundancy considerations, even though the issue has been essentially solved by requiring collector elements and their connections to be designed for OMEGAo times the design force. This usually results in having to use a reasonable number of shear walls to reduce the force in collector elements to a manageable level. Regardless, shear wall systems were added to the requirements to help ensure that an adequate number of wall elements is included or that the proper redundancy factor is applied.
ASCE 7-05 adds a new user-friendly feature of conveniently listing when p may be taken as 1.0. New section 220.127.116.11 reads as follows:
18.104.22.168 Conditions Where Value of p is 1.0. The value of p is permitted to equal 1.0 for the following:
- Structures assigned to Seismic Design Category B or C.
- Drift calculation and P-delta effects.
- Design of nonstructural components.
- Design of non-building structures that are not similar to buildings.
- Design of collector elements, splices, and their connections for which the load combinations with over strength factor of Section 22.214.171.124 are used.
- Design of members or connections where the load combinations with over strength of Section 126.96.36.199 are required for design.
- Diaphragm loads determined using Eq. 12.10-1.
- Structures with damping systems designed in accordance with Section 18.
S.K. Ghosh Associates, Inc., is a structural, seismic, and code consulting firm located in Palatine, Ill. and Laguna Niguel, Calif. President S.K. Ghosh, Ph.D., and Susan Dowty, S.E., are active in the development and interpretation of national structural code provisions. They can be contacted at firstname.lastname@example.org and email@example.com, respectively.