The 2005 edition of the American Society of Civil Engineer’s Minimum Design Loads for Buildings and Other Structures (ASCE 7-05) Section 11.2 defines “bearing wall system” and “building frame system” as follows:
Wall system: Bearing: A structural system with bearing walls providing support for all or major portions of the vertical loads. Shear walls or braced frames provide seismic-force resistance.
Frame structural system: Building frame system: A structural system with an essentially complete space frame providing support for vertical loads. Seismic-force resistance is provided by shear walls or braced frames.
The R-value assigned to “building frame systems” is greater than the R-value assigned to corresponding “bearing walls systems,” which makes the “building frame system” more desirable from a cost-efficient design perspective. Two questions are frequently asked by design firms: Can the gravity framing be integral with the lateral force-resisting shear walls? And, if some of the gravity loads are resisted by shear walls, can the structural system still be classified as a building frame system? These questions have been addressed in the past by two sources: Recommended Lateral Force Requirements Commentary published by the Structural Engineers Association of California (SEAOC) in 1999, and the National Earthquake Hazards Reduction Program (NEHRP) Recommended Provisions Part 2: Commentary published by the Federal Emergency Management Agency (FEMA) in 2003.
SEAOC—As stated in C407.1.1 UBC §§1921.6 and 1629.6.2-3 Structural System Type and R for Concrete Wall Buildings, concrete shear walls are employed in bearing wall systems (R=4.5) and in building frame systems (R=5.5). To see if the structure qualifies as a building frame system, with the higher R, apply the following test:
Assume that all portions of the walls that are not reinforced as columns or beams are removed, but that the self-weight of these wall portions is still present. If the remaining structure is capable of supporting the factored gravity loads and conforms to the detailing requirements for gravity frame members, then the structure qualifies as a building frame system.
It is not required that walls be physically separate from the building frame system. Building frame columns can, for example, be integral with a wall, forming boundary elements.
The distinction between the two different concrete wall systems and the two values for R is based on traditional practice and is not necessarily justified by technical principles or research results. The assumption of a building frame system may require the design to include elements—such as beams at each intersection of walls and floor slabs, or confined columns within the wall length—that, in some cases, are of no benefit to seismic performance.
NEHRP—On page 44 of the NEHRP document, it states: “A building frame system is a system in which the gravity loads are carried primarily by a frame supported on columns rather than by bearing walls. Some minor portions of the gravity load may be carried on bearing walls, but the amount so carried should not represent more than a few percent of the building area.”
Answers to FAQs
Q: Lately, there are several proposed high-rise concrete shear wall buildings in our jurisdiction with integral concrete columns and beams (within the shear wall) taking vertical floor loads. These buildings are for residential apartment and condominium uses. The buildings use flat post-tensioned slabs for floors. Design engineers would like to treat these buildings as building frame systems, utilizing an R factor of 6.0 in accordance with ASCE 7-05 Table 12.2-1, Item B. 5.
The SEAOC Commentary, although not written for the 2006 IBC, appears to support treating these buildings as building frame systems with R=6.0, provided the vertical floor loads are taken by columns and beams integrally built within the shear wall. However, intuitively we feel that any damage to the shear wall will also negatively impact the performance of the beams and columns within the shear wall. Therefore, we feel more comfortable that such buildings be treated as a bearing wall system utilizing an R factor of 5 in accordance with ASCE 7-05 Table 12.2-1, Item A.1. What is your opinion?
A: This issue has come up many times in the past. There are basically two key points made in the SEAOC Commentary. First, the gravity loads must include the weights of those portions of shear walls that have been rendered ineffective, simply because they physically exist and would contribute to the gravity loads. Second, all portions of the walls that are reinforced as columns and beams must comply with the special seismic-detailing requirements of the code. Gravity frame members must maintain their full-factored, gravity-load-carrying capacity under the design earthquake of the lateral-force-resisting system. If the webs of the shear walls would be so badly damaged in an earthquake as to be rendered ineffective, there should still be one or more load paths taking the gravity loads down to the soils underlying the foundation. Proper reinforcement detailing for the beams and the columns is key. Detailing requirements for gravity frame members of reinforced concrete are provided in Section 21.11 of ACI 318-05. You may find it easier to conform to Section 21.11.3 than to justify that you need only comply with 21.11.2.
Q: We have an 18-story tower project where approximately 46 percent of the tributary gravity slab load is carried by the bearing walls. We would like to classify our structure as a building frame system; however, the 2003 NEHRP Commentary does not support this classification. Can you please provide some guidance?
A: We disagree with the notion of “a few percent” in the 2003 NEHRP Commentary. The two requirements of a building frame system are that there is an essentially complete space frame carrying substantially all the gravity loads, and that there be shear walls carrying 100 percent of the lateral loads. Also, we have already established that the gravity frame members can be integral with the shear walls. We do not believe the definition requires that the shear walls carry only a few percent of the gravity loads.
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.