Lateral Force Distribution Procedures for Structural System Design

This project supports the technical development and refinement of seismic provisions in U.S. codes and standards through problem-focused, user-directed research and development. Two tasks will be undertaken in the project: (1) Development of improved rules for the distribution of design-level lateral forces based on structural horizontal and vertical irregularities, considering observed responses of specific buildings in the 2010 Chile earthquake; and, (2) Development of an improved approximate structural period relationship for use in model building codes. The present approximate relationships do not adequately model observed behavior, resulting in a potential lack of accuracy when applying model building code design rules.

Objective: The proposed research emphasis under this project plan addresses two distinct but related tasks:

(1) Development of Provisions for Structural Irregularities and the Corresponding Vertical Distribution of Lateral Forces. Standards and code provisions will be developed to accurately depict the effects of vertical and horizontal strength and stiffness irregularities on typical Seismic-Force-Resisting Systems (SFRS) response to earthquake ground motions. SFRS from mid-rise and high-rise buildings will be studied to examine this behavior and the relationship between irregularities on force distributions and system demands.

(2) Development of an Approximate Structural Period Relationship for use in Model Building Codes.  Model building codes, including ASCE 7, do not account for the impact of changes in structural system strength or stiffness in the determination of approximate building periods. This task will develop standards and code provisions that accurately depict this basic relationship, in a formulation easily implemented in design.

What is the new technical idea?

(1) Development of Provisions for Structural Irregularities and the Corresponding Vertical Distribution of Lateral Forces. Chile has uniquely referenced many of the U.S. model building code provisions in its earthquake-resistant code provisions. Building performance in the 2010 Chile earthquake showed damage not only from irregularities in building configuration but also damage that appears to be a result of higher mode response effects, which can amplify seismic forces and displacements in the upper stories of the buildings. This amplification is not addressed adequately in ASCE/SEI 7-10 when using linear analysis methods. The experience in Chile offers an opportunity to compare these damaged buildings with US design philosophy, to determine whether ASCE/SEI 7-10 would have correctly required more robust design of these irregular structures than was actually done. It also permits testing of code provisions developed through this study against these same buildings.

(2) Development of an Approximate Structural Period Relationship for use in Model Building Codes. ASCE/SEI 7-10 provisions do not recognize the effects of increased strength requirements on stiffness and thus on the fundamental response period of a building. Theoretical disparities exist with critical structures and limited redundancy structures where forces used for design are amplified by model building code rules; the resulting structures are typically stronger and therefore stiffer than would otherwise be required. Currently, natural structural period computed by the prescriptive procedures does not change for any of these structures, which is not consistent with behavior. Moreover, building codes also use strength forces as triggers for detailing improvements and design restrictions (e.g., boundary elements in structural walls). These inconsistencies in ASCE/SEI 7 may result in unconservative designs in some cases and, possibly, unduly expensive designs in other cases. This task will examine this behavior to suggest new code provisions that better capture behavior.

What is the research plan?

(1) Development of Provisions for Structural Irregularities and the Corresponding Vertical Distribution of Lateral Forces. Three buildings that exhibited significant structurally irregular conditions as defined by ASCE/SEI 7-10 and were severely damaged in the Chile earthquake will be analyzed using advanced high-fidelity Nonlinear Finite Element Analysis (FEA); as-built performance will be assessed by comparing analytical behavior with that observed in field reconnaissance. These same buildings will be redesigned using ASCE/SEI 7-10 and re-analyzed using advanced high-fidelity Nonlinear FEA to gauge the intended performance of ASCE/SEI 7-10; actual performance observed in the Chile earthquake will be used to benchmark the analytical results.

This task will also evaluate the vertical distribution of lateral design forces prescribed in ASCE/SEI 7-10. A design space consisting of five regular (no horizontal or vertical irregularities) building structures will be designed and analyzed using advanced high-fidelity FEA and evaluated against ASCE 7-10 requirements. The buildings used to investigate structural irregularities, described above, will be similarly analyzed to verify the effectiveness of the proposed lateral design force distribution for irregular structural systems. Ground motion data from the Chilean earthquake appears to be forthcoming to use in this project. Additional data from the ground motion database at the Pacific Earthquake Engineering Research (PEER) Center at UC Berkeley will be used to supplement the Chilean data.  New model building code provisions suggested by this research will be developed that incorporate the Chilean experience with the present code philosophy.

(2) Development of an Approximate Structural Period Relationship for use in Model Building Codes. The task will begin with the assembly of model building code resources from current and former US and foreign building codes, technical papers, and other resources on building period estimation. This initial phase will examine the progression of the structural period estimation procedures over time. Using the ASCE/SEI 41 NIST/EL NEHRP Building Model Library (developed in previous, parallel projects), the task will identify candidate buildings for study from these previously modeled buildings. A number of new candidate buildings will also be modeled and added to the Library. Based on this work, the parameters that are common to each building model and unique to each approach will be identified. These observations will form the basis for new code provisions for approximate building period determination. This task will be conducted in conjunction with a summer student, an advanced MS student, who will use the work performed during the summer in his or her thesis.

Recent Results: This is a new project. This internal project interfaces with ATC 94 (NEHRP Consultants Joint Venture Task Order 21), which was delayed somewhat because of problems in Chile in obtaining earthquake ground motions and building information from the appropriate Chilean authorities. The ATC-94 team has identified a solution and the Chileans have been providing information in a greatly improved manner. This solution will permit the needed data to be obtained to meet the schedule indicated herein. Related work has been continuing on model building analyses for other projects. These models are part of the Library that will be utilized in this work in both tasks.

Standards and Codes: The expected project outcome is a document that recommends new or updated code provisions related to structural irregularities and lateral design force distribution. The document will specifically provide needed updates to the NEHRP Recommended Provisions for Seismic Regulations for New Buildings and Other Structures; ASCE/SEI 7, Sections 12.3.3, 12.8.3, and 12.8.4; and, potentially, ASCE/SEI 41, Sections 2.4 and 3.3.1.3.