Seismic Design of Wind Load-Controlled Buildings

In some regions of moderate seismicity, particularly in the eastern U.S., design of lateral force resisting systems (LFRS) can be governed by wind requirements. If these buildings are assigned at least Seismic Design Category (SDC) D, as defined in model building codes, seismic detailing is required because of presumed seismic ductility demands on the LFRS. This study examines the relationship between the elastic strength provided by wind design to the ductile strength provided by seismic design detailing requirements for a collection of candidate buildings. Assessments of each building will be conducted to determine whether element and system capacities provided by wind load requirements are sufficient to meet seismic design requirements for the same building. If so, it may be possible to permit relaxation of the current stringent seismic detailing requirements by expanding the applicability of ordinary and potentially lower ductility structural systems into SDC D for wind load-controlled building designs. This relation would result in cost savings for buildings where wind loads control member and system strengths.

Objective:    To investigate the design of wind load-controlled buildings in areas that must also address moderate seismic hazards, to determine how the elastic strength provided by wind design compares to the strength provided by seismic design, assuming the same level of collapse risk as that in the current seismic requirements (i.e., 1% probability of collapse in a 50-year period). The objective is to assess more accurately low to moderate seismic demands on buildings in high wind hazard areas of the country and to determine if cost savings via less restrictive seismic connection detailing can be obtained by utilizing the strengths required to accommodate the wind loads.            

What is the new technical idea?  The model building code's [1] seismic design provisions have been developed primarily based on the long-standing knowledge of the high seismicity existing in the western U.S. (where design wind loads are also comparatively low). More recently, seismic hazards of the central and eastern U.S. have been highlighted, resulting in a new set of issues that are not necessarily addressed cost-effectively by the present code provisions. Most of the model building code provisions for the central and eastern U.S. have been extrapolated from western U.S. seismic design, based on expert opinion, but little research has been performed to validate these extrapolations. Therefore, it is possible that some aspects of current code provisions based on these extrapolations may be inadequate in terms of safety and/or cost-effectiveness (e.g., where seismicity demands are lower and other loads such as wind more prominent).

When a building is designated within Seismic Design Category (SDC) D, which is typical in California, the model building code1 inherently assumes a certain strength reduction factor (R) and a corresponding ductility demand.  However, this assumption may be invalid when non-seismic effects, such as wind, govern system design. This project will determine via analytical study, if there is a relationship between the elastic strength provided by a wind load-controlled design to the strength provided by seismic load-controlled design for the same building, where adequate robustness and performance is achieved. This evaluation consider both member-level and system-level strength and ductility demands.  The results of the study may justify relaxation of the stringent seismic detailing requirements by expanding the applicability of ordinary and low ductility systems into SDC D in wind-controlled areas,typical of regions in the Midwest and Eastern US. Cost savings can be realized if standard detailing can be permitted safely in lieu of seismic detailing in these regions of the country where wind controls design.            

What is the research plan? This project will focus on analyses of archetypical buildings. A systematic approach will be taken in investigating the problem. First, the relationship between wind-controlled and earthquake-controlled design will be investigated in terms of building heights and footprints. It is anticipated that three different building heights and three different footprints will be selected, for a total of nine different building geometries in the design space, with wind-to-seismic base shear ratios targeted at approximately 1.5, 2, and 3. These values may be adjusted in order to properly capture the range of relevant shear rations as this investigation progresses and will be based approximately on Midwestern and Eastern U.S. building exposures.

For each building, different lateral force resisting systems (LFRS) will be used in orthogonal directions; typically, a moment frame will used in one direction and a braced frame will be used in the other. The buildings will be designed per the latest model codes (i.e. ASCE 7-101 and AISC341-10 [2]). The buildings will be assessed for seismic strength using the methodologies presented in ASCE 41-06 [3] and FEMA P695[4] (ATC 63). Each LFRS will be modeled in detail, including material and geometric nonlinearities, in LS-DYNA, or an equivalent nonlinear FEA program. The seismic performance objective will be a 1% probability of collapse in a 50-year period, consistent with current design practice.  

For each building, if the performance objective is met, further analysis will be conducted using non-seismically detailed connections, which effectively reduces ductility capacities for seismic response – the reduced capacities will be simulated using analytical properties found in the above-mentioned literature. Each such building will be of the same basic design but with connection modeling following the response that would be expected from a non-seismically detailed connection. The original seismically detailed performance will then be compared to the non-seismically detailed performance.     

Twenty far-field earthquake records (distance from site > 5 km) from the Pacific Earthquake Engineering Research (PEER) Center Strong Motion Database will be selected for the dynamic analyses. Response histories will be selected for each earthquake intensity following rules and restrictions defined in FEMA P695, with specific consideration given to source magnitude and peak ground acceleration and velocity. Scaling of records for evaluation at the collapse limit state will follow the methodology used in FEMA P695.

 

Minimum Design Loads for Buildings and Other Structures, ASCE/SEI 7-10, American Society of Civil Engineers, Reston, VA, 2010

[2] Seismic Provisions for Structural Steel Buildings, AISC 341-10, American Institute of Steel Construction, Chicago, 2010

[3] Seismic Rehabilitation of Existing Buildings, ASCE/SEI 41-06, American Society of Civil Engineers, Reston, VA, 2006

[4] Quantification of Building Seismic Performance Factors, FEMA P695, Federal Emergency Management Agency, 2009

Recent Results: None, this project has yet to start.              

Standards and codes: The project will be formulated to facilitate incorporating its products into future editions of ASCE 7 and NEHRP Recommended Provisions for Seismic Regulations for New Buildings and Other Structures.