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Seismic Analysis and Design Procedures in Existing Standards Project

Summary:

This project provides proposed refinement of key prescriptive seismic provisions in U.S. model building codes and standards concerning:

  1. approximate structural period relationships;
  2. lateral force distribution rules; and
  3. design rules for buildings in high wind-moderate seismic areas.

Description:

Objective: Through detailed analysis of archetypical buildings and assessment of building performance in recent earthquakes, this project will refine three key prescriptive provisions that are used in the ASCE 7[1] standard by 2015:

  1. Develop an approximate structural period relationship that more accurately accounts for the impacts of variations in structural system strength or stiffness.
  2. Develop a prescriptive methodology for determining the vertical distributions of lateral earthquake forces in buildings that accurately considers the impacts of vertical and horizontal strength and stiffness irregularities on higher mode responses of low-rise and mid-rise buildings.
  3. Develop integrated structural design and detailing rules for areas in the Central and Eastern U.S. where both high wind loading and moderate earthquake loading must be considered in building design.

What is the new technical idea?

  1. Develop an approximate structural period relationship (Task 1): Disparities between structural dynamics theory and model building code provisions exist in some classes of structures, wherein the building codes amplify design forces based on structural importance or configuration and not on actual force demands. The resulting structures are typically stronger and stiffer than required based on demand alone. These inconsistencies may result in unconservative designs in some cases and, potentially unnecessarily expensive designs in other cases. Using the the NIST NEHRP model building library (developed in previous projects), and using current ASCE 7 – prescribed structural period formulations as baselines, finite element models of those buildings will be used to develop alternate and improved methods for approximating structural period.
  2. Develop a prescriptive methodology for determining the vertical distributions of lateral earthquake forces (Task 2):  Building performance in the 2010 Maule, Chile, earthquake showed damage from vertical and horizontal irregularities in building configuration. Moreover, damage in the upper stories of some buildings appears to have resulted from higher mode response effects. The experience in Chile offers an opportunity to evaluate buildings where damage was caused at least in part by structural irregularities, by higher mode effects, or by both. These buildings, taken together with those damaged by similar irregularities in other recent earthquakes, will be assessed using current ASCE 7 design philosophy and detailed nonlinear analysis, to determine whether ASCE 7 would have correctly required more robust design of these irregular structures than was actually provided and to develop the improved methodology. This topic was identified as an important technical need at the May 15-16, 2012 NEHRP Research Roadmap Workshop hosted by Building Seismic Safety Council (BSSC)iii.
  3. Develop integrated structural design and detailing rules for areas where both high wind loading and moderate earthquake loading must be considered in building design (Task 3): ASCE 7 seismic provisions are primarily based on knowledge of the high seismicity of the Western U.S. (WUS). Most seismic provisions for the Central and Eastern U.S. (CEUS) have been extrapolated from the WUS experience, with little research having been performed to validate the extrapolations. When a building is assigned to Seismic Design Category (SDC) D, typical in the WUS, ASCE 7 prescribes enhanced detailing that permits ductile response when severe ground shaking occurs. In the more moderate seismicity of the CEUS, buildings would typically be assigned to SDC C, with standard detailing that provides ductile response for lower intensity ground motions. The differences between SDC C detailing requirements and SDC D detailing requirements are significant, making SDC D construction more expensive.

In CEUS areas, design wind loads for mid-rise and taller buildings can be significant. The ASCE 7 design approach for wind loads results in structures that resist wind loads by essentially elastic response. If design wind loads exceed seismic design forces, then the wind load-designed structural system is stronger than that required for the anticipated earthquake, which may negate the real need for the seismic detailing. The wind load-required strength above that required for seismic demand, is not accounted for in ASCE 7, so that ductility via seismic detailing may still be code-required, even though there is little chance of reaching yield conditions in anticipated earthquakes. A series of analytical studies of archetypical buildings will be conducted for  locations in the U.S. where ASCE 7-specified wind loads may produce higher design forces than ASCE 7-specified seismic loads. Methods that permit using the seismic over-strength of wind-load-controlled buildings to achieve life safety performance objectives for seismic design will be developed. The study will permit the potential relaxation of stringent seismic detailing requirements by expanding the applicability of ordinary/lower-ductility systems in wind-controlled areas where moderate seismicity is present. The 2009 NEHRP Recommended Provisions Research Needs[2] report and the 2012 BSSC research roadmap workshop[3] both highlighted the need for more research on the performance of ordinary steel systems such as those described here.

What is the research plan?

  1. Develop an approximate structural period relationship: U.S. and foreign building codes, technical papers, and other resources on building period estimation will be assembled and analyzed. The progression of structural period estimation procedures over time will be analyzed. Using the NIST NEHRP model building library (developed in previous projects) and the EL Structures Group model building library, supplemented by other buildings with characteristics not found in the libraries, candidate buildings will be carefully analyzed to determine their dynamic properties. The parameters common to each building model and unique to each period estimation approach will be identified. These observations will form the basis for the development of new code provisions for approximating building period.
  2. Develop a prescriptive methodology for determining the vertical distributions of lateral earthquake forces: A suite of buildings with significant structural irregularities that were severely damaged in the Chile earthquake or other recent earthquakes, will be analyzed using Nonlinear Finite Element Analysis (NLFEA); performance will be assessed by comparing analytical behavior with data from field reconnaissance surveys where available. These same buildings will be redesigned using current ASCE 7 provisions and re-analyzed using NLFEA to gauge the intended performance of ASCE 7 as compared to the actual performance observed. The suite of buildings may be supplemented by purposely designed irregular buildings to test the ASCE 7 provisions using NLFEA to assess how well the provisions capture the effects of irregularities. Based on this work, a methodology will be developed to distribute lateral forces accurately among the building elements without having to perform detailed nonlinear analysis. Attention to higher mode effects in the distribution methodology will be an important feature of this development process.
  3. Develop integrated structural design and detailing rules for areas where both high wind loading and moderate earthquake loading must be considered in building design: The relationship between wind-controlled and earthquake-controlled design will be investigated for selected geographic locations and corresponding wind and seismic hazard assessments. A potential first case study area is Charleston, S.C., which has significant wind and earthquake demands. Three different building heights and three different floorplans will be selected as case studies, for a total of nine different building geometries. The ratios of wind-to-seismic base shear will be initially targeted at approximately 1.5, 2, and 3, but may be adjusted to capture the range of relevant shear based on analysis of CEUS building exposures. The study will initially focus on structural steel moment and braced frame buildings and will consider connection, member, and system strength and ductility demands under both wind and seismic loading. For each building, an ordinary steel system (SDC C) will be designed along with a second design assuming a special steel system (SDC D) for the same seismic hazard level. These buildings will be designed using the latest model codes (i.e., ASCE 7, and AISC 341-10[4]) and assessed for seismic capacities using the methodologies presented in ASCE 41-06[5] and FEMA P695[6]. If the results show that seismic detailing can be relaxed in high wind areas with no loss in earthquake life safety, efforts to develop cost estimates for the different systems may be undertaken.  A limited peer review will be conducted to ensure the approach in this study is consistent with current practice for wind and seismic considerations.

 


[1] Minimum Design Loads for Buildings and Other Structures, ASCE/SEI 7-10, American Society of Civil Engineers, 2010.

[2] 2009 NEHRP Recommended Seismic Provisions Research Needs, FEMA P-750.

[3] Development of NIST Measurement Science R&D Roadmap:  Earthquake Risk Reduction in Buildings Workshop, BSSC, 2012.

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

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

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

 

Major Accomplishments:

Recent Results:

There has been limited 2012 progress in the three listed task areas. This project was part of the FY 2012 portfolio of NEHRP research. The February 2012 peer review of the Assessment of First Generation PBSD Methods project necessitated additional, unplanned work on that project, which forced a delay on starting work in this project to FY2013.

Task 2 leverages work currently being conducted externally in project ATC 94 (NEHRP Consultants Joint Venture Task Order 21), which is analyzing buildings damaged in the 2010 Maule, Chile, earthquake for areas of improvement in reinforced concrete design requirements. Data collection efforts for Chile are also now concluded, with available electronic data now part of the Disaster and Failure Studies Program (DFSP) data repository.

A 2012 summer intern student has completed an initial comparison of the most recent ASCE 7 wind and seismic hazard maps to highlight areas in the Central and Eastern U.S. that will potentially contribute to Task 3.

Standards and Codes:

The project will provide recommendations for new or updated ASCE 7 provisions as follows:

  • It is anticipated that the results of Task 1 regarding approximate structural periods will be applied to Section 12.8.2, Period Determination, of ASCE 7.
  • It is anticipated that the results of Task 2 regarding vertical distributions of earthquake forces will be applied to Section 12.3.3, Limitations and Additional Requirements for Systems with Structural Irregularities; Section 12.8.3, Vertical Distribution of Seismic Forces; and Section 12.8.4, Horizontal Distribution of Forces, of ASCE 7.
  • It is anticipated that Task 3 regarding seismic design in areas of combined high wind load and low to moderate earthquake loads will be applied to Section 11.6, Seismic Design Category, and Section 12.2, Structural System Selection of ASCE 7.

The development and review process for new provisions ultimately incorporated in ASCE 7 begins with vetting via the FEMA-funded Building Seismic Safety Council (BSSC) through its development of the NEHRP Recommended Provisions for Seismic Regulations for New Buildings and Other Structures, recently published in 2009 and likely published next in 2014. The “Recommended Provisions” document forms the basis for ASCE to update ASCE 7, with the next ASCE 7 edition anticipated in 2016. NIST EL is represented on the BSSC NEHRP Provisions Update Committee (PUC) by Dr. Jay Harris, who will ensure that proposed provision updates from this project are brought before the PUC.