Seismic Performance of Steel Buildings in the Central and Eastern United States

Summary

In certain regions of moderate seismicity, particularly in the central and eastern United States (CEUS), member sizes of the seismic force-resisting systems (SFRS) can be governed by wind requirements—either wind-resisting strength or stiffness. If these buildings are assigned to Seismic Design Category (SDC) D, as defined in model building codes, seismic detailing of members and connections is required because of anticipated ductility demands on the SFRS resulting in a significant increase in construction resources and financial commitment. 
The effect of wind-dominant design on the seismic performance of steel buildings built in the CEUS has not been formally quantified. Nor has the effect of not including seismic detailing on these buildings been evaluated. This project will investigate the seismic collapse performance of steel buildings in the CEUS through two synergetic activities: 

(1) Develop the seismic fragility curve for new and existing (circa 1980) steel systems for use in community resilience efforts by the NIST-funded Community Resilience Center of Excellence (CoE).
(2) Evaluate the system strength provided by wind design that results in new steel systems that satisfy the current seismic collapse objective and determine if cost savings via reduced seismic connection detailing can be obtained.

Description

Objective - This project will assess the seismic performance of structural steel SFRSs in buildings located in regions of moderate seismicity in the CEUS. The design of these building systems is commonly governed by wind demands. Two synergetic activities will be undertaken in this project: (1) develop the seismic fragility curve for new and existing steel systems for use in community resilience efforts by the CoE, and (2) evaluate the potential for a wind-to-seismic strength ratio that result in designs that have the same level of seismic collapse risk as that in the current seismic requirements (i.e., 1% probability of collapse in a 50-year period) and determine if cost savings via reduced seismic connection detailing can be obtained. 

What is the technical idea? The model building codes have been developed primarily due to long-standing knowledge of performance requirements for buildings located in high seismicity regions of the western United States (WUS). More recently, the seismic hazards in regions of the CEUS have been brought to light, resulting in a new set of performance issues that are not necessarily addressed by the building codes. Most of the model building code provisions for the CEUS have been extrapolated from WUS seismic design rules, based on expert opinion, but little research has been performed to validate these extrapolations. Therefore, it is possible that some aspects of such extrapolations may be inadequate in terms of safety and/or cost-effectiveness. The central question will be how do buildings perform seismically when their “controlling” design load is from wind effects? This is a question affecting buildings in several regions in the CEUS where the combination of moderate-to-high seismicity exists together with substantial wind speeds from hurricane or other wind storm phenomenon.

Two synergetic activities will be undertaken in this project. 

Task 1: Ongoing community resilience efforts by the CoE require system-level fragility curves that accurately represent the seismic collapse performance of new and existing steel buildings. This project will collaboratively work with the CoE to develop and analyze a suite of archetype buildings designed in accordance with older building codes (circa 1980) and current building codes for various locations within the CEUS. Performance and seismic safety of each building will be determined by conducting a FEMA P-695 collapse probability study to develop the building fragilities.

Task 2: When a building is within SDC D as defined in ASCE/SEI 7 (ASCE 2016), a category typical in California, the building code inherently assumes a certain strength reduction factor, R, and a corresponding ductility demand. However, this assumption may be invalid when non-seismic effects (i.e., wind) govern system design. The effect of non-seismic strength of members and connections on the seismic performance of the building (i.e., probability of collapse) is unknown. Further, the effect of including seismic detailing on the collapse objective is also not quantified, which can be exacerbated by the changes in SDC since current code provisions use a step function in lieu of a functional interaction. Further, this study hypothesizes that there is a ratio of strength provided by wind design to that provided by seismic design where adequate robustness is provided to the building system (and the structural elements they include) from wind design such that the building code-required seismic performance is achieved.

Through the use sophisticated structural analysis software, this task will determine if there is a potential (at the member-level, story-level, or system-level) for wind load-controlled design, with and without seismic detailing, to provide the same level of collapse risk as that in the current seismic requirements (i.e., 1% probability of collapse in a 50-year period). Additionally, determine if there are impacts to the safety objective and whether there can be cost savings achieved in regard to changes in seismic requirements. Impacts of the results could potentially enable relaxation of the stringent seismic detailing requirements by expanding the applicability of ordinary and R = 3 (i.e. low ductility) systems into SDC D.  

What is the research plan?  Archetype building suites will be designed for each task, representative of steel buildings located in the CEUS where wind can govern structural design. The suite will be designed by a contracted design firm(s) experienced in building design for the CEUS, based on configurations identified by NIST in consultation with the Contractor and CoE. The relationship between wind-controlled and seismic-controlled design will be investigated in terms of building geometry (height and footprint). 

The two building suites are as follows:  

Suite No 1: Structural steel buildings located in moderate seismicity regions with high design wind loads will be developed and designed. Three prospective areas have been identified including Charleston, SC, Memphis, TN, and Long Island, NY. Soil properties shall be assumed Site Class D. The buildings will be designed twice, (1) using older regional building codes from the 1980-1985 era (National Building Code (NBC) or Standard Building Code (SBC)) and (2) using the 2018 International Building Code (IBC). Table 1 provides a tentative list of buildings to be included in the project. Steel connection types and detailing shall be determined with coordination with subcontracted design firm(s) and NIST. This suite of buildings will be assessed by EEG engineers working with the CoE to determine seismic safety levels and fragilities for each building, where wind loads will control design with seismic effects being considerable. 

Table 1 - Tentative Building Prototypes

Building Type	Occupancy or Risk Category	Number of Stories	SFRS	Floor Plan Area (square feet)
Emergency Facility	IV	6	Braced	20,000
Rescue Facility	IV	2	Braced	7,500
Educational Facility (with assembly)	III	2 or 3	Braced and Moment	15,000
Light Industrial	II	1	Moment	10,000
Office 1	II	4 to 8	Braced and Moment	15,000
Office 2	II	12 to 16	Braced and Moment	20,000

Suite No. 2: A second suite of buildings will be designed for one location in the eastern US in a moderate seismicity region with hurricane wind loads, such as Charleston, SC to study overall performance and the impact of seismic detailing rules on wind-controlled buildings. Two buildings will be selected from Table 1 that are governed by wind effects and also detailed using seismic design rules for Seismic Design Category (SDC) D, corresponding to regions of moderate to high seismicity. These buildings will then be redesigned using design rules for lower seismic zones connection modeling following the response that would be expected from a non-seismically detailed connection. All buildings will be designed using the 2018 IBC. This design work, performed by the Contractor, will enable Suite No. 2 to be assessed by EEG engineers to determine seismic safety levels and fragilities for buildings in the CEUS.

For each building, different lateral force resisting systems will be used in orthogonal directions: a moment frame in one direction and a braced frame in the other. The buildings will be designed per the latest model codes (i.e., ASCE 7-16 and AISC 341-16 (AISC 2016)). A complete detail of the building geometries, layouts, and properties will be established with the Contractor and with the CoE.

The collapse fragilities and performance objective of the buildings from both tasks will be assessed using the nonlinear dynamic analysis methodologies presented in ASCE/SEI 41-17 (ASCE 2017) (collapse prevention metric) and FEMA P-695 (FEMA 2009) (probability of collapse). Each SFRS will be modeled in detail, including material and geometric nonlinearities, in LS-DYNA (LSTC 2016) or an equivalent nonlinear FEA program such as ABAQUS (Dassault Systemes 2016). The seismic performance objective measure will be satisfying the collapse prevention performance level prescribed in ASCE 41 and the risk-targeted object of 1% probability of collapse in a 50-year period in ASCE 7. Evaluation of performance will be conducted in terms of collapse probabilities, fragilities and detailed nonlinear analysis in close cooperation with the CoE.