Objective - This project will assess the seismic performance of structural systems in buildings located in regions of moderate seismicity in the CEUS where wind loads govern the design of structural system. Further, this project will evaluate wind-to-seismic strength ratios 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 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 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 research will consist of having archetype buildings designed by an outside engineering firm(s), experienced in CEUS building design. These buildings will be studies to determine how they perform under these two very different design regimes where wind loading is resisted elastically while significant seismic loadings is resisted in a nonlinear manner. The design and detailing requirements are very different for these two types of loading. Performance and seismic safety will be determined by conducting a FEMA P-695 collapse probability study to identify the building fragilities. The central question will be how do buildings perform seismically when their “controlling” design load is from wind? This is a question affecting buildings in a number of areas in the CEUS where the combination of moderate to high seismicity exists together with substantial wind speeds from hurricane or other winds.
Another important question concerns the question of detailing requirements in the CEUS. 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.
Through the use high level analyses, this project will determine if there is a point (at the member-level, story-level, or system-level) where wind load-controlled design, with and without seismic detailing, can 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 regards 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? A systematic approach will be taken in investigating the problem. The relationship between wind-controlled and seismic-controlled design will be investigated in terms of building heights and footprint. Archetype buildings will be designed representative of steel buildings located in the CEUS where wind can govern structural design. This design work will be conducted by an outside engineering firm(s) through the EEG IDIQ contractor or via separate competive RFP; the outside firm(s) designing these buildings will be experienced in building design for the eastern US. Two related suites of buildings will be designed, these will be studied to determine the buildings designed to withstand controlling wind loads perform when subjected to significant seismic demands. Close collaboration with the Center of Excellence will be an important part of this project in determining the buildings for study and the resulting building performance assessments.
The two suites of buildings are as follows:
Suite No 1: Structural steel buildings located in moderate seismicity regions with high design wind loads will be designed and studied. Three prospective areas have been identified including Charleston, Memphis and Boston. The buildings will be designed using older building codes from the 1980-1985 era regional building codes (BOCA or SBC) and a second design using the 2012 IBC. This suite of buildings will be assessed by EEG engineers working with the CoE to determine seismic safety levels and fragilities for buildings in the eastern part of the United States, where wind loads will control design with seismic effects being considerable. The buildings will located in moderate seismicity regions with high design winds: Charleston, Memphis and Boston. Consisting of two steel building configurations, these structures will consist of
• Hospital – 6 story building
• Office Building – 12 story building
A total of 2 buildings * 3 locations * 2 building codes = 12 buildings is planned to be designed to study the impact of location and the effects of changes to the building code on the seismic performance. This work requested by CoE to assist in their resilience work for the central and eastern US, and this fits well with EEG priorities.
Suite No. 2: A second suite of buildings will be designed for one location in the eastern US in a moderate seismicity region with high design wind loads, such as Charleston to study overall performance and the impact of detailing rules. Consisting of two sets of six structural steel buildings, each set will consist of one immediate occupancy, four story office building and one four story office building designed to the standard life safety seismic philosophy, both designed using moment frames; and two office buildings of eight and 16 stories designed using moment frames and a second set of two designed using braced frames. One building set of six will be detailed using seismic design rules for Seismic Design Category (SDC) D, corresponding to regions of moderate to high seismicity. The second set of six will be designed using design rules for lower seismic zones where design is controlled by wind. This design work, performed by outside contractor, will enable Suite No. 2 to be assessed by EEG engineers to determine seismic safety levels and fragilities for buildings in the eastern part of the United States.
These buildings will be of the same design but with connection modeling following the response that would be expected from a non-seismically detailed connection. The original seismically detailed performance can then be compared to the non-seismically detailed performance. 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 at Colorado State.
The collapse objective of the buildings from Suites 1 and 2 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 LFRS 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.