Force-based Design Procedure Research Articles

Controlled Rocking CLT Walls for Buildings in Regions of Moderate Seismicity: Design Procedure and Numerical Collapse Assessment

ABSTRACTControlled rocking heavy timber walls are designed to rock on their foundations in response to earthquakes. For regions of moderate seismicity, it is proposed that this rocking behaviour can be adequately controlled using only post-tensioning, even with a large force-reduction factor and no supplemental energy dissipation. This article presents a force-based design procedure for controlled rocking cross-laminated timber walls without supplemental energy dissipation, including a method for estimating higher mode effects. Fragility analyses of three prototype walls demonstrate that the procedure can limit the probability of collapse to <10% during a maximum considered earthquake in a region of moderate seismicity.

Earthquake design of timber structures – Remarks on force-based design procedures for different wall systems

Timber structures exhibit excellent performance under earthquake impact if all structural members and details are designed and constructed correctly. As force-based design is most used in engineering practice, the decisive strength and stiffness parameters will be explained. Reference will be made to light-frame structures and cross laminated timber (CLT), which are most common for wall elements to transfer horizontal loading.The focus lies on the explanation of the post-processing of experimental data into input data for design calculations. In this context, the aim is for practical engineers and researchers to have a better mutual understanding. Stiffness, ductility and force reduction factors will be especially addressed. Recommendations are given how to handle several gaps within the code regulations in Europe about the earthquake design of timber structures.

Problems associated with direct displacement-based design of concrete bridges with single-column piers, and some suggested improvements

Currently available displacement-based design (DBD) procedures for bridges are critically evaluated with a view to identifying extensions and/or modifications of the procedure, for it to be applicable to final design of a fairly broad class of bridges. An improved direct DBD procedure is presented, including a suite of comprehensive design criteria and proper consideration of the degree of fixity of the pier top. The design of an overpass bridge (originally designed to a current European Code), applying the improved ‘direct’ displacement-based design (DDBD) procedure is presented and both ‘conventional’ and displacement-based designs are assessed using non-linear response-history analysis (NLRHA); comparisons are made in terms of both economy and seismic performance of the different designs. It is seen that DDBD provided a more rational base shear distribution among piers and abutments when compared to the force-based design procedure and adequately captured the displacement pattern, closely matching the results of the more rigorous NLRHA.

Evaluating Code Criteria for Regular Seismic Behavior of Continuous Concrete Box Girder Bridges with Unequal Height Piers

The seismic design and response prediction of irregular bridges supported on piers of unequal heights—a commonly adopted solution when crossing steep-sided valleys—represent a particularly challenging problem that is yet to be effectively addressed by seismic design code provisions worldwide. From a force-based design perspective, shorter piers are subjected to increased ductility demand, and consequently damage tends to localize in these relatively stiff piers at increasing seismic hazard levels. This paper presents an investigation of the seismic response of a few schemes of a three-span case-study continuous bridge (commonly encountered in practice), featuring two unequal piers with relative heights of 0.5, 0.64, 0.75, and 0.86, respectively. Static pushover and time history (under incrementally scaled-up actual records) nonlinear inelastic analyses are performed using OpenSees to check the validity of Eurocode 8 (EC8) and recently proposed AASHTO-LRFD provisions for regular seismic behavior of ductile bridges dimensioned per a force-based design procedure. It has been demonstrated that satisfying the EC8 regularity condition does not necessarily result in a near-simultaneous failure of unequal piers at the extreme hazard level, especially for a low ratio between pier heights (≤0.5). It has been further concluded that regularity criteria in both provisions fall short of providing the absolute regular seismic behavior of the bridge under investigation. Finally, a new design criterion is introduced (and verified) herein and is promoted for bridges with unequal height piers but with the same cross-section dimensions as may be typically dictated by some compelling aesthetical and practical considerations. However, it is worth mentioning that pier behavior (and, hence, failure) is assumed to be governed by flexure, and no shear failure and buckling are considered.

On the design and seismic response of RC frame buildings designed with Eurocode 8

Performance-Based Seismic Design is now widely recognized as the pre-eminent seismic design and assessment methodology for building structures. In recognition of this, seismic codes may require that buildings achieve multiple performance objectives such as withstanding moderate, yet frequently occurring earthquakes with minimal structural and non-structural damage, while withstanding severe, but rare earthquakes without collapse and loss of life. These objectives are presumed to be satisfied by some codes if the force-based design procedures are followed. This paper investigates the efficacy of the Eurocode 8 force-based design provisions with respect to RC frame building design and expected seismic performance. Four, eight, and 16-storey moment frame buildings were designed and analyzed using the code modal response spectrum analysis provisions. Non-linear time-history analyses were subsequently performed to determine the simulated seismic response of the structures and to validate the Eurocode 8 force-based designs. The results indicate the design of flexural members in medium-to-long period structures is not significantly influenced by the choice of effective member stiffness; however, calculated interstorey drift demands are significantly affected. This finding was primarily attributed to the code’s enforcement of a minimum spectral ordinate on the design spectrum. Furthermore, design storey forces and interstorey drift demand estimates (and therefore damage), obtained by application of the code force-based design procedure varied substantially from those found through non-linear time-history analysis. Overall, the results suggest that though the Eurocode 8 may yield life-safe designs, the seismic performance of frame buildings of the same type and ductility class can be highly non-uniform.

Effect of foundation flexibility on ductility reduction factors for R/C stack-like structures

The most important parameter used to determine force reduction factors in force-based design procedures adopted in the current seismic codes is the structural ductility. For a structure supported on a flexible foundation, the ductility factor could be affected by foundation compliances. The ductility factors given in the current codes are mostly assigned ignoring the effect of SSI and therefore the objective of this research is to assess the significance of SSI phenomenon on ductility factors of stack-like structures. The deformed configuration of stack-like structures is idealized as an assemblage of beam elements considering nonlinear moment-curvature relations, while a linear sway-rocking model was implemented to model the supporting soil. Using a set of artificial records, repeated linear and nonlinear analyses were performed by gradually increasing the intensity of acceleration to a level where the first yielding of steel in linear and nonlinear analyses is observed and a level corresponding to the stack collapse in the nonlinear analysis. The difference between inelastic and elastic resistance in terms of displacement ductility factors has been quantified. The results indicate that foundation flexibility can decrease the ductility of the system and neglecting this phenomenon may lead to erroneous conclusions in the prediction of the seismic performance of flexibly-supported R/C stack-like structures.

Performance-Based Procedure for Direct Displacement Design of Engineered Wood-Frame Structures

This paper reports on a study to extend a recently proposed direct displacement design (DDD) procedure for midrise engineered wood-frame structures and develop a set of factors for use in the procedure to meet specified performance levels with certain target probabilities. Representative index multistory building configurations were selected from the archetype buildings developed for the FEMA ATC-63. Seismic hazard levels and performance requirements recommended by ASCE 41-06 and modified for use in the National Science Foundation sponsored NEESWood project were used. The archetype buildings, originally designed using current force-based design procedures, were redesigned using the simplified DDD procedure (also described herein) with a range of nonexceedance (NE) probability adjustment factors (CNE). Specifically, the design interstory shear forces and the sheathing nail spacings were determined for each structure designed using CNE. Nonlinear time-history analysis was performed for each archetype structure under the 2%/50 year seismic hazard level and peak interstory drift distributions were developed. The NE probability at the 4% drift limit was then plotted against building height and design charts were developed for each different value of CNE. Given the building height and desired NE probability, engineers/designers can select the appropriate minimum value of CNE using these charts. Additional analyses could be performed to consider other hazard levels and performance requirements. Using design charts of this type, engineers/designers are able to specify a target drift limit as well as a target NE probability when using the simplified DDD procedure. Thus, a true performance-based procedure for the seismic design of midrise wood-frame structures is described.

A displacement-based strength reduction factor for high-rise steel moment-resisting frames

A preliminary study of the ‘displacement-based strength reduction factor’ for high-rise steel moment-resisting frames is presented in this paper. The base shear capacity required for a high-rise steel building in a displacement-based design can be estimated from the reduction of the displacement-based elastic response. The conventional force-based design procedure is still adopted as the initial stage of the displacement-based design. To establish an empirical formula of the proposed displacement-based strength reduction factor, non-linear time-history analyses of six moment-resisting frames are investigated. The conventional ‘equal displacement rule’ and ‘equal energy rule’ are no longer held when the displacement limitations are considered. As a result, a modification for conventional strength reduction factors is proposed for further applications in displacement-based design. An adjustment factor defined as ‘deformation energy ratio’, β, which is related to natural periods, is introduced. The final displacement-based strength reduction factor is defined as a function of ductility demand, fundamental period and the deformation energy ratio. Copyright © 2006 John Wiley & Sons, Ltd.

Preliminary detailing for displacement-based seismic design of buildings

This paper presents several approaches for practical engineers to accomplish a displacement-based preliminarily seismic design for buildings by employing the conventional analysis methods, design tools and current structural design codes. The performance objectives are pre-quantified by the inter-storey drift limit, allowable ductility ratio and the minimum vertical load carrying capacity. A simple yet effective displacement-based procedure is mainly employed. Although the procedure has been successfully applied to both two-dimensional and three-dimensional steel and RC frames without significant irregularity, the paper only focused on a two-dimensional three-bay five-storey steel moment resisting frame on a rigid foundation for the simplicity of illustrating the design approach step by step. A conventional strength-based design approach, a stiffness-based design approach and/or a graphical approach for structural detailing are investigated. If the inter-storey drift limit controls the design, the stiffness-based approach is more efficient, while for other cases, the strength-based approach is recommended. The feasibility of the proposed procedure is demonstrated through nonlinear pushover analysis. Factors affecting the difference between the final design model in terms of required design base shear and initial stiffness and the nonlinear pushover results are examined. It is found that nonlinear pushover analysis may not be necessary in the preliminary design but could be eventually performed to define the best design parameters that will meet the performance requirements. Its advantage over the traditional force-based design procedure is also highlighted.