Siegfried F. Stiemer

Object-oriented finite element analysis

Abstract This paper describes the problems with conventional finite element analysis software and the potential solutions offered by object-oriented programs. It introduces the basic concepts of object-oriented programming and of expandable applications. Finite element fundamentals are explained using a new perspective leading to the implementation of an object-oriented numerical analysis program. Objects, classes, methods, and inheritance are illustrated using a graphical representation. Implementation-independent descriptions are given for the internal operation of a generic class system, followed by some details regarding the design of object-oriented programs using class libraries from an expandable application framework. A theoretical foundation is laid for the implementation of an object-oriented program which uses isoparametric elements for the numerical analysis of two-dimensional linear problems in solid and structural mechanics. Class descriptions are given in parallel to a step by step formulation of the analysis solution. The analysis program is portrayed as an assembly of classes which control and organize, the solution process. The design, implementation, operation, validation, and maintenance of this program is compared to that of an equivalent procedural program to identify the advantages of the object-oriented approach. Practical applications of object-oriented finite element analysis are discussed with particular reference to the use of knowledge-based expert systems.

Improved arc length orthogonality methods for nonlinear finite element analysis

Abstract The path following technique known as ‘The Arc Length Method’ has evolved over the past decade into a commonly used tool in nonlinear finite element analysis. Variations of this method have orginated from a number of other workers. This paper presents a new general formulation for all arc length procedures. A derivation is given based on orthogonality principles which provides a new perspective illustrating the relationship between the existing and proposed path following techniques. A simplified procedure, obtained directly from the new general formulation, provides the same results as Crisfields explicit iteration procedure (Comput. Struct. 13, 55–62 (1981)) with a reduction in computational effort. Practical application of this theory is demonstrated using a numerical example of a reticulated shell structure.

CLT–Steel Hybrid System: Ductility and Overstrength Values Based on Static Pushover Analysis

AbstractA timber-steel hybrid system, where cross-laminated timber (CLT) shear panels are used as infill in steel moment resisting frames, is analytically investigated. The proposed hybrid system combines ductile behavior of steel moment frame with lighter and stiffer CLT panels. Initial parametric study is carried out with a single-bay-single-story model using pushover analysis to investigate effect of CLT panel thickness, crushing strength, and confinement gap. Results showed that the parameters studied have a significant influence on system’s ultimate strength, ultimate drift capacity, and post-peak behavior. Subsequently, parametric studies are carried out for three-bay hybrid buildings with three-, six-, and nine-story hybrid buildings for varying panel configuration (a given gap, panel thickness, and crushing strength) values. The parameters varied and are design ductility level (limited ductility and ductile) and infill pattern. A monotonic pushover analysis was performed to develop a preliminary d...

Damage Assessment of Connections Used in Cross-Laminated Timber Subject to Cyclic Loads

AbstractCross-laminated timber (CLT) products are gaining popularity in the North American market and are being used in midrise wood buildings, in particular, in shearwall applications. Shearwalls provide resistance to lateral loads such as wind and earthquake loads, and therefore it is important to gain a better understanding of the behavior of CLT shearwall systems during earthquake events. This paper is focused on the seismic performance of connections between CLT shearwall panels and the foundation. CLT panels are very stiff and energy dissipation is accomplished by the connections. A literature review on previous research work related to damage prediction and assessment for wood frame structures was performed. Furthermore, a test program was conducted to investigate the performance of CLT connections subjected to simulated earthquake loads. Two different brackets in combination with five types of fasteners were tested under monotonic and cyclic loading protocols. In total, 98 connection tests were co...

Seismic Vulnerability Assessment of Hybrid Steel-Timber Structure: Steel Moment-Resisting Frames with CLT Infill

In this article, seismic vulnerability assessment is carried-out on a novel hybrid structure (steel moment resisting frame (SMRF) and cross laminated timber (CLT) infill panels). For the seismicity of Vancouver, Canada, a three-bay, 3-, 6-, and 9-story height SMRFs are designed for two ductility levels (ductile and limited ductility). To study the seismic vulnerability CLT infilled building, parametric analysis was performed by varying infill configuration (bare frame, one-bay infilled, two-bay infilled, and fully infilled). The structure is modeled in OpenSees and nonlinear dynamic analysis is performed. Peak inter-story drift demand and corresponding FEMA performance limits (capacity) values are used to compute the corresponding fragility curves. From the analyses, it can be seen that as more bays are infilled, the fundamental period and seismic vulnerability is reduced significantly. The results highlight that, within the performance-based earthquake engineering, different objectives can be met with varying the CLT configuration.

Special Issue on Performance of Timber and Hybrid Structures

A change in timber building height limit, from short to midrise story, has been introduced in the British Columbia, Canada, building design code. Various research activities culminating from Network on Innovative Wood Products and Building Systems (NEWBuilds) (newbuildscanada.ca), funded through Natural Sciences and Engineering Research Council of Canada (NSERC), shows that through the use of hybrid structures, the building height can indeed be increased further. Hybrid (composite or mixed) systems utilize different materials in the design of structures (e.g., wood-concrete, wood-steel, steel-concrete systems). The different materials can be integrated at the component levels (hybrid slab/diaphragms, hybrid beams, hybrid columns, hybrid diagonals, hybrid shear walls) and/or at the building system levels (hybrid shear wall system, tube system, vertical mixed system) (Taranath 2005). The utility of using hybrid systems is for aesthetic purpose, and optimal use of different material properties. Though there are no clear design guidelines, hybrid structures are already used based on conservative design assumptions. For example, with the introduction of new hybrid systems for forcebased seismic design, strength and ductility reduction factors, along with estimation of fundamental period, are of importance. To increase the adoption and to show utility of using hybrid systems, concerted research in connections (e.g., Schneider, Karacabeyli, Popovski, Stiemer and Tesfamariam, in this special issue), design parameters (e.g., Nishiyama et al. 2004), modeling capability (e.g., Mehanny and Deierlein 2001; Noguchi and Uchida 2004) is needed. Moore (2000) reported the application of a hybrid 12-story building where a concentrically braced frame is used for lateral load resistance with a glulam timber floor slab. The use of glulam floor slab led to a substantially reduced self-weight, compared with the reinforced concrete slab option. The lighter structure has made wind the governing design load rather than earthquake requirements. Thus, for design adoption, multihazard risk assessment (with consideration of fire, wind, earthquake) and compatibility of the different materials (e.g., steel and timber) should be considered. The problem is further compounded in regions of high seismicity, where there is an exposure to recurrent earthquake hazard. Furthermore, the 2011 Tohoku earthquake in Japan and the 2011 Christchurch earthquake in New Zealand have highlighted that consideration of mainshock and aftershock earthquakes in the risk and loss assessment is important (Goda 2012). Recognizing the complexities and challenges faced by the industry, a number of authors with varying expertise and research areas were invited to contribute to this special issue that explores the vast area identified earlier. The different topics can be categorized into system and component responses, connections, and utilization of innovative slab/deck designs. The contributions provide further insight to the academic and practicing engineering communities. Dickof, Stiemer, Bezabeh, and Tesfamariam develop a new CLT-steel hybrid structure, where cross-laminated timber (CLT) is used as in-fill in a steel moment-resisting frame. An initial parametric study is carried out with a single-bay, single-story model using pushover analysis to investigate effect of CLT panel thickness, crushing strength, and confinement gap. Furthermore, using static pushover analysis, the authors have quantified the ductility and overstrength factors in congruence with the Canadian Seismic Design Code (NRC 2010). Their research findings highlight that the CLT does indeed significantly contribute to the seismic resistance of the steel frame, and should be considered in the design. Hafeez, Mustafa, Doudak, and McClure, through ambient vibration measurements, predict the fundamental period of light-frame wood buildings, and compare it with the National Building Code of Canada formulation. The authors highlight that the fundamental period calculation using the code is conservative. Finally, the authors have developed a building stiffness evaluated using the current Canadian Standard CSAO86 Engineering Design (CSA 2009) in wood deflection equation for wood shear-wall buildings. Finally, a simple analytical shear-building model is developed to study the effect of multistory sway deformations. Yang, Li, and Leelataviwat propose a performance-based design and optimization of a buckling-restrained knee brace truss moment frame (BRKBTMF). BRKBTMF is a novel steel structural system which combines steel trusses with buckling-restrained braces to form an alternative seismic-force resisting system. The authors, through analytical and experimental work, highlight the utility of using BRKBTMF: efficient design, with economical and designated energy dissipation devices. Finally, a performance-based plastic design procedure is derived to determine the member sizes of the BRKBTMF. In the fourth paper, Chen, Chui, Ni, Doudak, and Mohammad present a model to capture load distribution in timber structures consisting of multiple lateral load-resisting elements with different stiffnesses. The developed model utilizes multiple springs, whereby the translational springs are used to model the diaphragm stiffness and the stiffness of the lateral load-resisting element. The model is validated with tests and finite-element results of the Network for Earthquake Engineering Simulation Wood (NEESWood) benchmark building. Zhou, Ni, Chui, and Chen investigate increase in the story limit of wood frame buildings in British Columbia (BC), Canada, and the possibility of greater flexibility in these buildings. The authors then propose the consideration of a hybrid building consisting of a light wood frame and a reinforced masonry core. To highlight the impact of the stiffer core, the authors design a six-story light wood frame building with the consideration of 25, 50, and 100% shear resistance of the wood subsystem, and carry out a seismic

Knowledge-based control for finite element analysis

This paper shows that control logic may be separated from analysis software and that a knowledge-based expert system can use this logic to perform interactive computation. Heuristics that control a simple interactive finite element analysis program are represented using a rule-based format and are used by a goal-driven logic processor to invoke analysis activity.Traditional algorithm-oriented control and the proposed knowledge-based control are compared in a simple displacement computation scenario to identify the advantages/disadvantages of the two approaches. General activities and constraints, practical methods of reasoning and representation, and knowledge-based expert systems are discussed with emphasis on applications to interactive finite element analysis.An analysis control expert system has been developed for use in the numerical analysis of two-dimensional linear problems in solid and structural mechanics. An example problem is used to clarify the methods used to direct activity and to identify the problems associated with conditional task processing for interactive analysis.The main difference between the analysis program described in this paper and conventional analysis programs is related to the control architecture. The general conclusion of this paper is that knowledge-based control is more effective and flexible than algorithm-oriented control.

Object-oriented knowledge frameworks

This paper introduces a strategy for the construction of object-oriented knowledge frameworks. Global control programs, which are traditionally used for manipulating structured representations, have been replaced by domain-specific resources. This new approach offers increased efficiency and simplified development of knowledge-based expert systems for problem domains that deal with a variety of complex structured information.A theoretical foundation is laid for the representation of knowledge, starting with the adoption of a simple schema for structuring stereotypical information. Data and method abstraction is achieved by installing knowledge frames into part of an object-oriented class system that supports procedural attachment and object communication. This allows frame specialization using domain-specific resources and subsequent construction of modular systems with these specialist frames. Editors are introduced for the alteration of data primitives used in declarative representation. Algorithms, which operate on these data primitives, are given for frame instantiation, data access, and user-directed inference.Examples from structural engineering are used throughout the paper to illustrate the practical application of object-oriented knowledge frameworks.

Direct Displacement-Based Design of a Novel Hybrid Structure: Steel Moment-Resisting Frames with Cross-Laminated Timber Infill Walls

This study proposes an iterative direct displacement based design method for a novel steel-timber hybrid structure. The hybrid structure incorporates cross-laminated timber (CLT) shear panels as an infill in steel moment-resisting frames. The proposed design method is applied to design three-, six-, and nine-story hybrid buildings, each with three bays and a CLT-infilled middle bay. Nonlinear time history analysis, using 20 earthquake ground motion records, is carried out to validate the performance of the design method. The results indicate that the proposed method effectively controls the displacements due to seismic excitation of the hybrid structure.

Seismic fragility assessment of RC frame structure designed according to modern Chinese code for seismic design of buildings

Following several damaging earthquakes in China, research has been devoted to find the causes of the collapse of reinforced concrete (RC) building sand studying the vulnerability of existing buildings. The Chinese Code for Seismic Design of Buildings (CCSDB) has evolved over time, however, there is still reported earthquake induced damage of newly designed RC buildings. Thus, to investigate modern Chinese seismic design code, three low-, mid- and high-rise RC frames were designed according to the 2010 CCSDB and the corresponding vulnerability curves were derived by computing a probabilistic seismic demand model (PSDM).The PSDM was computed by carrying out nonlinear time history analysis using thirty ground motions obtained from the Pacific Earthquake Engineering Research Center. Finally, the PSDM was used to generate fragility curves for immediate occupancy, significant damage, and collapse prevention damage levels. Results of the vulnerability assessment indicate that the seismic demands on the three different frames designed according to the 2010 CCSDB meet the seismic requirements and are almost in the same safety level.