Ninth International IBPSA Conference Montréal, Canada August 15-18, 2005 THE USE OF ADVANCED COMPUTER SIMULATION IN STRUCTURAL DESIGN Caroline J Field1, Andrew Mole1 and Mark Arkinstall2 1Arup, San Francisco, USA 2 Arup, Sydney, Australia ABSTRACT Performance Based Approach Performance based engineering is establishing a set This paper covers the innovative use of advanced of performance criteria that is acceptable for the numerical simulation in the field of structural client for prescribed load scenarios and then engineering. It begins with a brief review of current quantifying the building performance through standard practice and then discusses the latest more appropriate methods and adjusting the design where advanced technologies and approaches that are necessary. available. The practical benefits of using advanced simulation as an integral part of the design process Simulation plays an important role in quantifying the are illustrated through a range of international project performance and is a necessary tool for enabling this case studies. type of approach. INTRODUCTION This approach is gaining ground in current practice and has been adopted by some modern guidelines. In Current Structural Design Practice the United States, publications sponsored by the Structural engineering is the design of building Applied Technology Council (ATC) and the Federal elements to resist a set of environmental building Emergency Management Agency (FEMA) discuss loads. Some of these loads are very straightforward – and provide guidance on the use of increasingly such as the dead weight of the building and the sophisticated analysis techniques within a weight of potential occupants, fitout, machinery and performance based design approach. furniture. These are static loads. Other types of load Standard Analytical Methods depend on location and are subject to statistical interpretation of their frequency and size, such as Standard practice in structural engineering is a wind or typhoon loading, earthquake and wave moving target as in all technical fields, but with loading. Others are unpredictable such as blast and regard to mathematical modeling is characterized by impact loading. These loads are dynamic and time the following: varying in nature. • One-dimensional (and increasingly two- Building Code Approach dimensional) representation of structural elements using finite element formulation Structural design is prescribed by a whole range of building codes, depending on location, building type • Matrix representation of the stiffness of a two- and loading. or three-dimensional structure or sub-assembly Building codes should be viewed as a minimum • Linear analysis (implicit analysis) by means of design standard. They are generally empirical in stiffness matrix inversion to generate the static nature and are an attempt to provide a set of stress state in the structural model resulting from simplified design rules that cover the majority of static loads or imposed displacements buildings. The fact that these rules have to be Since the models are linear elastic, the results from applicable to a very wide range of buildings, means any two loading conditions calculated separately can that for some buildings the design can be very be combined to provide the results that would be conservative and for others possible unconservative. obtained from analyzing the model stress state for the Often, buildings do not fit comfortably within these combined loads. rules and certainly, if one wants to take advantage of In cases where dynamic characteristics are new technologies and developments in the design, significant, for example for machine vibration or these will not be covered by the code. seismic analysis, linear elastic modal analyses are performed. This is relatively unusual in terms of global structural engineering, but is becoming - 303 - standard practice in places where seismic loads are Applications critical in design. Advanced structural simulation would typically be The methods outlined above are restrictive in so far used to address the following situations: as they limit the analyst-designer to design conditions that can reasonably be approximated by • Unusual buildings not defined by the code linear formulations, and these are the approaches that • Complex problems not addressed by the code are codified (e.g. in the Uniform Building Code or its successor the International Building Code). • Cases where the code is too conservative However, for situations where the structure responds • Virtual Testing & Prototyping outside of the linear elastic range of its materials, for example, seismic, blast and impact scenarios, these • Verifying Innovative Design Concepts approaches can provide only an approximate • Forensic Engineering indication of the building’s peformance by introducing “factors” to account for these • Value Engineering (design optimization) nonlinearities. In these situations, advanced Value simulation, which incorporates the effects of geometric and material nonlinearities, provides a far The benefits to be gained when appropriately utilzing superior representation of the actual performance. advanced simulation include the following: This must be informed by a fundamental understanding of the "first principles" that underlie • Enable Creativity the code and structural behavior in general. Creative and innovative design solutions can be virtually tested and developed during the design phase ROLE OF ADVANCED SIMULATION • Improve Performance Definition The building performance can be accurately In the context of structural engineering, advanced quantified and then optimized to improve the simulation is the application of mathematical performance under the load scenarios modeling to problems that are complex and are at a • Produce Cost Savings higher level than standard practice. By adopting a performance based approach and In this paper, "advanced simulation" will be used to utilizing advanced simulation the conservatisms refer to non-linear dynamic analysis of multi-degree- inherent in a simple code approach can often be of-freedom systems. "Advanced simulation" permits reduced to provide significant cost savings a much wider consideration of building types and conditions than can be reliably be designed using • Manage Risk standard practice. The client is able to consider the risk and cost Analysis in standard practice is characterized by the associated with various design solutions because definition of static load conditions (typically called he/she better understands the consequences of "load cases") which are summed. However, advanced each load scenario because it has been quantified structural simulation involves the analysis of "load through “virtual testing”of each design option scenarios", which must be defined to represent conditions that can reasonably be considered to occur • Increase Sustainability and are typically time varying. Thus multiple The ability to accurately predict building scenarios will be run for every structural model. behavior coupled with the ability to explore Tools more innovative design solutions often leads to a more economic solution that more efficiently The analyst-designer has increasingly greater choice and effectively meets the design criteria, thereby of tools for advanced simulation (based on implicit achieving a more sustainable design solution and explicit solvers or a combination). In many cases these are also "multi-physics" - able to perform • Enhances Communication "coupled" analyses - for example, coupling thermal Simulation can be a very compelling and analysis with structural analysis to model the effect invaluable visualization tool if using good of fire on structural steel frames. software. It facilitates a dialogue with your client These packages typically have powerful graphical and makes it easier for the engineer to user interfaces available to speed up the process of communicate complex concepts in a simple way. modeling and to aid the interpretation of results. In the same way it can facilitate a creative dialogue between the design team. - 304 - Getting in early permit the rear columns of the seismic frame to lift off their foundations when the column experienced One of the biggest challenges with structural tension. This innovative ‘stepping column’ would simulation is the difficulty of getting involved from enable the building to ‘rock’ to a limited degree the project conception and being able to utilize under strong earthquake actions. This behavior simulation as a creative part of the design process would reduce seismic forces in the building and and not just as a verification tool. hence cut steelwork and foundation costs. The lifting of the columns was controlled by visco-elastic dampers, which would provide preferential energy- dissipation over hysteretic deformation of the frames, which is the standard approach in building design. As a result less structural damage would occur in any seismic event. As would be expected the performance of this system cannot be satisfactorily predicted using linear elastic analysis and it clearly falls outside the normal bounds of building codes. As a result non-linear analysis was used throughout the design process, explicitly simulating the performance of the structure to various earthquake scenarios. Such studies were performed initially to evaluate the system as a concept, then subsequently to develop the design and Figure 1 Benefit of early use of simulation – from to assess proposed changes and finally to report the