The impact of invisible technologies on design
5.4 New Methods in Structural Analysis – Design for Seismic Areas
Structural analysis underwent a tremendous devel-opment during the twentieth century (Sebestyen, 1998). Probability-based models were offered in order to perfect deterministic models.
Strictly elastic and linear models gave way to plas-tic or partly plasplas-tic and non-linear models. Macro-molecular structural models remained to dominate structural analysis but fracture mechanics com-menced investigating better models of structures.
More accurate models were also introduced to analyse the impact of dynamic actions, the impact of earthquakes, wind and other forms of vibration.
Devices to reduce the damages to buildings during earthquakes have now reached the position when they include structural and dynamic tools for pas-sive and active dampening. The structural design that formerly was carried out with models of per-missible stresses has been succeeded by ultimate stress models in which the structure was pre-sumed likely to fail when it could no longer meet either the ultimate limit states causing failures, or the serviceability limit states comprising deforma-tions. Ultimate limit states correspond to the fol-lowing adverse states:
• loss of equilibrum of the structure or a part thereof
• attainment of the maximum resistance capacity
Figure 5.2 Municipal gymnasium, Odawara, Japan, designer: Kiyoshi Takeyama. The rectangular space frame was designed with the assistance of a computer
producing the best deformation for structural purposes.
of sections, members, or connections by rupture, fatigue, corrosion, or excessive deformation
• transformation of the structure or part of it into a mechanism
• instability of the structure or part of it
• sudden change of the assumed structural sys-tem into a new syssys-tem
• unacceptable or excessive deformation.
The two basic models for structural analysis are the partial factor method and the full probabilistic method, the first being considered as a simplifica-tion of the second. Whilst both models may be applied (as stated, for example, in the ISO 2394 International Standard), in practice it is the partial factor method that finds practical application, this being abundantly supported by calculation meth-ods, characteristic values, partial factor values and load combinations. Research is faced with the clear task to elaborate all necessary help for the full prob-abilistic method in order to make it also operational.
The most important progress in structural analysis has been the advent of electronic computation.
Eminent civil engineers (Torroja, Nervi, Arup, Iyen-gar, Rice and many others) and architect-engineers (Frei Otto, the late Ted Happold) figure in the list of structural designers who have contributed to the development of innovative structures (Rice, 1993).
Various forms of cooperation between architects and structural designers were experienced and such cooperation was often a decisive factor in progress, for example, in the cooperation among several of the designers listed above. Special com-puter-based methods were developed for protec-tion against earthquakes, wind, fire, smoke and building physics (heat, moisture, light, noise). Elec-tronic computation enabled designers to solve large-size calculations and design new types of structure, such as skyscrapers, long-span bridges and wide-span roofs.
Earthquakes are attributed to fault movement within the earth’s crust. They are a natural phe-nomenon occurring as a result of sudden rupture of the rocks, which constitute the earth (Dynamic Analysis and Earthquake Resistant Design,1997).
Throughout the history of mankind they have brought catastrophic results in their wake. It was not so long ago that scientific research into their
causes commenced, which has evolved into a solid science over the last hundred years. Vibrations, surface waves and ground motion of differing mag-nitude and intensity are generated by an earth-quake. Serious earthquakes cause damage to or failures of buildings and structures and can also result in heavy loss of life. Strong motion observa-tions have been recorded since the last century. By now much data on strong motion is available and theoretical models of strong motion could be cal-culated. As a consequence, earthquake hazard can be analysed, risk indices can be calculated and seismic zones; in which construction should be avoided or at the least special design guidelines should be applied, could be established. Building design profited from this progress so that currently design can assess in advance probable seismic force. For this purpose dynamic analysis methods were worked out.
The behaviour of a structure in an earthquake hinges on the intensity of the earthquake and the quality of the structure (Jeary, 1997). The quality of the structure in turn depends on the configuration of the building (very much a result of architectural Figure 5.3 Denver International Airport, USA, designer: Severud Associates, New York (Edward M.
DePaola, principal), principal design consultant:
Horst Berger. The tensile roof structure covers 428 000 square metres, it consists of a series of tent-like modules, supported by two rows of masts:
the roof mesh’s design was generated by computer using non-linear analysis methods to take account of possible maximum deformations.
intentions), the characteristics of the building ma-terials, the architectural and structural design solu-tions and the quality of the construction’s execu-tion (Penelis and Kappos, 1997).
The seismic design principles comprise the follow-ing guidelines:
• Structures must resist low-intensity earth-quakes without suffering any structural damage, which means that during such earthquakes all structural elements should remain in the elastic range.
• Structures should withstand moderate-intensity earthquakes with very light and repairable dam-age.
• Structures should withstand high-intensity earthquakes (whose frequency of occurrence should not exceed calculated periods) without collapsing.
The above-listed principles are formulated in EC8
‘Earthquake Resistant Design of Structures’ by some fundamental requirements, compliance requirements and some specific measures. Earth-quake-resistant structural analysis and design are based on the requirements. Methods based on structural dynamics are necessary for important buildings with a considerable height, wide span or other sensitive characteristics. For simpler build-ings static analysis may be satisfactory (Browning, 2001). In well-defined and limited cases seismic design may be restricted to structures with con-trolled inelastic response, assuming primarily elas-tic behaviour in earthquakes (CEB, 1998).
Traditionally, seismic design relied mostly on the ductile behaviour of the structure. The ductility of an element is its ability to sustain inelastic defor-mations without substantial structural reduction in strength and the capacity to absorb and dissipate seismic energy (Penelis and Kappos, 1997). Mod-ern design comprises the (static or dynamic) analy-sis of the superstructure and, very often, seismic isolation and passive and/or active dampening (Kelly, 1997, Soong and Dargush, 1997; Wada et al., 2000). The essence of the concept of seismic (or base) isolation is that uncoupling the super-structure by some type of support would allow the building to slide in the event of an earthquake. The
isolation system may be a system of elastomeric bearings (usually natural rubber) or sliders. The low horizontal stiffness of the seismic base isolation reduces the superstructure’s fundamental fre-quency below its fixed base frefre-quency and the predominant frequency of the ground. Most often, multi-layered laminated rubber bearings with steel reinforcing layers are used for seismic isolation. It is also possible to combine sliders with elastomeric bearings. Excessive displacement of the structure may be controlled by active damping counteracting the forces during the earthquake.
The seismic design is different for tall buildings and for light wide-span structures. Rubber-metal lami-nated bearings as seismic isolation proved to be a good solution for tall buildings and, therefore, find increasing application for such structures. Space structures do in general perform outstandingly when subject to severe earthquakes (Moghaddam, 2000). Translational pendulum and paddle isolators provide a better protection for wide-span struc-tures (Tatemichi and Kawaguchi, 2000). However, they are difficult to apply for lightweight structures.
Structural dynamics is applied also when designing buildings and structures to withstand strong winds (Simiu and Scanlan, 1996).
Seismic design, especially of important, tall or wide-span structures, requires specialized knowl-edge and experience, which means that the archi-tect’s and the structural engineer’s skill and work should be combined.