This paper presents some of the oral discussion by the author and others at the 2005 Annual Meeting of the Los Angeles Tall Buildings Structural Design Council. It also includes additional opinions added by the author after the annual meeting. These opinions address the development of a new building code for tall buildings and where the non-structural engineering decision makers can and must make contributions. It also addresses the very important topic of quality control. Copyright (c) 2005 John Wiley & Sons, Ltd.
A large-size, single-sided cover plate connection composed of a built-up BW36x670 column and a W36x359 rolled beam section was tested in a custom-made test fixture using the SAC Phase I loading protocol. A trapezoidal plate was used as top flange reinforcement and a rectangular pi ate was used as bottom flange reinforcement. The test specimen failed catastrophically at a story drift angle of 1.7% and a beam plastic rotation of 0.009 radian by fracture through the beam top flange and beam web. An ABAQUS nonlinear finite element model of the connection was prepared and analyzed. The ABAQUS data indicate that the likelihood of brittle or ductile fracture at the observed crack initiation site is very low. Fractographic and microstructure studies were conducted after testing, The fractographic analysis of the failure surface showed that the fracture initiated at the toe of a fillet weld that joined the top cover plate to the beam flange. The microstructure analysis around the crack initiation site revealed laminations in the base metal at the crack initiation site. Copyright (C) 2002 John Wiley Sons, Ltd.
This paper summarizes the results of a study that is to evaluate the structural response attributes of near-fault ground motion. Ground motion recordings from the Chi-Chi earthquake are used as inputs to the structural system. An improved nonlinear hysteretic model, based on the experimental study, was used to calculate the response of the single degree-of-freedom inelastic system. Comparison of the results of analysis with traditional elastic-perfect plastic mode calculations was made. Discussions on the inelastic design spectrum, particularly the code-specified base shear coefficients, using the improved nonlinear hysteretic model incorporated with the near-fault input ground motion are made. Copyright (C) 2002 John Wiley Sons, Ltd.
The application of a combined controlled stiffness and magnetorheological (MR) damping system in a base-isolated nonlinear multistorey structure with amplifying braces is described. Passive control theory is used to obtain the viscous characteristics of the MR damper fluid. Selective control of the proposed system is used to enhance the behaviour of a structure during earthquakes. The nonlinear equations of motion are solved using the Wilson 0 method. The control forces are obtained using instantaneous control theory with the predictive control approach. The efficiency of the proposed system is demonstrated by a numerical simulation of a seven-storey building subjected to four different earthquakes. The stimulation shows that the behaviour of the selective controlled structure with the proposed damping system is significantly improved compared to that of an uncontrolled structure. The energy required for the adjustment of the proposed system with amplifying braces is much lower compared to that of the case of MR dampers connected directly to chevron braces. The response of the predictive controlled structure is close to that of an instantaneous controlled structure. Copyright (C) 2002 John Wiley Sons, Ltd.
Reinforced-concrete (R/C) free-standing towers such as TV towers are often analysed using elastic analyses as fixed-base cantilever beams, ignoring the effect of soil-structure interaction. To take the capacity of structures after yielding into account, most designers usually prefer to decrease the peak values of the elastic response spectrum for the maximum credible earthquake (MCE) anticipated at the site by a factor called the ductility capacity factor, which varies with the design earthquake level and the structural characteristics of the structure neglecting the effect of supporting soil. To investigate the effect of foundation flexibility on the response of R/C free-standing towers deforming into their inelastic range during intense ground shaking, a linear sway-rocking model is applied in numerical modelling of the soil-structure system. The effect of concrete cracking and reinforcement yielding on the elements used in the structure modelling is taken into account by introducing a nonlinear model for R/C frame elements using the moment-curvature (M-phi) relation. A method called pseudo-dynamic analysis is presented to quantify the inelastic seismic response spectrum of a soil-R/C free-standing system using response spectrum analysis method and push-over analysis technique. The earthquake responses of cracked and uncracked systems for a practical TV tower and a practical range of soil shear wave velocity are calculated and compared with the objective of understanding how soil-structure interaction influences structural responses. Copyright (C) 2002 John Wiley Sons, Ltd.
A simplified elastic hand-method of analysis for asymmetric multi-bent structures with cores subjected to horizontal loading is presented. The structures may consist of combinations of framed structures such as coupled walls, rigid frames and braced frames with planar and non-planar shear walls. Results for structures that are uniform with height compare closely with results from stiffness matrix analyses. The method is developed from coupled-wall deflection theory which is expressed in non-dimensional structural parameters. It accounts for bending deformations in all individual members, axial deformations in the vertical members as well as torsion and warping in nonplanar walls. A closed solution of coupled differential equations for deflection and rotation gives the deflected shape along the height of the building from which all internal forces can be obtained. The proposed method of analysis offers a relatively simple and rapid means of comparing the deformations and internal forces of different stability systems for a proposed tall building in the preliminary stages of the design. The derivation of equations for analysis shown in this paper are for unisymmetric stability systems only, but the method is also applicable to general asymmetric structures with cores. Copyright (C) 2002 John Wiley Sons, Ltd.
Tuned mass dampers (TMDs) and active tuned mass dampers (ATMDs) are control devices added in tall buildings to suppress the response of buildings in irregular external excitations such as earthquakes and wind. The performance of both passive and active damper systems can be assessed by parametric studies. This paper presents a brief description of the theory of passive and active tuned mass dampers. In this work, the effect of TMDs has been studied by changing the frequency ratio of the damper to the first mode of the structure. It was observed that the extra damping provided by TMDs, called effective damping, can primarily be obtained from design charts developed for this purpose, A 25-storey building is designed and analysed using these charts as an example. The behaviour of a structure with an active tuned mass damper is investigated. The related parameters and equations of the active mass damper for the building are solved and its behaviour arising from external loads is studied. The conventional studies, which are based on white-noise excitations, usually overestimate the performance of TMDs and ATMDs. Hence, in order to study the behaviour of a building with a TMD and an ATMD in real excitations, El-Centro and Tabas earthquakes are selected. The results show a significant increase in the effect of TMD performance in controlling the structural displacements by using additional control devices. Copyright (C) 2002 John Wiley Sons, Ltd.
A simple hand method is presented for the three-dimensional stability analysis of buildings braced by frameworks, coupled shear walls, shear walls and cores. Sway buckling behaviour is characterized by three types of deformation: the full-height 'local' bending of the individual columns, wall sections, shear walls and cores, the full-height 'global' bending of the frameworks and coupled shear walls, which is associated with the axial deformations of the column and wall sections, and the shear deformation of the frameworks and coupled shear walls. Based on the stiffnesses associated with these three types of deformation, a closed formula is derived for the calculation of the sway critical load. An analogy between bending and torsion is used to carry out the pure torsional buckling analysis. The interaction between the bending and shear modes as well as among the basic buckling modes (sway in the principal directions and torsion) are taken into account. A worked example with step-by-step instructions shows the easy use of the method. The results of a comprehensive accuracy analysis involving 73 multistorey buildings are also given together with comparisons with other analytical methods. Copyright (C) 2002 John Wiley Sons, Ltd.
A simplified hand method of analysis is presented for the calculation of the overall critical load of planar lateral load resisting structures commonly used to provide stability in tall buildings. They may comprise three stiffness components: bending and shear stiffness in addition to a rotational spring stiffness at the base of the structure. The method requires the calculation of three individual critical loads which are then combined to yield the structural critical load. This allows a rapid assessment of second-order effects on the horizontal displacements and bending moments. The method is most accurate for structures with uniform geometry up the height and is therefore best suited for preliminary stages of design and checking the reasonableness of the results from computer analyses. Copyright (C) 2002 John Wiley Sons, Ltd.
This paper addresses the issue of horizontal overstrength in modern code-designed reinforced-concrete (RC) buildings. The relationship between the lateral capacity, the design force reduction factor, the ductility level and the overstrength factor are investigated. The lateral capacity and the overstrength factor are estimated by means of inelastic static pushover as well as time-history collapse analysis for 12 buildings of various characteristics representing a wide range of contemporary RC buildings. The importance of employing the elongated periods of structures to obtain the design forces is emphasized. Predicting this period from free vibration analysis by employing 'effective' flexural stiffnesses is investigated. A direct relationship between the force reduction factor used in design and the lateral capacity of structures is confirmed in this study. Moreover, conservative overstrength of medium and low period RC buildings designed according to Eurocode 8 is proposed. Finally, the implication of the force reduction factor on the commonly utilized overstrength definition is highlighted. Advantages of using an additional measure of response alongside the overstrength factor are emphasized. This is the ratio between the overstrength factor and the force reduction factor and is termed the inherent overstrength (Omega(i)). The suggested measure provides more meaningful results of reserve strength and structural response than overstrength and force reduction factors. Copyright (C) 2002 John Wiley Sons, Ltd.
A tall building comprising frames and shear walls coupled together is idealized as a shear-flexure cantilever through the continuum approach. The effects of axial deformation as well as axial force in the frames are considered and incorporated in the formulation of the governing equations. Numerical examples are solved through the Galerkin method and the results compared with finite element solutions. The study indicates that the effect of axial deformation in the frame should be considered for tall and/or slender buildings while the effect of axial force in the column should be included for buildings with soft-storeys resulting from the termination of core walls in the lower portion of the building. Copyright (C) 2002 John Wiley Sons, Ltd.
Earthquake vulnerability is an important financial issue in evaluating the acceptability of a property as a security for a loan or for an equity position. Such analyses are usually performed as part of the due diligence assessment of the property. Probable loss (PL) and scenario loss (SL), for both design basis and maximum capable earthquakes, were assessed to determine their utility in damageability analyses. Evaluations were made for a range of building damageabilities and for 14 sites throughout the United States that are representative of seismic hazard from very high to low. Portfolios from one to 16 buildings are considered. The analysis indicates that none of the damageability criteria is sufficient to distinguish between good and bad buildings to be securities for loans or to be good risks for equity investment for general use throughout the United States. Scenario upper loss (SUL) and probable loss (PL) values are consistently very stringent criteria for individual buildings in moderate-risk and high-risk regions, while scenario expected loss (SEL) values are too lenient in moderate risk areas. PL and SUL criteria are much too stringent at the 20% threshold level to be used generally, except for a group of well-designed buildings. They can be acceptable if threshold values for acceptance are set to be above 20%. The SEL criterion is evaluated as not adequate to discriminate between good and bad damageability buildings by itself. It can work well in combination with a stability criterion so that buildings with poor predicted earthquake performance are eliminated based on stability, not damageability. It is recommended that combined criteria be used for fiduciary evaluation that includes both an assessment of the stability of the buildings and a damageability assessment to achieve a balanced, consistent evaluation of the suitability of the buildings as securities or as investments. Four alternative acceptance criteria are recommended for due diligence assessments. It is recommended that all due diligence reports include evaluations of scenario and probable losses for each building and the group so that they can enter into the decision process for the property. Copyright (C) 2002 John Wiley Sons, Ltd.
This paper describes some results obtained from full-scale measurements of wind effects on a super-tall building, Di-Wang Tower, located in Shenzhen, China. This tall building has 79-storeys with a height of approximately 324 in. Field measurements including wind speed, wind direction and wind-induced acceleration responses have been made. The amplitude-dependent characteristics of damping are obtained by using the random decrement technique from the detailed analysis of the field acceleration measurements. The main objective of this paper is to present detailed investigations into the effects of nonlinear damping on the dynamic responses of the tall building subjected to various types of applied loads based on the measured amplitude-dependent damping characteristics. The predicted dynamic responses of the building obtained by using the measured damping characteristics were compared with those computed by using constant damping parameters assumed by the structural designers. It is concluded from the investigations that knowledge of actual damping characteristics are very important in the accurate prediction of the dynamic responses of a tall building when the major harmonic components of the applied loads overlap with the lowest natural frequencies of the building. The design damping level for tall building structures currently used by structural engineering practitioners appears to be high and not conservative. Copyright (C) 2002 John Wiley Sons, Ltd.
Seismic response of large concrete tower structures with passive or active damping is important in terms of performance under earthquake loads. The conventional finite element method has been used successfully in linear and nonlinear analyses in large concrete structures. The method can be performed by subdividing the large structure into small uniform elements having approximate shape functions. Although this replaces a single complicated structural system with a number of simple uniform elements, in cases of tall concrete tower structures with cracking and crushing behaviour in the concrete material and yielding in the reinforcement, the computer time and memory can be large. Hence, it is desirable to search for a procedure requiring fewer elements and also less computer time and effort to model the structure. In this respect, attention is paid to the advanced complex damped spectral element method, which benefits from the more accurate and also mathematically complicated shape functions. Use of the advanced spectral element method can help engineers to design a complex structure, such as a tall concrete tower, with lower cost and lower weight. Using a computer program, the proposed formulation has been used to derive the nonlinear dynamic response of the 435-m Tehran Telecommunication Tower. Copyright (C) 2002 John Wiley Sons, Ltd.
To economically enhance the performance of active/passive control devices for the seismic response reduction of tall buildings, this paper investigates the optimal placement and optimal parameters of control devices using the linear quadratic performance index as an objective function. The optimal placement of control devices is determined in terms of the sequence of the calculated performance index increments and the number of control devices to be used. With the control devices at their optimal places, the seismic response of the building is finally computed using the suboptimal control gain derived using the minimum error principle. The results from the numerical examples show that the suggested approach is quite accurate and effective in determining the optimal placement and optimal parameters of control devices if the number of removed control devices is limited to a certain range.
Framed tube structures are particularly suitable for tall buildings. Owing to flexural and shear flexibilities of the frame members, the overall bending of the frame is complicated by the occurance of shear lag. The shear lag phenomenon in framed-tube structures under lateral loads is explained in the literature, but explanation of its origin and comprehensive studies of it is lacking. In this paper an analogy between the shear lag behaviour of a cantilever box representing a uniform framed-tube building is used for exact analysis of stress and displacement components of perimeter columns. The coefficients of the power series used for the governing equilibrium and compatibility equations are functions of the height and width of the frame panels. Numerical examples are given to demonstrate the accuracy of the proposed method. Copyright (C) 2002 John Wiley Sons, Ltd.
An analytical solution for the bearing capacity of a large-diameter pile is proposed based on the load transfer function in this paper. The load-settlement relationship or the P-S curve of such a pile can be determined by the proposed method for practical application. Comparison is made between the results obtained by the proposed method and the test data, and close agreement has been found, thus verifying the accuracy of the proposed method. It is noted that the most reliable approach to the determination of the vertical load capacity of a pile is to use the P-S curve, which is usually obtained through on-site load test. However, for a large-diameter pile (e.g. a diameter over 3 m), this kind of test requires a thousand tons of test load, which sometimes is impossible to provide. It is expected that the proposed method can be used in this kind of situation. Copyright (C) 2002 John Wiley Sons, Ltd.
A new simplified model for the application of a shallow cylindrical tuned liquid damper (TLD) in structural vibration control is presented in this paper. The dynamic properties of a shallow liquid in cylindrical containers subjected to forced horizontal oscillation are analysed directly from the continuity and momentum equations of fluids. Following some practical assumptions, the nonlinear partial differential equations describing the wave movement of a shallow liquid in cylindrical containers is established and a numerical procedure for the solution of these equations is proposed using the finite element method. The formula for determining the control force provided by the shallow cylindrical TLD is presented and the effect of several parameters on the control efficiency of a shallow cylindrical TLD controlled structure under wind action is investigated. Copyright (C) 2002 John Wiley Sons, Ltd.
Drift design methods based on resizing algorithms are presented to control lateral displacements of steel-frame shear-wall systems for tall buildings. Three algorithms for resizing of structural members of the steel-frame shear-wall systems are derived by formulating the drift design process into an optimization problem that minimizes lateral displacement of the system without changing the weight of a structure. During the drift design process, cost-effective displacement participation factors obtained by the energy method are used to deter-mine the amount of material to be modified instead of calculating sensitivity coefficients. The overall structural design model with the drift design method for the steel-frame shear-wall systems is proposed and applied to the structural design of three examples. As demonstrated in the examples, the lateral displacement and interstorey drift of a frame shear-wall system can be effectively designed by the drift design method without the time-consuming trial-and-error process, Copyright (C) 2002 John Wiley Sons, Ltd.
The shear-lag phenomenon is a critical consideration in high-rise building analysis and design. The shear-lag behaviour of framed-tube (including tube, tube-in-tube and tubes-in-tube constructions) is investigated for the behavioural characteristics of the structures and their performance in relation to the various structural parameters. The stiffness factor in terms of the axial stiffness of the columns and the bending stiffness of columns and beams is chosen as a main parameter to explain the shear-lag phenomenon and the global behaviour of the structures, A simple numerical technique is also proposed for estimating the shear-lag behaviour of framed-tube systems with and without multiple internal tubes. Further work is carried out to demonstrate the simplicity and accuracy of the proposed method through the analysis of three framed-tube structures (of different heights) without internal tubes and three other framed-tube structures with single, two and three internal tubes. The shear-lag phenomenon of such structures is studied taking into account the additional bending stresses in the tubes. Copyright (C) 2002 John Wiley Sons, Ltd.
