T. Balendra

Effectiveness of tuned liquid column dampers for vibration control of towers

The effectiveness of tuned liquid column dampers (TLCD) in controlling the wind-induced vibration of towers is studied. The nonlinear governing equation of the TLCD is linearized to obtain the stochastic response of the towers due to along-wind turbulence. Through parametric studies, the optimum parameters for maximum reduction in acceleration and displacement are presented for a wide range of towers. It is found that for any tower of practical interest, almost the same amount of reduction in acceleration can be obtained by choosing an appropriate opening ratio of the orifice in the TLCD. The same opening ratio would give almost the maximum reduction in displacement. Generally, the opening ratio needs to be varied between 0.5 and 1.0, with smaller opening ratios for shorter towers.

Vibration control of various types of buildings using TLCD

The effectiveness of the TLCD in reducing the along-wind response of tall buildings is investigated. Variety of buildings with different mass-stiffness distributions (e.g. uniform, linear, abrupt variation) modeled as shear wall and rigid frame systems are studied. A continuum formulation is adopted, which provides response statistics along the entire height of the structure. The performance of the TLCD is discussed with respect to the mode shapes of buildings. A numerical example, illustrating that a second damper could greatly improve the overall response of certain type of buildings is also presented.

Evaluation of flow characteristics in the NUS-HDB Wind Tunnel

Abstract A new low-speed boundary-layer wind tunnel has been designed and constructed at the National University of Singapore. A series of flow-characteristic evaluations were performed in this wind tunnel to determine the uniformity of flow and to verify its adequacy to simulate the atmospheric boundary layer for wind load studies on structures. This paper presents the results of mean velocity measurements in the empty tunnel using a Pitot-static tube, and measurements of mean and fluctuating velocities and flow characteristics in a simulated boundary layer using a hot-wire anemometer. The measurements of mean velocity in the empty-tunnel condition showed a two-dimensional uniform velocity distribution and low turbulence intensities. The simulation of natural wind in urban terrain was carried out using the Counihan simulation method that consists of a castellated barrier wall, quarter-elliptic wedge spires and roughness elements. The simulated results were found to compare well with the atmospheric data for urban terrain at a scale of 1:375.

Preliminary studies into the behaviour of knee braced frames subject to seismic loading

Abstract A recently proposed structural system for earthquake-resistant steel-structures is investigated. This new framing system, called the knee braced frame (KBF), dissipates energy during severe seismic excitations through flexural yielding of the knee element. The lateral stiffness is provided by a conventional diagonal brace with at least one end connected to the knee element. As the performance of this framing system depends on the ductile behaviour of the knee element, this critical element was tested to confirm its behaviour under cyclic loading. An analytical model for the moment-rotation relationship of the knee element, derived from the bilinear stress-strain relationship obtained through tensile test, is found to predict the cyclic response of the knee element accurately. Using this analytical model, the energy dissipation characteristics of the KBF are presented.

FREE VIBRATION OF PLATED STRUCTURES BY GRILLAGE METHOD

Abstract Free vibration of plated structures such as plates, stiffened plates and cellular structures are analysed by the grillage method. The accuracy of the method is assessed by comparing with published results and those of the finite element method. Reasonably good results are obtained by the grillage method, which is less demanding on computer time and thus reduces the cost of analysis substantially. By using the grillage method the influence of the number of stiffeners in a stiffened plate and the number of webs in a cellular structure on the vibration characteristics of plated structures is investigated.

Effectiveness of TLCD on various structural systems

Abstract This paper examines the effectiveness of using the TLCD for vibration control of various structural systems under random wind loadings. The effectiveness is indicated by the percentage reduction in acceleration and displacement of the building. The structural systems include frame, shear wall and frame-shear wall systems. A wide range of building heights are considered. These buildings are modeled as shear-flexure cantilever beams, taking into account the effects of the higher modes. The sensitivity of the TLCD to changes in structural periods (which are assumed to be proportional to the structural heights) and damping ratios is also examined. Unlike commonly used lumped-mass studies, a continuous formulation is adopted here which allows for response quantities to be determined along the entire length of the structure. Also the effect of varying the damper position on the damper performance can be examined more readily.

Vulnerability of reinforced concrete frames in low seismic region, when designed according to BS 8110

The overstrength and ductility due to redistribution of internal forces are being investigated for three bay multi-storey reinforced concrete plane frames, using non-linear push-over analysis. These frames are designed to resist gravity loads, wind loads and a notional horizontal load in accordance with the British code BS 8110, which does not have any special provision for seismic loads. The results show that the overstrength factors for the three-, six- and ten-storey frames are respectively, 7·5, 5·6 and 2·2 times the design lateral loads, whereas, the ductility factors for the three frames are similar, and slightly greater than 2. These values yield a response modification factor of 18·0, 12·2 and 4·7 for the three-, six- and ten-storey frames, respectively. The effect of infill walls on the response modification factor is also being investigated, and a suitable response modification factor for assessing the vulnerability of reinforced concrete frames of about 10 storeys high is recommended. Copyright

Control of wind-excited towers by active tuned liquid column damper

Abstract An active control system involving a tuned liquid column damper (TLCD) is developed for the vibration control of a single-degree-of-freedom tower subjected to wind excitation. This new control system will be referred to as the active tuned liquid column damper (ATLCD) system. It is found that the wind-induced response of the tower can be reduced substantially by the ATLCD system. The most efficient type of sensor is found to be the acceleration sensor when compared with displacement- and velocity-type sensors. To assess the effectiveness of such a control system, a comparison study is made between the ATLCD and the active–passive tuned mass damper (APTMD). In seeking the optimal control parameters for both systems, the norm of the root-mean-square of the system output under actual wind excitation, ‖ H ‖ rms , is adopted as the objective function instead of the commonly used ‖ H ‖ 2 (i.e., the norm of the system output under a unit white noise). The results show that the proposed ATLCD system is a good alternative to the APTMD system. Considering the potential advantages of the liquid column damper, the ATLCD system may be a better choice for vibration control of wind-induced towers.

Earthquake-resistant steel frames with energy dissipating knee elements

Moment-resisting and concentrically braced frames have been widely used for earthquake-resistant steel buildings. However, neither of these structural systems alone, can efficiently provide the required stiffness and ductility simultaneously, which are required for structures subjected to severe seismic excitations. This paper presents the results of an investigation into a new alternative structural system for earthquake-resistant steel buildings, which aims to overcome the limitations of existing systems. This system employs a diagonal brace with one end anchored to a knee element. Stiffness is derived from the nonbuckling brace, while ductility under severe lateral loading is achieved through flexural yielding of the knee element. Large-scale model tests were conducted to assess the performance of this system. A nonlinear analytical model of the knee element has been presented for application in the dynamic analysis of the new structural system. Using this analytical model, the dynamic response of a multistorey frame with energy dissipating knee elements is compared with the corresponding eccentric braced frames.

Free vibration analysis of cylindrical liquid storage tanks

Abstract Cylindrical liquid storage tanks anchored to rigid base slabs are considered. By using finite elements for both liquid and tankwall, the natural frequencies of the coupled system are presented in graphical form for various tank dimensions, liquid depths and wall thicknesses. From the mode shapes it is concluded that during coupled vibration a large core of liquid is practically unaffected.