Roberto Leal Pimentel

Cases of Collapse of Demountable Grandstands

A significant number of accidents has occurred with demountable grandstands in the past as well as recently. The performance of such structures is the subject of this paper. First, structural details and some structural systems of actual grandstands are presented and discussed. Next, specifics of the design, erection, and inspection process are detailed. Finally, a survey of cases of collapse of demountable grandstands is presented and discussed. Ninety three cases of collapse were found between the years 1889 and 2008, in which more than 85 people died and 6,350 were injured. From these, 35 cases in which information about the causes of collapse were available were selected. An analysis of the causes showed that unexpected or excessive loads were the major contributing cause, followed by problems with the supports, connections, and bracing. Possible measures that could be adopted aimed at minimizing the risk of accidents with these structures are discussed.

Experimental Evaluation of Synchronisation in Footbridges due to Crowd Density

Synchronisation of movements of pedestrians implies a significant increase of the applied loads, and this has been appointed as the cause of excessive vibration in some footbridges. Experimental results on this phenomenon are scarce to support the development of design load models. Two aspects have been appointed as related to the phenomenon of synchronisation: interaction between pedestrians and structure (lock-in effect) and densification. In this paper, an experimental investigation of the potential synchronisation among pedestrians while walking across a prototype footbridge is presented, focusing on the contribution of densification. An additional walking test in a 25 m long corridor was included to evaluate the effect of the path length. In the footbridge, three tests with different pedestrian densities were carried out. The tests were filmed by a video camera and the processing of the images enabled the phase differences among the movement of pedestrians to be determined in both vertical and lateral (transverse) directions, by capturing the lateral movement of each pedestrians head. The pacing rates of pedestrians walking in groups of varied densities were also determined. Synchronisation was not observed in spontaneous walking in either of the analysed directions (vertical and lateral) for densities up to 0,9 pedestrians/m2. The results presented may be useful for the development of crowd load models for footbridges. However, further tests with higher densities and, ideally, longer pedestrian paths would be necessary to completely clarify the phenomenon.

Modal Tests and Model Updating for Vibration Analysis of Temporary Grandstand

Temporary grandstands are subjected to dynamic loading due to the movement of spectators, and problems associated with the excessive vibration of these structures have been observed. In this context, since they can be used during design stage to address such problems, reliable finite element models of the structure are needed. In this paper, a finite element model of a temporary grandstand is developed while modeling effects are investigated. To do that, a temporary grandstand for about one hundred people was assembled in a laboratory. Modal tests were conducted to calibrate the numerical models of the whole structure. These models were developed by using various degrees of refinement in order to allow comparing the effect of modeling simplifications. The results evidenced that the modeling of connections and seat planks caused significant changes in natural frequencies. The complexity of the structure, with a significant number of parameters affecting dynamic behaviour, requires that modeling ideally start from individual structural components tested in separate.

Structural Integrity Assessment of Precast Concrete Slabs Employing Conventional and Recycled Coarse Aggregates via Vibration Tests

Precast slabs were tested, consisting of ceramic blocks supported by concrete ribs and with a concrete topping a few centimeters thick. These structures are very common in Brazil, being employed for the construction of houses and small buildings. In one of the tested slabs, recycled coarse aggregate was employed while conventional aggregate (granite gravels and quartz sand) was employed in the other tested slab. This study is part of a broader experimental programme which was designed to assess the applicability of concrete containing recycled aggregates as conventional aggregate replacement. In order to get insight into the structural behavior of slabs with recycled aggregates, the main focus of this paper is to assess the effect of structural load level on vibration parameters (Resonance Frequency, Damping Ratio and Transit Time). Full scale slab structural integrity was assessed by following changes in these parameters via a vibration (modal) test employing impact excitation. The structures were tested for several static load ratios (load to ultimate loading ratios (55%, 82% and 98%)). Visual inspection of induced cracking was also performed. The results obtained indicate a reduction of stiffness in both slabs up to 50% of the stiffness of the respective uncracked structure, which followed a similar trend despite the aggregate types. As for the damping ratios, the recycled aggregate containing slab showed lower values in comparison to the control slab with conventional aggregate. Whereas the former results might indicate similar load capacity, the latter, in principle, may have implications for the vibration serviceability limit state of structures. However, although structures with lower damping ratio can potentially present vibration problems, these studied elements are not often subjected to excessive vibration in its service life.

Synchronization among Pedestrians in Footbridges due to Crowd Density

AbstractCrowding is a critical condition for footbridges that are prone to vibration problems from pedestrian loads. Synchronization of pedestrian movements has been identified as the cause of the excessive lateral vibration in some footbridges. Two conditions have been named as the cause of synchronization: increase in crowd density and pedestrian-structure interaction. The former would be related to the onset of the phenomenon; the latter takes place after structural vibration reaches a certain level. This paper focuses on the former condition. A test program to investigate whether this condition occurs was carried out for a range of pedestrian densities, complementing data previously published on this subject. The head movement of pedestrians walking both in groups and in a flow was recorded by a video camera, and the examination of the video indicated no synchronization from densification. However, it was observed that the lateral sway of the pedestrians’ bodies increased with the increase of density....

Automatic Directing System for Controlling the Vibration of Frame Structures in Any Horizontal Direction Using a Hybrid Mass Damper

AbstractThis paper presents a hybrid system for controlling wind-induced flexural vibration of a frame structure in any vertical plan. The system was implemented and tested in a prototype reduced-scale two-floor building, which had steel decks supported by four columns of aluminum. The actuator consisted of a direct current motor that slid on a rail placed on top of the structure. The displacement of the motor was synchronized with lateral displacements of the structure obtained from the deformations of the columns, which, in turn, were measured by strain gages. A proposed system to automatically adjust direction was incorporated to the actuator to position it at the plan in which the structure oscillates, thus enabling a single mass to be used to absorb vibration in any vertical plan. The fuzzy control technique was employed in this control system, which was shown to be adequate for attenuating vibration.

Simplified Expressions for the Vibration Serviceability of Beam-like Footbridges

Simplified expressions presented in the British code BS 5400 and Eurocode 5 for the vibration serviceability of beam-like footbridges in the vertical direction due to pedestrian load were analyzed. Adjustments made in such expressions improved their accuracy. In addition, 2 proposed expressions are presented. One of them followed the line of thought of the British code and is based on the use of a table and chart, which is simple but limited to predefined footbridge configurations. The other expression is not restricted to predefined configurations nor to predefined values for the speed or weight of the crossing pedestrian and damping of the structure. Numerical simulations and results from measurements show that the proposed expressions generally improved the estimate of peak accelerations and also presented better performance than the code guidelines, particularly for the frequency range of the second harmonic of the pedestrian load.

Generation of electricity using tubular PZT

Currently possible solutions for alternative electric power generation have been the subject of interest of many researchers. More recently, the possibility of generating low power electricity has been studied, being enough to meet the demand of some electronic systems. Systems such as wireless sensors or remote communication which has low power consumption can be benefited. Among various technologies for producing alternative electricity, the use of vibratory energy and deformation of structures can be used to generate electricity. In this conversion, piezoelectric materials that convert mechanical strain energy into electrical energy are employed. Thus, this work presents a characterization study of a piezoelectric material (PZT), Lead Zirconate Titanate, with the purpose of generating electricity. For the characterization experiment a cylindrical PZT was employed, subjected to compression in a cyclic manner in the axial direction. An experimental apparatus was designed and instrumented to capture the force, acceleration, voltage and electric power generated due this piezoelectric cell. Initially simulations were developed in order to guide the experimental set of actions. From the experimental results with a piezoelectric cell, a piezoelectric generator was designed with three cells and characterized. Results of the physical parameters related to the characterization are presented.

Sound pressure levels in food courts of shopping centres

Two potentially critical food courts were selected from a group of twelve of such eating establishments, and sound pressure levels (SPLs) were measured and analysed with regard to acoustic quality. Initial measurements of background noise with the food courts almost empty revealed high SPLs in such places. Measurements taken with varied number of users showed an increase in SPLs in accordance to the cocktail party effect, ending up in values of 75 dBA and 81 dBA for each food court. Design aspects of the courts were discussed in order to improve the acoustical quality.