Friction of Polymers Sliding on Smooth Surfaces

Friction plots of polymers sliding on smooth metal surfaces are generally characterized by two regions of distinct dependency on the normal load, with low sensitivity at low stress levels followed by a sharp change in the rate of decrease of friction with increasing pressure at levels above the plastic flow limit of the polymer. A simplified model is proposed to describe this behavior which accounts for the effect of the normal load on the growth of the real contact area and the shear stress at the interface between the polymer and the mating surface. The model has a wide generality when expressed in terms of dimensionless variables, allowing to rationalize the friction behavior of different polymers within a single framework

Experimental and Numerical Investigation of a Dissipative Connection for the Seismic Retrofit of Precast RC Industrial Sheds

Past earthquakes have highlighted the seismic vulnerability of prefabricated industrial sheds typical of past Italian building practices. Such buildings typically exhibited rigid collapse mechanisms due to the absence of rigid links between columns, beams, and roof elements. This study aims at presenting the experimental and numerical assessment of a novel dissipative connection system (DCS) designed to improve the seismic performance of prefabricated sheds. The device, which is placed on the top of columns, exploits the movement of a rigid slider on a sloped surface to dissipate seismic energy and control the lateral displacement of the beam, and to provide a recentering effect at the end of the earthquake. The backbone curve of the DCS, and the effect of vertical load, sliding velocity, and number of cycles were assessed in experimental tests conducted on a scaled prototype, according to a test protocol designed accounting for similarity requirements. In the second part of the study, non-linear dynamic analyses were performed on a finite element model of a portal frame implementing, at beam-column joints, either the DCS or a pure friction connection. The results highlighted the effectiveness of the DCS in controlling beam-to-column displacements, reducing shear forces on the top of columns, and limiting residual displacements that can accrue during ground motion sequences

High damping curved surface sliding isolators for bridges

n this study, material characterization and full-scale assessment of a curved surface sliding isolator with high damping capacity for a viaduct structure have been conducted to investigate its effectiveness and durability. Characterization tests conducted on the PTFE composite used as the sliding surface confirm the durability of the material with no significant deterioration of the coefficient of friction after more than 2400 m of sliding. Prototype tests performed on the isolator show its ability to provide high damping capacity at seismic velocities, and to accommodate slow service movements of the structure with minimum resistance. Despite of the large energy dissipation and relevant temperature increase, the isolator behaves stably during cyclic loading, with variations of stiffness within 10% and damping within 20% with respect to the design values. The proposed design and the advanced PTFE composite are promising in terms of upgrading the seismic resistibility of bridge structures under near-fault motion

CONTROLLO PASSIVO MEDIANTE CONTROVENTI DISSIPATIVI, Principi generali, requisiti normativi ed evoluzione dei principali dispositivi a comportamento dipendente dallo spostamento

La dissipazione di energia è una tecnica di mitigazione sismica che consiste nel dissipare la maggior parte dell’energia proveniente dal terremoto mediante appositi dispositivi inseriti in una costruzione e non facenti parte del sistema resistente ai carichi gravitazionali, al fine di limitare i danni a carico degli elementi strutturali. Nel primo contributo di questa serie (Structural 237 – settembre/ottobre 2021 – paper 25) è stata presentata una procedura progettuale semplice e veloce per la progettazione dell'adeguamento sismico di edifici a telaio attraverso dispositivi dissipativi a comportamento non lineare dipendente dallo spostamento. In questo secondo contributo viene presentato un inquadramento teorico generale, dove sono richiamate le nozioni di base del controllo passivo delle vibrazioni mediante dissipazione di energia, seguito da un approfondimento sui requisiti normativi previsti dalle Norme Tecniche e dalla norma Europea EN 15129, con un focus sui dispositivi a comportamento dipendente dallo spostamento. Viene infine descritta l’evoluzione dei dispositivi di dissipazione dipendenti dallo spostamento, mettendo in luce vantaggi e svantaggi delle soluzioni proposte e presentando alcune applicazioni pratiche in riferimento anche ai dispositivi attualmente disponibili sul mercato

School as Community Center / La scuola come centro per la comunità.

Within Design Studio 2, students engaged with the theme of educational architecture working on the design of the elementary school in via Massaua in Milan and its relationship with the adjacent urban space of Piazzale Tripoli. Students were asked to think creatively about innovative pedagogical approaches and its spatial translation into classrooms, circulation spaces, playgrounds, and the importance of nature in the school and in the city. The school was seen as a landmark and a civic center in the neighbourhood, spatially designed to offer some facilities (gym, library, auditorium) to be shared with the local community

High performance materials for pendulum sliding bearings

At Politecnico di Milano a novel thermoplastic material for pendulum bearings operating in seismic isolation of buildings and structures has been recently developed. The material properties were characterized in small scale tests, and the dependence of the coefficient of friction on operating conditions like pressure, temperature and velocity was determined at speeds and durations typical of seismic events. The effect of extended durations of excitation on the stability of friction was assessed. Two prototypes of pendulum bearing operating with the new material were manufactured and tested. The results confirmed the good performance of real scale isolators in terms of low horizontal stiffness, high damping and good stability of the dynamic propertie

Thermal stabilization of highly crosslinked UHMWPE:A comparative study between annealed and remelted resins

Purpose: An important issue related to the use of highly crosslinked ultra-high molecular weight polyethylene (HXLPE) in arthroplasty concerns the long-term oxidation of the polymer and related degradation of its end-user properties. Although in very recent years several procedures have been introduced into the manufacturing of prosthetic components to overcome this problem, the risk of long-term oxidation has not been completely eliminated. The aim of this study is to compare the effects on the physical and mechanical properties of HXLPE from two different thermal treatments used to promote oxidative stabilization. It also gives a description of the effects of oxidative degradation on the mechanical performance of HXLPE. Materials and Methods: Virgin medical grade UHMWPE GUR 1020 was irradiated at 100 kGy and submitted to heat treatments at either 110°C (annealing) or 150°C (remelting). Oxidation analysis, Wear tests, Tensile tests and Charpy impact tests were carried out. Results: The temperature of the thermal treatment affects both oxidation resistance and impact strength of HXLPE, whereas wear resistance is not affected. Conclusion: The study provides a confirmation that oxidative degradation is a serious issue for the long-term performance of HXLPE prostheses

Effect of the Static Coefficient of Friction of Curved Surface Sliders on the Response of an Isolated Building

Effective implementation of the static friction of sliding isolators in object-oriented software for structural design has not yet been achieved, and use of the dynamic friction only for design is common practice. A modeling strategy to account for the contribution of static friction under unidirectional ground motion histories has been developed and used to assess its effects on the response of a building isolated with curved surface sliders. Under low-to-moderate intensity earthquakes, disregarding static friction can lead to unpredictable response of the isolation system and result in a non-conservative evaluation of accelerations and forces transferred to the superstructure

Comparison of Linear and Nonlinear Procedures for the Analysis of the Seismic Performance of Straight Multi-Span RC Bridges

The assessment of the seismic performance of transportation infrastructures is of primary importance for the management of the aftermath of an earthquake. To perform such an assessment, various modeling approaches characterized by different levels of accuracy are available and modern seismic design codes provide recommendations about their use. Non-linear time-history analysis (NLTHA) is acknowledged as the most reliable method, but is difficult to implement and is computationally expensive. This paper aims to investigate the viability of less complex methods, but with low computational cost, for the assessment of straight, multi-span bridges and compare their performance against the results of NLTHA in order to quantify the expected accuracy. The study is developed considering three bridge archetypes with either simply-supported or continuous-deck layouts, representative of typical features of the Italian bridge stock. The bridges are analyzed first through nonlinear dynamic analyses, to define the benchmark solution; then linear dynamic analyses, such as Linear Time-History and Response Spectrum Analysis, nonlinear static analyses, such as MPA (Modal Pushover Analysis), and Equivalent Static Analysis are considered. A comparison among the examined procedures is eventually proposed, highlighting the strengths and weaknesses of each approach

Seismic Upgrade of Steel Frame Buildings by Using Damped Braces

Supplementary energy dissipation has proved to be an effective way of protecting structures from the disastrous effects of earthquakes and has been used in the last decades both in new and in existing constructions. In this regard, various procedures for the design of the damping system for the seismic retrofit of buildings have been formulated over the years, mainly focused on reinforced concrete (RC) constructions, which represent the largest part of the existing stock in many seismic-prone countries. The study deals with the assessment of a displacement-based design procedure for proportioning the damping system recently proposed in the literature for RC framed buildings, with the goal of establishing a good practice for the application of the procedure to steel buildings as well. The method was applied to three case-study frames, regular in plan and in elevation, which were assumed as being representative of old structures designed without consideration of seismic requirements. The retrofit was performed by using chevron braces equipped with dampers with an elastic-perfectly plastic behavior. The method aimed at defining the properties of the dampers to achieve a target performance in terms of the maximum lateral deflection for a specific level of seismic intensity. The effectiveness and reliability of the proposed procedure was eventually assessed by evaluating the seismic performance of the upgraded steel structures in static and dynamic non-linear analyses