Traffic Signal Structures Research Articles

Feasibility Study of Capacitor-Based Smart Washer Sensor Systems for Highway Sign, Luminaire/Light, and Traffic Signal Structures

This paper aims to provide an improvement on the current inspection methods for anchor bolts used in highway sign, luminaire/light, and traffic signal (SLTS) support structures to reduce the risk of structural failure as well as human labor costs. A feasibility study on a capacitance-based smart washer system was conducted. The capacitor-based sensor system design combined with customized polylactic acid three-dimensional printed attachments was proposed and tested. Laboratory tests on multiple sets of sensors were performed through initial testing and tightening–loosening testing; results have shown that the performance of the sensor system follows the working principle of the capacitor and, with a less than 5% error tolerance, a capacitance–pretension relationship was developed in the laboratory. Then the sensor system was installed at a sign structure specimen for long-term evaluation; the 12-month consecutive monitoring results show that the sensor system was able to perform steadily with a proper waterproof cover applied. The change of the capacitance was found at ±5% of the reference value, and torques were applied to verify the measurement results from the sensor system, indicating that the minor capacitance change was caused by a change of temperature. The results have proven the feasibility of this capacitor-based smart washer system that detects the pretension change inside the anchor bolts and performs steadily in a controlled on-site environment with a high durability. In summary, this study provides the possibility of improving the monitoring methods for anchor bolts in highway SLTS structures.

Physics-informed long short-term memory networks for response prediction of a wind-excited flexible structure

Slender and flexible infrastructures such as sign supports, cantilever traffic signal structures and high mast lighting towers are sensitive to wind force and were reported to have fatigue-related issues due to the large-amplitude vibrations throughout thier life. Simulating wind-induced structural response can be an important step to evaluate their fatigue life and reliability. However, wind simulations are usually quite complicated. A comprehensive wind force model was usually developed by conducting multiple wind tunnel tests. However, due to the high cost of wind tunnel tests and the limitation of a wind tunnel, aerodynamic and aeroelastic coefficients were usually extracted only at certain wind speeds and wind directions. Interpolation or extrapolation methods were commonly used when coefficients were not available, which makes the simulation result questionable. In this study, a methodology was proposed to simulate wind-induced structural response with lower costs. The proposed method uses monitoring data in the field to develop long short-term memory (LSTM) networks. In training LSTM networks, only the monitoring data in regular wind condition was used. However, the trained LSTM network can still predict the wind-induced response in high and extreme wind conditions observed during the monitoring of the structure. The proposed method can be useful when simulating wind-induced structural response in a wide range of wind speeds and can be widely used on other structures suspected of having fatigue damage due to wind-induced vibrations.

Creating an asset management plan for traffic signal structures through interactive explorations of wind induced fatigue damage

This paper presents an asset management framework for remote monitoring of traffic signal structures for fatigue damage under wind loads, using historical wind measurements from nearby weather stations. Estimate wind profiles developed from the wind data are used to calculate fatigue damage at welded connections for a wide variety of common traffic pole configurations. The analysis is embedded within a novel remote monitoring application that performs the fatigue life analysis in real time for a user defined location and traffic pole configuration. Such a framework is the first of its kind that allows operators to make real time asset management decisions though interactive explorations of the relationship between traffic pole configuration and fatigue life due to local wind conditions. Results are demonstrated using several traffic poles located around Logan, Utah, which has complex topography that creates drastic changes in wind conditions over short geographical distances. The results illustrate the ability of the remote monitoring application to detect small-scale differences in wind conditions that result in significant differences in the estimated fatigue life of traffic structures within communities. Such results highlight the need for, and feasibility of developing site-specific asset management frameworks at a much finer spatial resolution than previous attempts.

Evaluation on stress intensity factors of octagonal tube-to-transverse plate joint for sign and traffic signal structures

Sign and traffic signal structures are susceptible to fatigue damages caused by wind, temperature change, and road-induced excitation. This study focuses on the stress intensity factor (SIF) ranges for octagonal tube-to-transverse plate joints widely used in sign and traffic signal structures. The finite element (FE) models were established and calibrated by using Newman and Bowness equations. The SIF ranges of the joints with fillet weld and full-penetration groove weld were compared. The effects of initial crack location, transverse plate thickness, tube diameter and thickness, taper ratio of tube, weld size, and number of bolts, on the SIF ranges of tube-to-transverse plate joint with fillet weld were intensively investigated. The suggestions for improving the remaining fatigue life of the joint were offered.

Studying the wind-induced vibrations of a traffic signal structure through long term health monitoring

Recent failures of traffic signal structures have showcased the vulnerability of such structures to wind loading. During the wind-induced vibration of the mast arm, the large mass of the mast arm creates high-level stress at the pole-to-arm connection. Additionally the low mechanical damping results in accumulationof stress cycles ewhich could cause fatigue damage at the pole-to-arm connection. In this study, a traffic signal structure with three vertical traffic lights was selected for long-term health monitoring. The monitored acceleration data at the mast arm tip was used to study the wind-induced behavior and was compared with the theoretical analysis results. The monitored strain data at the pole-to-arm connection was used to evaluate the fatigue damage. Low stress ranges were recorded most of the time, which indicates that the wind-induced fatigue damage might not be a significant concern for the monitored traffic signal structure. However, high stress ranges have been recorded at high wind speeds, indicating fatigue damage can still be an issue at higher wind speed regions. The performance of monitored traffic signal structure was compared with those in the literature. An apparent discrepancy was noticed between their wind-induced behaviors, and the orientation of the attached traffic lights was identified as a key parameter to influence the wind-induced behavior. It was found that at low wind speed, attached vertical traffic lights can reduce the likelihood of vortex-induced vibration and, as a result, lower the stress range at the pole-to-arm connection.

Study of the aerodynamic damping of wing plates on traffic signal structures based on the 2D one-way FSI analysis

ABSTRACT To improve the fatigue performance of these traffic signs, luminaries and signals structures, the wing plate, a type of aerodynamic damper, is adopted to control the dynamic responses due to wind-induced fatigue load. In this study, one-way 2D fluid-structure interaction (FSI) analysis is conducted to investigate the mitigation ability of the wing plate. Parametric study is involved to investigate the influence on the damping effect caused by the different configuration of the wing plate, as well as the wind speed. The different configurations of signals and signs applied in Maryland, USA, have been studied to find the potential aerodynamic instability due to galloping. The required length of wing plate for the 23-m mast-arm signal structure in Maryland have been analyzed. The study shows that small space between the wing plate and the arm will reduce the mitigation ability. Wing plate may lose mitigation ability due to stalling. Large vortex shedding is observed in the FSI analysis. The wing plate with dimension 610 mm × 610 mm applied by PennDOT can effectively prevent galloping for signal pole structures in Maryland. This research can be used as a reference for the selection of mitigation devices to satisfy the “effective mitigation devices” stated by AASHTO.

A numerical simulation method in time domain to study wind-induced excitation of traffic signal structures and its mitigation

Traffic signal structures, commonly used to control traffic, have exhibited fatigue failure in their connections due to wind-induced vibration. This study focuses on developing a generalized method to predict the coupled across- and along-wind responses of a traffic signal light structure in normal or yawed wind by combining the aerodynamic properties of such a multi-component structure with a numerical simulation technique to generate wind loads and resulting responses in the time domain. The methodology and data presented here can be applied to any signal structure to explore its dynamic behavior, estimate its fatigue life and devise mitigation means such as dampers. The tip response of the mast arm and the maximum stress at its joint were calculated for a given signal light structure and validated with field measurements to demonstrate the methodology for a range of wind speeds and yaw angles. Results indicate that yawed wind below a speed of 9 m/s from the backside of the signal light and vortex shedding of the mast arm are the primary reasons of large-amplitude vibration, and the maximum stress exceeds the endurance limit that can cause fatigue failure over time. A modified signal light design to mitigate this problem is proposed.

Damage avoidance solution to mitigate wind-induced fatigue in steel traffic support structures

Non-redundant, cantilevered traffic signal support structures undergo frequent wind-induced excitation; the subsequent vibrations result in stress reversals that lead to fatigue, and possibly fracture, particularly at the welded connections. To accommodate more lanes and reduce roadside hazards, spans continue to increase—as do the number of connection failures. Presented herein is a low-cost damage avoidance approach to mitigate wind-induced fatigue effects for cantilevered traffic support structures. Load-balancing is provided to relieve the dead load tensile stresses in the tube-to-transverse plate connections, thereby increasing fatigue capacity. The proposed damage avoidance system adds a beneficial fail-safe, load-balancing redundancy for cantilevered traffic signal structures. The benefit of the proposed system is quantified using a probabilistic fatigue assessment framework. Full-scale prototype testing is conducted in an ambient wind environment to serve as input and statistically describe response. Fatigue performance is modeled as mean stress dependent from which a dependable service life is derived. The efficacy of the proposed damage avoidance technique is assessed for a variety of wind environments where it is shown the dependable service life increases by an order of magnitude.

Analysis of Free Pendulum Vibration Absorber Using Flexible Multi-Body Dynamics

Structures which are commonly used in our infrastructures are becoming lighter with progress in material science. These structures due to their light weight and low stiffness have shown potential problem of wind-induced vibrations, a direct outcome of which is fatigue failure. In particular, if the structure is long and flexible, failure by fatigue will be inevitable if not designed properly. The main objective of this paper is to perform theoretical analysis for a novel free pendulum device as a passive vibration absorber. In this paper, the beam-tip mass-free pendulum structure is treated as a flexible multibody dynamic system and the ANCF formulation is used to demonstrate the coupled nonlinear dynamics of a large deflection of a beam with an appendage consisting of a mass-ball system. It is also aimed at showing the complete energy transfer between two modes occurring when the beam frequency is twice the ball frequency, which is known as autoparametric vibration absorption. Results are discussed and compared with findings of MSC ADAMS. This novel free pendulum device is practical and feasible passive vibration absorber in the mitigation of large amplitude wind-induced vibrations in traffic signal structures.

Fragility analysis of wind-excited traffic signal structures

Traffic signal structures are prone to fatigue failure due to wind-induced excitation that is often difficult to characterize due to the variable response arising from a combinations of along- and across-wind effects and the uncertainties associated with the fatigue and fracture phenomena. A probabilistic framework is proposed to predict structure component fatigue life. To solve the problem of wind-induced fatigue, the model is introduced to inter-relate wind hazard to structural response and hence fatigue damage. Stress range demand variabilities are considered separately from the randomness associated with the fatigue capacity. To characterize the natural wind response of a representative structure, full-scale structural monitoring is conducted, and the data were incorporated into an example application of the framework. To successfully demonstrate the efficacy of the proposed framework, resulting wind-induced fatigue life distributions are compared against compiled inspection records for a large traffic signal structure population in Wyoming.

Full-scale investigation of wind-induced vibrations of a mast-arm traffic signal structure

In previous model- and full-scale studies, high-amplitude vertical vibrations of mast-arm traffic signal structures have been shown to be due to vortex shedding, a phenomenon in which alternatingly shed, low-pressure vortices induce oscillating forces onto the mast-arm causing a cross-wind response. When the frequency of vortices being shed from the mast-arm corresponds to the natural frequency of the structure, a resonant condition is created causing long-lasting, high-amplitude vibrations which may lead to the fatigue failure of these structures. Turbulence in the approach flow is known to affect the cohesiveness of vortex shedding. Results from this full-scale investigation indicate that the surrounding terrain conditions, which affect the turbulence intensity of the wind, greatly influence the likelihood of occurrence of long-lasting, high-amplitude vibrations and also impact whether reduced service life due to fatigue is likely to be of concern.

Wind-induced traffic signal structure response: Experiments and reduction via helical arm strakes

Traffic signal structures are prone to fatigue failures due to a combination of along- and across-wind dynamic response. Stress reversal magnitudes with respect to wind speed and direction are experimentally investigated in a natural wind environment for a typical cantilevered mast arm traffic signal structure. Results show a prevalence of in-plane response induced stresses due to vortex shedding for wind speeds in the range of 4–6m/s. To better study the role of the tapered arm in the wind-induced response, the mast arm is retrofitted with helical strakes. Although the diameter of the mast arm is not directly related to the adverse stress reversals observed, arm strake installation significantly reduces the across-wind response attributed to vortex shedding. Results indicate that the slender mast arm section is critical in flow separation and vortex formation on the back side of the signal cluster backplates. As a result of the findings herein, properly designed helical arm strakes can be considered for reducing vibrations that can be attributed to vortex shedding, thus have the potential to reduce fatigue-induced damage in critical wind environments.

Full-scale experiments of cantilever traffic signal structures

Wind-induced vibrations of mast arms of cantilever traffic signal structures can lead to fatigue failure. Two such structures were instrumented each with a sonic anemometer and a camera that records the motions of the tip of the arm. It was observed throughout this experiment that large amplitude vertical vibrations of mast arms with signals with backplates occur for the most part at low wind speed ranges, between 2 to 7 m/s, and as the wind speed increases the amplitude of the vertical vibrations decreases. The results of these experiments contradict the generally accepted belief that vortex shedding does not cause significant vibrations of mast arms that could lead to fatigue failure, which have been attributed to galloping in the past. Two damping devices were tested with mixed results.

Wind tunnel studies of cantilever traffic signal structures

The wind-induced vibrations of the mast arm of cantilever traffic signal structures can lead to the fatigue failure of these structures. Wind tunnel tests were conducted on an aeroelastic model of this type of structure. Results of these experiments indicated that when the signals have backplates, vortex shedding causes large-amplitude vibrations that could lead to fatigue failure. Vibrations caused by galloping were only observed for one particular angle of attack with the signals having backplates. No evidence for galloping, previously thought to be the dominant cause of fatigue failures in these structures, was observed.

Performance-based response evaluation of reinforced concrete columns subject to vehicle impact

Structures are often subject to vehicle collision. A collision can be accidental, in the case of a vehicle going astray, or intentional, as in a terrorist attack. Bridge columns, building columns, traffic signal structures, and electric poles are the most vulnerable structural members to vehicle collisions. Columns are typically constructed from reinforced concrete (RC); therefore design and protection of RC columns against vehicle impact is an important consideration.The current analysis methods and experimental procedures to estimate the capacity of and demand on RC columns do not capture the complex mechanism of an impact event. Current code specifications have only provisions for collapse prevention and do not take into account different performance levels. The categorization of the different damage states and defining appropriate performance levels during various vehicle impacts is important to minimize damage and provide an economical design. The estimation of the dynamic shear force capacity and demand at different performance levels becomes key factor for design and protection of the structure.This research aims to develop a framework for estimation of the dynamic shear force capacity of and demand on an RC column subject to vehicle impact for different performance levels. The performance levels are related with the different impact levels of the vehicle for design. The proposed methodology is an improvement over the existing static or quasi-static analysis to the dynamic analysis which is a more realistic representation of the vehicle impact with structures. The proposed procedure can be used for the design of RC columns to minimize damage and meet a set of performance objectives during different vehicle impact scenarios. The current work can be extended to estimate the capacity of and demand on other members such as prestressed columns, steel columns, and beams and other hazards such as high velocity impacts due to blasts or missiles impact.

Fatigue Testing of Traffic Signal Structures Using an Eccentric-Mass Oscillator

Abstract Traffic signal and other ancillary structures have failed due to fatigue caused by wind. Such structures are typically lightly damped and have low natural frequencies. The fatigue cracks begin in the base metal near welds. Because of geometric and functional requirements, welds are often category E or lower. Additionally, transportation agencies use unique connection details for a variety of reasons including aesthetics and economy. Fatigue resistance is difficult to categorize because of the uniqueness of the geometries, member sizes, and manufacturing processes. A low-cost performance based procedure is needed in order to test joints as manufactured. Such a procedure would permit the many tests required to develop reliable fatigue performance of a connection. This research focuses on the development of an economical fatigue test procedure for mast arm connections and base plate connections. A full-scale cantilevered traffic signal structure was fitted with an eccentric-mass oscillator, and the oscillator was used to excite the structure near its natural frequency. Different stress ranges were achieved by controlling either the frequency of the motor or the mass-eccentricity of the oscillator in a reliable and repeatable manner. The observations indicate that near-constant amplitude and direction control are possible for in-plane, out-of-plane, and bi-directional load effects. The proposed test is conducted at a fraction of the cost of traditional actuator-controlled methods.

Development of hurricane resistant traffic signal structures

Performance of traffic signal support systems during hurricanes has indicated that the current dual cable system used in Florida often experiences substantial damage. Because there is widespread use of single cable support systems in hurricane-prone regions of other states, this project investigated the performance of both dual cable and single cable support systems in high velocity winds. The weight, orientation, and type of signal support hardware were modified to show the effects of various signal configurations. Test results indicate that (1) the difference in signal rotation is insignificant between dual cable and single cable systems, (2) the tension in the catenary cable does not significantly increase for either system; however, in the dual cable system, the increase in tensile force is substantial in the messenger cable resulting in increased flexural loading in the pole support structure, and (3) the cable supported traffic signals should be designed using both the drag and lift coefficients rather than just the drag coefficient to account for signal rotation. The single cable configuration was found to be the preferred option for structural response to wind.

Fatigue Performance of Groove-Welded Tube-to-End-Plate Connections in Highway Sign, Luminaire, and Traffic Signal Structures

Fatigue performance of full penetration groove-welded tube-to-end-plate connections was evaluated by parametric studies verified by laboratory testing of full-scale galvanized cantilevered highway sign and signal and high-level luminaire structures. In the absence of sufficient experimental basis, the fatigue resistance of this connection detail in the existing AASHTO specification is defined as Category E. Fatigue performance of tube-to-end-plate connections in thin-walled tubular structures is governed by secondary out-of-plane bending stress, the magnitude of which depends on relative stiffness of the tube and end plate. In some groove-welded connections, the backing ring is welded to the tube wall, adding more variability in fatigue performance of the connection. Twenty-three full-size specimens composed of five configurations of the groove-welded connections were fatigue-tested under constant amplitude loading simulating wind-induced aeroelastic oscillations, such as galloping and vortex shedding, that are typically experienced in service. Variation in six parameters of the connection geometry were considered: diameter of tube, thickness of tube, thickness of end plate, diameter of end plate opening, number of anchor bolts, and height of backing ring. The research results demonstrated that with a suitable combination of the geometric parameters, the constant amplitude fatigue threshold of the groove-welded tube-to-end-plate connections can exceed that of AASHTO Fatigue Category D for sections widely used in highway sign, luminaire, and traffic signal structures. The local stress-based approaches provided reasonable lower bound estimates of fatigue performance of the connection detail for different geometric configurations.

Damping in Cantilevered Traffic Signal Structures under Forced Vibration

The dynamic characteristics of a cantilevered traffic signal structure were determined experimentally and theoretically. The experimental procedures involved forced vibration of the structure by an eccentric-mass oscillator. Theoretical characteristics were determined using computer modeling. Several devices were developed to provide additional damping. The response spectra of the retrofitted structure were compared to the response spectrum of the non-retrofitted structure. Theoretical response curves were developed and compared to the experimental results. The most viable devices included a dual-strut system equipped with shock absorbers, and a tuned-mass impact damper. In the former, the damping was increased by an average factor of 6.3 in the in-plane direction, and 1.8 in the out-of-plane direction when considering the response at the box connection. In the latter, the most promising orientation yielded a 3.5 and 2.4 fold increase in damping in the in-plane and out-of-plane directions, respectively.

Damping of Cantilevered Traffic Signal Structures

Fatigue failure of cantilever (mast arm) supports for traffic signs and signals due to wind-induced vibrations has occurred in several states. A preferred method of reducing and perhaps eliminating these failures is to decrease the number and magnitude of load cycles by installing a damping device. Several types of damping devices were developed and tested on a mast arm at the University of Florida Structures Laboratory and field mast arm structures in Gainesville and Tampa, Florida. Both free-vibration and forced-vibration techniques were used to evaluate the responses of the structures. In addition, response under actual wind conditions was determined for the preferred damping device on the Tampa installation. Damping devices tested included tuned mass dampers, liquid dampers, friction dampers, damping devices installed at the arm-pole connection, and impact dampers. The preferred device was a tapered impact damper that functions in both the horizontal and vertical directions. This device was selected b...