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Hvor meget støjer dilatationsfuger?

Biler, der kører over broer, skaber ekstra støj, når de passerer dilatationsfugerne. For bilisten er det hurtigt overstået, men mange naboer er generet af impulslydene. Vejdirektoratet og vores akustikere har vurderet støjgenen for naboerne ved Gudenåbroen. Resultaterne viser, der er grund til at tage denne støj alvorligt.

Af Per Finne

De fleste kender til en biltur over en motorvejsbro. Der kommer nogle bump, hver gang hjulene passerer én af broens fuger. Som bilist tænker man nok ikke så meget over lyden, men naboerne vil ofte blive berørt af de mange tusinde impulslyde døgnet rundt. Det kan være særligt generende, og der er ikke grundlag og tradition for at tage hensyn til det, når man måler eller beregner vejstøjen.

Støjens særlige karakter

Når bilens hjul passerer dilatationsfugen, opstår en kortvarig ”klik-” eller ”klonk-lyd” oven i den almindelige støj fra bilerne på vejen. Denne ”klik-lyd” kaldes i støjterminologien en impuls. Den er kortvarig og udbreder sig væk fra vejen sammen med den almindelige brusende motorvejsstøj. Hvorfor og hvordan den opstår, er ikke helt indlysende, men i alle tilfælde er menneskers oplevede genevirkning af støjimpulser større, end den er for almindelig motorvejsstøj.

Ved andre former for støj – fx industristøj – bliver virksomheden ”straffet” med 5 dB, hvis støjen indeholder tydeligt hørbare impulser ved naboerne. Derfor er det standard-procedure at undersøge støjens karakter ved naboerne, når man skal støjkortlægge en virksomhed.

Dilatationsfuger er uundgåelige, når man bygger broer, for broen skal kunne give efter uden at ødelægge vejen og konstruktionen (se figur 1).

Figur 1. Dilatationsfuger. Fugerne er bevægelige, så betonbroen har plads til at udvide sig og eller trække sig sammen, uden at der dannes sprækker i vejen. Fugerne er lavet som store metalfingre, der glider ind og ud mellem hinanden. Kilde: Vejdirektoratets pjece: ”Broer og tunneler”.

Vejdirektoratet modtager jævnligt klager over vejstøj – også over støjen fra motorvejsbroer, hvor særligt impulser fra dilatationsfuger kan være til gene for naboerne. Senest har der efter renovering af Gudenåbroen ved Randers (E45) været klaget over, at der er opstået nye impulslyde (beskrives som ”klonk”- og ”klik”-lyde) i området. Nogle af dem, der klager, bor op mod 1.000 meter fra broen.

Målinger omkring Gudenåbroen

På baggrund af de gentagne nabohenvendelser besluttede Vejdirektoratet at undersøge nabostøjen i området omkring broen for tydeligheden af de impulser, der opstår ved kørsel over dilatationsfugerne. Der blev udført målinger og lydoptagelser ved en række nabopositioner vest for broen en dag med svag (op til 5 m/s) sydøstlig vind. Det vil sige, at vinden bar støjen fra motorvejen hen mod målestederne. Det er standardprocedure ved måling af støj. Støjen blev målt flere steder bl.a. tæt ved vejen og på 450 meters afstand.

Resultaterne viser en rimelig god sammenhæng mellem impulsernes tydelighed og afstanden fra kontrolpositionen ved broen. Jo større afstand fra broen, desto mindre er impulsernes tydelighed.

Subjektiv vurdering

Udover de objektive analyser af impulsernes tydelighed blev impulserne også bedømt subjektivt ved afspilning af lydoptagelserne via hovedtelefoner. Denne nyttige, men ikke særligt videnskabelige bedømmelse viste, at der er god overensstemmelse mellem den subjektive bedømmelse og de objektive analyser af tydelig hørbarhed.

Hvad har vi lært ved Gudenåbroen

Det er nyt, at Vejdirektoratet har afprøvet en metode til at vurdere støjgener fra kørsel over dilatationsfuger på en motorvejsbro. Resultaterne viste, at der er grund til at tage støjen fra dilatationsfuger alvorligt, særligt for brostykker tæt ved boligområder.

Ved Gudenåbroen vil man forsøge at justere asfaltbelægningen, så der er mindre niveauforskel mellem asfaltslidlaget og dilatationsfugerne. Det er håbet, at det kan nedsætte generne fra impulsstøjen.

Denne artikel er et uddrag fra artikel bragt i TRAFIK & VEJE, august 2017, side 62-62. Læs hele artiklen, hvor metoden til at vurdere støjgenen ved naboerne fra kørsel over dilatationsfuger på Gudenåbroen beskrives.

Mere info

For yderligere information om projektet og om vejstøjsberegning kontakt Per Finne, Senior Specialist, DELTA Akustik, pfi@delta.dk, tlf. 72 19 46 28.


ENGLISH VERSION

Are expansion joints noisy?

When cars drive over the expansion joints on bridges they generate additional impulsive noise to the ordinary vehicle noise. This noise is quickly forgotten for the car drivers, but for those living nearby the bridge, impulsive noise can be very annoying. The Danish Road Directorate, together with our acoustic specialists, have assessed the noise annoyance for neighbours of the Gudenå road bridge. The results show that there is good reason to take this additional noise seriously.

By Per Finne

Most of us are familiar with driving over a motorway bridge. You feel a small bump each time the wheels move over one of the bridge’s joints. You don’t tend to think much about the sound when you’re inside a car, but neighbours to the bridge will often be affected by the many thousands of impulsive sounds they are subjected to 24 hours a day. It can be annoying, yet there is no tradition or basis for taking this into account when measuring or calculating road traffic noise.

The special character of noise

When the wheels of a vehicle pass over an expansion joint, a brief “click” or “clonk” sound is emitted, in addition to the usual vehicle noise on the road. In noise terminology, this “click” sound is called an impulse. It is brief and spreads from the road together with the ordinary motorway noise. It is not entirely clear where or how it occurs, but regardless, people appear to be more annoyed by impulsive noise than they are by ordinary motorway noise.

For other types of noise – e.g. industrial noise – companies are “punished” at 5 dB if the noise includes impulses which are clearly audible (prominent) to their neighbours. It is therefore standard procedure to study the character of the noise as it sounds to those living in the neighbourhood when measuring noise from an industry.

Expansion joints are unavoidable when constructing road bridges, as the bridge must be able to contract or expand without destroying the road or its own construction (see figure 1).

Figure 1. Expansion joints. The joints are movable, so the concrete bridge has room to expand or contract without causing cracks in the road. The joints are made like large metal fingers which glide in and out between each other. Source: Danish Road Directorate pamphlet: “Bridges and tunnels”.

The Danish Road Directorate often receives complaints about road traffic noise, as well as complaints about noise from motorway bridges, where impulses from expansion joints can be particular annoying to neighbours. After a recent renovation of the Gudenå Bridge near Randers (part of the E45 motorway), there have been complaints that new impulsive sounds have been generated (described as “clonk” and “click” sounds) in the area. Some of those making the complaints live up to 1,000 metres away from the bridge.

Measurements around the Gudenå Bridge

Based on the repeated complaints from neighbours, the Danish Road Directorate decided to study the noise in the neighbouring area to the bridge for audibility of the impulses that are generated by vehicles driving over the expansion joints. Measurements and sound recordings were made at neighbouring positions west of the bridge on a day with a weak (up to 5 m/s) southeasterly wind, i.e. in a downwind situation where the wind carried the noise from the motorway against the measurement locations. This is standard procedure for noise measurements. The noise was measured in several places, including close to the road bridge, and at a distance of 450 metres.

The results show a reasonably good correlation between the audibility of the impulses and the distance from the bridge. The impulses become less audible with increasing distance from the bridge.

Subjective assessment

Besides the objective analyses of the audibility of the impulses, they were also assessed subjectively by playing back sound recordings on headphones. This useful, but not fully scientific assessment showed that there is a high level of correlation between the subjective assessment and the objective analyses of impulse audibility.

What we have learned from the Gudenå Bridge

It is a new initiative for the Danish Road Directorate to test out a method of assessing impulse noise annoyance from driving over expansion joints on a motorway bridge. The results show that there are grounds for taking the impulsive noise from expansion joints seriously, especially on bridges close to residential areas.

Attempts will now be made on the Gudenå Bridge to adjust the asphalt surface so that there is less difference in gradient between the asphalt wearing course and the expansion joints. It is hoped that this will reduce the problem of impulsive noise generation.

This article is an extract from an article published in TRAFIK & VEJE, August 2017, page 62-62 (in Danish). Read the entire article, where the method for assessing the impulsive noise annoyance for neighbours from driving over expansion joints on the Gudenå Bridge is described.

More info

For more information about the project and calculating road traffic noise contact Per Finne, DELTA Acoustics, pfi@delta.dk, tel. +45 72 19 46 28.

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