Analysis of Footbridge Comfort Vibrations
(2023) In TVSM-5000 VSMM01 20222Structural Mechanics
Department of Construction Sciences
- Abstract
- The aim of this master's thesis was to investigate and increase the knowledge of how different parameters affect the dynamic properties of steel beam bridges used for pedestrian traffic.
Steel footbridges are commonly used due to their high strength and low weight. Due to the lower static loads caused by pedestrians, the dynamic loads are usually decisive. For lightweight bridges with a low ratio of stiffness to mass, the natural frequencies can coincide with the walking frequency of pedestrians. The dynamic properties of a bridge are usually more difficult to determine at an early design phase than for the static case. To be on the safe side, this generally results in a bridge with a low utilization ratio, and oversized dimensions.... (More) - The aim of this master's thesis was to investigate and increase the knowledge of how different parameters affect the dynamic properties of steel beam bridges used for pedestrian traffic.
Steel footbridges are commonly used due to their high strength and low weight. Due to the lower static loads caused by pedestrians, the dynamic loads are usually decisive. For lightweight bridges with a low ratio of stiffness to mass, the natural frequencies can coincide with the walking frequency of pedestrians. The dynamic properties of a bridge are usually more difficult to determine at an early design phase than for the static case. To be on the safe side, this generally results in a bridge with a low utilization ratio, and oversized dimensions. This comes at a great cost, both environmentally and financially.
There are different guides and methods on determining the dynamic properties of a bridge. The one used in this thesis, the Sétra technical guide on footbridges, is commonly used in the industry. It is based on different bridge classes depending on crowd density and use. Depending on the natural frequency of the bridge, different dynamic load cases are achieved.
This guide was used to evaluate a large number of footbridges for their dynamic properties. The different bridges varied both in geometry, stiffness, and mass. This was to see if there were any thresholds for where the dynamic criteria could be met reliably, and to compare the dynamic and static criteria. The analysis was achieved by writing and using a Python script to create and analyze each numerical model in the finite element analysis tool Abaqus. This allowed for a study of the different parameters.
The analysis concluded that the choice of bridge class had the individually largest impact on the results according to the Sétra guide. It also concluded that for bridges with a relatively small bearing area of less than 140 square meters, it is not possible to achieve a high utilization ratio. The natural frequencies had to be close to threshold values for the acceleration criteria to be met. (Less) - Abstract (Swedish)
- Syftet med detta examensarbete var att undersöka och öka kunskapen om hur olika parametrar påverkar de dynamiska egenskaperna hos balkbroar i stål som används för gångtrafik.
Stålbroar används ofta på grund av sin höga hållfasthet och låga vikt. På grund av de lägre statiska lasterna som orsakas av fotgängare är de dynamiska lasterna vanligtvis dimensionerande. För lätta broar med ett lågt förhållande mellan styvhet och massa kan de naturliga frekvenserna sammanfalla med fotgängares gångfrekvens. De dynamiska egenskaperna hos en bro är vanligtvis svårare att fastställa i projekteringsfasen än för det statiska fallet. För att vara på den säkra sidan resulterar detta väldigt ofta i en bro med mycket låg utnyttjandegrad, och för stora... (More) - Syftet med detta examensarbete var att undersöka och öka kunskapen om hur olika parametrar påverkar de dynamiska egenskaperna hos balkbroar i stål som används för gångtrafik.
Stålbroar används ofta på grund av sin höga hållfasthet och låga vikt. På grund av de lägre statiska lasterna som orsakas av fotgängare är de dynamiska lasterna vanligtvis dimensionerande. För lätta broar med ett lågt förhållande mellan styvhet och massa kan de naturliga frekvenserna sammanfalla med fotgängares gångfrekvens. De dynamiska egenskaperna hos en bro är vanligtvis svårare att fastställa i projekteringsfasen än för det statiska fallet. För att vara på den säkra sidan resulterar detta väldigt ofta i en bro med mycket låg utnyttjandegrad, och för stora dimensioner. Detta resulterar i en stor kostnad, både miljömässigt och ekonomiskt.
Det finns olika guider och metoder för att bestämma en bros dynamiska egenskaper. Den som används i detta examensarbete, Sétras tekniska guide “Footbridges - Assessment of vibrational behaviour of footbridges
under pedestrian loading” används ofta i branschen. Den är baserad på olika broklasser beroende på fotgängardensitet och användning. Beroende på brons egenfrekvens uppnås olika dynamiska lastfall.
Denna guide användes för att utvärdera ett stort antal gångbroar för deras dynamiska egenskaper. De olika broarna varierade både i geometri, styvhet och massa. Detta var för att se om det fanns några trösklar för var de dynamiska kriterierna kunde uppfyllas på ett tillförlitligt sätt, och för att jämföra de dynamiska och statiska kriterierna. Analysen åstadkoms genom att skriva och använda ett Python-skript för att skapa och analysera varje numerisk modell i datorverktyget Abaqus med finita element. Detta gjorde det möjligt att studera de olika parametrarna.
Analysen kom fram till att valet av broklass hade den individuellt största inverkan på resultaten enligt Sétra-guiden. Slutsatsen kunde dras att för broar med en relativt liten användingsarea på mindre än 140 kvadratmeter är det inte möjligt att uppnå en hög utnyttjandegrad. De naturliga frekvenserna måste ligga nära tröskelvärdena för att accelerationskriterierna skulle kunna uppfyllas. (Less) - Popular Abstract
- Footbridges are usually statically designed to be slender, lightweight, and flexible structures. This means however that when pedestrians use the bridge, their footsteps may induce resonance. If the bridge is not designed for these phenomena in early stages, costly measures must be used afterwards to secure the criteria of pedestrian comfort. Knowing in the project stage what bridges are sensitive to vibrations will make early adjustments possible. This way material inefficiency can be reduced, which is not only cost efficient, but also makes bridge construction more sustainable.
Please use this url to cite or link to this publication:
http://lup.lub.lu.se/student-papers/record/9112276
- author
- Thuresson, Niklas LU and Ruuth, Henrik LU
- supervisor
- organization
- alternative title
- Analys av komfortvibrationer i gång- och cykelbroar
- course
- VSMM01 20222
- year
- 2023
- type
- H3 - Professional qualifications (4 Years - )
- subject
- keywords
- footbridges, dynamics, vibrations, accelerations, Sétra, comfort vibrations, natural frequencies
- publication/series
- TVSM-5000
- report number
- TVSM-5262
- ISSN
- 0281-6679
- language
- English
- id
- 9112276
- alternative location
- http://www.byggmek.lth.se/english/publications/tvsm-5000-present-2014/
- date added to LUP
- 2023-05-03 09:29:40
- date last changed
- 2023-05-03 09:29:40
@misc{9112276,
abstract = {{The aim of this master's thesis was to investigate and increase the knowledge of how different parameters affect the dynamic properties of steel beam bridges used for pedestrian traffic.
Steel footbridges are commonly used due to their high strength and low weight. Due to the lower static loads caused by pedestrians, the dynamic loads are usually decisive. For lightweight bridges with a low ratio of stiffness to mass, the natural frequencies can coincide with the walking frequency of pedestrians. The dynamic properties of a bridge are usually more difficult to determine at an early design phase than for the static case. To be on the safe side, this generally results in a bridge with a low utilization ratio, and oversized dimensions. This comes at a great cost, both environmentally and financially.
There are different guides and methods on determining the dynamic properties of a bridge. The one used in this thesis, the Sétra technical guide on footbridges, is commonly used in the industry. It is based on different bridge classes depending on crowd density and use. Depending on the natural frequency of the bridge, different dynamic load cases are achieved.
This guide was used to evaluate a large number of footbridges for their dynamic properties. The different bridges varied both in geometry, stiffness, and mass. This was to see if there were any thresholds for where the dynamic criteria could be met reliably, and to compare the dynamic and static criteria. The analysis was achieved by writing and using a Python script to create and analyze each numerical model in the finite element analysis tool Abaqus. This allowed for a study of the different parameters.
The analysis concluded that the choice of bridge class had the individually largest impact on the results according to the Sétra guide. It also concluded that for bridges with a relatively small bearing area of less than 140 square meters, it is not possible to achieve a high utilization ratio. The natural frequencies had to be close to threshold values for the acceleration criteria to be met.}},
author = {{Thuresson, Niklas and Ruuth, Henrik}},
issn = {{0281-6679}},
language = {{eng}},
note = {{Student Paper}},
series = {{TVSM-5000}},
title = {{Analysis of Footbridge Comfort Vibrations}},
year = {{2023}},
}