Table Of Contenti
Capacity(cid:2)assessment(cid:2)of(cid:2)
arch(cid:2)bridges(cid:2)with(cid:2)backfill(cid:2)
Case(cid:2)of(cid:2)the(cid:2)old(cid:2)Årsta(cid:2)railway(cid:2)bridge(cid:2)
ANDREAS(cid:2)ANDERSSON(cid:2)
(cid:2)
Doctoral(cid:2)Thesis(cid:2)in(cid:2)
Structural(cid:2)Design(cid:2)and(cid:2)Bridges(cid:2)
Stockholm,(cid:2)Sweden(cid:2)2011
(cid:2)
Capacity assessment of
arch bridges with backfill
Case of the old Årsta railway bridge
ANDREAS ANDERSSON
Doctoral Thesis
Stockholm, Sweden 2011
TRITA-BKN. Bulletin 107, 2011 KTH School of ABE
ISSN 1103-4270 SE-100 44 Stockholm
ISRN KTH/BKN/B--107--SE SWEDEN
Akademisk avhandling som med tillstånd av Kungliga Teknisk högskolan framlägges
till offentlig granskning för avläggande av teknologie doktorsexamen i Brobyggnad
fredagen den 20 maj klockan 10.00 i sal D2, Kungliga Tekniska högskolan, Lindstedts-
vägen 5, Stockholm
© Andreas Andersson, April 2011
Preface
The present thesis was prepared at the Department of Civil and Architectural
Engineering at the Royal Institute of Technology (KTH) at the Division of Structural
Design and Bridges. The project was initiated by the Swedish National Rail
Administration (former Banverket) and the research work has been carried out in
collaboration between Banverket and KTH. The field measurements presented in this
thesis was financed by Banverket and performed in collaboration between KTH and
former Carl Bro AB. The field measurements performed during 2005 were carried out
by Mr. Claes Kullberg, Mr. Stefan Trillkott and the author, at the Department of Civil
and Architectural Engineering and PhD Jonatan Paulsson-Tralla and Lic. Engr.
Rickard Johnson at former Carl Bro AB. Additional field measurements during 2007 -
2011 were performed by Mr. Claes Kullberg and the author, instrumentation and
surveillance were performed by PhD Gerard James who also performed other field
measurements on the bridge.
I wish to express my gratitude to my supervisors Professor Håkan Sundquist and
Professor Raid Karoumi for giving me the opportunity to work in this research area.
Thanks are also due to my colleagues at the Division of Structural Design and Bridges
and the Division of Concrete Structures at KTH for valuable discussions during the
research progress. I also thank Professor Johan Silfwerbrand for valuable comments
during reviewing of this thesis.
An extensive part of the presented research work has been performed during my
service at Banverket and I wish to express my gratitude to my colleagues at the Bridge
Division of former Banverket, for providing guidance and valuable discussion. Special
thanks go to my mentor M.Sc. Bo Eriksson-Vanke for his guidance and indefatigable
interest in the research project and Civil Engineering in general.
Stockholm, April 2011
Andreas Andersson
i
Abstract
The work presented in this thesis comprises the assessment of existing arch bridges
with overlying backfill. The main objective is to estimate the load carrying capacity in
ultimate limit state analysis. A case study of the old Årsta railway bridge is presented,
serving as both the initiation and a direct application of the present research. The
demand from the bridge owner is to extend the service life of the bridge by 50 years
and increase the allowable axle load from 22.5 to 25 metric tonnes. The performed
analyses show a great scatter in estimated load carrying capacity, depending on a large
number of parameters. One of the factors of main impact is the backfill material, which
may result a significant increase in load carrying capacity due to the interaction with
the arch barrel. Based on theoretical analyses, extensive conditional assessments and
the demand from the bridge owner, it was decided that the bridge needed to be
strengthened.
The author, in close collaboration with both the bridge owner and the persons
performing the conditional assessment, performed the development of a suitable
strengthening. The analyses showed a pronounced three-dimensional behaviour, calling
for a design using non-linear finite element methods. Due to demands on full
operability during strengthening, a scheme was developed to attenuate any decrease in
load carrying capacity. The strengthening was accepted by the bridge owner and is
currently under construction. It is planned to be finalised in 2012.
The application of field measurements to determine the structural manner of action
under serviceability loads are presented and have shown to be successful. Measured
strain of the arch barrel due to passing train has been performed, both before, during
and after strengthening. The results serve as input for model calibration and
verification of the developed strengthening methods.
The interaction of the backfill was not readily verified on the studied bridge and the
strengthening was based on the assumption that both the backfill and the spandrel
walls contributed as dead weight only. The finite element models are benchmarked
using available experimental results in the literature, comprising masonry arch bridges
with backfill loaded until failure. Good agreement is generally found if accounting for
full interaction with the backfill. Similarly, accounting for the backfill as dead weight
only, often results in a decrease in load carrying capacity by a factor 2 to 3. Still,
several factors show a high impact on the estimated load carrying capacity, of which
many are difficult to accurately assess. This suggests a conservative approach,
although partial interaction of the backfill may still increase the load carrying capacity
significantly.
Keywords: Concrete arch bridge, soil backfill, spandrel walls, field measurements,
finite element method, load distribution, ultimate limit state.
iii
Sammanfattning
Arbetet i föreliggande avhandling omfattar analyser av befintliga bågbroar med
ovanliggande fyllning. Huvudsyftet är att uppskatta bärförmågan i brottgränstillstånd.
En fallstudie av gamla Årstabron redovisas, vilken utgör både initieringen och en
direkt tillämpning av föreliggande forskning. Kravet från broförvaltaren är att öka
brons livslängd med 50 år, samtidigt som axellasten ska ökas från nuvarande 22.5 ton
till 25 ton. Utförda analyser visar på stor spridning i uppskattad bärförmåga, beroende
på ett stort antal parametrar. En av de främsta faktorerna är fyllningens egenskaper,
vilken kan resultera i en markant ökning av bärförmågan p.g.a. samverkan med bågen.
Baserat på teoretiska analyser, tillståndsbedömningar och krav från broförvaltaren
beslutades att bron skulle förstärkas.
En förstärkningsmetod har utvecklats i nära samarbete med broförvaltaren och
personer som tidigare utfört tillståndsbedömningarna. Analyserna visar ett utpräglat
tredimensionellt beteende, vilket har föranlett användandet av icke-linjära finita
elementmetoder. Krav på full trafik under samtliga förstärkningsarbeten har resulterat
i att dessa utförs enligt en föreskriven ordning, som ska reducera minskning i bär-
förmåga under samtliga etapper. Förstärkningsförslaget godkändes av Banverket och
är för närvarande under byggnation. Enligt plan ska dessa slutföras under 2012.
Fältmätningar har använts för att bestämma det statiska verkningssättet under
brukslaster, vilket visas ge goda resultat. Resulterande töjningar från passerande tåg
har uppmäts i bågen, både före, under och efter förstärkning. Resultaten har använts
både för att kalibrera beräkningsmodeller och att verifiera utförda förstärkningar.
Samverkan mellan båge och fyllning har inte kunnat verifierats för den aktuella bron
och de utvecklade förstärkningarna baseras på en modell där både fyllning och
sidomurar endast utgör yttre verkande last. De framtagna finita element modellerna
har jämförts med experimentella resultat från litteraturen, omfattande tegelvalvsbroar
med ovanliggande fyllning belastade till brott. Generellt erhålls god överensstämmelse
om full samverkan mellan båge och fyllning antas. Om fyllningen istället endast
betraktas som yttre last, minskar lastkapaciteten ofta med en faktor 2 till 3.
Fortfarande uppvisar ett antal faktorer stor inverkan på bärförmågan, vilka ofta är
svåra att med säkerhet bestämma. Ett konservativt betraktningssätt rekommenderas,
även om delvis samverkan med fyllningen fortfarande kan öka bärförmågan avsevärt.
Nyckelord: Betongbågbro, jordfyllning, sidomur, fältmätningar, finita element
metoder, lastspridning, brottgränstillstånd.
v
Description:The present thesis was prepared at the Department of Civil and Architectural. Engineering at the Royal Institute of Technology (KTH) at the Division of Structural. Design and Bridges. The project was initiated by the Swedish National Rail. Administration (former Banverket) and the research work has
