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- Nilsson, Martin, et al.
(författare)
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Partialkoefficienter för hållfasthet i betongbroar längs Malmbanan
- 1999
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Denna rapport redovisar två metoder att bestämma nyanserade värden på betonghållfasthetens partialkoefficienter för befintliga betongbroar längs Malmbanan. Rapporten presenterar därefter dimensionerande drag- och tryck-hållfasthetsvärden för sju broar mellan Luleå och Boden. Hållfasthetsvärden är baserade dels på de nya partialkoefficienterna, dels på provresultat, Thun et al (1999). Provresultaten ligger till grund för karakteristiska hållfasthetsvärden som bestämts enligt BBK94 (1995). Partialkoefficienter för hållfasthetsvärdena har bestämts med vad som i rapporten benämns tabellmetoden och sannolikhetsteoretisk metod. Tabellmetoden är hämtad från NKB (1978) och NKB (1987). Metoden bygger, som namnet indikerar, på tabeller varur fyra delfaktorer till partialkoefficienten bestäms. Faktorerna behandlar (1) osäkerheter i karakteristiska värden och i förhållandet mellan hållfasthet i konstruktion och i provkropp etc.; (2) osäkerheter i beräk-ningsmodeller; (3) brottkaraktär (segt eller sprött) och (4) omfattningen av kontroll vid uppförandet av broarna. Den sannolikhetsteoretiska metoden för att bestämma partialkoefficienter tar i sin tur också hänsyn till osäkerheter i karakteristiska värden, i förhållandet mellan hållfasthet i konstruktion och i provkropp och osäkerheter i beräkningsmodeller. Metoden tar även med partialkoefficienter för laster, vilka här har satts till normenliga värden förutom tåglastens partialkoefficient som satts till 1,1 enligt en utredning av Östlund (1997) i bilaga A och används här endast vid statistiska beräkningar och godtas därför inte av Banverket. Inverkan av tåglast och last av betong och ballast ingår. Även broarnas individuella längder beaktas då tåglasternas dynamiska förstoringsfaktorer bestäms. Resultaten av bestämning av partialkoefficienten med tabellmetoden och den sannolikhetsteoretiska metoden ger för de sju broarna mellan Luleå och Boden något skilda resultat. Tabellmetoden ger generellt högre värden än den sannolikhetsteoretiska metoden. Tabellmetodens värden varierar mellan 1,44 och 1,95 för draghållfastheten och mellan 1,44 och 1,48 för tryckhållfastheten. Värdena från den sannolikhetsteoretiska metoden ligger ca 15% lägre. Vi förordar i första hand, på grund av sin enkelhet, att tabellmetoden används vid bestämning av partialkoefficienten. Den sannolikhetsteoretiska metoden är något mer komplicerad och måste ofta kompletteras med kvalificerade bedömningar. De dimensionerande hållfasthetsvärdena som bestäms i rapporten är betydligt högre än de värden som föreskrivna hållfasthetsklasser ger. Den största inverkan på detta har de provade hållfasthetsvärdena, som ligger väsentligt över de föreskrivna värdena. Partialkoefficienternas inverkan är mindre. Rapporten avslutas med en diskussion om inverkan av hållfasthetens variation inom konstruktionsdelar, vilken kan ha stor inverkan på bärförmågan. Likaså har läget för uttagna provkroppar i förhållande till aktuellt beräkningssnitt betydelse. Båda dessa faktorer bör ingå vid en bestämning av hållfastheters partialkoefficienter i befintliga konstruktioner.
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- Ohlsson, Björn, et al.
(författare)
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Follow-up after curative surgery for colorectal carcinoma - Randomized comparison with no follow-up
- 1995
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Ingår i: Diseases of the Colon & Rectum. - 0012-3706. ; 38:6, s. 619-626
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Tidskriftsartikel (refereegranskat)abstract
- PURPOSE: This study investigated the value of intense follow-up compared with no follow-up after curative surgery of cancer in the colon or rectum. METHODS: One hundred seven patients were randomized to no follow-up (control group; n=54) or intense follow-up (follow-up group; n=53) after surgery and early postoperative colonoscopy. Patients in the follow-up group were followed at frequent intervals with clinical examination, rigid proctosigmoidoscopy, colonoscopy, computed tomography of the pelvis (in patients operated with abdominoperineal resection), pulmonary x-ray, liver function tests, and determinations of carcinoembryonic antigen and fecal hemoglobin. Follow-up ranged from 5.5 to 8.8 years after primary surgery. RESULTS: Tumor recurred in 18 patients (33 percent) in the control group and in 17 patients (32 percent) in the follow-up group. Reresection with curative intent was performed in three patients in the control group and in five patients (four of whom were asymptomatic) in the follow-up group. In the follow-up group two asymptomatic patients with elevated carcinoembryonic antigen levels were disease-free three and five and one-half years after reresection and were the only patients apparently cured by reresection. No patient underwent surgery for metastatic disease in the liver or lungs. Symptomatic metachronous carcinoma was detected in one patient (control group) after three years. Five-year survival rate was 67 percent in the control group and 75 percent in the follow-up group (P >0.05); the corresponding cancer-specific survival rates were 71 percent and 78 percent, respectively. CONCLUSION: Intense follow-up after resection of colorectal cancer did not prolong survival in this study.
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- Ohlsson, Ulf
(författare)
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Fracture mechanics analysis of concrete structures
- 1995
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis presents results from numerical and analytical analyses of concrete structures as well as results from laboratory tests. Numerical methods based on the finite element method and fracture mechanics are used. The thesis comprises five papers and a general introductory part. The five papers deal with the following topics: Material properties for concrete at low temperatures, Paper A: This paper presents results from an investigation of the fatigue strength and the fracture energy of water saturated unreinforced concrete. The tests were carried out both at room temperature and under cold conditions. The experiments show that the load bearing capacity of water saturated specimens is strongly affected of low temperatures. The water in the pores freezes and generates a load bearing ice skeleton. The compressive and splitting strength of the concrete will as a result increase. Also the fracture energy increases at low temperatures. Both the absolute and the relative fatigue strength increase for water saturated concrete. Bond properties of concrete, Paper B: The paper discusses the influence of splitting failure on the bond properties of deformed reinforcement bars. Laboratory tests with normal and high strength concrete are presented. The tests were designed to obtain a high degree of confinement with the intention to establish the upper limit of the bond strength. The bond strength has been normalized with regard to the concrete splitting strength. The high strength concrete has a higher normalized bond strength than the normal strength concrete for bar diameters of 8 and 16 mm. A theoretical model, based on fracture mechanics, is presented. The model gives relationships between the pressure generated by the mechanical interaction and the length of the splitting cracks. Modelling of concrete shaft in a gravity based offshore structure. Influence of hydraulic pressure on fracture mechanics parameters, Paper C: The paper describes an analysis carried out in order to predict the risk of concrete delamination in the shaft of the Draugen Gravity Based Structure (GBS) platform situated in the northern Atlantic. The analysis formed part of an independent verification of the platform by VERITEC in Norway and was performed using a nonlinear finite element program (DIANA) with a material model based on fracture mechanics. The analysis shows that there was no risk of progressive cracking or delamination in the shaft of the studied gravity based offshore structure. Modelling of mixed mode fracture, Paper E: This paper describes numerical modelling of fracture formation under combined tensile and shear loading. The numerical model used is based on the Inner Softening Band Approach, see Klisinski et al. (1991). The results are compared with test results presented in Hassanzadeh (1992) and Nooru-Mohamed (1992). The model reproduces crack pattern and nominal stress displacement curves. The compressive stresses due to dilation can also be modelled. The stress rotations are generally small. In the final stage of the test, the stress rotation increases in some elements, especially in the smallest elements, so that more cracks should be introduced in each element in order to avoid interlocking effects. Anchor bolts in concrete, Papers D and E: The first paper [D] presents results from anchor bolt tests, comprising both plane and axisymmetric specimens. Different design formulas are discussed. Results from finite element analyses are presented. The second paper [E] presents analyses and tests on a plane anchor bolt specimen. The analyses are based on the Inner Softening Band Approach, see Klisinski et al. (1991). The analysis reproduces deformations at maximum load and crack pattern at an early stage of the test. Peak loads are overestimated. The post peak behaviour is not properly captured due to stress interlocking even before the peak load is reached. The possibility to introduce more then one crack in an element seems to be an essential step for further development of the analysis.
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- Ohlsson, Ulf, et al.
(författare)
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Influence of hydraulic pressure in fracture mechanics modelling of crack propagation in concrete
- 1998
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Ingår i: Materials and Structures. - 1359-5997 .- 1871-6873. ; 31:3, s. 203-208
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a nonlinear fracture mechanics analysis of a part of an offshore concrete structure. The analysis focuses on the risk of cracking between the prestressing ducts in the shaft of the submerged platform. The influences of water and grout pressure in prestressing ducts have been taken into consideration. The analysis has been performed using both discrete and smeared crack analysis.
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- Ohlsson, Ulf, et al.
(författare)
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Mixed-mode fracture and anchor bolts in concrete analysis with inner softening bands
- 1997
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Ingår i: Journal of engineering mechanics. - 0733-9399 .- 1943-7889. ; 123:10, s. 1027-1033
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Tidskriftsartikel (refereegranskat)abstract
- The paper presents a nonlinear fracture mechanics analysis of mixed-mode fracture and anchor pullout in concrete. The analysis is based on elements with inner softening bands (ISB). A major advantage of the ISB approach is that discrete cracks can be introduced anywhere and in any direction within the finite-element mesh. Calculated loads, deformations, and crack patterns are compared to experimental results. A good correspondence is achieved.
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