Study On Robustness Of Suspended Floor System In Half-through And Through Arch Bridges

Accidents of bridge floor system collapse due to hanger fracture occur in half-through and through arch bridges from time to time.Hence,current researches have mainly focused on the design of hangers.Based on the state-of-the-art structural design concept,this thesis investigates robustness of suspended floor systems in half-through and through arch bridges for the purpose to eschew the disasters of continuous bridge collapse.Main contributions and research outcomes are as follows:(1)Based on the basic principle of structural robustness and the common design method,the robustness analysis on suspended floor systems of half-through and through arch bridges is carried out with regard to hanger fracture accidents to achieve qualitative results and general design principles for floor systems.Since cross girders in these floor systems are the main components to carry loads,they have relatively poor robustness.Hence,this study aims to develop a design calculation method for the robustness of the floor systems.(2)Experimental studies are conducted on a test model for a suspended floor system of a through arch bridge with broken hangers.Through experiments,its dynamic responses are investigated under different initial tensile forces of hangers,which are obtained with the deck of different weights,and different locations of hangers.A finite element model is established to model the floor system with its calculation results agreeing well with the testing results.From the results of testing and finite element analysis,it is indicated that(i)the initial tensile forces in hangers have effects on their fracture patterns and time to break;(ii)the less time to break the hangers increases the dynamic response of the system due to the hanger break;(iii)the system dynamic response with the fracture of longer hanger is larger than that with the fracture of shorter hanger;(iv)the most unfavorable stress sections due to the static effects produced by broken hangers includes the sections in broken hangers as well as the section of stiffening girder between two neighbouring hangers.(3)Parametric analysis using the finite element method shows that the ratio(At/T)between the time to hanger fracture △t and the fundamental period T of the remaining structure has higher impact on the dynamic response of the floor system with broken hanger.When the ratio △t/T is not larger than 1%,i.e.△t ≤ 0.01T,the dynamic magnification factor ηd tends to be constant,which is the worst case for hanger facture.Therefore,it is suggested that △t = 0.01T be taken for analysis.Based on the field measurements from 14 half-through and through arch bridges,the modal damping ratio for the first in-plane mode of the bridges is in the range of 0.025-0.060.Considering that the damping ratio can mitigate structural responses,it is recommended that the modal damper ratio ξ = 0.02 is set in the finite element dynamic analysis under the situation of hanger fracture.(4)Vulnerability analysis is also performed on the remaining structural components with the hanger fracture.Analysis results show that the sequence of possible structural failure in the remaining structure due to hanger fracture is from stiffening girders,remaining hangers to arch ribs.Accordingly,the designing of stiffening girders and remaining hangers should be emphasized in the robustness analysis for suspended floor systems in arch bridges.(5)An equivalent static calculation method taking account of dynamic effects due to hanger fracture is proposed in this study.Parametric analysis is executed on dynamic magnification factor by considering the remaining structure dynamics after hanger fracture that is affected by the time of hanger fracture and structural damping.It is indicated that the dynamic magnification factors for hangers and stiffening longitudinal girders can be set to be 2.0 and 1.6,respectively,in the equivalent static calculation.(6)Based on the elastically supported continuous beam method,a simplified equivalent static calculation model is developed for the suspended floor system with broken hangers.In addition,the parameter analysis is conducted to evaluate the influences on the developed model.It is found that the simplified equivalent static model can be built using continuous beams of 3 spans or 4 spans with elastic supports under the most unfavorable conditions,which are the fractures of the first and second outermost hangers.The flexibility coefficient of the elastic supports can be taken to be that of the corresponding hangers.(7)Based on the principle of failure-safety limit state design,analysis on the selections of values for coefficients of variation of load and material are carried out under the situation of hanger fracture.It is suggested that a combination of dead and live loads be taken as input for the robustness design of suspended floor systems,in which the partial safety factors for dead load,live load and pedestrian load are taken as yGj=1、2,γQ = 0.9 and Ψc = 0.80,respectively.The reduction coefficients for lateral vehicle load on single lane,two lanes,and three or more lanes are taken as 1.00,0.75,and 0.52,respectively.Strength of material is set to be 1.0-1.25 times of the standard value.Finally,robustness design and analysis are performed on the suspended floor system of three arch bridges by employing the developed simplified equivalent static calculation method.Moreover,an improved robustness design scheme is proposed for the existing stiffened longitudinal girders in Lanxi Bridge.