Research On The Force Mechanism And Design Method Of The Prestressed Fish-belly Beam Composite Steel Bracing

Foundation pit engineering has always occupied an important position.With the complication of foundation pits and the strict restriction of surrounding environment,the traditional foundation pit support systems are increasingly difficult to meet the increasingly stringent requirements.In respond to the environmental policies and the weakening of impact on the surrounding environment,Innovative Prestressed Support System(IPS)was introduced from abroad as a new type of foundation pit supporting.However,there are few relevant researches at home and abroad.And there are still some key technical problems that have not been solved in the practical projects.This paper intended to establish a three-dimensional finite element model of the IPS and verify the reliability of the three-dimensional simulation results.On this basis,the sensitivity analysis of the IPS and the abilities of the IPS to resist the risk of continuous collapse were explored by selecting different parameters.The specific research content is as follows:First,most of the domestic and foreign studies are based on plane analysis and two-dimensional modeling.But the two-dimensional model is difficult to reflect the changes of displacement along the depth direction and is hard to simulate the multichannel support construction condition.Therefore,this paper prepared to establish a three-dimensional model of the IPS by using Midas software and compared the simulation results with experimental data.It showed that the three-dimensional whole model results was relatively close to the measured data and the trends of displacement and internal force were consistent with the working mechanism of the IPS.It proved the rationality and reliability of the three-dimensional model of the IPS which can be used for further analysis below.Secondly,the three-dimensional numerical simulation was carried out for a proposed construction project that adopted the IPS as the retaining scheme of foundation pit.And it was found that the results all met the requirements of monitoring and related specifications.Taking the above numerical model as a control group,sensitivity analysis of the IPS was carried out about the soil parameters,the presence of groundwater and the magnitude of prestress.The following conclusions were obtained: a).pile displacement and surface settlement decreased with the increase of elastic modulus,internal friction angle and cohesion and that increased with the increase of the gravity.b).The uplift decreased with the increase of elastic modulus and increased with the increase of gravity.The influence of internal friction angle and cohesion on uplift was negligible.c).Groundwater had a remarkable influence on pile displacement,surface settlement and bottom uplift and the difference could be twice or more.d).For every 10% increase of prestress,the displacement decreased by 1?3mm,and the influence of prestress on the displacement is limited to the depth range above the excavation surface.Then according to the above finite element analysis results,some design suggestions are provided for the IPS.Finally,redundancy is introduced as the basis for evaluating the ability to resist continuous collapse of the IPS.Partial failure simulation analysis was carried out through the "dismantle components method".The failure of the lower component of the IPS caused a greater increase in the deformation and internal force of the foundation pit than the failure of the upper component.It means small redundancy system was poor to resist the risk of continuous collapse.And the failure of bracing was more likely and harmful to cause the continuous collapse than the failure of the strut and steel wires.