Behaviour of prestressed steel girder bridges

Prestressing was one of the recent technical advantages in construction anddesign of reinforced concrete and steel structures. Prestressing reinforced concretewas mainly required to overcome the tensile stresses imposed due to loading.Prestressing steel structures aimed to counteract the stresses caused by external loadsso that the final steel sections become more economic. Prestressing steel structuresmay be applied by tensioning tendons anchored to girder ends or by bending of girdersin a reverse direction to that resulting from the applied loads and welding highstrength cover plates. In this research the second technique is studied.The research is divided into three main parts. The first part is concerned withthe effect of different characteristic cross section parameters on the final prestressedsection using non-dimensional parameters. Non-composite as well composite (shoredand unshored type) sections were studied. The different geometrical relationships aswell as the governing equations were derived for the non-dimensional parameters andthese parameters were changed to study their effect on the equivalent allowablestresses and on the design of non-composite and composite sections. The effect ofprestressing cost was handled approximately to clarify the effect of the cost on theprofit of prestressing. A computer program was developed to facilitate the design ofprestressed as well non-prestressed sections. The program can extract the values ofdifferent cross section parameters that yield the minimum depth and weight.Prestressing was found to be an effective means of increasing the beam allowablestress and reducing the required cross section dimensions and weight.The second part is aimed to study the behavior of prestressed members underloading and comparing it with its conventional counterpart of non-prestressedmembers. Another goal was the investigation of this technique in strengthening ofexisting structures. The experimental work includes testing of six specimensrepresenting composite and non-composite sections. It was found that providingprestressing by using this technique will enlarge the elastic range and can effectivelybe used as a strengthening technique.The third part is aimed to study and evaluate the real profit of prestressing in thereduction of steel weight for bridges taking into account the different aspects ofbridge design i.e. web buckling, deflection limitation and fatigue requirements.Therefore 42 bridge models were chosen including simple and continuous two andthree spans bridges, composite (shored, unshored) and non-composite bridges. Thesebridges were designed for minimum weight and analyzed under truck loading usinggrid analysis. It was found that prestressing using the suggested technique reduces theweight considerably even when taking the different bridge design aspects intoconsideration.