AASHTO LRFD Construction Stages
In AASHTO-LRFD design of prestressed precast girders, the construction stages need to be analyzed to monitor overstresses during production. Overstresses due to tension will result in cracking of concrete and overstresses due to compression will result in cracking or crushing of concrete. Observation of cracks along the girder typically results in rejection of girder. Construction steps are explained in detail in the following slides.
The main tasks involved in the production of prestressed precast girders including shipping and handling has been investigated by designers. Any problem in one of the steps usually ends in rejection of the production.
Precast prestressed girder production starts with preparing the formwork and assembling the reinforcement cage. Reinforcement cage is typically assembled close to the formwork.
Reinforcement cage has been positioned to its location by use of overhead cranes. The formworks have been jacked to their true coordinates to give the right geometry. The strands will be aligned to their design path. The strands will be jacked to the proper tension forces from the anchor ends of the casting bed. Each casting bed may accommodate five girders.
Concrete will be placed after stabilizing the formworks at their position. The concrete needs to be properly vibrated. To speed up the production time, concrete needs to be cured with steam.
Formworks will be removed once the concrete reaches to the target compressive strength to transfer prestressing force. The strands will be cut to provide compressive stress into girders as required by AASHTO-LRFD design. The girder will be stored before shipping to the site. At this stage, stresses of girder need to be checked for concrete with early strength properties.
Girder camber needs to be checked prior to placing the girder at the site. Each girder will be shipped by trucks. Before positioning the girder to its position, the bearings are usually placed to its position. If there is a big difference, an investigation can be made.
The girders can be placed either by cranes or launching system. Launching system is usually used for bridges with more than five spans with high piers.
Loading and unloading of girders can be made by use of overhead cranes of the launching system. Cranes will pick up the girder after launching system is moved over the span. Girder will be moved longitudinally within the launching system to their position. Once it has been positioned, the girders will be lowered to touch down on bearings.
In multi-girder system, deck concrete will be placed after all girders are positioned. The camber on girders needs to be monitored at this time of instant. The deck concrete will be cast as planned. At this stage, we need to check the stresses on the girder. At service stage, we will have the super imposed dead load and live load acting on the composite section. Stresses during service life need to be checked considering the long-term effects.
The sections that need to be analyzed for stresses are summarized. Girder section needs to be used in stress-check computations at the transfer of prestressing steel where concrete has an early strength of about 80% of 28-day strength. At the time of deck casting, the stresses again need to be checked on girder section due to additional loads. At service case, the composite section will be used in stress computations.