In bridge engineering with prestressed precast concrete beams there are three major forms of constructions. I’ll show you what they are. First one with shallow, when you have a requirement to have a shallow construction depth, you use infill decks. They would look like something like this. So you have these prestressed precast units and then you have this in-situ concrete as the infill. One of the major governing points is the construction depth because they’re quite shallow and one other thing is when you want to avoid permanent formwork, you will see what that means in the next two forms of construction. When you want to avoid permanent formwork then you could use this form of construction but this would ideally or economically work with shorter spans. You cannot really use with longer span or even medium. Anything more than 15 – 17m because you’re adding quite a lot of additional self-weight with the infill rather than using a slab on top of the beams. And the second major form would be precast beams with slab on top. It would look something like this. as you can see here when you compare this one the beams are bit deeper but the slab on top of bems, they’re not as big as this one. It’s quite thin. This sort of arrangement, we use typically in medium span bridges. Because you can get rid of the infill in between the prestressed beams like in the first form construction. However one important thing is you need do ideally use permanent formwork to support your slab on top of these beims. The first form of construction you don’t need to use permanent formwork, you use a filler in between these beams and It’s not necessarily… when you want to minimise the amount of inspection required underneath the slab you would use the first form of construction. And when you want to go for longer span you would use a voided slab. It would look like look like something like this. Slab on top. Again this would be permanent formwork. You would use this form of construction for longer span because you can get rid of lot of lot of self-weight and of course you can see when you compare this form construction with the first two, the beams look quite deep. Again this form of construction is good for longer span. However you need to have access to carry out inspections. Not necessarily this area, this particular area. Because most of the it isn’t accessible.