Cable-Stayed bridge (Beginner)
The bridge deck is made of timber, and the frame from aluminium, so the solid parts of the structure are quite flexible. The cables are high strength steel, so you should be able to guess already some things about where the load’s going.The great thing about this bridge is that you can play around with the connections and loads, and get quite different behaviour. I’d strongly advise going to see it for yourself, walking along the deck and seeing how it behaves.
APPLIED LOADS
As with any structure we need to consider the effect of self-weight, but with this bridge the lightweight materials used should minimise this.
The most significant loads are going to be people walking across it. It’s important to be aware that with this bridge there will be cases where adding weight in certain places is actually going to improve the structural stability. We call these ‘favourable loads’, meaning they’re good for the structure, but not necessarily for the designer!
There could also be dynamic effects caused by exuberant students or lecturers jumping up and down on the bridge. This will mean designing for larger, but very short term ‘shock’ loads.
CONCEPTUAL/QUALITATIVE BEHAVIOUR
Supports:
In fact for this bridge there are two support cases. The central bracket which the two longest cables attach to, can be replaced by two separate brackets. What this does is to turn the bridge deck from a continuous beam into two cantilevers.
Removing the central connection obviously makes the bridge less stable, so why do it? Couldn’t we just design the bridge to have a continuous deck and be done with it?
It would be great to be able to do this. Unfortunately, for life-sized, 1000m long bridges you can’t get trucks big enough to carry them ready made, so they have to be constructed in-situ.
Cantilevering a structure out from the bank of a river may seem an inefficient way to go about building, but there’s really no better method, so each half-span has to be self-supporting.
DESCRIPTION AND LOCATION
In this tutorial we’re going to look at a creation of the University’s very own Department of Civil and Structural Engineering. Our Cable-Stayed Bridge model.
You may have seen this structure in the structure’s lab or the Engineering workrooms.
Now, when modelling supports to analyse, we have to do a bit of mental gymnastics. You might notice that although the supports at the abutments are supposedly rigid on the diagram, our Cable-Stayed Bridge is actually just placed on the floor. There’s technically no connection at all.
This is because we’re designing for failure. To make an efficient structure, the rule is to work out what will just about fail, and then pick something slightly stronger (with a factor of safety of course).
So we model them as encastres. Then we can analyse the rest of the bridge with the assumption that the supports won’t move or rotate. We don’t actually know if they will or not yet, but we can forget about that until we’ve found out the loads on them, and can design them properly.
Imposed Loads (Variable Actions):
Let’s look at half the bridge by itself. Imagine standing on the end. The cables will carry your weight by going into tension, and this force will be transferred through to the cables on the opposite side of the piers. The piers themselves will have to go into compression to stop the cables sagging, and finally the cables will transfer the tension to the abutments. Seems pretty simple. There’s a problem here though. If we stop for a second and take moments around the piers there’s something funny going on.
taking moments about the pier to find the direction of the reaction forces
We’ve only got the external force from your weight acting the bridge, so to balance the moment the supports at the abutments will need to pull down, not push up. This phenomenon of supports being pulled upwards like this is called ‘uplift’.Basically, the bridge is behaving just like a seesaw. You’re standing on one end and there needs to be a force on the other end so it doesn’t overbalance. But there’s no connection to the ground at the base, nothing to provide a downwards reaction force like the one we need. The solution with a bridge is the same as the one for a seesaw: we add weight. For a full-scale bridge we would use the weight of the soil and concrete foundations. For this model we use metal bars.
There’s half a ton of weight that can be added to both ends in total. How many people do you think that would support standing at the mid-span of the bridge?
OTHER THINGS TO THINK ABOUT
What is going to happen to the bridge deck? If you look at the connections between the deck and the cables and try to resolve the forces at these points you should be able to work it out. Another method is to actually see the model for yourself.