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Tesco Canopy (Beginner)
DESCRIPTION AND LOCATION
This canopy, made with circular hollow steel sections can be found in the Abbeydale Road Tesco car park, just outside the main entrance (postcode S7 2QB).
It’s a very interesting design because on first glance there seems to be a very large overhang. How can such a slender structure be supporting the canopy?
APPLIED LOADS
In most cases the loads on an everyday structure are easy to think of. You have the self-weight of the frame. There may be some uplift caused by pockets of wind collecting under the canopy. There will also be horizontal wind loads on the structure, and occasionally it might have to deal with snow on the roof, or other temporary loads of that nature.
However, the most significant design load case will almost certainly be a car or other vehicle crashing into one of the columns. Imagine the column buckling and the whole structure crumpling on top of a car!
As designers it is important to recognise all potential failure modes/scenarios and consider them in the design process. Take some time to look at this frame, and how the engineer may have thought about this load case in their design.
CONCEPTUAL/QUALITATIVE BEHAVIOUR
Supports:
The first and most obvious thing to start with is the base supports. Hopefully we can all see that they must be rigid connections, otherwise the canopy would just keel over.
Less obvious are the supports at the horizontal and diagonal struts. Are they pins, or rigid connections?
There are two ways of looking at this.
The first is to say: how could the support act? If we removed the diagonal strut, would the frame immediately collapse? Well, there’s a haunch there, and some thick steel sections, so I think that connection probably could support the weight, at leasttemporarily. But we can also say: how does this support actually act? In some ways this is more useful, as the path a force takes is dependent on the stiffness of the members it is travelling through, the stiffer the path, the more force.
In this case, the struts can carry the external force axially (with tension and compression) so there’s no need to have rigid connections. The fact that the two members connect to the column at different positions effectively creates a couple, which is a (very efficient) way of resisting bending moments. They will probably act as pins.
Dead Loads (Permanent Actions)
Let’s look at how the self-weight is carried to the ground.
Support with haunch
The weight of the roof is transmitted to the structure at its corners as point loads. These forces need to be carried through the structure to the ground.
The left hand force must be resisted by tension in the diagonal strut. We know this because it is the only member at that joint that can resist a vertical force. Tension and compression can’t go round right angles (since cos90°=0).
So we have tension in the diagonal which balances the vertical forces. The horizontal strut also needs to go into compression to balance the horizontal part of the tension force in the diagonal strut.
When these forces get to the column the vertical parts of the force will compress the column, but the horizontal components (circled in yellow) will have to be resisted in bending. Again, the axial forces can’t turn through right angles.
If we set up a simplified system and look at the bending of the column drawing the bending moment diagram is fairly straightforward.
bending increases from the top of the column as the lever arm of the blue force increases. After this the effect of the two forces cancel each other out and the moment remains constant
As we move away from the top of the column the moment will increase linearly due to the blue force generated from tension in the diagonal strut. When we get to the green force the moment will become constant. We’re back to the situation of a ‘couple’, which will generate no net moment.
From that point the moment will remain the same all the way down to the support.
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