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St. Pancras Lighting (Advanced)
Suggested Viewing: St Pancras Lighting (Intermediate)
QUALITATIVE ANALYSIS
Dead Loads (Permanent Actions):
Another way to look at the lighting system is as a catenary; a catenary is the curve that forms when a chain hangs under its own weight (i.e. a UDL). Catenaries are very important because they experience absolutely no bending moment; all the forces are perfectly transmitted in tension or compression.
The lighting in St. Pancras station is supported on a catenary, a cable that hangs under a UDL; presented by the weight of the cable and the lighting. One could argue that the load is not a UDL but it can be approximated to a UDL and safety factors added during the design process.
Since there will be no bending in most of the structure, visualizing the deflected shape might not be much help. Another approach we could take is tracing the load path throughout the structure and then analysing the forces. If we take a look at the lighting system from a side view and model it we end up with the figure below.
schematic diagram showing lighting gantry
diagram showing assumed loading
The figure above shows how we have a fixed connection at one end and a pin connection at the opposite end, the horizontal member, the cable and the little struts between them.
If we assume the dead load is a UDL, as shown above, how can we begin to analyse the structure and load paths?
The UDL is supported by the horizontal member, which is supported by the struts, the struts are supported by the catenary cable and the cable is supported by the columns at the supports. Therefore, we know that the struts between the horizontal member and the catenary are in compression and that the catenary is in tension.
The tension in the catenary will pull down on the columns at the supports putting them in tension as well; we can now put all this information into a diagram where green symbolizes tension and red compression.
The next step is to make sure all forces are resolved; by looking individually at each part of the structure. The force of the UDL is resisted by compression in the column that in turn exerts a force on the catenary.The diagonal tensile force in the catenary can be resolved into a vertical resisting the compression from the strut and a horizontal force, as shown in the diagram adjacent. The diagonal forces in the catenary all build up to one big tensile force; this force has two components. The vertical component is resisted by tension in the vertical columns at the supports, which in turn are resisted by an upward force in the roof structure. The horizontal components are resisted by forces in the horizontal member, putting it into compression; as shown below. These forces will be equal and opposite, so no external thrust will be generated.
Imposed Loads (Variable Actions):
Wind is the variable load of interest in this structure, let’s imagine it hits the structure from two sides.
One hitting the lighting along its length and another along its width. It’s apparent that the more critical one is the wind hitting the lighting along its length; as the other wind loading is resisted by the supports. The pin connection is more rigid in the dimension along the length of the lighting. Furthermore the aerodynamic shape aids in reducing the wind loading.
Considering the wind loading along the length of the lighting; we can see that the fixed connection is capable of withstanding the wind load, the problem occurs at the near connection (pin connection).
Let us model it so it is easier to analyse:
We can see that as the wind hits the side of the lighting this causes bending in the column at the far end, but the column is thick and sturdy, capable of withstanding that moment.
Furthermore, the holes between the columns in the catenary allow lower wind loading.
However at the pin connection, if we imagine the vertical strut was there alone it wouldn’t be able to withstand the moment, so the other choice is to make it a truss system and avoid any moments at all.
The diagonal strut resists any horizontal load that one may apply; it’s also important no notice that it is not made of wire. If it was made of wire then it would only be able to withstand wind from the other side (out of the page), but if it’s a solid rod then it can go into both tension and compression and resist horizontal loads from either side.
OTHER THINGS TO THINK ABOUT
Looking at the whole system from the side we can see that the pin connection isn't directly in the middle. It's closer to one side of the 'aeroplane wing' type section than the other. Why might this be?
How will that wind load get to the supports when it is applied across the entire length of the lighting?
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