The dog days are behind us, and as summer turns into autumn, for many summer is really turning into football season. For those not as familiar with the pigskin, college football has a longstanding tradition of rowdy students, after being thrown into more of a frenzy immediately following a big win, storming the field in victory, and oftentimes tearing down the upright field goal posts at either end of the field. In the (highly) unlikely event my hometown Gophers upset TCU this week, I’d love to do the same.
But the practice can often be dangerous - even fatal - and many football stadiums, like the Gophers’, are going to collapsible posts that can be taken down by a small crew within a minute of a game’s end. Lots of stadiums have at least semi-permanent post installs, however, so this blog will look at a few static simulations to see what kind of stresses these posts are put under, highlighting the different Simulation element types along the way.
There are umpteen vendors out there with slight variations on their field goal post designs and specs, but a pretty standard post design centers around a 90° angled "gooseneck" tube. This gooseneck then supports three slightly smaller aluminum tubes that make up the frame for field goal attempts. Why use steel in the gooseneck? Manufacturers recognized this as the area of highest stress in the post assembly, and so that is where we’ll focus our analysis.
I designed my post assembly as a weldment part, using 6" OD schedule-40 galvanized steel for the gooseneck, 6” 1060 aluminum for the crossbar, and 4” aluminum for the uprights. Because this was a weldment, SOLIDWORKS Simulation automatically created beam elements out of the bodies, which we will bond throughout our analyses. For our first pass, we’ll just apply a gravity load (remember: crawl-walk-run!).
Easy enough! Probing around the result, however, we see a fairly even distribution of the maximum stress in the vertical portion of the steel beam, something we may not expect. In order to dive further into the point where yielding would occur, we need to apply solid or shell elements. A common path many Simulation users will go down is to try to use solid elements, regardless of the model geometry being analyzed. This is one such situation where solid elements are not appropriate, and a quick run with a default solid mesh shows why.
Our max stress went down significantly, along with our result accuracy, as we’ve broken one of the cardinal rules of solid elements: having at least two elements across any body’s thickness. Beefing up the mesh to achieve this, we now have more than four million elements to be analyzed – obviously not an ideal way to run the study…
Instead I used shells, as we have a thin-walled tube well suited for that element type. An initial run turned up results similar to our beam study, but refining the mesh makes clear the singularities on the edge bonds of the gooseneck body.
So what do we do with our gooseneck boundaries? In actuality, there is no true “bond” that can be made on either end of the tube – rather, some connecting hardware or mounting apparatus would be used. The installation manual from this manufacturer shows a few of the most common mounting methods: straight into poured concrete, using a sleeve buried in concrete, and using a base plate with connecting hardware.
It is at this interface that goal posts failures are seen before any yielding occurs in the gooseneck, and for good reason: if a crowd mobs a post, a fracture occurring about six feet high might do even more damage than it otherwise could. If the ground mount fails, however, the failure location is at least known and people can brace themselves for it.
An assembly accounting for these extra parts would need to be modeled to fully examine failures. Then, the loading for people hanging on the crossbar and on the ground shaking the gooseneck (as a worst case) would provide meaningful results. That said, first finding out what element types should be used can save work down the road and give confidence as these pieces of the problem are brought in.
SKI-U-MAH!