On Camping

Some observations about tents

A recent camping trip got me thinking about how tents work; specifically my own. I think it seems obvious to most casual observers that tents are tension structures, but perhaps there is also more to it than that.

I like camping, but I am not obsessive about it. I am therefore not sure if there is a specific name for the type of tent that I own. I shall simply be referring to it as a horseshoe tent, because its cross-section is roughly that shape.

The horseshoe shape is created by an exoskeleton consisting of three arches, two larger ones at the front end and a slightly smaller one at the rear. The smaller one is inclined a little towards the rear of the tent. The rear of the tent is also the location of the sleeping compartment, which is hung between the two arches closest to the back.

The arches themselves are formed of thin plastic tubes, which slot together to form long flexible rods. These are held together by an elastic chord which passes through their center and allows them to be folded away without coming apart. Each rod is inserted into a long pocket stitched to the tent’s waterproof flysheet. Each end of the rod is then inserted over a brass pin attached to the base of the flysheet. Each pin is connected to its neighbour via a nylon strap, which sits beneath the tent’s ground sheet. In the photo below you can see them on the grass before the ground sheet has been laid. 

This is the first structural feature of the tent. The flexible arches have a natural tendency to spring apart, but the tension strap prevents them from doing so. It locks the springiness of the tent rods into place and keeps the rather thin structure rigid.

The next structural feature of the tent is its guylines, which we shall divide into two. The transverse guys i.e. those in the same plane as the arches have only one role, at least so far as I can tell. They anchor the tent to the ground, via the arches, and prevent uplift in stormy weather. I believe this to be the case, because the tent will stand happily without them in calm weather. 

The second set of guylines we shall call longitudinal guys. They are located to the front and rear of the tent. I am sure that they help anchor the tent in stormy weather too, but they also have another, perhaps more important role, as I suspect the transverse guys could do most of the wind resisting work themselves.

It seems to me that the primary role of the longitudinal guys is to stretch out the shape of the tent and to provide it with longitudinal stiffness. The more taught they are made, by adjusting the tensioners, the more rigid the tent becomes. 

It is self evident that to ensure equilibrium i.e. to stop the tent being pulled either forward or back the front and rear guylines must balance each other. What is perhaps less obvious, and is therefore more interesting, is what happens to the tension force after it leaves the guylines. Put another way, what happens between the front and rear guylines to ensure equilibrium. This is what caught my eye and is the underlying reason for this post.

While erecting my tent I had become a little frustrated that the flysheet appeared to be wrinkled and was not sitting flat and smooth as I thought that it should. Then I noticed that the wrinkles had a pattern and that pattern was structurally significant. 

I realised that in any other context an engineer would describe a structure like a tent’s flysheet as a stressed skin. Stressed skin structures resist out of plane loads, for example the wind, by being stretched tight. 

Normally a stressed skin will be fixed along the edges either with a continuous seam or with fixings at close centres. This is because you want the load to be imparted into the thin stressed skin in an even manner. This is not, however, the case with tents.

The tensile loads, which convert the flexible flysheet into a taught stressed skin structure, are imparted at discreet points by the guylines. In the case of my tent via the thin rods that form the horsehoe arches.

The slender rods have little stiffness perpendicular to the arches they form and are of little assistance in distributing the tensile load from the guylines. It is therefore no coincidence that the observed creases extend diagonally from the guyline connections on one arch to the base of the adjacent arch, where it is pegged to the ground.

On closer inspection there is a second set of creases, which start midway between the guyline connection and ground level. This happens to be the point where the rods are connected to the flysheet with plastic clips. 

The question arises, why does the fabric crease and what does it indicate? 

As the flysheet has little thickness it is not good a distributing concentrated load across its own surface. For this reason load applied at discreet points by the guylines causes localised stress in the fabric between points of restraint. Since these parts of the fabric are subject to more stress they stretch more than neighbouring fabric causing the observed creases to appear.

What this phenomena indicates is none-other than the arrangement of internal forces, one might say the load path, in the flysheet. We can visibly see that it has taken up the form of a truss with node points at the parts of the tent that are held stiff. Structures do not often reveal their load path in this way, which makes it all the more interesting.

It is also interesting to note that there are diagonal creases near the top of the tent too. These appear to be the result of the additional section of flysheet that helps ventilates the roof and provides additional stiffness.

One can also detect some creases in a rectangular section of the flysheet at mid height of the first bay. It is difficult to see in the photo, but at this location there is a clear plastic window, covered on the inside by a canvas flap that is used to blank the window for privacy. The clear plastic is a stiffer material than the flysheet around it.

Something else that interested me was why the direction of the creases was in one direction rather than the other. The answer I believe lies in the different geometries, which exist at the front and rear of the tent, and thus vary the angle at which the guylines apply load to the structure. That said, I am also curious to know whether the order in which the guylines were tightened also plays a role. Next time I pitch the tent I shall reverse the sequence and see what happens.

The final observation that ought to be made about my tent, or any tent for that matter concerns the effectiveness of tent pegs. These are short lengths of wire, which are pushed into the ground in order to provide resistance to the the tensile loads in the guylines, including those generated by the full force of the weather. This seems remarkable given their small stature. 

While the angle at which the pegs are pushed into the soil is surely important, friction between the soil and the circumference of the pegs is the primary guard against the pegs being pulled free of the ground. Thus, we can see the ability of friction to resist load is not to be underestimated. It is this same force by which many buildings are supported in soft ground on concrete piles, though in this case the piles are in compression rather than tension like tent pegs.

So it turns out that I needn't have been concerned about the creases in my tent; there doesn’t seem to be anything I could have done about them. This of course wasn’t the main reason I enjoyed my camping trip. Good humour and good company had something to do with that too, however it was quite interesting.