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Tuesday, April 1, 2008

Wooden Beams for Tents

Note: This is adapted from a post to NORSEFOLK.

When planning the materials for a period tent, either A frame or Geteld type, consider something about the structural nature of wood as it applies to the loads imposed by tents. Then work backwards against availability and cost (rather than the other way round).

Now several of my old friends will be rolling their eyes. Repeating my own mantra back to me : " Metal good - wood EVIL " !

The thing to consider is grain in the wood - and how grain effects strength.
You are applying two load types to the wood used in a tent. One is compression - the weight of the tent itself applied straight down the long axis of the beam. This aspect effects the uprights, especially the vertical pieces on a Geteld style. The second load is a sideways thrust, contributed by both tent and the force of wind against the horizontal cross beam.

Against these forces, consider the direction of the grain in the timber. Also the relative density of the wood, and how fine the grain is. Individual species will also differ in their basic flexibility (I suspect largely determined by the relationship of density and grain structure - but ask a wood guy.) Fast growing soft woods ( southern pine) are going to have the weakest grain structure. Slow growing hard woods (northern oak) are going to have the strongest. Certain species (as noted by others) will combine strength and flexibility - ash for example.

Modern timber is almost without exception cut to maximize production. This most often results in individual pieces having diagonal lines of the grain that run against the line of the beam: ///////// : Imagine applying force to either the end or the side of a stack of these diagonal lines...
On the compression members, there will be less effect (leading to failure). Practical experiences (related by many here) show that even smaller sizes of cheaper, lighter materials are less likely to fail. You can likely get away with 2 x 2 pine for the uprights.
The greatest effect will be seen on the horizontal beam. The sideways stress from wind can easily shear sawn lumber. We all have seen the warping effect that even the weight of a tent cover can exert on sawn planks.

If you could fix a plank used for the horizontal beam so that it rested with the largest dimension at 90 degrees to the ground : l : this would put the greatest mass of wood against the line of greatest stress. (It would not help with the sideways stress of wind load however). In practice, this would require the use of some kind of quite strong mortise and tendon arrangement where the upright joined to the horizontal. The tenancy is for the tall thin cross section to try to 'roll over', so there is a great sideways force applied to the pin used to join the two members.

Historically, timber was not sawn - but split. This results in the grain running in straight lines along the length of the timber, so this diagonal weakness in the grain is not present. Think of the thin split timbers used for ship construction.

The other way to avoid the whole problem is to use whole saplings. This gives the gain an uninterupted set of circluar 'tubes' that provide the maximum strength for the size and weight of the material. This is by far the superior solution to the problem of stress loads.

Getting the required saplings, for the longer horizontal beam especially, may prove a bit of a problem for urban dwellers. I'd suggest contacting your rural living friends. My own A frame uses a spruce sapling cut from about a 16 foot tree down to the required 12 foot length. It runs about 4 inches to about 2 inches. This tent is over 15 years old, and has weathered repeated violent storms (including one legendary one at L'Anse aux Meadows in 96). The main overhead beam still remains solid and straight.
In practical terms, you want to look for a small grove of trees growing too close together, This tends to force them upwards - rather than spreading outwards. The trees towards the centre are likely to have few sideways branches as well. (Imagine a single pine cone hitting fertile soil.)

One other important effect of the use of natural saplings is that you are also duplicating the type of wood used by the Norse. No cut 2 x 2 in the Viking Age. We have replaced all our commercial uprights with saplings for just that reason.

There are a set of simple plans for a classic Norse A frame tent available on the Norse Encampment series:

oh - one piece of advice:
Strip off all the bark. With the increasing contamination from alien insects, many regions do not allow transport of any timber with the bark on. You can NOT transport bark on timber across the US / Canada boarder for example. Easy to do with the wood green.


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