I have developed a system for bending wood for my arch bridges, and though I do not claim it is the best way, it works well. It does require some materials and equipment that hopefully you will already own and not need to get.
I have a pot, or basin rather, that is about 18″ long and 12″ deep. I fill it with about two cups of water (I have found that more water just takes longer to start boiling, and I usually don’t boil all of it anyway) and stick it on a stove burner. Currently I am using a stove that has a glass top and the burners are underneath, but in the past I have used one that had burners above the top of the stove.
I place any wood I am steaming on top of the basin, I usually have at least one or two pieces that are longer then the basin and some that are shorter, but the shorter ones lay on the longer pieces so that nothing is actually touching the water at this time, for the goal was to steam and not soak the wood. I turn the burner on high, and wait for the water to start boiling. I also place a shallow cookie sheet on top, to hold in the steam.
I have never steamed my wood for over a half hour, perhaps I am too impatient. But with this amount of time the wood is very bendable.
Beforehand, I have prepared two “molds” that consist of two sheets of wood, with a form of nails hammered into each, to mold the steamed wood. I have taken to making the mold a half inch or so taller then what I want the actual height of my arch, because the wood always bends a little less then the form afterward. You may want to use more or less extra height depending on the overall height of your arch.
This form is fairly easy to make, I just set the height and length of what I want with nails, and then bend a stick of wood (usually 1/16″ by 3/16″) and hold it in place by clamping the wood to the nails with clothespins. I then add as many additional nails to the form as I see fit, along the arch of the wood.
After steaming the wood I clamp the pieces to the form and wait another thirty minutes to an hour for it to dry. Again I may be impatient, but it works for me. I then apply glue to the wood, and wait 4-5 hours for it to set (that is the amount of time my glue takes to set). I then do a little sanding, and finish building my bridge. So far, I have built an 8.5 gram bridge (as of Jan/19/04) that held 16k and broke. However, the testing apparatus was not ideal, in the fact that I forgot to bring an S hook to the place I was testing, and had to make do with something else, which I believe may have unevenly applied the load to my bridge. But then again, that may not have been the case.
I usually make my arches into the shape of an L beam, but have used a T beam in the past. I have never used an I beam, though that would most definitely be stronger but heavier. It is worth experimenting with.