Sunday, July 18, 2010
Whenever I see an artist’s conception of something futuristic I always have to muse at the attributes they give the materials of the future. While up at the cabin, I stumbled on this old magazine. Note the total lack of support for the track of the main vehicle. (I won’t even get into the giant ULTra vehicles)
While this is common practice, it’s pretty tough to compete with designs made out of super alloys straight out of science fiction. In the case of PRT advocacy, the main job is to sell the concept of a whole new infrastructure, overlaid on the existing one. This is obviously much easier to do if it is minimalized. On the other hand, there is also a credibility issue here. If the system you are sold on bears no resemblance to what you are really going to get, how can any further claims by a PRT advocate (or vendor) be trusted?
I have given a lot of thought to PRT trusses, and I just want to make an observation. I have never seen any system meant to carry people that is more than about 35 times as long as it is high. Box beams, I beams, trusses, whatever. They rarely approach this mark and are usually much less. There is some pretty good reading on the subject in the Wikipedia entries on beams, bridges and trusses.
The bottom line is that PRT needs to have as skinny a support structure as possible, much more so than any other application I can think of. In other situations, generally, something needs to be built and the buyer has little choice but to take the recommendations of the architect or engineer. Here they can just decide not to involve themselves with PRT in the first place.
Next time you pull down your wooden attic stairs or have a wooden stepladder handy, note how they reinforce the steps. You will see that they have a thin metal rod beneath each step that can be tightened, squeezing the step end-to-end. The step then becomes a compression member resting on a tension member, resulting in a very strong “beam”. Tension members (cable) are also employed to great effect on pre and post-tensioned concrete in a very similar way. Below are some drawings of cables that are integrated into a traditional truss. I am quite confident that this type of addition would increase span substantially, beating that 35 to 1 ratio.
Fig. 1 shows geometry similar to a suspension bridge. Support points can be had by periodic attachment along the length of a stretched cable. In a Suspension bridge secondary cables can hang straight down to support the bridge decking. Suspension bridges must have the ends of the cables be anchored to the ground, however, although multiple spans can attach together instead. Pulling the cable tighter makes the decking arch upward. Figs. 2 and 3 show how cable could be stretched within a truss and terminated in a spool, which could be tightened. Tightening both spools equally (with a BIG torque wrench) would be essential, or the support would be pulled over. Straight runs would need to terminate by ground anchoring, just like a suspension bridge. The cable could also be continuous. (Between ground anchors) Fig. 4 shows how cable-stayed bridges differ from suspension bridges. Instead stretching a cable from two ground anchors and hanging a bridge off of the cable, the cable-stayed design balances cantilevered loads on a support column. At these slight angles there would obviously be a lot of compression put on the truss itself, pushing it toward the support posts. What is interesting to me though, can be seen in FIG. 5. Note that the truss also becomes a tension member, because the cables work to pull the structure in opposite directions in the middle of the span. This gives it characteristics similar to the continuous cable in Fig 3.
Finally, to really confuse the reader, figure 6 is a full crossbreed. Depending on how the cables are tensioned, this can be either a suspension or a cable-stayed design. Confused? I know I am. But I have used similar techniques to make impossibly long and thin unsupported shelves with great success, and even took the sag out of a roofline once. I know it would work to some degree. Also, never underestimate the wisdom and practicality of the farmer. They use cable to keep stuff from sagging all the time, like this irrigation system.
One final photo. It is nearly impossible to down a single telephone pole, since they are all cabled together at the top. I have seen them broken in half by trucks, and the snapped pole just hangs there. Such a quality would seem to be ideal for PRT from a safety point of view. By the way, cable is, relatively speaking, dirt-cheap. Each half-inch steel cable has a tensile strength of over 20,000 lbs. It would sure make ME feel better in an earthquake!