Can someone please explain what a A-Frame girder is?
Can you give a context? What are you referring to?
There were simple A-frame bridges used in high watershed areas, where timber was planted along each bank of the water course and tipped to an A-frame. This kept the pylons out of where the water would run strongly after a rain.
Is this what you are asking?
The sketch at lower right, minus the X brace, is an A frame Bridge.
That appears to be the same thing as a kingpost bridge. Are there differences between the kingpost and the A frame?
Enjoy
Paul
I found a place on the net where thay said that the D&RGW hanging bridge is a A-frame bridge. That made me curious. What do you say about that, true or not?
Here is a simple version of the A-Frame
I would say it would be closer to what Jeffers posted although I am not familiar with the bridge you mention. The a-frame I posted is used in quick and dirty logging and mining opreations. D&RGW was a class 1 and would have put more into the construction.
The A-frames that support the hanging bridges are actually more like what Spacemouse offered. They are steel girders that bridge the very narrow (10 meters, approximately) bottom of the Royal Gorge at right angles to both water flow and rails, meeting at a peak where the suspender for the outer (canyon center) edge of the bridge is attached. At the time they were built that was the only practical way to put a track through that part of the gorge, since putting anything in the river (fast all the time, and subject to sudden increases of depth and speed of flow after every rainstorm) was, and still is, impossible. The canyon walls are solid stone and close to vertical for over 100 meters above track level.
If the same line was being built today it would be located about 200 meters to the north, in a circular (cross section) tunnel about a kilometer long drilled by a tunnel boring machine.
Chuck
Since I haven’t seen a pic of the RG bridge lately, I can’t answer the direct question, but there is a difference between a simple arch and an A frame.
In a simple arch, an A frame without the crossbar, lateral thrusts are handled by the abutments, whatever the diagonal beams rest against.
In an A frame, the cross bar goes into tension to resist the lateral thrusts, and the structure is inherently stable without assistance from how and where it is sited. It can theoretically be dropped anywhere and still carry its rated load.
Well, not in this case. “Kingpost” is a word that be used in a lot of ways. The most common I’ve heard is that the vertical at the center of the truss is referred to as the kingpost, meaning that any triangular truss, even one with multiple panels similar to the sketch above, would still have a kingpost and therefore possibly be a kingpost bridge. By one strict definition of the word, a multi panel truss could have several kingposts, though some are shorter and one is tallest.
This is going to vary from region to region, and there may even be a registered design somewhere whose inventor or originator designated a kingpost bridge. Even so, the word means different things in different places. I’ve seen it used to refer to the longer member in a wood framed doorway, the stud which supports a cripple that in turn supports one end of the header. I’ve also heard that assembly called a “jack”, a “cripple”, a “cripplejack”, a “trimmer”, etc. in addition to “kingpost”.
An ‘A’ frame girder, as opposed to a bridge, is a girder whose cross section resembles an ‘A’. It is intended to increase the load carrying capacity without a corresponding increase in weight. A lot of new synthetic materials have extremely high tensile strengths, so the utilization of composite girders is increasing.