Part of Lecture 18

The homework question:

Question

ISS

Answer excerpts

The first structure is a simply supported bridge with the greatest moment occuring between the two supports. To compensate for this moment, the depth of the truss is greatest at these center points.	Very good analysis of the first truss.
The bridge that is thinner at the center works similar to a combination of a suspension bridge and an arch in which case the load is distributed fromand by the supports. It is less massive at the center because the support system is doing most of the work and it is better to have less weight in the center to support. (note that the very center segment of the second bridge is very similar to one segment of the first and they act in much the same manner at this point.)	"Combination of suspension bridge and arch" gets close to the heart of the matter, although there are differences. Very good observation about the center portion of the second bridge being similar to the first bridge.
The truss on the left is deepest at center span because a moment diagram of the structure would show that the maximum moment is at center span. Making the truss deeper there helps to resist the moment at that point.The truss shown on the right is shallower at mid-span because in this structure, a moment diagram would show that the maximum moment occurs at the supports of this bridge, and so more material needs to be at the supports in order to resist the moment there.	This does a good job of relating the form to the moment diagram, and is completely correct, although it fails to explain why the moment diagrams are different for the two cases.
In both examples the vertical supports are at the nodes where there is the littlest amount of "truss material".	This reflects a major misreading of the second bridge: the masonry tower does not support the large truss.
I don't think this is right but its all i could really think of...	Fair enough.

Question: The images below show a truss supporting a long escalator; the image on the left is an overall view and the image on the right is a close up looking from underneath. Note that the top chord of the truss is a double member, while the bottom chord is a single member. Write a paragraph formulating a hypothesis to explain this difference.

Answer excerpts

The top chord is wider because it is in compression and the thickness helps it resist buckling. The bottom chord is in tension, which is why the extra thickness is not needed.	This clearly addresses the basic issue, although there is more to read from the picture.
Since the two chords along the top are side by side designers probably included the left one to deter buckling along the strong axis of the top chord. Since weak-axis buckling would be braced by the web members of the truss, the second top chord prevents the buckling from moving to the strong axis.	This gets into more detail, explaining not only why the chord has more material, but also why it is arranged as it is.
Since the truss is diagonal, and most of the weight of the escalator is resting on the top of the truss, there is more force on the top truss. Also the top truss might have to be longer to reach what it is resting on, or it may help connect the truss to the wall.	This answer is exploring several possibilities, but not evaluating them very thoroughly. Many people said that the top chord had more force because that is where the load is applied. Statics reveals that this makes very little difference; moving the loads from the top to the bottom chord will have very small local effects, the global patterns and magnitudes of forces will not change.
The thicker double member on the top servers as added width helping to prevent objects or kids from falling or jumping over the edge of the escalator. The double member also gives extra support to resist compression on the top cord of the beam.	Great for Hot Wheels...