Engineering Design: Truss Bridge Questions

[WatermelonPig]

Hey.

I am currently in an engineering class designing a truss bridge and had a couple questions. (Note all truss members are considered to be two-force members)

1) Suppose the truss is symmetric and a force is applied through the middle of it. Is it normal in industry for any individual member to have an internal force greater than this external force? Would such a case be considered a design flaw?

2) How necesary is it really to remove ZFMs? (Again, in industry)

 

[Studiot]

These are strange questions.

Firstly you should realize that in the real world self weight imposes a significant load on a structure. In the case of bridges, which you mention, the self weight plus the dead load is usually far greater than the live loads, which vary.

Secondly real world structures are not loaded at one point. Most carry a distributed load of some sort and special arangements are made to transfer these to joints in trussed structures. For instance for roof trusses, distributed loads such as snow loads and roof tile weights are arranged to bear on special members, known as purlins.

Particularly these days, roof trusses are made (many) of exceedingly slender floppy members to save material.
Such trusses often have redundant members for transport and assembly bracing. Further bracing is added to the roof as a whole to lock it into a stable three dimensional structure.

go well

 

[AlephZero]

1) Suppose the truss is symmetric and a force is applied through the middle of it. Is it normal in industry for any individual member to have an internal force greater than this external force? Would such a case be considered a design flaw?

Yes it is perfectly normal. It is only a design flaw if the internal forces are greater than the load carrying capacity of the member!

2) How necesary is it really to remove ZFMs? (Again, in industry)

This situation doesn't really apply to "real world" structures, where there are many different load cases to be considered. For instance a truss bridge would have to withstand its own weight, loads from snow and ice, sideways loads from wind, dynamics loads from earthquakes, etc, as well as the loads from things crossing the bridge.

It is very unlikely that any member would have zero force in every load case. If it did, then you would have to find another reason why it was included in the design!

 

[WatermelonPig]

I suppose it is true that "ZFMs" actually support the weight of the other members. And that's good to know that an individual member could have such a high tension without failure. It's just a bit counterintuitive, that's all. Thank you for the replies.