Steel Bridge Truss

[brokenpile]

I have a plan showing a pedestrian steel truss bridge showing a bottom chord of 8"x 8" steel tube.Specifications read "The steel truss shall be designed to include a redundant bottom chord comprised of double channels"
Can anyone tell me whats that means since the drawing doesn't depict what the specifications are asking for?Or am i missing something here?

 

[ishvaaag]

A bad project or contract in that. Anyway usually the written documentation use to oblige to expose any findings on inconsistences in the project prior to contract, to resolve about before undersigning, and as well provisions as to the authority to decide what to build when contradiction appears.

Usual defence of contractors about these issues is to firmly state that they only are compromised to build what they offered, in a list of works made by themselves. On no agreement, you know the path. And not always the changes to the project made in such lists are innocent, or based on project error.

So seek who has to decide, who can support your point, and state it firmly before all the involved.

 

[rb1957]

won't it mean welding two C-channels together to fabricate a tube section ? granted to should say a lot more (unless it's obvious how to ... which i guess it isn't !?)

 

[hokie66]

You would have to ask the designer what he means.

 

[XRS]

The redundant requirement seems to an effort to avoid having a "Fracture Critical" bridge. Theoretically, to do that the double channels would need to be separate elements of the member such that a crack could not propagate through both elements of the member. That is to say they would have to bolted together instead of welded together. This is called internal redundancy.

According to one interpretation of the internal redundancy concept you would have to have three or more separate elements in each bottom chord member to avoid having a fracture critical bridge. According to another even more strict interpretation (promoted by the FHWA) you should not consider internal redundancy regardless of how many separate elements the member has when deciding whether a bridge is fracture critical. The FHWA also denies the validity of structural redundancy (span continuity) and only wants State DOTs to consider load path redundancy when classifying highway bridges as fracture critical.

The requirement for double channels on the bottom chords seems a bit arbitrary and unusual as well as unnecessary for a pedestrian bridge since by their nature pedestrian bridge do not receive the kind of significant fatigue loading that would lead to fatigue fractures. Moreover, most of the ususal steel truss pedestrian bridges are made with HSS sections for the chords and or web members as you described. For some unexplained reason almost all of these bridges are manufactured by one of several companies in Minnesota that specialize in steel truss pedestrian bridges. This type of pedestrian bridge is considered attractive and is used all over the country. I inspect bridges and have seen hundreds of these mostly in city parks, hike and bike trails, and golf courses.

 

[brokenpile]

Thank you for your comments which we agree with,from what i found out the design Engineer drew the plans for a pedestrian bridge and then the County put the specifications together using their boiler plate highway bridge specification.The pedestrian bridge is being made from a company that specializes in these bridges and is trying to resolve the issue with the County by attaching a channel to the truss member so the County Engineers can save face.

 

[dhengr]

Maybe you should paint on each visible surface of the channels; “the A$$hole who speced. these channels and is now trying to save face deserves full credit for spending your tax dollars so wisely, please search him/her out and thank them.” If they question this extra paint job, just tell them that in those particular area you wanted to provide a third layer of redundancy against corrosion, in keeping with their conservative design philosophy.

 

[hokie66]

Sounds like redundant in this case has another one of its meanings...superfluous.

 

[brokenpile]

Well the redundant issue has been resolved,the Owner's Engineers who were so adamant that it was required and they couldn't live without it have decided we don't have to suppy it.When asked to supply us with a detail drawing of this redundant chord which differs from the oringnal standard steel tube truss detail that was on the drawings,the request for it was dropped.Another case of buying a design and plans from a private engineering firm and then the local county RE's inserting specifications to the design that they had no part in to tickle their fancy.