When to kingpost

[gwynn]

After spending some time looking at a project and possible design methods (or remediation measures in terms of an existing structure), I can tell whether or not kingposting is the best option. I was wondering if anyone knew of some rough guidelines as to when to look at this first. I can generalise and say that kingposting is only attractive when live loads are much greater than dead loads, but I haven't been able to develop anything more concrete than that.

 

[Dinosaur]

Well, I suspect you need to consider the clearance under the bridge. Most structures I have designed would not have the clearance for this style. My guess is there are other practical reasons concerning the relationship between the P/T in the tie rods and temperature changes and things. However, I also know many railroad cars were designed with Queen-post strengthened floors in the early days of railroads. Maybe this is a system that deserves a new look.

 

[bridgebuster]

When you say "kingpost" do you mean


------- or a prestressed steel girder?
\ l /
\ l /
\l/

You can go to books.google.com and find numerous 19th/early 20th Century books with info on a kingpost truss.

 

[gwynn]

Dinosaur

Clearance is definately an issue. I have seen a fair number of bridges with enough clearance to make this option possible lately. Temperature changes can be a problem, as can fatigue detailing.

Bridgebuster

It would be as you have pictured, though typically the steel girder is also pre-stressed due to tensioning of the ties.