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Re: Tubular Steel Bridge Review

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Ashraf,
 
The chord size you indicated (4") appears small for the span (157 ft).  If you are following the American design standards, your bridge should be designed per AASHTO Guide Spec for Pedestrian Bridges.  In this case, the minimum loads (live and wind) would force the use of larger components, that also need to be verified for the actual (pipeline) loads.
 
The aforementioned spec mainly outlines the design of the half-through bridges.  If this is a box bridge (bracing at top and bottom chords), you would need to design the sway and the portal frames along with all other components.  For your particular structure, I think there won't be any "minor" braces - all of them would be of major importance for the structural performance of the bridge. 
 
The bearing movement of about 11" appears of concern, and will require a special design case and detailing.
 
For all other questions, you may want to follow AISC Manual 13th Edition, Design Guide for Hollow Structural Section: Connections and Trusses (Parker and Henderson), and AASHTO Bridge Design Specifications. 
 
Good luck,
 
V. Steve Gordin, SE
Irvine CA 
 
----- Original Message -----
Sent: Wednesday, April 08, 2009 09:18
Subject: Tubular Steel Bridge Review

Hello,
I am reviewing the design of a simply supported rectangular welded tubular truss bridge of 48.0m to support crude oil pipes. The bridge is supported by two single piled supports one is in offshore (sliding) and one in onshore (pinned) . The major concern is the top chord in which they used 10.75" dia tubulars. I am worried about the global buckling of the top framing in the horizontal plane. We are mainly following American standards such as API RP 2A, AISC etc.
 I would like to get some feed back on the follwoing issues.
1. Is there any limit on the Span/tubular dia limit on Bridges?
2. Is there any minimum size to be used in Tubular bridges for chords/braces?
3. How can I disinguish between major and minor brace?.
4. The design shows 30cm lateral deflection for the offshore support frame. What is the limit on the allowable deflections?.
5. What are the major things to be looked when reviewing tubular bridges design?.
6. Which is the preferred location for pinned end (whether onshore support or offshore support) ?.
7. What is the effective length to be considered for top chord (compression) if there is no knee/sway bracing in the cross sections ?. Sufficient braces are provided in between the top chords and bottom chords. But knee braces in cross section is provided only at the ends.
Thanks in advance,
regards
Ashraf