Bracing compression chord in a Bridge Truss
Bracing compression chord in a Bridge Truss
(OP)
I am working on a truss design for Highway bridge. I needed some help in lateral bracing of the compression chord. AASHTO 6.7.5.4 Trusses states "the member providing lateral bracing to compression chords should be as deep as practical and connected to both flanges." Can I use W12 member aligned and connected to top flange with a full depth stiffner plate to brace a W36 compression chord? Are there any articles I can refer to for this requirements?






RE: Bracing compression chord in a Bridge Truss
This must mean that such detail by whatever the reason (overload, impact, limit capacity) has been of superior behaviour. For, in one aspect, it needs not necessarily be, for deeper members have less rotational capacity for the same flange welds (you need to have more rotational distortion in the less deep member to get a Fy stress at flanges), and, depending on the detail, a capable design could show superior performance in fatigue with a less deep brace. Yet anyway a deeper member also restricts the intervening rotation, and it seems the experience at AASHTO is that deeper braces perform better.
RE: Bracing compression chord in a Bridge Truss
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RE: Bracing compression chord in a Bridge Truss
The Standard Specs require a strut to be as deep as the compression chord. it's been this way for decades. However, the diagonals in the horizontal plane could be shallower.
In the LRFD the phrase "connected to both flanges" is ambiguous. I think it's referring to the bracing member because wide flange sections (for chords) are sometimes used with their flanges in a vertical position.
"as deep as practical" I'm with ishvaag on this. Again, it looks like the LRFD spec dropped the old standard spec requirement about depth.
RE: Bracing compression chord in a Bridge Truss
If its new in the US I am shocked, thought steel truss bridges were living history at this point...
RE: Bracing compression chord in a Bridge Truss
RE: Bracing compression chord in a Bridge Truss
I mean out of pure interest, I am not being argumentative. I love bridges but all my structural engineering is with buildings and misc. structures. As a kid we'd go to the Louisville area where my mom is from and there are some great steel truss bridges over the Ohio River there.
My understanding is that these types of bridges are largely inefficient in terms of material useage and the labor intense connections. With cable stayed technology and pre-cast concrete box girders taking the place of bridges in the mid to long span category. Plus all that exposed steel must be a maintenance nightmare.
RE: Bracing compression chord in a Bridge Truss
Check this link for the AISC 2009 Prize Bridges
http://www.aisc.org/contentNSBA.aspx?id=20728
You could probably search for previous years as well. I do remember some award winning trusses in recent years.
Also look at
http://
Generally, they are not used for the reasons cite but in some cases they are still practical. One drawback with older trusses: They're non-redundant. Today, we can use the bracing system to provide redundancy.
I worked for a company that designed numerous long span trusses, including several over the Ohio River between OH & WV.
I love the old 19th & early 20th C trusses. On one occassion, I came across one with cast iron Phoenix Iron Co. compression members. alas, these old relics are going by the wayside.
RE: Bracing compression chord in a Bridge Truss
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And there are several more large trusses being built around the US right now. Some states like them more than others. I suppose trusses put you in a similar span range as arches, but you don't have to reach as high to erect?
Hg
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RE: Bracing compression chord in a Bridge Truss
RE: Bracing compression chord in a Bridge Truss
RE: Bracing compression chord in a Bridge Truss