Bracing compression chord in a Bridge Truss

[mshani]

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?

 

[ishvaaag]

Most likely the words that you quote mean that no. When you look at the deformed shape of the transverse section of the loaded through bridge the top chord will get induced deflection and this with transient load; hence it seems AASHTO has seen good to limit the deformation as well through this qualitative clause mandating or expressly recomending that the bracing members be deep.

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.

 

[BAretired]

The words "as deep as practical" do not sound very compelling.

BA

 

[bridgebuster]

"as deep as practical" is AASHTO for: We really don't know what to do.

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.

 

[a2mfk]

Curious- Is this in the US? A repair job?

If its new in the US I am shocked, thought steel truss bridges were living history at this point...

 

[bridgebuster]

trusses are still alive a well in the US. Although for new bridges becoming few and far between.

 

[a2mfk]

Bridgebuster- do you know of any new steel truss bridges? Where in the US? Sorry if this is off topic... Not sure about the rules on that.

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.

 

[bridgebuster]

a2mfk

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://www.contech-cpi.com/Products/BridgesandStructures/Truss.aspx

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.

 

[HgTX]

Little pedestrian trusses are very popular, and the companies that make those also provide somewhat larger "prefab" vehicular bridges. For larger truss briges, here's one that just opened:

http://www.ny1.com/content/top_stories/126444/new-willis-avenue-bridge-opens-to-traffic

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

Eng-Tips policies: faq731-376

 

[a2mfk]

HG- interesting, wonder if that was a constructability issue? Staging a bridge project in NYC may be impossible, I am guessing they built this thing in a shipyard in NJ and then floated it in place? Brilliant!

 

[bridgebuster]

a2 - In NYC every bridge project has to be staged to maintain traffic. The swing span (truss) of the Willis Ave bridge was built off-site and floated into place. As was done recently with two adjacent swing bring bridges - 3rd Avenue & East 145th Street.

 

[bridgebuster]

a2mfk - if you pick up a copy of the November issue of Roads & Bridges you'll see two new trusses under construction.