Pedestrian Bridge Design

[grogannc]

I am designing a pedestrian bridge (only 30 ft span but over a small creek) and I have initially designed it with ASD since its timber. But in doing some more research it appears that the bridge code makes you use LRFD (since 07) even for timber.

This is behind a proposed apartment complex in the woods but will provide access to a greenway at some point. Currently using 100psf for LL, also a 4000lb vehicle, and 1000lb (over 2.5ftx2.5ft area) for deck loading.

But I guess the main question is what governs. Also its 6ft wide. Main members are treated V4 1.7E 5.125x20.625 glulams (three of them). Planning to meet building code railing and ADA ramp requirements on approach. Interesting the bridge code uses a 6 inch sphere still for railing openings while building code uses 4 inch sphere.

 

[jayrod12]

It doesn't matter which method you use, ASD or LRFD, it should result in essentially the same members.

Otherwise I'm not sure your question. I would bet the wheel load for the vehicle (or the 1000lb) will govern the decking design. For the other members, we'd need to know more of the framing plan in order to comment further.

 

[grogannc]

Yeah sorry-that was a confusing post. I have already designed it using ASD. You are right typically it should be very similar to what I would get with LRFD but I haven't done that. My question is do I need to? Seems like a waste of time.

 

[Lomarandil]

What "bridge code" is requiring you to use LRFD? AASHTO? Sounds like your bridge doesn't fall in the scope of AASHTO -- not to say that its provisions aren't still sound guidance, but you may not be required to follow it the way you would for a highway bridge.

It really boils down to what jurisdiction will be approving the design (if any), and what they want to see.

Even if the plan reviewer or project specs nominally call for an LRFD design, a reasonable person would possibly still accept an ASD equivalent. As Jayrod points out, you should end up with the same physical bridge in the end. Your luck finding a reasonable reviewer may vary though.

 

[jayrod12]

To be honest, I'm north of the border, so to us no it technically wouldn't matter as long as we know it would work if we were asked to prove it.

South of the border however, I believe you are required to submit calculations, therefore I would say yes you likely would have to. There are some slight nuances that may result in slightly different sizing but if you were conservative enough with your design I can't see anything changing drastically.

 

[BUGGAR]

In SoCal it is common to design pedestrian bridges relating to buildings to CBC/AISC using ASD. The reviewing agencies are not that familiar with AASHTO. Most pedestrian bridges here are done by companies like Continental Bridge and you can let them do the design.

 

[grogannc]

AASHTO Code 6th edition is what I have been looking at. It does include some material for pedestrian bridges and then there is the supplemental LRFD Guide Specifications for the Design of Pedestrian Bridges. From what I can find, it appears some states modify chapter 1 of the IBC to specifically exclude bridges. The state the bridge is in does not so I'll probably end up using the more stringent 4 inch sphere requirement. Also this is private property and unlikely to be maintained by gov't. It is however in the most restrictive county in the state so I suspect they will let me know what they want. Thanks for the responses.

Its funny-the contractor who hired me wants "a simple wood bridge". That's an oxymoron.

 

[KootK]

Not wood specific but perhaps useful if you haven't yet stumbled across it: Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

Maybe some wood ideas here: Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[medeek]

I've never understood why they have a 5-1/2 glulam width and then a 5-1/4 width, maybe someone could explain that one for me. Sorry for posting off topic.

I am actually quite interested in pedestrian bridge design after my unfortunate experience with a previous renovation project that went south. Is the AASHTO Pedestrian Bridge Guide the definitive (best) reference? Where should one start to get more education on the subject?

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[bridgebuster]

Medeek - there's not much to the AASHTO guide spec, although it does have good examples regarding vibrations and analysis of trusses with unbraced top chords.

 

[StructuresDesign]

I second the comment that AASHTO will typically not be the governing code for pedestrian bridges on private property. Also note that the AASHTO Guide Specifications document is not a code per se, it is what AASHTO consider "good practice". Also note that it uses AASHTO, not ASCE 7/IBC/CBC, load combinations that the local plan checker may not be familiar with.

Regarding the glulam widths: the 1/2-widths are unfinished, i.e., suitable for conditions where the member is hidden by finishes (or the rough look is desired), whereas the 1/4-widths are finished. It is recommended to check with local suppliers to determine the availability of both "flavors", but I would suggest using the 1/4-widths for design. Then it is usually not a big deal if the contractor asks if he can substitute for the larger member.