Live load moment equation 1

[sopher]

I'm a junior at a local consultant and I'm tasked to check a design for a timber bridge that was design using AASHTO LRFD (2007)specs. Firstly, i don't understand the application of the format conversion factor Cfk. Secondly, the designer used an equation to compute the max. moment due to the live load on the bridge that i've never seen before. He/she used a live load distribution factor which i cannot find in the specs either. Can anyone help me with the equation to compute the max. moment due to the live load of 40 kips for the bridge?

 

[rowingengineer]

This may help you with working out the worst moment.

http://www.steeltools.org/STEELTOOL...umentKey=67a8bad2-5782-40de-b97a-f733046a3e0c

An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[crossframe]

How about:

http://www.dot.ca.gov/hq/esc/techpubs/manual/bridgemanuals/bridge-design-aids/page/bda_9_1.pdf

 

[tumbleleaves]

The Caltrans Chapter 9 of "Bridge Design Aids" was tremendously helpful back in its day. AISC also had that publication with tables of reactions for various bridge span configurations.

Has anything this detailed been published yet for the LRFD loadings? Or is it now sufficiently computerized where detailed tables, influence lines, etc are not as necessary?--Still seems like they would be nice, at least for good reality checks.

Just curious what people are using out there and what's new.

 

[bridgebuster]

tumbleleaves,

On occasion I still use the AISC tables but for the most part they've gone the way of the buggy whip along with Kleinlogel. :-(

 

[graybeach]

The AISC tables are out on my desk now. I used the influence coefficients in the back to do a quick check for a continuous non-bridge structure with a moving load. You could probably do the same with the LRFD live load.

 

[tumbleleaves]

Or I could use the influence lines in Griot's book, bottom shelf right between Pucher's and Kleinlogel's.

Bridgebuster's comment made me chuckle (only an engineer).

 

[rscassar]

I don't know what a Kleinlogel is but I am assuming it has something to do with the upcoming Oktoberfest.

When peer reviewing a colleague's work, rather than going through their calculations line by line. I perform sensibility checks. For example, I would calculate the total moment on the bridge and make an assessment of what I believe the distribution factor should be. From experience, I know that the distribution factor for a prestressed girder bridge with cast-in-place deck overlay would be roughly 15%. And I know that computer analysis will prove that this number will come down to around 10% with more complexity.

Does your colleague's work fall in the realm of sensibility?

 

[bridgebuster]

Kleinlogel is a book of rigid frame formulas.

 

[sopher]

hello everyone. thanks for the help so far. i have one more problem. i need to scrutinize the initial methodology used for the bridge design. i still don't know these structural analysis equations they used for calculating moment shear and deflection for the live load. does anyone know it. please have a look.

 

[sopher]

sorry didn't know the files upload one at a time. the following are the remaining. its start at file 13 and ends at 16.

 

[sopher]

this is the second one

 

[sopher]

this is the third file

 

[tumbleleaves]

Sopher I suggest you adopt a method similar to kikflip's. Run your own independent checks. That is the best way to catch blunders. If there are blunders call the consultant out. If the results agree contact the consultant and ask him to educate you on his methods, people are generally helpful and friendly. If there are "severe" problems with the work you should double-check to make sure the responsible licensed engineer in charge of the work is given notice that there is a problem.

I worked at a production style office where people that IMHO were under-qualified did complex calculations via very professionally looking Mathcad templates. Nothing against Mathcad, one of the best programs and highly recommend it. I also have worked with several engineers that had very fancy degrees from foreign institutions (and even fancy licenses), but were clueless, not sure what they teach over there, but there idea of engineering doesn't translate.

"Timber Bridges: Design, Construction, Inspection, and Maintenance" can be downloaded here:

http://www.fpl.fs.fed.us/documnts/misc/

The book always had everything I needed for timber bridges, but I have never designed a bridge using LRFD, and have limited experience.

 

[Qshake]

bridgebuster - You'll still find the Kleinlogel's and several other valuable references in my office library!

As to AISC for LRFD - The loading hasn't changed the vehicle just required to be in combination with the lane load now. SO you can still use AISC tables of shear moments and reactions so long as you can use superposition.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[bridgebuster]

Qshake - I'm not surprised. We're probably the only guys in this forum with a 1953 AASHO. I'll bet you have the old PCA booklet "Analysis of Rigid Frames Without Higher Mathematics".

 

[tumbleleaves]

Thanks for the info Qshake: But the tables won't work when the lane load controls, and will have to be adjusted by 5% when the alternative loading controls--can't remember if the AISC tables even included the alternative loading. I mainly used the Caltrans 1971 "Manual of Bridge Design Practice".

I guess most of the time they work or work close enough. Seems like some tables should be made for LRFD, at least for training purposes. Being able to draw moment diagrams, and know where to place the loads, provides insight and confidence. The State DOTs don't appear to be filling this void.

You fellows are about the age of crusty old bridge engineer that taught me. Certainly didn't make me rich. The recession hit and now I'm back to bridges, just more management and less design than before.

 

[TampaBridgeDesign]

Please post the MathCAD file.

 

[msquared48]

What about using ILD's here for the max moment/shear determination?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[DeL5]

The S/6.7 LLDF is from LRFD Table 4.6.2.2.2a-1. For commentary on the conversion factor Cfk, see LRFD C8.4.1 (bizarrely, LRFD brews its "own" allowable stresses before using Cfk to revert to NDS' allowables). To check moment and shear, just do it from scratch with a pencil. For deflection, try

http://www.awc.org/pdf/DA6-BeamFormulas.pdf.

It's nicely laid out.