Live load moment equation
Live load moment equation
(OP)
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?





RE: Live load moment equation
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An expert is a man who has made all the mistakes which can be made in a very narrow field
RE: Live load moment equation
RE: Live load moment equation
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.
RE: Live load moment equation
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.
RE: Live load moment equation
RE: Live load moment equation
Bridgebuster's comment made me chuckle (only an engineer).
RE: Live load moment equation
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?
RE: Live load moment equation
RE: Live load moment equation
RE: Live load moment equation
RE: Live load moment equation
RE: Live load moment equation
RE: Live load moment equation
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.
RE: Live load moment equation
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,
![[pipe] pipe](https://www.tipmaster.com/images/pipe.gif)
Qshake
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.
RE: Live load moment equation
RE: Live load moment equation
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.
RE: Live load moment equation
RE: Live load moment equation
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Live load moment equation