# Bridge Design Question



## IlPadrino (Dec 15, 2011)

Can someone point me to the right AASHTO manual that will help me classify a steel stringer bridge with an open grid steel decking? None of the military guides address decks other than timber or concrete to get "effective stringers" (which I think the rest of the world calls "distribution factor").

And does someone know the answer?

Thanks!


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## ADB (Dec 15, 2011)

AASHTO LRFD Bridge Design Specifications, 5th edition

Chapter 4 deals with distribution factors for grid decks with stringers


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## McEngr (Dec 15, 2011)

Because this is probably not addressed in the distribution factor tables (ge or something like that), you'll probably have to use the lever-rule method. I don't think the analytical method even qualifies for this. Perhaps someone could correct me on this.


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## IlPadrino (Dec 15, 2011)

ADB said:


> AASHTO LRFD Bridge Design Specifications, 5th edition
> 
> Chapter 4 deals with distribution factors for grid decks with stringers


Can you quote the relevant sections?


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## McEngr (Dec 15, 2011)

I think it's 4.6.1.2.


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## IlPadrino (Dec 15, 2011)

McEngr said:


> I think it's 4.6.1.2.


Sorry... I should have been more clear: I don't have AASHTO LRFD Bridge Design Specifications, 5th edition. What does 4.6.1.2 say about open grid steel decking?


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## ipswitch (Dec 16, 2011)

IlPadrino said:


> Can someone point me to the right AASHTO manual that will help me classify a steel stringer bridge with an open grid steel decking? None of the military guides address decks other than timber or concrete to get "effective stringers" (which I think the rest of the world calls "distribution factor"). And does someone know the answer? Thanks!


You're in luck. I just designed one. The only sections you'd need that are different from a regular bridge is the determination of the GDF for moment and shear, Table 4.6.2.2.2 &amp; 3.

Section 9.8.2.2 applies too if you have an open floor, which I did. It might help to have NCHRP Report 592 in the calcs for your lever rule. Hope this helps a bit.

I also designed this as a non-composite bridge


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## ADB (Dec 16, 2011)

I would be glad to scan and post the exact AASHTO references, but that is probably a copyright violation.


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## IlPadrino (Dec 16, 2011)

ipswitch said:


> IlPadrino said:
> 
> 
> > Can someone point me to the right AASHTO manual that will help me classify a steel stringer bridge with an open grid steel decking? None of the military guides address decks other than timber or concrete to get "effective stringers" (which I think the rest of the world calls "distribution factor"). And does someone know the answer? Thanks!
> ...


Thanks! Very helpful to guide me in the right direction. The military pub (FM 3-34.343 Military Nonstandard Fixed Bridging - http://www.sachs.us/nsfb.pdf) has a table for determining the number of effective stringers:




And I'm trying to reconcile the open grid steel decking. The 3-34.343 does not consider shear determination for steel stringers so I'm just looking for the moment classification. It would seem I can just use the table for "Distribution of Live Loads Per Lane for Moment in Interior Beams" and go with

S/7.5 if tg&lt;4.0 or

s/10.0 if tg&gt;4.0

for one-way traffic.

Then I just need to convert the distribution factor to number of effective stringers. From http://www.dtic.mil/...df&amp;AD=ADA408337:



> The factors in Table 3-3 were taken from Table 3.23.1 of Reference 1, where they are referred to as "Distribution Factors" (DF). The DFs represent the fraction of a wheel line load that is carried by a single stringer. Reference 1 rates/designs bridges based on a single stringer capacity and the portion of total load carried by that member. This is the more conventional method within the structural design community. The simplified procedures set forth in the FM 3-34.343 rate/design bridges based on the total capacity of the bridge to carry the entire vehicle (i.e. axle loads instead of a wheel line). The number of members sharing in the total load are referred to as "Number of Effective Components", N. Knowing these differences in design/analysis concept, the DFs in Reference 1 can easily be converted to N values for military usage. This is done by taking the inverse of the DF to convert from a portion of load carried by one member to the total number of members contributing to carrying that load. This value is then multiplied by 2 to convert from a wheel line load to an axle load as required for the FM procedures (an axle load consists of 2 wheel line loads).


My only confusion is the "multiply by 2" to convert from wheel line load to axle load, because when looking at AASHTO's "Distribution of Live Load Per Lane for Moment and Shear in Interior Beams with Wood Decks" for planks (S/6.7 for one lane and S/7.5 for two or more lanes) there doesn't seem to be a factor of two needed. Does AASHTO consider per lane to be axle load?

I'd like to ignore the exterior stringers and avoid using the lever rule to determine... which the FM3-34.343 seems to do.

Anyway, I greatly appreciate your help. This is part of an exercise to do an expedient field classification of a bridge if reachback support to the "real" structural engineers isn't available.


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