Use of AASHTO Distribution Factors for bridges w/ narrow lanes (<12')

[aspultor]

Hello, I have a question regarding the appropriate usage of the AASHTO LRFD distribution factors for a bridge with narrow lanes. For bridges in an urban environment, lane widths of less than 12' are commonplace. I am currently designing a bridge where the roadway engineer has specified a lane width of 11.5'. AASHTO 3.6.1.1.1 says in part: "in cases where the traffic lanes are less than 12 ft wide... the width of the design lane shall be taken as the width of the traffic lane". But, when it comes to the design of the girders, it's not clear to me what impact (if any) design lane width has on the DFs in 4.6.2.2.

How do you handle the case of a narrow design lane? Do you still use the 4.6.2.2 DFs, or do you calculate your own? If you know of code that speaks to this please include a reference. Thanks!

One additional thought... clearly the single lane DFs are not affected by the width of the lane. My concern is the 2+ lane DF.

 

[BridgeSmith]

The distribution factors should not be affected unless the girder spacing is extremely wide, such that loads in more than 2 lanes affect a single girder. It's still a 10 ft wide loading pattern, with 2 ft from the center of each wheel line to the edge of the loading pattern (6ft axle gauge, centered within the 10' loaded lane), which cannot infringe on an adjacent lane. All that to say, the minimum distance between the centers of wheel line loads remains 4', which is what the multiple loaded lane DF is based on.

 

[IDS]

I disagree. The load per unit area is increased so why wouldn't the load per girder change?

I don't know of any code other than AASHTO that still uses distribution factors. At least a grillage analysis is standard practice for highway bridges outside USA.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[BridgeSmith]

"The load per unit area is increased so why wouldn't the load per girder change?"

The load per unit area doesn't change. The lane load is 10' wide and the wheel loads from a single axle are 6' apart, centered within that 10' loaded width. The design lane being 11.5' instead of 12' doesn't change the critical positioning of the loads in 2 adjacent lanes (4' center to center of wheel lines, each 2' from the edge of the lane). As I said, it would only change the possible positioning of a truck load in a 3rd lane, assuming the girder spacing was wide enough (approximately 13' or more) for that 3rd truck load to affect the girder of interest.

"I don't know of any code other than AASHTO that still uses distribution factors."

AASHTO considers them adequate to estimate the loading within the limits of applicability. Outside those limits, Grillage analysis is one of the options to estimate load effects, although it is rarely used in the US anymore. When the DF method cannot be used, either the simpler and more conservative lever rule or a FEA is used, depending on whether a detailed analysis is economically warranted. Sometimes other considerations dictate the girder size, so it is pointless to go to the time, effort or expense of doing a FEA rather than using the lever rule, only to end up with the same sized girder.

 

[ukbridge]

FWIW we use grillages all the time in the UK for structures - they're simple, give reliable results and can be easily validated. Exception of course is where a structure is highly skew and integral or ladder decks (they're not so great at those).

From my experience when you deal with a lot of envelopes and smart combinations 3D shell models become a lot of hassle, slow to extract results from and not worth the additional "accuracy" you'd get over a grillage.

 

[BridgeSmith]

I'll agree that for most bridges, a FEA is not warranted. The exceptions would generally be large bridges (for which small decreases in member sizes add up to big savings) and/or very complex bridges for which other methods are inadequate to predict the behavior.

Distribution factors are also simple, and give reliable results. They have been validated by numerous FEM models. They also work for highly skewed structures by implementing adjustment factors.

 

[aspultor]

Thank you everyone for your thoughts. I agree w/ HotRod10's comment "it would only change the possible positioning of a truck load in the 3rd lane". Girder spacing would have to be unusually wide for that third truck to influence the live load.

Since this thread also started a discussion on the merits of distribution factors, I'll add my 2 cents: DFs are beneficial for load rating existing bridges. Since the math is so simple, every rater should get essentially the same result. This means that the rated load for an existing bridge shouldn't change due solely to the engineer who rated it.

 

[BridgeSmith]

DFs are also very beneficial for design. They provide a simple, conservative approximation of the load effects for a fairly broad range of the most common bridge types in the US. Europe may be different, and the Eurocode I'm sure is written to cover the design methods best suited to the common practice there.