Legal Load Rating Question 1

[OSUCivlEng]

I am performing an AASHTO LRFR legal load rating (Type 3, Type 3S2, 3-3, etc.) for a single span bridge less than 200 feet in length. Should I include the lane load (0.64 klf) with the legal truck? In my opinion the wording in the MBE isn't clear if the lane load should be included or not.

 

[bridgebuster]

My reading of the MBE is the lane load isn't required in your case. I looked at what was done on a recent project in our office - single span bridge, 60' long - no lane load with the 3, 3-3, & 3S2 trucks.

On NYSDOT-owned bridges those trucks aren't required but municipalities are free to include them.

 

[BridgeSmith]

For our load ratings, none of the legal load trucks are combined with lane load for any bridge.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[OSUCivlEng]

In my opinion the wording underlined in red could be written in a more clear manner. It's clear that there is a lane-type legal load model that is also used, but the wording makes it appear that there is some kind of lane load that is applied with the Type 3, 3S2 and Type 3-3 truck.

 

[BridgeSmith]

...the wording makes it appear that there is some kind of lane load that is applied with the Type 3, 3S2 and Type 3-3 truck.

Except that if that were the case, there would be no need for the load combination in the bullet point following the red underlined portion, since it would never control over the Type 3-3 + lane at their full scale.

The red underlined statement only means that the lane load is not applied differently as a state legal load, an AASHTO legal load, or an AASHTO design load.

Also notice the position of the comma in the sentence before the underlined one; you take the largest of the [AASHTO legal trucks] or [the state legal trucks + lane]. If the comma was after "state legal loads, then it would be the [AASHTO trucks or state legal loads] + [lane].

Rod Smith, P.E., The artist formerly known as HotRod10

 

[OSUCivlEng]

Nah it's poorly written and a good example of something a committee created. "Common to all truck types" could be easily understood as the three types of trucks mentioned in the previous sentence.

 

[AS_PEng]

Would the lane load even govern?? In Canada, dynamic load allowance (up to 1.30 factor)is not applied to lane loads and on short spans it won't be the determining factor.

 

[BridgeSmith]

Would the lane load even govern?? In Canada, dynamic load allowance (up to 1.30 factor)is not applied to lane loads and on short spans it won't be the determining factor.

On bridges with longer spans, it certainly can for design. The standard lane load (0.64 klf) is not combined with the standard rating vehicles for load rating. Only the reduced lane load (0.20 klf) is used in combination with the state legal load trucks as specified in the excerpt above:

- With the 75% of the Type 3-3 truck train when considering negative moments at interior piers, or
- for spans greater than 200 ft, a single Type 3-3 or state legal load truck.

Commentary on the excerpted section above:

Usually bridges are load rated for all three AASHTO trucks and lane loads to determine the governing loading and
governing load rating. A safe load capacity in tons may be computed for each vehicle type (see Article 6A.4.4.4). When
the lane type, load model governs the load rating, the equivalent truck weight for use in calculating a safe load
capacity for the bridge shall be taken as 80 kips.

AASHTO legal vehicles, designated as Type 3, Type 3S2, and Type 3-3 are sufficiently representative of
average truck configurations in use today, and are used as vehicle models for load rating. These vehicles are also
suitable for bridge posting purposes. Load ratings may also be performed for state legal loads that have only minor
variations from the AASHTO legal loads using the live load factors provided in Table 6A.4.4.2.3a-1 for the AASHTO
vehicles. It is unnecessary to place more than one vehicle in a lane for spans up to 200 ft because the load factors
provided have been modeled for this possibility.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[bridgebuster]

got to be careful with commas. About 30 years ago I was closing out a project. I told the contractor's super he was not entitled to extra payment for an item. He pulled out the spec book and under method of measurement he pointed out "...that's a semi-colon, not a comma. If it were a comma you'd be correct. Let's look at the definition of semi-colon...". He made his point. I paid him. Off-topic but I agree, AASHTO doesn't always write with clarity.

 

[Settingsun]

"Nah mate, you've got a spec of dirt on your copy."

 

[BridgeSmith]

Off-topic but I agree, AASHTO doesn't always write with clarity.

I agree that sometimes AASHTO is unclear, and oftentimes, even when things are clear, they are overly detailed and nitpicky about things that don't matter. In this case, I think they got it right (though not very understandable) with respect to the wording of the sentence I was referring to.

Rod Smith, P.E., The artist formerly known as HotRod10