Truck loading with lane load 2

[lsmfse]

I have a single 42' span one lane (13' wide) bridge located on private property. It has a concrete deck supported by wide flange stringers.
The requirements are to design for a 20T truck. I want to design for tandem axles (akin to HL-93). Since this is a private bridge, low volume low speed and will likely never see a traffic jam, I was going to neglect lane loading and add a front axle to the tandem. any opinions?

 

[BridgeSmith]

You're already deviating from the AASHTO LRFD spec, but if it's a privately-owned bridge, it's up to the owner to decide the design loads. It would be wise to post the rated loads if it's less than the HL-93 loading.

 

[bridgebuster]

A 20T truck is H20, two axels spaced at 14;'. The closest recognized legal load to a tandem 20T truck is AASHTO Type 3, which is 25T. The first axle is 16k, the next axle15' away is 17k, the third axle is also 17k, & 4 feet away. You could use that configuration and proportion the loads down to 20T, then compare the moment from that with H20. I think the H20 truck would govern slightly of the full Type 3 truck. You could also try Alternate Military Loading (Standard Specs) , which is 2 axles 4' apart, each weighing 24k.

I agree with BridgeSmith about putting up a load posting sign with the axle configurations.

 

[lsmfse]

As I mentioned, private bridge. Client wants to build a new home and concrete supplier wants to know if they can take a full load(9 yards of concrete)over the bridge. I designed using 2 tandem 26k axle spaced 4' apart and 10k front axle 14' from rear tandems. The plans do show a sign posting the load. In full disclosure, I'm a building engineer more than a bridge engineer but I'm the "old" guy that people refer. Maybe I could refer to you?

 

[BridgeSmith]

I'm not licensed in California, so I couldn't accept the referral. I'm sure there are qualified bridge engineers around, though. Seismic considerations are minimal if the abutments are fully integral, but if they are not, I highly recommend you involve a bridge designer familiar with seismic design for bridges.

 

[question_asker]

I agree with your approach, and the loading / axle configuration you have proposed sounds reasonable. I have worked on several smaller private (and sometimes public) bridges where the AASHTO loading provisions are not applicable and engineering judgement must be used as to how the bridge will be loaded, what restrictions to loading there will be (physical barriers/geometry vs. posted signage, etc). As BridgeSmith states, it is up to the owner to stipulate what design vehicles and loads they would like to use, but typically most private owners of this nature won't be able to tell you more than something generic such as "dump trucks and concrete trucks," or "legal loads" and rarely will you get any defined axle loadings from them - they are looking to you as the structural engineer to provide this guidance.

Use engineering judgement and owner's input to determine the design loading, check a handful of load combinations that may control, develop an envelope for the loads to be resisted, and design accordingly. It sounds like this is your approach. The process is simplified from your typical DOT bridge design and lends itself to hand calculations, engineering principles, and sound judgement.

To BridgeSmith's second point, if seismic is a big consideration, the need for a more rigorous analysis and consultation with a bridge engineer is advised.

Final thought - load posting would be a good idea as mentioned by others. And often times we will have a "one vehicle at a time" sign coupled with a 5 mph speed limit sign to minimize impact loading (I would still design to account for impact - generally limited to ~30% increase to the axle loads).

 

[question_asker]

One more thing - a check of emergency vehicle loading is recommended. Don't think it would control over the load case you have proposed, but I would perform a quick check for good measure. The owner will take assurance in knowing there is no restriction to fire trucks and/or ambulances.