Single HS20 Live Load in 1 Lane. How to relate that to load on multiple Bearing seats

[dangengineer]

Hello, first time poster here. I am an entry level Project Engineer/Foreman for a bridge company. We have a bridge repair project at the moment that requires a live load calculation before we can begin. To summarize:

The Bridge needs to be jacked up half an inch along its bent 3 abutment. To do this we have 100-ton jacks under each(4) beam. The Highway Dept has given us dead load numbers to use so we know our jacks can handle this no problem.

We will flag the bridge and only allow 1 vehicle at a time across either way. What I need to do though is find out how much a single HS20 (80k lb) truck loading will increase the load on each bearing pad as that will be our worst case estimate. It's been a couple years since I had traffic in school so any help would be greatly appreciated.

Drawings are attached for clarification.

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[bridgebuster]

I'm assuming Bent 3 is at the right of your elevation view. Let's assume you have a truck going west and this is a bi-directional bridge. place one line of wheels directly over a stringer with the rear axle at the bearing; assume the deck acts as a beam one foot wide in the transverse direction; for the other wheel line calculate a simple beam reaction to increase the wheel loads for the line above the stringer. You can also treat the deck as a continuous beam one foot wide if the simple span gives overly conservative results. Then for the longitudinal wheel line above the stringer calculate the reaction at the bearing. Unless you plan to restrict truck speed to 5mph increase the reaction by 30% for impact. That's your live load reaction.

BTW HS20 is a 72k truck, is 80k an Arkansas DoT requirement?

 

[dangengineer]

Thanks for the quick reply! Once I calculate my load over the bearing itself, say I used the outside(north) stringer and bearing as my beam would I then have to treat the deck along the top off all the bearings as another simple beam to show the increases to each bearing? And we will probably only slow traffic to 15-20 mph so I will indeed throw in the 30% extra impact.

When I was checking into HS20 everything I found said it is a 3 axle truck with the front axle at 12k and each of the back axles at 34k each for a total of 80k. 80k is also the legal limit for AR highways so I figured that would be the best to use for a conservative estimate.

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Thanks again.

 

[bridgebuster]

Sorry I didn't get back to you earlier; a long day. I attached a sketch to explain what I meant. I was tired last night; hopefully what I wrote is clear. I think you get the drift. You calculate a reaction for all four stringers. However only two will see any significant increase in load. Typically, we jack with the maximum load because it's easier to jack simultaneously from the same pump. This way it's also easier to control differential displacement. If you're just removing bearings you may only need to pick the structure up 1/8".

Also, I sketched out an HS20 truck. The one you have in your sketch is a variation of AASHTO Type 3-3, which is a real truck whereas HS20 is an imaginary vehicle. To play it safe check your loads with both trucks, unless ADOT specifies the 80k truck.