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AASHTO Load Rating - HS-20 But no Design Tandem??

AASHTO Load Rating - HS-20 But no Design Tandem??

(OP)
I am attempting to calculate the Load Rating on an reinforced concrete bridge from 1962.

I am going to run the numbers using the Allowable Stress method and Load Factor method. For both these methods I will calculate an Operating Level Rating and an Inventory Level Rating.

My question is on the Live Loading. My reading of AASHTO's Manual For Bridge Evaluation (2008) is that I should apply an HS-20 load OR the design lane load.

Am I correct that.............

1) We apply ONLY the HS-20 or ONLY the design lane, but NOT both concurrently?

2) We do not apply the Design Tandem truck at all? (two 25kip axles @ 4ft spacing)

My simply supported bridge only spans 34.5 ft, so the design tandem definitely generates the largest moment load. It gives me pause to ignore this loading.



Thank you!

RE: AASHTO Load Rating - HS-20 But no Design Tandem??

Use the truck load only. The lane load wouldn't control on a short span. FWIT: The design truck is never applied concurrent with the lane load. The lane load uses single concentrated loads for shear and moment. Regarding the design tandem, some agencies require it; some don't.

RE: AASHTO Load Rating - HS-20 But no Design Tandem??

(OP)
Thank you Bridgebuster!

I have another question.......

I'm not seeing how we account for pedestrian loads.

The rating formula below seems to only account for live truck loading. If I include pedestrian loads in the 'L' variable, then it will get hit with the impact factor, which does not seem correct.

Maybe I need to modify the denominator to look like this: A_2 * [Live Ped + Live Truck(1+I)]

I'm open to input.

RE: AASHTO Load Rating - HS-20 But no Design Tandem??

JoelTXCive: Since you asked about pedestrian loads, I am assuming that there is a sidewalk on the bridge. The denominator in the rating equation is the effects (moment and shear) due to live loads. Take a look at the AASHTO Standards specifications 17th edition for how to account for sidewalk loading. AASHTO LRFD Design specs also has verbiage on how sidewalk loads are applied and how does the loading needs to be considered along with the truck/lane loading BUT since you are not doing a LRFR Rating I would stay away from LRFD code.

RE: AASHTO Load Rating - HS-20 But no Design Tandem??

(OP)
Thank you. Yes I will check the Specs.

The 1962 concrete bridge has 2 fracture critical main beams carrying the vehicular live load along with a pedestrian live load.

The pedestrian load is picked up from a wood deck attached to the side of each main beam. Due to the configuration, this wood deck could not ever take a vehicle load, so I do not need to consider it as a potential vehicle lane.

It does have pedestrian live load on it though. ~6ft wide * 75psf = 450lbs/ft along the beam or 67kip*ft of moment.

I feel this load needs to be incorporated into my load rating.

RE: AASHTO Load Rating - HS-20 But no Design Tandem??

JoelTXCive- normally the ped load isn't considered but I agree with your approach. Wrote too soon last night. You should take the ped load out of the numerator. The rating factor is a multiple of the design truck only.

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