LRFR Load Rating 1

[pbardoff]

When computing live load maximum moments on an interior or exterior beam due to standard loading (H15, HS 20, etc.), are the axle loads and wheel loads to be distributed per AASHTO 3.24.3.2 (E = 4 + 0.06 S, where S = Span Length)? My rating factors are coming out way high (4+), when I know they should be closer to 1.0.

Currently I am taking the calculated maximum moment multiplying it x 1/2 (1 wheel gets 1/2 the axle load) x 1.3 (Impact Factor) x LLDF (Since beam spacing is less than 5 ft, LLDF < 1).

Am I doing something wrong or is this bridge just over-designed?

Any help would be awesome - my company does not own a copy of the AASHTO MBE.

Thanks.

 

[bridgebuster]

First, since you are using the Standard Specs you are not doing an LRFR rating.

Section 3.24.3.2 is for the calculation of moments and live load distribution width for designing a concrete slab. The term "S" is the span of the slab.

Table 3.23.1 lists the distribution factors for longitudinal beams. IN the back of AASHTO there are tables of live load moments (for simple spans).

 

[ztengguy]

you might be right using LRFR distribution with Standard spec trucks, we have a few DOTs that require it that way.

What is your span? Check your design truck case, and see what that comes out, you might just be over designed for the smaller trucks, which makes sense.

 

[bridgebuster]

An LRFR rating has to be performed in accordance with AASHTO's Bridge Evaluation Manual. The live load distribution factors - the are several - would be calculated in accordance with Chapter 4 of the LRFD Manual.

 

[pbardoff]

Thanks - eliminating AASHTO 3.24.3.2 from my calcs helped bring everything down to where it should be.

One other question:

Does the 640 pound/ft lane load apply to all truck loadings (H15, Type 3, HS-20, 25, 27, etc.) or is it only applied for HL 93 loading?

Thanks

 

[bridgebuster]

Which specs are you working with Standard or LRFD? There is no section 3.24.3.2 in LRFD.

In the Standard Specs, the HS-20 lane load is 640 plf plus 18k concentrated load for moment and 26k concentrated load for shear.

HS-15 lane load is 480 plf, and 13.5k & 19.5 k concentrated loads, similar to above.

In the LRFD spec, HL-93 is the HS-20 truck plus 640 plf. However, impact (dynamic allowance) isn't added to the uniform load.

 

[JennyNakamura]

No disrespect intended, but your original questions and follow up responses don't make any sense. And your questions regarding application of the lane live load is very basic. I would recommend that you consult with your senior engineer because it sounds like you are getting yourself, and possibly your company, into trouble. You don't even have the AASHTO MBE! It is honorable to try to solve things on your own, but there comes a time when you need to ask senior engineers for help. Sounds like you need a face-to-face with someone more knowledgeable. There is nothing wrong with that. All the background research you have done should make for a very productive and intelligent discussion with your supervisor, and I'm sure he will be pleased with your efforts thus far.

 

[Qshake]

Good Advice JennyNakamura

Regards,
Qshake

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