AASHTO Bridge Question Regarding Adjacent Box Beams (prestressed concrete) Wheel Distribution Factor

[Bridgeman44]

It would be great to hear what other designers/raters are doing regarding this issue.

I’m working on a load rating for an adjacent prestressed concrete solid slab/beam bridge and have a question regarding the Load Fraction (or wheel distribution factor) provisions in AASHTO (LFD). Specifically, the sentence in Article 3.23.4.3 that reads "The Load Fraction, S/D, need not be greater than 1.0” is giving me heartburn(from AASHTO Bridge 2002 Edition). I’ve been calculating the Load Fraction as LF = S/D (per equation 3-11 on that page) for years with no issues regarding design or load rating (because the S/D value was always less than 1.0). However, this latest bridge I’m rating that specific sentence comes into play; for this bridge I get a LF value of greater than 1.0 for S/D. All past projects I’ve always gotten something less than 1.0 so I didn’t have to worry about that limiting 1.0 value. From my rough calculations and analysis, it seems that this limiting 1.0 value only comes into play if the width of the beams/slabs are about 6 ft or greater. This is why I never had an issue before because all of these that I’ve designed or rated in the past have typically used 3 ft or 4 ft wide beams/slabs. This particular bridge has beams/slabs that are 8 feet wide (5 total beams/slabs side-by-side).

To me, that AASHTO sentence that says “The Load Fraction, S/D, need not be greater than 1.0” is indicating that no matter what your bridge configuration is you never have to use more than 1 wheel load per beam. This does not seem to be a very conservative approach if you are using beams wider than 6 ft, where technically you can have a truck width entirely within the limits of one beam/slab. I know the theory is the adjacent beams are supposed to act as one unit and distribute the load through the shear keys and tie-rods, but limiting the wheel load fraction to just 1.0 still seems low to me. I thought I would see something in AASHTO that says these equations only apply for beams less than a certain width or some other limitations and then direct you to more detailed analysis.

Thanks in advance if you have any advice, additional information, or past experience that you can offer.

 

[bridgebuster]

I'm reading that section the same as you. I don't believe they only meant it to apply to skews. I've never encountered this situation; never seen adjacent boxes greater than 4'. One thought, why not compare your value from the Standard Specs with the LRFD spec? The latter is supposed to be more realistic; it could serve as a sanity check.

 

[miecz]

I've never had this condition either. For what it's worth, the PCI design manual reads the same as this section, but does not include the paragraph that has this sentence. As bridgebuster suggested, I would compare the result to the LRFD Spec. Also, I would find the section in Table 3.23.1 that most closely matches, and compare to that. I would be interested in seeing your cross section.

 

[Bridgeman44]

Bridgebuster & Miecz, thanks for taking the time to think this question through and respond with your thoughts. yeah, this is the first time I've seen adjacent beams greater than 4' wide too. I like the thought of comparing the results to the LRFD equations for the distribution factor, that's a good idea. I see the LRFD spec has some restrictions for beam widths to be between 3' and 5' and also no limit of 1.0 anymore. That is interested to hear that the PCI manual has the same info minus the 1.0 limiting sentence. I guess it's been a while since I've done a prestressed bridge, somehow I forgot about the PCI manual. It's beginning to seem like that 1.4 is reasonable and not to go with the 1.0 limit. I don't think I would be able to sleep at night going with the 1.0!! Some additional information, this single span bridge only has a span of 26 feet and has one tie-rod to post tension the slabs together at about midspan; I'm used to seeing at least 2 or 3 transverse tie-rods per span to help these adjacent beams act as one unit and distribute loads. Plus, the longitudinal joint between Beams 4 and 5 (5 Beams total this bridge) shows some signs of leakage and could mean deterioration of the grouted shear key above that is also supposed to be helping the units act as one. All this tells me limiting the DF to 1.0 is not a good idea for this particular scenario. miecz, I did want to include a scan from the bridge drawings of the typical section for this bridge in the original post but I'm not too familiar with this site yet and couldn't find an easy way to attach a picture to the post? It looks like all you can do is post the picture somewhere in cyberspace and post the link to the site where it is located? Is that correct as far as you know?

 

[miecz]

I haven't posted a picture here in quite a while. I believe it was easiest to upload the file to ENGINEERING.com. at the bottom of the post box.

 

[Bridgeman44]

Cool. Thanks for your tip miecz. The link should now have the scanned typical section of this bridge for your viewing pleasure. That will be handy to know how to upload pictures now. I didn't see that at the bottom. Thanks again!