Help understanding structural steel strength from existing bridge plans

[JHart_11]

I need to analyze an existing girder to determine if it can support an additional load my client wants to be placed on it in the form of a new larger concrete barrier wall. My existing plans are from 1971 and the bridge inventory says the bridge was built in 1973. The design data from the existing plans indicate the following "Structural Steel: ASTM A36, Unit Stress 20,000 psi". I thought that if it was called out as "ASTM A36" this indicated that the yield strength was 36 ksi, is this incorrect? In addition, the "Unit Stress 20,000" has me confused, what does this mean? Any information would be greatly appreciated.

 

[PT99]

Since this is a bridge, AASHTO code governs, not AISC. Allowable stress design was used in the 1970's.
Allowable stress values are not the same in AASHTO as they are in AISC, and are always lower. The plans are stating that allowable bending stress used was 20 ksi (vs 24 ksi in AISC for buildings)

 

[bridgebuster]

The yield strength is 36 KSI. The allowable bending stress (unit stress) is 20 KSI - 0.55FY (yeah, I know it works out to be 19.8 KSI but most of us don't split hairs). Back then steel design was all Working Stress/ Allowable Stress Method.I assume all compression flanges are fully supported; otherwise you'll need to refer to the AASHTO Bridge Evaluation Manual for the bending stress formula.

A bridge designed in 1971 would fall under provisions of the AASHO Standard Specifications -10th edition (1969).

 

[JHart_11]

PT99 & bridgebuster thank you for the information. I actually calculated out the 0.55*Fy = 19.8 ksi and was hesitant to make the assumption that this was the 20,000 psi I was seeing. Thanks!

 

[bridgebuster]

@JHART 11 - I forgot to mention thing. You can evaluate the girder using the LRFD specs, if your client permits.

 

[PT99]

Don't forget to consider if there was any section loss due to corrosion in the bridge members. If so you would have to re-determine section properties and re-analyze. Thickness verification is done with electronic gadgets (after removing coatings) but of course this is no small task if you are dealing with a large structure with access issues.

 

[JHart_11]

I am in the process of obtaining the most recent inspection reports which should indicate if the structure has any section loss and what the extent of that loss is if any. Thanks!

 

[JAE]

One other thought - if you get desperate - is that in the past A36 steel actual yield was a bit higher than 36 ksi. This started occurring more into the 1980's I think, when more recycled steel content was included in the A36 manufacturing process...but in 1973 there could be an actual higher yield present.

You could take coupon samples from the girders - carefully located to avoid any fatigue issues going forward - and test them to see what is actually there.
This costs money, and time, and may not end up getting you much - but like I said - if you are desperate to get something to work.



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[dhengr]

JHart_11:
Who does/did the bridge belong to, who was it built for? If it was build for any gov’mt. agency they may have a treasure throve of info. in their bridge files, including mill certs. for the materials for critical components, calcs., design drwgs. and shop drwgs., and the like. Who was the bridge designer, an who fab’ed. the bridge, they may have some files, probably no longer on the front shelf, of course. I agree with JAE that the actual yield strength is very likely greater than 36ksi, which was the guaranteed min. yield strength for that ASTM Spec., but you must test almost every critical mat’l. piece unless you can be assured that all of the mat’l. came from the same melt, and same mill order, which is unlikely. Could you shore the girder while the wall is being built, and design the wall as a deep beam, to do part of its own spanning and DL carrying, thus lessening the added load on the old girder? Maybe even post tension the wall as its own girder.

 

[bridgebuster]

JHart_11 - out of curiosity, is this a larger parapet or median barrier? One thing came to mind, check if the deck is composite with the steel. If so, your load is all superimposed dead load. Depending on the agency policy, it's most likely distributed equally to all members. Also, if you're doing the check per the allowable stress provisions of the Standard Specs and you come up short, use the LFD provisions of the Standard Specs. That usually gives better results.