Railroad Bridge 2

[yjung]

I am designing a UPRR underpass, multi-tracks, the superstructure is steel plate girder with steel deck, located in Arizona.

I have some question on the ASTM.

I designed the steel girder using 50ksi steel, but i am having hard time with what kind of ASTM steel to be used for this bridge.

I am very confused which one to be used between ASTM A709 GRADE50 no wearthering and no-FCM, and ASTM A572 GRADE50.

Per AREMA code, page 15-1-10, table 15-1-3, the ASTM A709 GRADE 50 is equvalent to ASTM A572 GRADE 50.

My main concern is the unit price per Ibs. According to the fabricators which i called, ASTM A709 steel is more expensive than ASTM A572 steel. If the two ASTM are equivalent per AREMA, why we should even think of using ASTM A572, because it is cheaper?

My 30% estimate is based on ASTM A572. I have to increase the estimate cost by minimum 25% if i have to use ASTM A709 for 60% submittal.

Please help!!!

 

[TTK]

I will offer the "quick" reply and hopefully others can expand on this further...

A good way to think about is:

ASTM A709 Grade 50 is the BRIDGE steel specification and ASTM A572 Grade 50 is the BUILDING steel specification

This is a simplified summary since you could have a need to use the A709 spec for non bridge structures.

The A709 spec was deveoped for structures such as bridges with high live load to dead load ratios and repitive loading cycles.

Since 90% of the total stress in your RR bridge is likley to be from live loading you will want to use the A709 spec (I beleive this is also what the AREMA code requires).

I am without my reference material but maybe others can provide the specific diferrences between the A572 and A709 specs.

 

[HgTX]

The chemistries are identical. ASTM is careful to keep A 709 synchronized with its equivalent non-brige standards. The big difference between A 709 and A 572 is that A 709 has mandatory Charpy testing for tension components. To order A 572 to meet the same requirements you'd have to specify supplemental requirement (S5) and further specify what values they need to make.

AREMA lists what CVN values they require, which are exactly the same as the ASTM A 709 requirements.

So regardless of which you order, A 572 with supplemental CVN testing or A 709, you're getting the same stuff and will probably pay the same price. A 572 *without* the CVN testing might be cheaper, but not with, and I'd be surprised if A 709 without CVN testing (say for a compression member) would be any more expensive than regular A 572.

Hg

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[yjung]

No, AREMA said, two ASTM are the same. We can use any.

 

[yjung]

HgTX

Which company are you working at?
I should work for you.

 

[HgTX]

Yes, but you need to order the ASTM A 572 to meet all the CVN toughness requirements given in the tables in the Materials section of the AREMA code.

Once again...

The two grades are chemically identical. They have identical tensile requirements. The difference is the impact test requirements. With A 709, those come automatically. With A 572, you need to specify them if you want them, and AREMA requires you to do so.

With those impact test requirements met, the two grades are completely identical. I would be surprised if at that point you would see any price difference. Your fabricators are probably accustomed to seeing A 572 used where there are no CVN requirements, and A 709 used where there are.

Hg

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[TTK]

Good summary HgTX.

yjung,

You are much safer using the A709 callout since everyone will immediately understand this to be a bridge steel.

As HgTX mentioned the cost should be the same as A572 (with supplemental requirement) so the cost factor is a non-issue.

Also, make sure you are comparing INSTALLED cost instead of just raw material cost or fabricated cost. I suspect the INSTALLED cost for A709 will be closer to your 30% estimate than you think.

 

[HgTX]

(I just wanna clarify that I didn't see yjung's 17:59 post before I sent my 18:05 post. I certainly didn't mean to imply that I should be yjung's boss!)

 

[yjung]

I had to decide a # for the 60% submittal yesterday,due day. My total steel weight is 1.5 million #. Initally i decided to use $1.8/# for the 60% submittal, and i am in Arizona, which was the same $ as 30% estimate.

Last night, i could not sleep with that #. So today i am going to increase to $2/#, and resubmit the estimate. I hope people understand my concern. They may blame on me because 60% estimate is more than 30% estimate.

I used to work under a superviser, and I thought i can do a project myself, and moved to my current firm to be a solo engineer. I had confidence before starting a project, but i am getting afraid of things and the issues that i have to make a decision in order to finish a project. I am realizing how hard it is to do everything myself. I now miss my ex-supervisor.

Just like i could not understand my parent' roll, not until i had a baby boy.

Thanks and Please help me in the future with any issue i may ask.

 

[HgTX]

$2/lb. for railroad bridge sounds sorta reasonable or even low. Railroad bridges are quite labor-intensive in the fabrication phase, much more so than typical plate girder highway bridges. How much is a typical highway bridge in your experience?

Hg

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[yjung]

This is my first RR bridge. So, I got the unit price from my highway bridge experience.

I thought the total weight, 1.5 million #, may reduce the unit cost? I called around but i could not find any fabricator who can give me a good # in the state. We may have to order from out of state.

 

[HgTX]

Do NOT take my word for this, as I do not handle bids for a living, but my understanding is that a railroad through-girder bridge can be twice as much per pound as a conventional medium-span straight plate-girder highway bridge.

Don't expect bulk discounts. Price will go up for particularly small orders. Price will not go down for large orders. And 1.5 million lb. is 750 tons, not all that huge for highway bridges. Just one highway interchange will dwarf that.

AISC can help you locate Major Bridge certified fabricators. Keep in mind that shipping costs may offset fabrications & material costs, but that would be part of their bid.

Hg

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[yjung]

Thanks HG

The bridge type for this job is steel plate girders with steel deck, fortunately it is not steel throught-girder brg. We do not need thousands of the shear studs for this project compared to concrete deck bridges. I will do more research on this matter for 90%.

 

[yjung]

FYI

You will find a good summary about the Steel bridge spec through Oregon state bridge manual.

(This is for myself in the future.)