Tubular Fracture Critical Member

Date 06-16-2004 21:41

Is your bridge being built to AWS D1.5?
That code requires main material to be CVN tested and weld PQRs are CVN tested (along with other test) to qualify. Fracture critical (FC) test results are required to have higher impact strengths than non-FC. D1.1 does not have the same provisions but in either case, the field and fabrication welds themselves would not be CVN tested. More likely, you would do ultrasonic (UT) testing of the welds. RT (radiography) could be done but would be more difficult. D1.5 or D1.1 have reasonable acceptance criteria for both RT and UT.

As for full pen welding, the only way I know to do that is with a backing bar fitted inside the tube joint. If you can allow access holes near the joint somewhere, you could weld inside, then backgouge and weld the outside. But that still is not easy to do with a HSS 8, not much room internally.

I assume the weld would be a field weld? to make shipping easier? Why not design a bolted splice with access holes? It would be simple to provide a weather cover over the access holes to limit corrosion problems. For that matter a joint without access holes could work, and field welding would not be a concern.

Fracture critical members are defined as main members that are subject to tensile loading and are not structurally redundant, the failure of which could cause collapse of the structure. I am not an engineer, but it would seem to me that a mid span splice would be a compression area and not necessarily fracture critical? If welded, the joint would likely have backing bars left inside, which would cause stress raisers. Not really good for a true FC condition but probably would be fine in a compression loading.

Chet Guilford

By swnorris (****)

Date 06-16-2004 22:10

Hi LL,

I can't help you with FCM criteria, but maybe I can explain the Charpy V-Notch Impact Test. It is used to determine fracture toughness in metals, and is used in specifying minimum acceptance criteria for base metal and filler metal for fabrication and for welding procedure qualifications.
For metals such as carbon and low alloy steels that show a change in failure mode with decreasing temperature, tests are conducted at several temperatures. The most commonly reported result is the absorbed energy, but the percent shear fracture may also be reported. Metals showing high CVN energy absorption are resistant to brittle fracture in service. There is no threshold value of energy absorption that will assure ductile behavior in service. High strength metals can store more elastic energy than low strength metals, so to provide the same ductile fracture behavior contained in lower strength metals, high strength metals should exhibit a higher CVN energy absorption. Specifications for most metals that include CVN impact tests normally require minimum energy absorption at a particular temperature. Some fabrication codes may require that the CVN energy absorption of the weld metal and the heat affected zone of the welding procedure qualification test plate be determined. These codes also prescribe conditions that require making the test, the test temperature, and the minimum acceptance result. In addition, some particular codes require impact testing of production welds. The production test plates may be prepared as runoff plates from a seam weld, or may be separate test plates, but they should be fabricated at the same time as the production welds and follow the same weld procedure specification.
In the CVN test for impact strength, there are two types of commonly used specimens, those prepared with a keyhole notch, and those with a v notch. However, there is no reliable method to convert v notch data to keyhole values, or vice versa. The test specimen is 55mm long and has a cross square section of 10mm. The test specimen is positioned with the ends of the notched surface straddling two supports and is struck opposite the notch by a wedge shaped hammer at the end of a pendulum, which swings from a standard height, The testing machine indicates the amount of energy in ft.-lbs. required to fracture the specimen. This is a measure of the notch impact strength. As I previously indicated, some steels show a considerable loss of notch impact strength at low temperatures. This is the reason that tests are made at different temperatures. The results of which provide information about the overall toughness of the metal and the temperature at which it can be expected to fail in a brittle manner.

By LL_biggs Date 06-16-2004 23:42

Thank you for the responses. Would you (or anybody else for that matter) be able to expand on your replies? Namely, how does one perform a CVN test on a tubular specimen; specifically, sections with curved surfaces? All the literature I have read through seems to discuss CVN testing on flat sections (e.g., plates and bars). Does such criteria for tubes and pipes even exist?

By CHGuilford (****)

Date 06-17-2004 19:09

I am at another jobsite today, so my info is not handy.

Essentially, tube steel is nothing more than plate formed and welded. Take a look at ASTM A673 for all you will need on CVN testing info.

Chet Guilford

By swnorris (****)

Date 06-18-2004 21:55

Hi LL,

I agree with what Chet is saying about the welding, and will add that if CVN testing is required, in order to perform CVN testing, you have to remove test specimens from the weldments. As far as welds go, you can only perform the CVN test on test pieces welded in the exact same conditions to represent the same properties finally available in the finished bridge, with the exception of the curves, not specimens from the actual finished product.
I don't know what design you're using on the bridge, and I am definitely not an engineer, but if there's a single tube arch on each side, and the splice is at mid-span, it seems to me that the weld on the top of the tube would be in compression and the weld on the bottom would be in tension, and subjected to tensile stresses. I say this because tensile stress is the stress that is applied to an object by either pulling on it or attempting to stretch it, which would seem to apply to the bottom weld, and compression is the opposite, which would seem to apply more to the top weld, as the compression forces are trying to compact or crush the weld. As for the other two sides, it seems that it would be a gradual shift from compressive to tensile stresses, because it seems that the forces on the top half of the weld would be trying to compress it, and the forces on the bottom half of the weld would be trying to pull it apart.
Just curious here. Is it a requirement in the specifications or bid documents that the fabricator be an AISC certified company in Simple (Sbr) or Major (Mbr) Steel Bridges? I think that if Fracture Critical members are to be produced in accordance with AAASHTO or AREMA codes the fabricator must also have the (F) Fracture Critical Member Endorsement.

Here's a link that might be of use to you if you don't already have it:

https://www.transportation.org/download/SBFQC-1_AASHTO.pdf

It's to the Steel Bridge Collaboration Guide Specification, which is a guide specification for bridge fabrication, quality control, and quality assurance. It is an AASHTO/NSBA publication with loads of information.

By CHGuilford (****)

Date 06-19-2004 14:13

I apoligize, I was out of town and could not give your question my full effort. I mean this with all due respect, but you really should do some more research on the subject. But I will give you a nutshell explanation.

CVN testing is used to determine the low temperature impact strength of the steel being tested. It gives a value relative to the material that is being tested to see how it can be expected to perform in comparison to other materials. This gives a relative indication as to how the steel will behave at depressed temperatures under an impact loading.

The test itself consists of machining specimans from a sample of the steel. Commonly, 3 specimens are made for each test. The specimens are a certain size (usually 10 mm X 10 mm x approx. 75mm) and have a precisely shaped notch cut into them. The specimens are cooled to a certain temperature, depending on the Zone specified. Specimens are placed in a (calibrated & certified) machine, a weighted pendulum swings and breaks them, and energy absorbed by the specimen is recorded; usually in foot/ lbs for the USA.

Usually specimens are extracted parallel to the grain, or rolling direction, of the steel. Some job specs require some testing done transverse to the grain, which is steel's weakest axis (much like the grain within a piece of wood)

Zone 2 testing requires 15 ft/lbs avg min at +40F for non FC materials (T2) and 25 ft/lbs avg min at +40F for fracture critical (F2). You will see there are T and F test designators. Zones correspond to geographic areas of the steel's end use (1 is for warmer areas such as Florida, 2 is used in most of USA, and 3 is for arctic conditions).
There is also "H" frequency where one CVN test represents all material produced from the steel heat number, and "P" frequency where each plate or batch of shapes is tested. Fracture critical work usually requires "P" frequency.

Welds are tested in a similar way, specimens are extracted from a test plate such that the notch will be located within weld itself. CVN test results for welds usually require a colder test temperature. Fracture critical, zone 2 is 25 ft/lbs avg min @ -25F (or somewhere around there, I don't have my D1.5 handy).

Naturally, all this is destructive testing.
It is not useful for testing actual production welds or production materials.

CVN tests (and several other tests) are done on weld test plates to prove the filler metal/base metal combination works when welded according to test parameters. As long as the testing was done, there is no need to do it again. QA/QC oversight makes sure the work is welded correctly.

Materials are usually purchased to comply with CVN specifications. The mill will cut off a piece of material and perform CVN testing on it. Results are then given to the customer. Sometimes a vendor or a fabricator will cut off a sample for testing when the original mill order did not specify otherwise. That is usually accepted by the owner (assuming satisfactory test results).

Rectangular HSS steel is formed from plate and welded together. CVN specimens can be easily cut from a piece of stock, remember, 10mm X 10mm. When the sample is too thin for full sized CVN specimens, sub-sized specimens are used.
Mills that produce HSS obtain their raw materials from plate producing mills. If requested, the HSS mill can usually produce a mill test report for the plate they used. So an A500 Grade B could have started as A572 Grade 50.

Hopefully this clears up what you asked. I would not ask for production welds to be CVN tested. Instead, I would require that FC welding be qualified by testing per AWS D1.5 section 12. I would specify that materials must meet F2 CVN requirements (assuming you are not in Alaska).
I would require MT (magnectic particle) testing of the root passes of full penetration welds, and UT and/or RT testing of the completed full pen welds. MT testing of fillet weld (usually 10% of each weld or 12" minimum) is a good idea. 100% visual is already required if D1.5 is invoked.

But also important, please realize, the "FC" designation is very over used when the actual conditions are not fully understood. FC fabrication is much more expensive and time consuming, and doesn't automatically mean the product will be better than "regular" fabrication would be. That's why I asked if you situation is a true FC condition. You might save a lot of time and money and still have a very satisfactory bridge that will last a long time.

Chet Guilford

By akottens Date 06-20-2004 01:27

a couple thoughts, not necessarily answering your question:

1. Tubular Members often are manufactured with longitudinal welding seams (sometimes not though, check the ASTM specification in question). Technically, this welded seam, made at the pipe mill, was not made to Fracture Critical requirements (at least per D1.5 requirments). From that limited standpoint, if you have welded seam pipe, the material itself cannot be considered FCM compliant. Perhaps this is a technicality. I would be interested in anyone's thoughts on this.

2. Since this structure will apparently not see any vehicular traffic, and therefore will not necessarily undergo the cyclical loading seen by vehicular bridges, perhaps the FC designation is not appropriate here. I recognize that the arch tie is in tension and is non-redundant, though. Any chance of simply re-categorizing the structure as non-FC?

3. It has always been a dilemma when working with tubular members on bridges. The D1.1 code deals with tubular but does not deal with Fracture Critical. The D1.5 deals with Fracture Critical but not tubular members. Hopefully the project specifications recognize this and have somehow melded them together.

4. Aside from the above...as you have laid this out, I suggest that the fabricator qualify the welding electrodes on ASTM A709 Grade 50W material meeting the minimum chemistry of D1.5 Section 5. This welding test would include the charpy v-notch testing of the weld metal. Chapter 12 of the D1.5 will apply also - especially for testing temperature for the Charpy's. The fabricator should submit a welding procedure for approval using a prequalified joint detail from AWS D1.1. The aforementioned PQR should also be submitted for approval as support for the welding procedure.

In any case, for procuction welding, a backing bar will be required and will have to permanently remain the joint. You don't have a choice on that even though the D1.5 does not allow leaving the backing in there on tension members. The backing bar should be continuous with no gaps. NDT should be done on both the backing bar and final welded joint. I won't specify here, as to my opinion on what NDT to do. Welders should be appropriately qualified to weld tubular members in the positions encountered.

I would be encouraged if anyone had differing ideas to share and compare.

Thanks