A325 vs A490 Bolts used as a pin. 6

[RARWOOD]

I have an application where I am overlapping steel plates at a truss joint in a timber truss. I am using a 2" dia. A490 bolt so that the joint behaves like a pinned connection. The nut is shop installed finger tight so the bolt doesn't develop any clamping action.

In this application A490 bolts are used instead of A325 so that a smaller bolt can be used.

Neglecting the cost difference is there an advantage of using a slightly larger A325 bolt instead of a A490 bolt?

 

[HgTX]

The cost difference may actually work in your favor, depending on the relative costs of A 325 and A 490 bolts.

A 325 bolts are more ductile, if that's of any concern.

Hg

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[kbarnett]

The only advantage I can see for using a lower strength bolt is that the larger diameter bolt required would increase the bearing area of the connecting plates (for a given thickness). A larger bolt of lower strength may allow you to use thinner plates.

 

[RARWOOD]

In working on my problem one concern I had was that I have heard that A490 bolts are brittle. Since my application is a little unusual and since for the last 10 years I have only worked with A307 bolts I thought I would ask about A325 and A490 bolts.

Since the major load at this joint is snow the load will be applied gradually without any dynamic loading. Also because the truss has a low span to depth ratio there will only be a very small amount of rotation at the joint.

From college thirty years ago I remember that brittle failures occur quickly. With my joint there is no redundancy, if the 2" bolt fails the truss comes down.

My current field of experience is the design of wood connections so its been a long time since I've worked with A490 bolts. So in working out this design I want to make sure that I am not overlooking something that might cause a brittle failure in the joint.

 

[HgTX]

I now have a dumb question. If A 325 bolts are so much more ductile than A 490 (the rational for allowing reuse of A 325s), how come both have the same elongation requirement of 14%? Am I completely misconstruing the relationship between ductility and elongation?

Hg

Eng-Tips policies: faq731-376

 

[MintJulep]

Brittle failures occur quickly. This has little to do with the speed of the load application.

Just as a point of principle, try to avoid designs where a single-point failure allows the whole thing to fall down.

 

[metengr]

HgTX;
Ductility is the ability of a material to absorb plastic deformation or strain before final fracture. Elongation and reduction of area are measures of ductility. Strength is probably the key above because different strength levels can be specified with a minimum elongation requirement.

 

[CoryPad]

Another possible advantage of A 325 vs. A 490 is resistance to Environment Assisted Cracking (Stress Corrosion Cracking, Hydrogen Embrittlement).

Regards,

Cory

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[SteelMover]

HGTX, RARSWC-

Even if A325 and A 490 bolts had the same fracture toughness, (which I don't have time to research) you have a lower saftey factor against fracture failure by the Griffith equation if you are applying a higher stress to the A490 than say the A307. You will genearlly be stressing A490's to a higher level or you would just be using a lower strength bolt. In designing pinned connections we we always turn our own using lower strength, more ductile steel, and we retain them with snap rings or spiral retaining rings. Like you said if it fails the whole thing comes down (increase safety factor here). You may also find the 490's to be expensive. I just had a number of 6" dia x 12" pins turned for $130 and 3" dia x 6" turned & plated for $50.

 

[RARWOOD]

SteelMover

I have several questions about your use of 6" diam. pins.

1. Do you take bar stock and machine it down to make 3" - 6" pins?

2. Also do you do this in your own machine shop or do you order them from a specialty company like Dyson?

3. Is the $130 amount per 6" dia x 12" pins.

Generally the company I work for, orders all their hardware from a supplier like Building Fasteners or Fastenal. The only time I get involved in ordering, is when an item is a specialty item that our normal suppliers are not familair with.

 

[SteelMover]

Just about any machine shop can do pins with a wide range of steels. I would bet you are within 10 miles of multiple shops that will do this. We typically spec AISI 1018, 1045, 4140, 4150, 8620; all readily available in bar stock. Check a Ryerson catalog for material properties & availability.

Make a drawing of what you want email them for a quote. Send it to more than one shop because they vary widely. Some shops sub out this type of work so they come in higher. Typical tolerance on pins that size is +.000", -.001" on the diameter. If you get it plated spec the OD after plating. Price was each piece.

 

[RARWOOD]

Thank you every one for your advice.

 

[MrStohler]

FYI

The ASTM A490 specification is limited to bolts between 0.5" and 1.5" (inclusive).

A 2" pin would need a different material specification than A490.

 

[LEEMARVIN]

A449 would might be the spec for 2".

 

[Dinosaur]

The larger material may also provide a higher factor against corrosion loss. If the rate of corrosion is X inches per year, then it would take longer to compromise a larger piece of material. I believe this is a bit of logic used in the AASHTO Standard Specification for minimum plate thickness.

 

[Metalguy]

"Brittle failures occur quickly. This has little to do with the speed of the load application."

Not true. Fast rates of loading can make steels change from ductile to brittle fractures.

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