Possible damage on Aluminum Members due to pretensioning of A325 Bolts

[Surya77]

Hello Frs,

To counter weight concerns on slab, I am design a braced frame structure with Aluminum. Initially for all the fasteners I specified Al alloy 2024 but due to lead time/procurement issues, I have to use A325 Bolts.

Now the issue is, for slip critical connection considering 3/4 in dia fastener the required torque is around 350 ft-lbs. So if the bolt is torqued around 350-400 ft-lbs will it cause permanent indentation/or deformation on the AL members thus loosening the bolts?

Any thoughts in countering this issue

 

[Tmoose]

Maybe adequately covered in a code somewhere, but I just don't know.

What What is the fastener grip length? What is the range of ambient temperatures?
With the potential for thermal expansion/contraction I think I'd be stacking the deck by protecting the surfaces against the ~30,000 lb clamp load and sliding.
I'd try a couple of test assemblies to evaluate interface damage. I like to use generous diameter, thick, good quality, even hardened washers under the heads of fasteners clamping steel components with decent surface finishes and geometry. They add some grip length, protect against surface damage from tightening and clamping pressure. Some resources warn that using washers causes various "bad" things, but the luxurious smooth feel when tightening a lubricated fastener combo pleases me.

 

[JAE]

Do you need to use slip critical? And does slip critical even work on aluminum? I doubt that it does.

Why not just snug tight bolts for your braced frame?

Also - aluminum and steel react to one another (rusting). Need to keep the materials separated or coated.

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[Surya77]

Thanks for your time
@Tmoose

grip length is 7/8 in. Ambient temperatures are 70-120 F

@JAE

In recent past I ended up dealing with fabricators driller oversized holes and then PM asking me to qualify the bolts. So I am trying to be conservative.

Slip critical works on AL if we prepare the surface (coal slag blasted). But I need to talk o Fab shop if that can be done in time and with in the budget.

YEs the bolt and AL is isolated (painted/coated) to avoid corrosion.

But looks like designing a bearing connection and mentioning snug tight is preferred to avoid other issues

 

[racookpe1978]

IF - Big "IF" there - this is a temporary slip-tight connection used during construction for a lift or a tilt operation, then not left in place, then corrosion will be a minor issue for a typical short construction period. A nuke plant or a dam or tunnel 10 year construction period is a whole different time frame.

Also, it depends on the specific shape of the two Al shapes at each bolted connection: Is the bolts go through a hollow square tube steel shape to clamp, then compression is very likely, almost certain. But two flat plates compressed together by a bolts through both? Not so much at the same torque.

 

[wannabeSE]

I don't have experience with this. But, the 2010 Aluminum Design Manual has provisions for slip-critical connections that use galvanized A325 bolts with washers on each side.

 

[SAIL3]

slip critical connections are expensive....the fabricator should not be drilling oversized holes without a good reason in doing so....

 

[Surya77]

@racookpe1978 (Nuclear)

No they are two flat plates together. I am concerned is about severe indentation due to heavy torque. like bolt punching through the softer metal.

@wannabeSE (Civil/Environmental)

YEs that's true, but they refer back to AISC pretensioning values. That is where I feel there is a disconnect

@SAIL3 (Structural)

Ya u r right.

Now to answer all my issues, I changed the design to Bearing Type. This relieves me from spending too much time. Thanks to all for your inputs

 

[boo1]

check the plastic deformation that occurs under the nut face, size washers to prevent permanent deformation