Slip critical bolts_Steel Structures 2

[Veer007]

Hey Guys, If slip critical bolts failed coz of loss of friction on the faying surface, can this connection work as bearing type? Awaiting for your more wise and intellectual answers.

Thanks in advance!!

 

[Veer007]

Guys, is there any chance that bolt where carrying tensile load can be slip critical? i.e for Hanger with 10mm thick base plate and 4-19 dia A325 bolts.

Thanks in advance!!

 

[r13]

Yes, you can. If you can design the connection, under full load, without losing contact on surfaces required to provide friction resistance. I find it is difficult, if not impossible.

 

[klaus.]

Guys, is there any chance that bolt where carrying tensile load can be slip critical?

yes of course... no difference to a not slip critical connection....

 

[BridgeSmith]

...is there any chance that bolt where carrying tensile load can be slip critical?

As others noted, yes, but you have to subtract the tension on the bolt from the clamping force that provides the frictional resistance to slippage. In AASHTO, it's a straight reduction factor - 1 - (applied tension / min required bolt tension). (Equation 6.13.2.11-3)

On a subject of discussion earlier, this is from the AASHTO spec (8th Ed. C6.13.2.1.1, commentary):

"In bolted slip-critical connections subject to shear, the load is transferred between the connected parts by friction up to a certain level of force that is dependent upon the total clamping force on the faying surfaces and the coefficient of friction of the faying surfaces. The connectors are not subject to shear nor is the connected material subject to bearing stress. As loading is increased to a level in excess of the frictional resistance between the faying surfaces, slip occurs, but failure in the sense of rupture does not occur. As a result, slip-critical connections are able to resist even greater loads by shear and bearing against the connected material. The strength of the connection is not related to the slip load. These Specifications require that the slip resistance and the shear and bearing resistance be computed separately. Because the combined effect of frictional resistance with shear or bearing has not been systematically studied and is uncertain, any potential greater resistance due to combined effect is ignored."

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Rabbit12]

The noise created when a slip critical connection snaps into bearing can be loud and very unnerving. I personally never spec slip critical connections unless I need them for some specific reason. For one, as discussed they generally have a lower capacity.

 

[JLNJ]

I believe that bolts in direct tension are required to be fully pretensioned, unless you are sure the load is static and the bolts are A325. No faying surface prep is required.

If you lose, or never had, proper pretensioning of bolts in direct tension, then the joint is out of spec.

 

[ozPE]

Here is a brainstorming question, is there a way to increase the coef of friction in the faying surfaces beyond what is allowed in the codes? i did this for a steel clamp that was clamping a timber post with weld buttering, approx 0.5mm high low strength weld applied on the clamp surfaces in lines at +/-10mm spacing perpendicular to the load. this was ok because it was steel against timber, so 0.5mm ridges biting into the timber. Conservatively i used 0.6 for coef of friction, i bet it was more. I would think that for a steel on steel we would need to do a load test to verify and justify the coef friction if we treat the faying surfaces with something other than the standards specified in the codes. Any thoughts?

 

[phamENG]

ozgur - is that really increasing friction, or is it altering the load transfer mechanism? Seems like you're creating lots of small bearing points to transfer the load in addition to the friction between your weld lines.

 

[DrZoidberWoop]

Yes. We joke that we should charge the client extra for allowing the steel beams to sit outside and gather small amounts of rust in the fab yard. Thin rust layers increase the coefficient in a lot of cases. Aging coatings like wine haha

 

[Veer007]

yes of course... no difference to a not slip critical connection....

you mean no difference..then why we call slip critical?

Thanks in advance!!

 

[dik]

If it's loaded in tension... there's not much slip...

Dik

 

[klaus.]

you mean no difference..then why we call slip critical?

I mean no difference in tension capacity if connection is slip critical or not

 

[dik]

Yup... unless combined with shear...

Dik

 

[ozPE]

phamENG,
It is kind of altering i guess if you zoom in,

 

[BridgeSmith]

...steel against timber, so 0.5mm ridges biting into the timber.

...is that really increasing friction, or is it altering the load transfer mechanism?

In the case of the steel against timber, what you effectively created was shear keys. With steel on steel, the effect would be minimal, possibly a worse condition overall if the internal shear capacity of the weld metal is exceeded.


Rod Smith, P.E., The artist formerly known as HotRod10