Moment connection types 4

[Mohamed Maher]

Dear all,

If I use moment connection or splice is there any potability to make bolts snug tightening if i use AISC Code or IT should be Slip critical?

and what is the difference between Snug tightening bolls and pre tension bolts in design procedure or in bolts capacity?

Thanks
Maher

 

[Agent666]

No difference in the capacity in all codes I'm familiar with for snug and tensioned (slip critical) bolts. However tensioning help address joint rigidity and fatigue.

 

[Mohamed Maher]

@Agent666

Ok thanks
is the splice connections should be slip critical or it can be bearing connection?

 

[klaus.]

Bolt tension (pretension) will/can reduce the deformations and make the connection 'stiffer'
but the load capacity of the connection is (theoretically) the same ...

 

[Mohamed Maher]

My problem now that i had splice and I designed it as bearing connection and the two plates had already painted almost for the all project and all quantity .. consultant till now not accept design as bearing and required to be slip critical which means that painting should be removed from all quantity ..my question is that a must is the splice should be slip critical? or it can be accepted as a bearing connection knowing that moment values are small like 41 KNm and shear 82 KN?

THanks

 

[klaus.]

well basically the design should always be bearing .... not slip critical
slip critical is an addition to make connection 'stiffer' and reduce slip ( deformations/rotations )

But for a good slip critical connection of course paint must be removed...
special treatment must be done for the slip surface ==> see codes and regulations

 

[Guest090822]

Mohamed,
It depends on the structure. I’m in the bridge design business and memebers that carry live load are required to be designed as slip-critical. It will be hard to know whether or not your splice is adequate without knowing the loads. Sometimes you need more bolts when considering slip-critic, sometimes you don’t.
I suggest reading AASHTO LRFD section 6.13.
AISC is virtually silent on splices.

 

[Agent666]

Probably also some differences in practice regarding requirements for tensioning in some parts of the world if the splice is in a beam or a column, post lacked any context or actual detail of the splices or the type of structure that is being discussed.

Some types of primer are ok on the faying surfaces (in my part of the world at least) as the primer achieves a friction coefficient similar to a bare metal surface. Can't remember the type off the top of my head but it's only one specific type that's acceptable. Otherwise for slip critical, bare metal surfaces are generally required as klaus noted. Might be cheaper to refabricate the loose plates than remove the paint, but for sections/fabricated items you don't really have a choice but to remove it depending on what paint system is specified. Like I noted certain primers might be ok, but if the top coats have been applied then it's going to need to be removed.

 

[MotorCity]

Bolted moment connections and bolted splices should be made using slip critical bolts because movement (slip) in the connection would induce translation in a moment connection (which by definition is not permitted) and joint displacement in a splice (which will almost certainly result in serviceability issues).

In AISC, there IS a significant difference in bolt shear strength for snug tight vs. slip critical bolts. The difference is due to the fact that slip critical bolts require a certain pre-tension force which reduces the allowable shear strength under combined loading.

For example, for 3/4" bolts in single shear, in a standard hole, threads included in shear plane.......Shear capacity = 11.9k (snug tight) vs. 6.33k (slip critical)

 

[Agent666]

The force to slip (I.E, serviceability criteria) is lower as its based on friction and zero slip occurring, however the ultimate capacity in shear (after it slips) should be the same for snug and slip critical installations.

Pretension shouldn't affect the shear strength of bolts, they are relatively independent based on real world testing/research as I understand it. Basically irrespective of the pretension the full shear capacity can be attained in practice and many codes reflect this, AISC obviously are free to take a more conservative approach (not particularly familiar with AISC).