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!!

 

[BridgeSmith]

AASHTO gives friction values for slip-critical connections as follows:

For Class A surface conditions 0.30
For Class B surface conditions 0.50
For Class C surface conditions 0.30
For Class D surface conditions 0.45

Class A Surface: unpainted clean mill scale, and blastcleaned surfaces with Class A coatings,

Class B Surface: unpainted blast-cleaned surfaces to SSPC-SP 6 or better, and blast-cleaned surfaces with Class B coatings, or unsealed pure zinc or 85/15 zinc/aluminum thermal-sprayed coatings with a
thickness less than or equal to 16 mils,

Class C Surface: hot-dip galvanized surfaces, and

Class D Surface: blast-cleaned surfaces with Class D
coatings.

From the commentary:

"On clean mill scale, this research found that even the smallest amount of overspray of ordinary paint, i.e., a coating not qualified as Class A, within the specified paintfree area, reduced the slip resistance significantly. On blast-cleaned surfaces, the presence of a small amount of overspray was not as detrimental."

"As a result of research by Frank and Yura (1981), a test method to determine the slip coefficient for coatings used in bolted joints was developed (RCSC, 2014). The method includes long-term creep test requirements to ensure reliable performance for qualified coatings. The method, which requires requalification if an essential variable is changed, is the sole basis for qualification of any coating to be used under these Specifications."

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

 

[lexeng18]

Very related question, I work in an office where we design many small, low rise steel x-braced structures. I could've sworn I remember reading an AISC excerpt or SteelWise article that said short-slots are allowed in collector/chord elements, but not in the actual bay of the braced frame? Maybe only so long as the structure can tolerate the additional movement from bolt slip, not sure what the specifics were.

However I've not been able to locate this. Intuitively, I can't think of a reason why it wouldn't be ok if the collectors slip along the line of resistance under a design-level wind or seismic event? One could even go so far as to check slip critical condition for unprepared faying surface against 10-year winds to ensure bolt-slamming wouldnt be a common concern.

 

[STrctPono]

To piggyback off of dik, sbisteel, and Bridgesmith, yes, there are primer coatings that can work in a slip critical application. I am not familiar with any topcoats that will work in a slip critical joint which is why you need to provide a paint blocking detail for slip critical joints. After the joint has been bolted and pretensioned, then you can come back and field touch up with final paint.

Whenever I spec out slip-critical joints I always have the Contractor choose a qualified system from the following NEPCOAT website.

https://www.nepcoat.org/qualprod.htm

To answer your original and very simple question, yes, if a pretensioned bolt in a standard size hole slips, it still has capacity in bearing.

 

[Veer007]

Guys, i too know structurally it's never acceptable to allow slip or improper faying surfaces for the slip critical connection, But What I'm saying as few of above are told that " at ultimate load condition slip happens if its standard hole then the connection is no more slip critical whereas it would not fail due to allowing slip" right?

Very interesting answers I got.. Thank u guys...

Thanks in advance!!

 

[BridgeSmith]

It all depends on what constitutes failure. If the connection slipping produces unacceptable displacement of the structure, or redistributes the loading to other parts of the structure that do not have the capacity for the extra load, then slip of the connection would be considered failure. Otherwise, slipping of the connection generally would not be considered failure.

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

 

[Veer007]

It all depends on what constitutes failure. If the connection slipping produces unacceptable displacement of the structure, or redistributes the loading to other parts of the structure that do not have the capacity for the extra load, then slip of the connection would be considered failure. Otherwise, slipping of the connection generally would not be considered failure.

That makes sense.

Thanks in advance!!

 

[Blackstar123]

Well technically speaking, the difference between slip critical and bearing type connection is the "slip". So if the connection is slipping it will behave as bearing type connection. Slip is not permitted in connections where load reversal could produce fatigue on the fasteners. So if the connection was originally design to resist the slip and if its not resisting it anymore then you should also be worried about the state of fastener that will be after repeated slipping.

Euphoria is when you learn something new.

 

[phamENG]

Question for the OP - why are you asking? Is there a particular situation you're evaluating, or is this just a general curiosity? If it's the latter, I'm not sure how many other ways there are to say it. If the former, lay it out there and maybe we can apply our thoughts to the specific case and help with that.

 

[Veer007]

It's my clarification regarding steel connection, how they are actually working? By different cases...

Thanks in advance!!

 

[ozPE]

i tend to focus on the bolt stresses in a slip situation.
in slip critical joints when slip occurs (say due to increased load) and the bolts start to bear on the holes, then the shear is shared by the faying surface and bolts (in what ratio that is another interesting topic). prior to slip the shear is taken up by the friction in the faying surfaces. so if you check your bolts for combined tension from pretension plus new shear you will reach to some conclusions. available shear strength in bolts for slip critical connections is less than bearing type connections.

 

[phamENG]

ozgurPE - true, but after your last deleted post, I ran the numbers on A325 assuming minimum pretension and shear equal to phi(rn) for slip critical connections in standard holes per the design tables in the SCM. If the connection slips into bearing with all tension in the bolts preserved, the available tension (F'nt) for a bolt loaded to phi(rn) is greater than the minimum pretension for all bolts. The gap widens as the bolts get bigger. So an overzealous iron worker on a 5/8" SC bolt may push you close to the limit, but in general I'd say it would be ok - especially if tension controlled bolts or "squirter" DTIs were used.

 

[dik]

I often encounter connections where the engineer has specified slip critical where snug tight would be OK... just a force of habit, I think.

Dik

 

[BridgeSmith]

phamENG, if the tension in the bolt is still present after it slips, the friction is still present as well, so the capacity is the slip resistance + the shear capacity of the bolt, isn't it?

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

 

[DrZoidberWoop]

I wouldn't consider the scenario at all. If you have enough bolts to handle a particular load via slip-critical conditions, you have more than enough to handle it in bearing. A pretensioned bearing bolt has the same tensile force on it as a slip-critical bolt, and we don't bat an eye. The old Von-Mises equation could be considered, but I wouldn't bother unless I was in a field of work that relied on high precision.

 

[phamENG]

Rod - yes, it would be. That adds an additional level of resistance to the connection. I think the moral of the story is that, if properly designed as a slip critical connection is loaded beyond its capacity and slips, it is highly unlikely that you'll have a localized rupture of the connection. What happens to the rest of the structure is case by case.

 

[ozPE]

phamENG,
using table 7-4 and J3.7 as per J3.8. A325, 7/8" bolt.
plugging the available shear from table 7-4 into equation J3-3a, i get F'nt=73ksi > min pretension=65ksi so yes i agree with you there is 12% extra tensile capacity beyond the min pretension using the shear capacity from the tables.

but 3 things could use up this 12%:
the perception of the word "MIN",
as you said overzealous iron-workers,
engineers omission.

if the shear is more than the capacity then there would be a problem. as i said it is a function of ratio of shear distribution between the faying surfaces and bolts once slip occurs.

however personally i would not base my design on available shear capacity in a slip critical connection, for serviceability as many mentioned above and for strength reasons.

 

[phamENG]

ozgur - I wouldn't be so concerned. As BridgeSmith pointed out, the if the tension is still there, the friction is still there. Though you'll get a transient drop while the slip is occurring (static to kinetic friction), and the assembly may experience some level of shock as the bolts impact the edge of the holes, the resulting steady state situation would include both frictional resistance (probably close to the original shear capacity), and so the bearing (and therefore bolt shear) will only be resisting the shear above the capacity of the slip critical connection. So to arrive at a combined tension and shear failure from bearing and pretension in the bolt, you'd likely have to see twice the design shear in the fastner.

 

[r13]

SC connection is usually designed to serve a purpose, once slip occurs, the function is lost. I agree there is no problem strength-wise, but service concerns.

 

[BridgeSmith]

If I understand the check for bearing/bolt shear in a slip-critcal connection, it assumes there is no tension in the bolt, and also no friction. This presumably would represent the worst-case condition of the connection. So then, any tension in the bolt would provide frictional resistance exceeding the loss in bolt shear capacity due to the tension stress. I believe this is why the strength load combination check has no reduction in bolt shear capacity for tension stress on the bolt.

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

 

[dauwerda]

from the RCSC commentary (section 5.1):

When required, pretension is induced in a bolt by imposing a small
axial elongation during installation, as described in the Commentary to Section
8. When the joint is subsequently loaded in shear, tension or combined shear and
tension, the bolts will undergo significant deformations prior to failure that have
the effect of overriding the small axial elongation that was introduced during
installation, thereby removing the pretension. Measurements taken in
laboratory tests confirm that the pretension that would be sustained if the
applied load were removed is essentially zero before the bolt fails in shear
(Kulak et al., 1987; pp. 93-94). Thus, the shear and tensile strengths of a
bolt are not affected by the presence of an initial pretension in the bolt.