slip critical connections 4

[Heldbaum]

Hello folks. I have a question, is there any way to justify slip-critical connections for typical steel frame in residential building? I prepared calculations for all connections in the building (shear and moment connections) treating them as normal bearing connections but EoR rejected the calculations saying all of them shall be slip-critical. And yes, I found this information in his notes but I am just wondering what is the reason for those connections..? It's a 6 story, 25'x100' steel frame with sloped roof..Thank you for any thoughts.

 

[BAretired]

Slip-critical connections are normally used where slip in the joints under service loading would jeopardize serviceability of the structure. I don't know how this would apply to the structure in question.

BA

 

[kipfoot]

I think they're justified if the EOR is relying on them in conditions with a possible load reversal. If it's a normal braced frame then possibly one could make the argument for bearing connections. You could talk it through directly with the EOR.

Outside of strict engineering necessity, there are cases where the EOR may not want to spend additional time and effort to approve the change, issue revised drawings, and take additional liability for something that only saves the builder money. If the builder is offering a credit, you might find the conversation goes more smoothly, but I can't remember ever having that happen.

AISC has had some discussion about this, for example at this link.

 

[dik]

I would normally use slip critical fasteners for x-bracing, and that's about it. Do the drawing notes stipulate slip critical, and if so, for what items? Do the specs? Slip critical connections generally require added fasteners, but, at a cost. If not stipulated, then snug tight may be all that is required to supply, anything more may be an extra to contract, plus added work on your part.

Dik

 

[Heldbaum]

To make a long story short, EoR have not specified connections, his details are very poor, he just says in steel notes that connections shall be designed as slip-critical class A.But I overlooked this note since I was not expecting such a requirement and I calculated all connections as bearing. There are X braces at roof level plus moment connections , I don't know what EoR expects to happen there but in my opinion designing slip critical connections for such a frame, where no fatigue is expected, is an overkill. 2 times more bolts plus inspection is different money-wise and contractor has to know how to do these kind of connections etc. I am not much experienced but these are just my thoughts.

 

[Heldbaum]

Kipfoot thank you for the link

 

[dik]

Moment connections are also something I spec as slip critical.

added (from my drawing notes):

SLIP CRITICAL CONNECTIONS SHALL BE USED FOR ALL DIAGONAL CROSS BRACING, ALL BEAM SPLICES WITH BOTH DESIGN SHEAR AND MOMENT STIPULATED, AND ALL BEAM TO COL CONNS LOCATED AT EXTERIOR WALLS

SLIP CRITICAL CONNECTIONS SHALL BE USED FOR ALL CRANE AND HOIST SUPPORTS AND RELATED BRACING ELEMENTS

SLIP CRITICAL TYPE CONNS ARE TO BE DESIGNED FOR THE ABSOLUTE VALUE OF THE DIFFERENCE BTN MAX AND MIN LOAD


Dik

 

[Ron]

Heldbaum....kipfoot nailed it....if you have a potential of a load reversal, a slip-critical connection is required. Assuming this is for a project on Long Island, you have a wind load consideration (relatively high) that will require design for reversals.

 

[wannabeSE]

For high seismic, AISC 341-10 §D2.2 requires slip-critical detailing on seismic force resisting system connections. But, the shear capacity at standard holes is calculated as bearing bolts. The EOR may have additional reasons for requiring slip-critical connections. If the contract documents require slip critical, the EOR needs to approve any change.

 

[WillisV]

Some EORs specify SC bolts at typical to avoid "banging bolts"

 

[KootK]

I'm with OP on this one. I think that slip critical connections tend to be grossly over-specified in situations where they have no business being specified. And, without knowing all of the details, this sounds like a prime example of that where I suspect that EOR ignorance has led to unnecessary SC specification. The extra bolts and the attention to the faying surfaces are just so much urine in the jet stream. It's that extra inspection that'll kill 'ya.

That said, the drawings say what they say and the EOR is under no obligation to make any changes unless a material benefit to the owner can be tabled. This is the same reason that so may firms still spec connection shear forces as % uniform load capacity. Specifying precise load requirements may save somebody money but it's not the EOR and it's rarely the owner.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Istructeuk]

Another reason: it may be important to eliminate slip in connections. In some situations, accumulated slip in a number of connections may lead to additional deflection of the bolted assembly.

 

[Heldbaum]

Ron the building is in the middle of Manhattan..residential..

Thank you guys for all your comments. I appreciate it very much!

 

[cliff234]

The only time that AISC requires the use slip-critical bolts is when you have connections using OVS holes or slotted holes with the load parallel to the slot. In fact AISC 341 even permits the use of bearing bolt shear values (when bolts are installed in STD holes or slots with the load perpendicular to the slot) when designing connections in the lateral load resisting systems of high seismic structures (R>3) – however those connections must be detailed as slip-critical.

How about if you ask the EOR if you can detail the connections as SC, but design them as bearing bolts – and point out that this is what AISC permits in the Seismic Specification? This way you get the economy of the bearing bolts and the EOR gets their SC bolted connections.

 

[Heldbaum]

I had GC ask EoR for an explanation why he requires SC and this is his response:

"We always request slip critical bolts, because they're easy to inspect and they should become standard on all steel jobs. The shop drawings already show slip critical bolts, the calculations should match what is shown on the shop drawings"

 

[RWW0002]

I sure hope that was a GC interpretation of an engineer's response and not the actual response..

It sounds like the engineer may be referring to TC bolts in the response, but mistakenly calling them "slip critical bolts". Requiring pre-tension in high strength fasteners is a very different thing than requiring slip critical connections and I would hope the EOR would understand that.

 

[Heldbaum]

RWW0002 - No this was EoR's response, GC simply forwarded me the email he got from EoR.

 

[Ingenuity]

...and all the welds are CJP's

 

[dik]

I've had to correct a few engineers in our firm about the use of snug tight... seems to be a simple way of doing things without thinking...

Dik

 

[RWW0002]

dik - Out of curiosity, what have you had to correct about the use of "snug tight". I feel like snug tight joints are under-specified by engineers for many connections (the default being to require pre-tension for all high strength bolts regardless)