ASD vs LRFD, SC vs Bearing type and Service loads vs Factored loads 1

[Veer007]

Hey Guys,

How these terms are interconnected with each other?

ASD method of design using service load/unfactored right? Whereas LRFD is using factored or ultimate loads, right?

Also generally slip critical connection designed using service capacity of member (unfactored) whereas bearing type using its ultimate loads, right?

What was my concern is, do slip critical connection be connected for ultimate loads?

Similarly,

Do bearing type connections use service loads?

I know, we can use slip critical connection for both ASD and LRFD, but

Something looks Messy, anyone can assist me?

Thanks in advance!!

 

[r13]

Simply to say, ASD is using un-factored loads and un-factored load combinations, the results are to be checked against an allowable stress that is factored down from the material ultimate/yield strength. ASD can be, and has been, used for the design of both bearing and slip critical connections. I'll let others to point out the use of LRFD.

 

[phamENG]

The answer to your question depends on the desired performance of the structure. If the intention is to preclude slip at ultimate loads, then the slip critical connection needs to be designed for ultimate loads.

I would say that it is rare in building construction to intentionally design a connection to be slip critical at service but not ultimate loads. There may be specific cases where this becomes common that I have not encountered. I also believe that this is a method used in bridge connection design, though I'm not personally familiar with that.

This doesn't mean that a connection won't transfer some moment at service loads - a properly erected, snug tight connection will prevent slip up to a certain point, and it's important to know that when considering how a structure will actually behave, but it's rarely considered in design.

 

[jayrod12]

My opinion on why slip critical is to service, versus ultimate loads, is because generally speaking you should only need slip critical connections in the event of load reversals to prevent the bolts from banging back and forth in the oversized holes. However, that top me is strictly a use and occupancy type requirement and therefore falls under the service load category. As long as the building doesn't fall down in the event of a ultimate load situation, I don't care one bit if there's a bit of banging in the connection.

Generally speaking, if a building ever saw the ultimate load scenario, there's going to be repairs required. So what if they need to repair a couple of connections. This is also why I have a beef with the people who's general notes indicate all connections are slip critical, or even that all moment connections are slip critical (I can partly get behind the latter, but not the former).

I think you're not fully understanding the design methods.

ASD, allowable strength design as it is now called not stress, compares actual loads (service level) to a reduced strength.

LRFD, Load and resistance factor design, compares factored loads to actual strengths (with some reductions).

It's essentially a terminology and process thing. ASD you only have to run your analysis once, but perhaps have to use different allowable stresses depending on what item (or portion of item) you are designing. LRFD you get to use the same material strength (yield stress for steel for example) and apply it to either the factored loads when checking an ultimate limits state, or service loads when checking a serviceability limits state. I know I did a fair hack job of explaining this, but it's the best I've got since I've never truly designed anything to ASD except foundations

 

[r13]

Let me provide a numerical example to depict the role of SC connection.

A maximum service axial load (or Ultimate) = 100 kips
Selected beam axial load capacity = 110 kips
As a rare event, an overload of 125/130 kips could occur without notice.
Connection is SC, bolts tightened in according to beam capacity (say 110 kips) < bearing capacity.

At overload, the beam broke free when reached its capacity (110 kips) and rest in bearing position, with a remaining load of 15/20 kips to be resisted by bearing.

The bolts were not re-tightened after the event, and plant operation went back to normal. With bearing capacity was greater than the maximum service load, everything went fine until next overload event had occurred. At this time, without the buffer of the SC preload, the 125/130 kips axial force directly pushed the beam into bearing, thus the beam buckled, as its capacity (110 kips) was lower than the overload. However, the connection survived, as the bearing capacity was in the range of 130 kips and above.

I think there are many other examples. The point here is "the SC connection usually serve a special function, and it needs attention/work after it has encountered/executed the special function". So it is not a good idea to have SC connection at hidden location, or a location that is difficult to reach.

 

[Veer007]

This makes me wonder, can a beam transfer force more than its capacity when we provided SC connection?

So however the beam remains in slip critical connection, the beam will not buckle under service load condition?

Thanks in advance!!

 

[r13]

Veer007,

I hope the chart below can eliminate the confusion over designs based on LRFD and/or ASD.

Note that the SC connection will be tightened per the capacity of the selected beam, which in term is based on the required strength from design (the nominal strength).

 

[jayrod12]

Retired,

Up here in Canada I believe the SC design would still be for the 150, not the 230. The 230 would be used for the ultimate limits states checks like bearing, block shear etc. For SC as I indicated in the other SC thread is for service level loads, therefore unfactored.

Perhaps the LRFD design in the US is different than the LSD (Limit States Design), but I can't imagine it be significantly different.

 

[HTURKAK]

The answer is NO!.. The 2016 AISC specification requires the use of slip-critical connections ; in case of oversized holes,slotted holes with the direction of the load parallel to the slot ,and when bolts joining the extended portion of bolted, partial-length cover plates.
More over, RCSC specification requires slip-critical connections for Joints that are subject to reversal fatigue loading and when the slip at the faying surfaces could be detrimental to the performance of the structure.

My favorite code BS EN 1993-1-8 defines 3 categories for bolted type shear connections ; I copied and pasted below ;


The design of SC connection shall be based on ultimate loads in case of slip will be detrimental to the performance of the structure. For instance, slip of the faying surfaces could cause excessive P-Δ effects.

 

[phamENG]

jayrod - I'm a little skeptical of your approach. I certainly think it would work fine for a one or maybe two story building, but I'd be concerned if you get any higher than that. By allowing it to slip at ultimate loads, you're structure is going to be a lot more flexible. If you're depending on moment frames for lateral support, your going to see quite a bit of P-Delta if you let your moment connections slip. And if your ultimate load is seismic, then you'd be looking at a reversible ultimate load, right? That would demand a SC connection at ultimate.

Not saying you're wrong, I just don't think it applies universally. AISC's LRFD spec has equations for slip critical connections considering ultimate loads, and the RCSC has some pretty good info, as HTURKAK posted above.

 

[r13]

 

[jayrod12]

Thanks for that retired. I'm only discussing what I read in my local code which I posted an excerpt in the other thread. To me, slip critical connections are intended almost solely for situations where there is load reversal expected. At ultimate limit states, I don't particularly give a shit if the bolts slip as long as nothing falls down. And drift calculations are based on service loads, not ultimate, so there's that to keep in mind.

A caveat to my answers, I don't practice in any seismic regions, so that may have an effect on my opinions and experience.

 

[r13]

Jayrod,

You are correct, SC connection should be used for a "special/specific concern" only. I don't really concern at what load level (service/ultimate) of load it is specified, as long as it fits the need. However, keep in mind the high maintenance requirement attached to it, just specify it wisely to avoid sudden failure as described in the example provided.

 

[Veer007]

I just mess up with CISC and AISC, that may the reason for my confusion

In CISC they are following only one method named LSD, right? And LRFD and LSD looks similar?

As "jayrod12" states, in CISC, we check slip critical connection under service load but we also have to check that connection will pass at ultimate load using bolt Bearing/shearing resistance

Thanks in advance!!

 

[Veer007]

Anyone can picturise how slip critical connection deforms at reversible load and how bearing type was? If possible

All I can is just imagine how can this works

Thanks in advance!!

 

[jayrod12]

There's negligible deformation in a slip critical connection. That's the point. The intent is that it doesn't move or deform under it's design loads, regardless of load reversal. However if the design load is exceeded and the bolts go into bearing, then you could have the standard yielding at the bolt/steel interfaces that you see on all types of bearing connections.

As retired has indicated, once the slip critical resistance has been exceeded, then you cannot call it a slip critical connection again without some serious re-working and repair of the faying surfaces.

 

[r13]

jayrod's explanation is in line with this quote.

High-strength bolts in slip-critical connections are permitted to be designed to prevent slip either as a
serviceability limit state or as a strength limit state. The most common design case is design for slip as a
serviceability limit state. The design of slip as a strength limit state should only be applied when bolt slip can
result in a connection geometry that will increase the required strength beyond that of a strength limit state, such
as bearing or bolt shear. Such considerations occur only when oversized holes or slots parallel to the load are
used, and when the slipped geometry increases the demand on the connection. Examples include the case of
ponding in flat-roofed long span trusses, or the case of shallow, short lateral bracing.

 

[BAretired]

Another quote:

1.Introduction

Slip-critical (or friction type) joints are joints that have a low probability of slip during the life of the structure. Because of their increased cost compared to connections with snug tight bolts, slip-critical connections should be used only when it is expected that slip in the joints would jeopardize serviceability of the structure, or would result in a reduction of the ultimate strength of the structure. The RCSC guide suggests that slip-critical joints should be designed to prevent slip under service load conditions and prevent rupture at factored loads, thus making the slip-critical joint behave as a bearing-type joint at the factored load level. This requirement was clarified in the latest edition of the RCSC bolt specification (Schlafly, 2004). It is also possible that slippage in the joints may result in significant second order effects in the structure, which could reduce the stability and strength of the structure. In this situation, the AISC specification has included in its 2005 edition provisions to design against slip at the factored load level. The design of slip-critical joints has traditionally been performed to prevent slip of a joint at the service load level. The consequence of slip at the service load level is usually minimal.

BA