Long-slotted holes as expansion joint 7

[zoparrat]

What is your opinion on the use of long-slotted holes to accommodate thermal/wind induced movements (not temporary but as a permanent expansion joint)? The long slot is in the horizontal direction; gravity load is normal to the direction of the slot. I'm looking for references that advise against this practice.

 

[slickdeals]

It is a bad idea to use long slots as a permanent expansion joint.
See attached article.

 

[Ron]

They are usually detailed and installed incorrectly to work for the long term. If you use it, make sure there is a slip pad between steel surfaces and that the bolts are finger tight but with a jam nut.

 

[zoparrat]

slickdeals: thank you!

Are there any codes, design guides etc that doesn't allow the use of long-slotted holes for this purpose? As far as I can tell the AISC manual and the AISC high strength bolt design guide doesn't say anything about accommodating movement along the slot; just that the loads should be close to normal to the direction of the slot.

 

[nutte]

Slickdeals beat me to it. See also

http://failures.wikispaces.com/David+L.+Lawrence+Convention+Center.

I've always thought these connections violate the specification anyway. The spec doesn't make allowance for shear being tranferred through bolts in a joint that are not at least snug tight. These bolts are only finger tight, and they are being used to transfer shear load.

 

[rowingengineer]

I use them, detailed them similar to Ron’s explanation, and I require a hold and inspect for the contract. I don't think an article about a bridge expansion joint should be used for all structure types for recommendation on expansion joints. I have three buildings over 200m long that have expansion joints, have reviewed the buildings over the past 5 years and as of yet they are working great. As for the length of building that requires an expansion joint, while that is another question.

ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION.”

 

[racookpe1978]

I think you need to address all of the "awe-sh*ts" and "I didn't think that would matter" and "what-else-could-go-wrong" problems very, very carefully.

For example: Rust (or dirt, slime, paint, barnacles (?) and biologic trash like pigeon poop or the like) in the joint, slot, and between the bolts and the supposedly sliding part of the three pieces. Galvanic corrosion, flooding or roof leaks, new insulation in ten years, new paint, sand-blast residue not removed, grit and sweeping from later renovations will foul the supposedly "open" slots and prevent movement. (Each part ( the bolt, the beam, and the supported/supporting steel) have to remain free for the life of the building - and that's very hard to guarantee in the real world for even four to ten years.

Assume your design is perfect and will be kept perfect over the life of the building. But this means the mechanical parts of your assumed design need to be verified, then re-inspected regularly during construction of every joint. Unfortunately, without very, very diligent inspections, today's installers won't do it themselves, nor will they always ensure their newbies and apprentices will do the assembly with the care and attention to exact detail that your drawing must assume will be followed. Too much day labor, temp labor, and unskilled labor is used today to avoid precautions that - in year's past - could be taken for granted.

It is hard enough to even get all the bolts, washers, and nuts mounted in conventional steel joints even made up when they are all identical, much less fully torqued properly. And here you are demanding that some bolts are snug, some torques tightly, some with backing nuts ......

The mechanical details above are good. They are proper for designing such a flexible joint in the building. But if you can avoid using them - all the better.

 

[msquared48]

Ron didn't mention it, but do not get peanut butter on the Jam nut.

Mike McCann
MMC Engineering

 

[zoparrat]

nutte: "The spec doesn't make allowance for shear being tranferred through bolts in a joint that are not at least snug tight. "

what is this spec you are referring to?

 

[JAE]

I've never allowed slotted holes to be used in an expansion joint as the direct shear support mechanism. Grinding of the bolts with cyclical building expansion-contraction just doesn't "feel" right and I'd always use a seated connection with teflon/slide bearings instead....or preferably a double column system.

 

[ajose]

I do not agree the statement of the slot holes to build expansion joints have to be discarded.

I have read the paper and it like the joint was not the proper one and was not carefully designed.

The sloted hole is normally used on seated beam to void unseated. So the bolts are locked with two nuts and the suface is the one that takes vertical forces. This is a tipical detail on expansion joints on buildings.

I think the way it is said on several reports "the failure of structures is 50% by poor detailing, the other 50% are other reasons".

 

[Ron]

I agree with JAE that a double column system is the better way to go.

Mike...some of the frozen joints I've seen could have used a little peanut butter...and I prefer grape jam.

 

[connectegr]

I think everyone else has said it... DO NOT USE LONG SLOT AS AN EXPANSION JOINT.

Long slots used in the stability angle of a seat connection is intended to avoid rigidity in the connection. The bolts are not in bearing, which can resist slippage. The connection shear is taken in the bearing seat connection only. With the addition of slide bearing pads, you have an expansion joint.

I have reviewed the forensic report on the Pittsburg Convention Center, and several factors contributed to the poor movement in the knife connections that failed. The actual failures were in the welds of the knife angles to the columns. The lack of slippage in the angles induced large axial loads and prying in the welds of the outstanding angle legs. As a practical connection it is impossible to determine the effects of surface prep, bolt plowing, and connection eccentricity on the expansion/contraction movement require by the connection.

http://www.FerrellEngineering.com

 

[connectegr]

The condition and failures in Pitsburg were not limited to the exterior bridge area only. All expansion joints were retrofitted with slide bearing seat connections. The most significant failure was due to the extreme temperature changes and contraction of the concrete, while supporting the load of a semi truck. But nearly all the expansion joints showed distress and many had cracked welds.

http://www.FerrellEngineering.com

 

[hokie66]

Just one more vote for prohibition of this type joint for support of large gravity loads, e.g. floor beams. However, the OP's query was originally about thermal and wind induced movement with no mention of the type members involved. I think slotted connections can work in light roof structures, especially where snow loads are not involved.

 

[IJR]

This is a great thread.

I never never never use slotted holes for expansion. Some old book I can not find anymore suggested not to use these when vertical reaction exceeds 12 tons metric(26kips).

Next I wouldnt use anything that is not likely to pull itself back gracefully when the expansion is gone.

If my boss forces me to use them(he is a proponent), then I would put a plate with rounded top at contact, to cut down friction and sticking, or wait until he doesnt watch and go on with PTFE or elastomer

respects
ijr

 

[zoparrat]

Thanks to everyone who shared their opinion. I'm on the camp that says a shear connection with long slotted hole for expansion is not a good idea.

Now I need to find code references, provisions etc to back this up. Has anyone seen something to this effect anywhere?

 

[ToadJones]

The connection described by the OP is a bad idea.

Because some have used it successfully means absolutely nothing to me.

I have recently done some investigation into thermal effects on connections in some industrial buildings. The thermal effects are very hard to quantify.
IMO, if you cannot accurately determine the effects of thermal and wind on your connection, you have to eliminate that force....best way is a double column row.

I agree with JAE's description.

How can one depended on a direct bearing shear connection to slide?
How do you determine the force at which the connection will slide?
This detail screams of fatigue problems.

 

[WillisV]

Another brief article on the conference center connection from ENR attached.

zoparrat, there is no code reference to back not using this detail, its just good engineering judgement. The code cannot specifically exclude every poor detail anyone can dream up.

 

[JAE]

The code cannot specifically exclude every poor detail anyone can dream up.

Agree with that. The codes identify good practice. Not too many sections of codes and specs spend time identifying bad practices.