Friction grip collar on CHS

[STpipe]

Typically we use welded shear tabs on CHS columns which we use to connect beams, braces etc.

For various reasons which I won't get into, some of my colleagues are looking to move away from this detail, and instead use something similar to a friction collars (custom part) and have the shear tab welded to it. I have been asked to provide an opinion on this new detail.

They have friction collars for concrete (see links below), however I have never seen anything like this for steel. I have no experience with the ones for concrete.

http://www.proscaffna.com/frictioncollar.php

http://www.cffeco.com/#!friction-collars/c112d

At first glance, this type of connection seems like a bad idea because of unknowns like the actual coefficient of friction between the surfaces, how much of the collar is actually in contact with the CHS wall due to potential fabrication errors and just the general concept of having a connection held up entirely by friction.

Does anyone have any experience with this type of connection (either with steel or concrete) and how it actually works in practice? Are my concerns entirely valid?

 

[jayrod12]

My first feeling is yuck. why bother with that route if you're still going to weld the shear tab on in the end?

 

[BUGGAR]

If the connection is subject to wind or seismic forces, it may be subject to load reversals. These can reduce or eliminate frictional resistance. Vibrating or impact loads, too.

 

[STpipe]

jayrod12,

That's my general opinion as well, but I understand what my colleagues are trying to achieve in terms of developing an alternative connection detail. The general idea is that we have a large number of these columns that need to be produced, and each of them will have multiple shear tabs welded to them at different orientations - resulting in a lot of unique parts that require their own shop drawings.

The intent is to reduce the number of unique parts down to 2, one for the regular CHS, and one for the collar with a shear tab welded to it. Then multiple collars could be installed on the CHS columns and oriented in the field.

Buggar,

These will likely be subject to wind and seismic forces.

However, friction based connections are typically required in situations where there is load reversal. These are not assumed to experience any sort of reduction (or elimination) of the frictional resistance when they experience load reversal, at least not at service load levels.

Is this because in this particular scenario, the faying surfaces providing the frictional resistance would be away from the fasteners? Do you have a reference in mind that I could look into that sheds some light on this?

 

[KootK]

Field welding is out?

You could weld shear tabs to the columns at 0,120, and 240 and then bolt a part between tabs to pick up the odd angle connections. I did something similar for some utility poles that have a lot of possible attachments.

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.

 

[hokie66]

Those collars for concrete are for temporary works. Using friction to resist permanent loads in a building sounds foolhardy. If you use collars, put some bolts through.

 

[XR250]

What about welding some stops to the column to prevent the collar from sliding?

 

[jayrod12]

Although you're correct that friction based connections (slip critical) are required where there are load reversals, there is the fail-safe part of the connection (standard bolts) that prevent the entire thing from coming apart.

Unless you're planning on providing some sort of fail-safe backup I wouldn't be doing this.

What's the application anyway? What's the result of the connection failing?

 

[Ussuri]

If it helps, friction clamps are commonly used to retrofit steelwork (risers, caissons etc) to existing offshore platforms. and for also repairing the tubulars

Unfortunately, I know of no standard guidance document. They are always designed from first principles.

http://www.google.co.uk/url?sa=t&rc...iss5y0teHNAhXlA8AKHWiTClgQFggcMAA&url=http://

http://www.hse.gov.uk/research/othp...k6oAkFDaysc25DcvYUPqPg&bvm=bv.126130881,d.ZGg

 

[STpipe]

KootK,

Field welding is likely out. The parts are galvanized, so any welding should preferably be done in the shop.

XR250,

That might be the ideal solution and should be easy enough to design and implement.

jayrod12,

The collars with welded shear tabs would connect to beams or members that support small platforms for pedestrian traffic. Failure of that connection would ultimately lead to the collapse of that platform with the potential for injury to the people standing on the platform. I think having a welded tab as XR250 suggested might be a workable solution.

Ussuri,

Thanks for the document. I'll be able to review in more detail later.

 

[KootK]

Is it the case that all of the connections will be at the same elevation on the columns?

Another option might be to simply install galvanized blind bolts in field drilled holes.

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.

 

[STpipe]

KootK,

Using blind bolts was my first suggestion when my colleagues and I discussed this. Their concern is with field drilling the holes and damaging the galvanized coatings layer which could lead to the onset of corrosion. Although they could always touch up those areas in the field to avoid any potential problems.