Slotted holes in bracing connections

[gonutsdonuts]

Hello All,

I'm currently working on some bracing connections and I'm concerned about relative deflection between the two points we're bracing between. Through doing some hand-calcs it looks like there will be 0.239" of relative deflection (conservative) which would require the brace to elongate/compress something like 0.06". In order to resist that I'd need my connection to account for 100kips of load, and that just isn't going to happen. Instead I was thinking I could allow for slotted holes and maybe have the connection designed as slip critical, although I'd really love to keep it as a bearing connection.

If that's the case, I'm wondering if slipping is even allowed in slip-critical connections; and if slipping occurs does it regain it's resistance once movement has stopped? Also, would the slip resistance induce a large unwanted moment in the connection?

Other than that, are there any allowable ways of keeping a bearing connection while still allowing for a little axial movement?

Thanks!

Donuts

 

[SteelPE]

I don't quite understand your question. Maybe a sketch would help. Why would you want to provide slots in your bracing, wouldn't that defeat the purpose?

 

[gonutsdonuts]

You're right. This is my first project and it's stressing me out a bit. It's also a really unusual connection for a really unusual retrofit. I've attached an old sketch of the connection I did. The relative elevations are really exaggerated, but the general idea is that the concrete and steel girders will deflect a bit relative to each other after the braces are installed. I'm worried the elongation needed to accommodate those deflections is going to exceed what the brackets can handle. I was thinking though.. the elongation needed would be less than 1/16th of an inch, and this may be accounted for in the clearance from the standard bolt hole size. What do you think?

 

[kingnero]

Recently I've seen lots of attachments, uploaded to filesharing sites that are blocked my office desktop.
As far as I'm aware, the standard (button below => engineering.com) upload service works great, and is never blocked.
Why won't you use that one?

 

[dik]

With most cross-bracing, the steel erector typically 'forces' the connection a tad with his spud wrench and drops a bolt into the adjacent hole (2 bolt min connection). Slotted connections are not normally used for bracing (In my experience). I usually use slip critical connections for bracing... and these can be used with slotted holes if required.

Dik

 

[SteelPE]

What is the purpose of the bracing? Are you trying to brace the steel section against buckling (either axially or laterally)?

 

[gonutsdonuts]

Yes, SteelPE, I'm trying to brace the steel section from buckling laterally. The existing concrete girders are connected to a deck at the top. We are trying to brace the steel girders to the adjacent concrete girders.

Only one bolt is used because we needed to allow for rotation. Is the 2 bolt minimum a code requirement or is it something contractors will universally ask for? The contractor on this project hasn't been willing to talk to us.

 

[SteelPE]

I think other comments are based upon the bracing being used to resist lateral forces vs member stability that you are using it for. If you are that concerned about the differential deflection, why not make the bracing horizontal and not worry about it transferring vertical loads. I believe horizontal bridging, while capable of transferring loads, is pretty flexible which would allow for more movement.

Also, what it the purpose of the steel beam?

 

[JLNJ]

I don't think single bolts which are allowed to pivot in their holes will provide a stable system to brace the girder as you have it.

 

[Agent666]

Agree with JLNJ, your detail offers no rotational restraint. If near horizontal the beam can rotate to some degree.

Do you have to have the double braces, as opposed to a beam of sufficient depth to provide both lateral and rotational restraint on one side only between the steel and concrete girder?

 

[msquared48]

I would run a single horizontal angle brace between the concrete girders above the steel beam and attach a plate from the wide flange beam to the angle brace above with vertically slotted holes. This should restrain the top flange of the steel beam, but allow for vertical movement.

Mike McCann, PE, SE