Bracing Connection at base 1

[Leftwow]

I'm having a hard time finding a good detail for a bracing connection at a base. Even AISC DG 29 is not giving me a good detail of this. Is anyone familiar with vertical bracing connections?

Background: We are designing connections for bracing, we do not usually do this. The brace is a W6X15 and the column is an indian section "ISHB 250". We are designing the brace in a chevron format. If anyone could help me with this it would be greatly appreciated.

 

[JedClampett]

Is there a reason you're not using double angles? They're a lot friendlier to connect for bracing. Plus you get double shear on your bolts.

 

[KootK]

Can you post a sketch so that we can see:

1) Column orientation.
2) Brace orientation.
3) Concentricity of connection or lack thereof.
4) Approximate angles involved?

One of the options shown below might suit. If the loads are light, as I would expect, it may be cheaper to use web plates on their own rather than cope the flanges.

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.

 

[Leftwow]

Kootk, Please look at attachment.
Either column orientation, after I get a detail I can engineer it by DG29 design of vertical bracing.

Jed, I will try that scenario... Just never used it before... I'll calculate and see if it works.

**it looks like all the column bracings at base that I found are somehow bearing on the base plate of the column.

 

[KootK]

Thanks Leftwow. I stand by the diagrams that I posted previously as reasonable ways to attach the bracing to the gusset plate.

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.

 

[TehMightyEngineer]

I agree with KootK's details. His first will look the cleanest and his second will be the strongest and cheapest (but looks messy).

I also agree with Jed; double-angles are much easier. If you need higher strength then I'd look at HSS braces slotted onto the gusset. I feel that field-welded HSS braces are easier to connect than bolted W-section braces and typically result in lower steel weights for the brace.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[Leftwow]

Ok thanks for the input guys. Another question I have is, in order to reduce eccentricities does my bracing plate have to bear against the base plate?

 

[KootK]

If your column were wide and your angle steep you might be able to stay concentric without hitting the base plate. That's rare-ish though and, based on your sketch, I suspect that you'll need to grab a little base plate.

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.

 

[mike20793]

In short, yes. You want the work point of the brace connection to be at the intersection of the centerline of the brace and the centerline of the column at the top of the baseplate, or you have to deal with the eccentricities. AISC does a good job showing the calculations in the manual chapter 13.

 

[Leftwow]

I have 55 kips of compressive force acting on this "brace" all the time. What are your thoughts about this? I tried double angles... it couldn't handle the stress. I could use larger angles but then size matters on operating platforms for walking room. W6x18 works seems the most economical when it comes to space and stress.

 

[KootK]

Makes sense. I don't see a lot of double angles in chevron systems where there's real gravity load on the beam. Is another brace configuration an option? Or just roll with your W6. With a double web plate shipped loose, it should be easy enough to erect.

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.

 

[mike20793]

Why not HSS slotted into the gusset? Seems pretty easy if it's a chevron bracing configuration and not x-bracing. The HSS also makes it easier to infill with studs if needed.

 

[Leftwow]

Mike, my superior doesn't like to use HSS sections... Our stuff is exposed to harsh chemical and weather corrosive parameters. The problem comes in when it starts deteriorating from the inside and these clients could give a poop about observing the structure a couple years down the road. They also do not hire us to keep an eye on it. There are lots of residential homes around these industrial areas. I honestly think he makes a good point and I do concur. In addition, all our steel must be either painted or galvanized... it's not a common practice for us to use hollow sections, I believe it to be used more in commercial construction where the beams are typically protected from outside conditions...

 

[mike20793]

Oh, gotcha. Thanks for sharing. Seems like a pretty good reason not to use HSS. There are things that can be done to protect it, but it seems like it's a lot of extra work that can be avoided.

 

[TehMightyEngineer]

Why not HSS slotted into the gusset?

Hey, that was my idea. (kidding!) Glad you think it makes sense to you as well.

Obviously I can't change your bosses mind but my previous boss/mentor had similar reservations about HSS sections in industrial settings. After doing a lot of work with steel corrosion in these settings I found that other than surface rusting issues, most of the major corrosion issues were leaks; direct steam/condensate exposure (such as condensate traps), direct exposure to process chemicals, and areas where water could pond against the steel. Overall, if left with no maintenance or in a bad environment, no coating will hold up. Here's a picture of a typical mill column/brace:

It was quite typical to find rust on almost every surface of a column/beam with no coating left. However, a lot of steel had very little section loss. The real corrosion issues were caused by poor setup of the process. In the picture there was a process drain that they let flow on the floor around the column and splash up onto the brace (note the missing column web). Also, note how the brace is a wide-flange turned flat. Terrible idea, every time I saw this the web was rotted out at the bottom end where condensation collected (avoid flat/enclosed surfaces where anything can pond).

Long story short; I don't believe the lack of a coating on the interior of a HSS is going to significantly impact your corrosion resistance. However, if you do expect direct exposure to water/steam/chemicals then either hot dip galvanized or stainless steel will probably be what is required regardless of your section shape. But, if you do go to a high-end corrosion resistance material/coating then you should use an HSS anyway for least weight (stainless HSSs are quite common in paper machine frames). It's also worth noting that I don't recall any time I saw a carbon steel HSS used that it seemed to perform any worse than a wide-flange. I think most people in the industry just shared that mentality of HSS = hidden corrosion.

It's entirely worth noting that I would not put end plates and seal up the HSS. Any condensation left inside would undoubtedly pool on the bottom and corrode it. Leave it open; better, cheaper, plus easier to inspect.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[Leftwow]

Ah that's a good post.

I got some funny ones. This is still in operation lol

 

[TehMightyEngineer]

That's almost as bad as my worst one; column folded over on itself. Was very lucky it didn't collapse. Can't share due to an NDA, though. Looks like we worked for the same mills.

Oh, and I did manage to find a picture of the infamous industrial HSS. Don't get too close, it could collapse at any minute! This column was splattered with pulp at the top but the wide flange column right next to it looked slightly worse.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[Leftwow]

Hey, that looks like it's inside a building. Not fair.

 

[KootK]

I saw something like this at the Seattle public library and thought it was slick. Do you really need the weak axis stiffness inherent in the channels? Surely not. Is it awesome and cost neutral? Hell yeah.

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]

Hollow sections, hot dipped galvanized, are in my opinion the best choice for corrosive environments. Protected inside and out, there is less area exposed, and normally less inaccessible areas at the connections.