support condition in model (urgent)

[Dawaab]

I am working on stiffening an existing profile deck to address snow drift, and I plan to introduce a single angle at the bottom of the rib, spanning perpendicular to the ribs across the entire deck span.

I intend to use a shot pin to connect the single angle to the bottom of the rib of the deck. Could you advise on how to specify the support condition for this shot pin in RISA3D? I am considering using a spring boundary condition, but I’m not sure if this is the correct approach. The goal is for this angle to act solely as a stiffener, reducing the bending of the rib and allowing the deck to bear the drift load.

Additionally, I want to ensure that this setup does not generate significant negative moments, as I was unable to verify the rebar inside the rib.

 

[JAE]

How are those angles going to do anything?
They will just be additional dead weight on the deck and take zero load, correct?
They certainly can't span all the way across the building and as far as I can see only perhaps help spread out any concentrated loads to adjacent ribs.

Can you better explain what you're attempting to do here?

 

[XR250]

I don't get this and the fact that you are using FEA to solve this baffles me.
I probably need to start a new thread on "why so many people on this forum use FEA for simple problems"

 

[Dawaab]

I am looking to stiffen the deck by attaching a steel angle to the bottom of the ribs using shot pins.

Initially, I considered placing a channel between the ribs, but that option turned out to be too expensive.

Do you have any suggestions for an alternative way to stiffen the deck?

 

[Greenalleycat]

Agreed with JAE, not sure what this angle is doing here other than giving a boost to load sharing
I would be dubious about using shot pins for this

 

[SWComposites]

"why so many people on this forum use FEA for simple problems"

Simple; They can quickly generate fancy colorful contour plots of plausible (but often wrong) results to impress their management. And it often avoids having to think about complicated things like load paths, free body diagrams, etc.

 

[Dawaab]

The drawing attached was simply meant to illustrate what I was trying to accomplish.

 

[Greenalleycat]

To answer your question: I don't have experienced in Risa3D but this is how I'd do it in SpaceGass (the software I use)
I would draw the angle offset say 200mm vertically below and then put a pinned member in from the bottom of the decking members down to my angle
This is a good enough model for what I understand you're looking to accomplish (deck pushes down on angle -> angle pulls the adjacent decking down to share load more effectively)
Make a custom profile for the 200mm stick that is very stiff (1000x1000 or whatever) so that axial effects in that member don't meddle in the results
You will see compressions in some areas and tensions in others as it pulls the decking down - it's not pure, but it's OK

I would look to a more robust connection than a shot fired nail though - site weld or bolting if possible
I doubt you can manifest the tension into a nail

 

[phamENG]

The drawing attached was simply meant to illustrate what I was trying to accomplish.

What's the relative stiffness of the deck with slab compared to the angle?

Also - the shot pin wouldn't be a support condition in Risa. It's a connection you design based on the output of the model.

 

[Dawaab]

Thank you, Greenalleycat! You really grasped what I was aiming for. I'm having some trouble figuring out how to represent the connection between the angle and the deck in RISA. I'll give your suggestion a try in RISA 3D.

 

[Dawaab]

Does anyone have any suggestions on how I can go about stiffening the existing deck? Please see the attached PDF.

 

[RenHen]

To answer your question: I don't have experienced in Risa3D but this is how I'd do it in SpaceGass (the software I use)
I would draw the angle offset say 200mm vertically below and then put a pinned member in from the bottom of the decking members down to my angle
This is a good enough model for what I understand you're looking to accomplish (deck pushes down on angle -> angle pulls the adjacent decking down to share load more effectively)
Make a custom profile for the 200mm stick that is very stiff (1000x1000 or whatever) so that axial effects in that member don't meddle in the results
You will see compressions in some areas and tensions in others as it pulls the decking down - it's not pure, but it's OK

RISA3D has a shortcut for the custom profile approach you're outlining. If you set the member Material Type to General, Section/Shape to Section Set & Section Set to Rigid, it gives you a rigid link member.

 

[Greenalleycat]

If you main issue is 'stiffening' the deck then I doubt the angle will accomplish it
The angle will only really do 2 things:
1/ Support load sharing between adjacent areas of the deck
2/ Could provide improved buckling support for uplift cases where the bottom of the deck is in compression (not relevant here for snow)

To give better ideas on what you could do we need more info (is stiffness your issue or is it strength? what's the deck made of? what are the access constraints? how much snow are we talking about? what is this thing even used for?)

But, in isolation, here are some thoughts
1/ Put a post under the existing main beam

2/ Add new beams at the 1/4 and 3/4 points of the span - these would have to be much stiffer/stronger than your angle as they would be intending to span the full width, rather than just improve load sharing
Note that these would only really be useful for live load and snow load portions, so you'd need to work through your load cases carefully
Unless you prop the floor and force it back to level the gravity load will continue to be taken by the main beam

3/ FRP is a potential solution to some concrete floor but not helpful here if it's a steel deck

4/ Prop the existing floor, weld a doubler plate to the bottom flange (full width x 20mm or whatever) and unload it to reduce deflection of the existing system

 

[Once20036]

I don't think anyone has said it yet, but you cannot shoot a pin through an angle into the deck.
The pin is shot from above, through the deck, into the thicker steel element. The deck gets sandwiched between the head of the pin and the angle.
If you shoot the pin from below, it will lose all of its energy going through the steel element and not penetrate the deck. Alternatively, if it did penetrate the deck, there is nothing for it to "bite into". I`d expect it to detach under its own self weight.

 

[phamENG]

@Once20036 - disagree. You'd be shooting into the concrete, not the steel deck, but you could shoot it up. I wouldn't really want to hang anything from the concrete like that, but if you were going for some sort of unit shear I could get behind it.

Edit - depending on the steel thickness, you might need to drill it first.

 

[Once20036]

@phamENG - i didn't see anything in there about concrete.
If that's the conversation, I agree you could do something if you predrill the steel, shoot something through the hole, into a concrete deck. If nothing else, it wouldn't fall under its own self weight.

 

[Greenalleycat]

I've raised several times that I don't think shotfired pins are good here, especially as they will be needed to transfer tension if OP wants to use this for loadsharing across the deck