Composite Steel and Concrete

[jplay2519]

There are precast panels being supported by columns and we're doing a renovation where we need to open it up as much as possible. I'm going to bolt MC channels to both sides to provide 3 spans 30', 31'-6" and 36'. The panels have joints at each of the existing columns so I'm not sure if I can use them to provide a composite member to span the new supports. Also if I were to try to engage the panels for support would I have to epoxy the channels to the panels along with the through bolts?

 

[KootK]

Is your situation that you are trying to make deep, simple beams of multiple precast units by bolting two channels to the bottom as positive flexural reinforcing? If so:

1) gives some serious consideration to the impact that channel bolt slip and compression zone gap closing may have on the panels. If ultimate strength works but you have to open up fist sized cracks in the precast to mobilize it, there could be trouble.

2) Depending on the panel to panel connections, all of your vertical shear may need to be carried locally be the channels at the joints.

3) where joints occur in the panel compression zones, there will be potential for lateral instability. Make sure those locations are braced laterally.

4) I don't like through bolting for this. You should be able to get more capacity from adhesive or expansion anchors and those will give you less slip and be more field friendly.

5) One hopes that bolt tension will provide some composite action at service loads via shear friction. That's tough to count on though as it's difficult to reliably pretension bolts in situations like these.




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.

 

[jplay2519]

KootK- Thanks for your reply. Yes I am trying to provide reinforcing at the bottom, however the channels are designed to support the panels alone. I want more stiffness to ensure that my panels don't deflect so much that they start to crack. This configuration already supports a roof so the tops of the panels are braced. Concrete epoxy or expansion anchors are likely the best option here, not sure why I bypassed that option in the first place. So there is no need to apply a layer of epoxy between the steel web and precast panel you think?

 

[KootK]

I had misunderstood your intent with the epoxy. That actually sounds like a pretty great idea from a stiffness perspective. Another thing you could do is install channels at the top of the wall too to bridge any compression zone gaps. They wouldn't need to be full length. I'd design the epoxy for both vertical load transfer and composite action as it will be the stiffest load pathetic for both. Do you have a product in mind?

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.

 

[Shu Jiang PEng SE]

Once removing the bottom sections of the panels, there will be concrete column formed from the panel sitting on the top end of the steel column. The connection between the steel and concrete column, and the strength and stability of the steel and concrete column may need attention.

 

[jplay2519]

KootK- I don't have any product in particular in mind just trying to determine if it's even something worth trying, which now that I know it is I'll try to find some resources, if you know of any please feel free, I know SIKA has a lot of products that may work.

jiang46602 - We will bolt the channels on and put in our new columns, then remove the ones in the photo. There will be a few panels between spans more than likely so do you think it will be an issue if at the column location and at panel joints we put extra bolting to ensure the vertical loads are giving extra attention? I think there will be approximately two panels per span for each of my beams, two panels will be attached to one span of bolted/epoxied channels.