Cutting openings in Hollow Core

[SocklessJ]

I need to put some new 18" wide openings in a precast hollow core mezzanine at a manufacturing facility. The planks span nearly 50'. Here's what I'm thinking:

Option 1: FRP

Since I'll need to cut strands, I assume an FRP overlay could restore the lost moment capacity. However, I don't have the original structural drawings, so how much capacity needs to be restored is unknown. I also haven't dealt with FRP before, so I should probably call a specialty contractor to see if they'll even touch this.

Option 2: Cut full plank width and support from below

I could frame out the opening with new steel beams and 4 columns below. Then I could cut out the full width of a plank, creating two new simple spans on each side of a new 4' wide opening. The only problem I see is deflection compatibility. Since the adjacent planks tend to act as a unit, I'd be putting the un-cut planks into negative bending.

Has anyone done this before? Any thoughts are appreciated.

 

[KootK]

I've designed a fair bit of hollow core in my day, most of it with penetrations through it.

Much depends on where your opening are located but, in many instances, I would expect to be able to install 18" penetrations without any reinforcement at all.

A good place to start is with the document below which describes how load distribution is typically handled in plank design.

 

[ANE91]

I don’t do any hollowcore, but I’ve done FRP, and I seriously doubt that you can substitute the effects of prestressing strands with FRP. The strength gain is on the order of 10%, and there are limits based on the unstrengthened element (ACI 440.2, 9.2; ACI 562, 5.5.2).

Also, fire is a real concern, given that it’s a manufacturing facility.

You might be better off with bonded steel plates or external strands + a reaction frame, instead.

 

[SocklessJ]

Thanks @KootK . It looks like the long span works to my favor with the 0.5L effective width mentioned in the PCI hollow core manual. My section loss ends up being around 7%, so I'm thinking as long I keep my opening away from midspan, I should be ok for flexure. I'll still need to think about the implications for shear.

The manual also says "when considering flexure, an opening that extends less than 0.125L and 4ft from the slab end into the span may be neglected."