Replacing Damaged Precast Concrete Slabs in Florida

[OLDSOUL22]

Hey everyone,

I hope you're doing well. I'm facing a challenging structural issue in Florida following the damage caused by Hurricane Ian's surge last year. I'd love to hear your opinions and insights on how to approach this problem. Please note that this is more of a discussion thread, and I'm not looking for official help but rather the benefit of your diverse perspectives.

The Problem:
We need to replace 4" precast prestressed concrete slabs that sit on a concrete frame with infill CMU walls. Unfortunately, we don't have access to any legacy structural details or drawings for this particular structure.

Key Details:
The damaged slabs are 4" precast prestressed concrete.
These slabs are supported by concrete beams.
The CMU walls continue vertically on top of the slabs.
Legacy structural documentation is not available.

My Questions:
1- Given the absence of legacy structural details, how would you approach the design and engineering aspects of replacing the damaged slabs?
2- How would you go about removing the portion of the slab that support the CMU? would you leave it in place?
3- would you go with precast slabs for the replacement (for the balconies)? if so, I guess we will need new corbels to support the slabs? how would you design the corbels with the absence of structural details, reinforcement, grout, etc...
4- for the ground floor slabs, there will be no chance of accessing inside the building with a crane to erect precast slabs, I think a composite concrete slab with W beams is fairly simple to design and install there. would you provide a steel angle at slab edges"?
5- What do you think would be a potential challenges or pitfalls I should be aware of and plan for?

Feel free to share your thoughts, experiences, or any resources you think might be helpful. Thanks in advance for your input!

MSc in Structural Engineering, PE,SE,PMP®

 

[Aesur]

This is the type of project that should be taken on by someone who is well versed in destructive testing and structural repair. Is insurance involved in this? The owner/insurance carrier couldn't offer me enough fee to take on such a project as this, it appears the rebar is corroded, I suspect you will be required per IEBC to bring the entire structure up to code, which means massive amounts of destructive testing to figure out the entire structural system, completely re-engineering the building based on the results of testing trying to make what is there work for codes that have changed over time, strengthening as required with FRP or other sim products.

Have you taken on such a project before? If not, do you have peers whom have?

To answer some of your questions I think you will want to discuss with a local contractor on their preferred means and methods.

I'm not sure how I would proceed with framing as I would need to run multiple design options to determine the best solution.

Hopefully you got a massive fee for this workload, good luck.

 

[hokie66]

Based on those photos, looks like a knockdown to me.

 

[1503-44]

TKO

Charge by the hour.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[XR250]

Based on those photos, looks like a knockdown to me.

Same here.
At what point does insurance stop covering these things?
My guess is now.

 

[OLDSOUL22]

@Aesur - Florida Building Code and FEMA requires bringing entire building to code only if cost of repairs are beyond 50% of value, otherwise depending on the work classification it can be as simple as the new structural elements be designed to current codes. I've done many rehab and restoration projects in the commercial and industrial fields. However, almost every single one is unique. the fee is not massive by any means but I'm not the one to worry about that.

@hokie66 - Looking at the big picture here, the damage is not that significant. 1st floor lost (30%) of slabs, 2nd floor lost balconies (10%), 3th to 6th only cosmetic damages.

@1503-44 - I'm on W-2, so will get paid anyway!

@XR250 - not really sure, but in this case between the Wind Insurance (2ND - ROOF)and Flood Insurance (GF), they should be nearly fully covered. They pay massive premiums.

MSc in Structural Engineering, PE,SE,PMP®

 

[Aesur]

Florida Building Code and FEMA requires bringing entire building to code only if cost of repairs are beyond 50% of value, otherwise depending on the work classification it can be as simple as the new structural elements be designed to current codes.

I admit I'm not well versed in the FBC, however it has been my experience that typically state amendments are more stringent than the IEBC and this is close to IMO being considered "substantial structural damage" based on the pictures and your description, which I believe would require more than simple replacement of what is missing. This being said, you know the percentages of damage and therefore are able to adequately make this determination and I am only able to go off what I have seen here.

 

[Enable]

1- Given the absence of legacy structural details, how would you approach the design and engineering aspects of replacing the damaged slabs?
2- How would you go about removing the portion of the slab that support the CMU? would you leave it in place?
3- would you go with precast slabs for the replacement (for the balconies)? if so, I guess we will need new corbels to support the slabs? how would you design the corbels with the absence of structural details, reinforcement, grout, etc...
4- for the ground floor slabs, there will be no chance of accessing inside the building with a crane to erect precast slabs, I think a composite concrete slab with W beams is fairly simple to design and install there. would you provide a steel angle at slab edges"?
5- What do you think would be a potential challenges or pitfalls I should be aware of and plan for?

1 - GPR the existing CMU and relevant connections on a random basis. Confirm scan with destructive openings.

2 - Yes. If it is of sufficient integrity don't mess with it lest we have a lot of vertical load to shore off. Patch and replace grout as needed.

3 - I live in a CIP world so I would go CIP. But presumably pre-cast would be lighter so may be advantageous in keeping loads similar to what they were originally. In situations where we replace concrete slabs either side of a shearwall we will often through-bolt an angle either side and weld the bottom slab reinforcing steel to the angle. This provides continuity across the joint that acts somewhat like integrity steel. You can also just through-epoxy bars to do the same.

4 - How heavy are the slabs? You may be able to beam and lift from above. I've done that for a number of interior slabs and such items. Don't have a good picture of a slab lift sadly but here's one below of us lifting some beams under an underground parking garage ramp to give the idea (admittedly way lighter). Or if you can fit an interior forklift that may do the trick. Either way I feel a beam/composite slab configuration also works.

5 - You need to be rather confident in how the original building was intended to work so that you know if your replacement methods will restore the intended load paths. Looking at the global structure is as important as is looking at your individual component.

 

[XR250]

@XR250 - not really sure, but in this case between the Wind Insurance (2ND - ROOF)and Flood Insurance (GF), they should be nearly fully covered. They pay massive premiums.

I imagine they will get paid out for this one but when do the insurers decide this is a losing battle. I believe they already have in many parts of FL and CA.

 

[1503-44]

With prices today replacing 20% or more might get close to 50% of building's value.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[OLDSOUL22]

@Aesur - you're correct with your reasoning. However, in this case, the building is on a prime location by the beach and the appraised value and the building size is high enough for the work be less than substantial improvement.

@Enable - 2- Thanks for input, I think I will saw cut the slab at the face of the CMU or couple inches beyond and do more inspection.
3- I like the through bolting idea, but I think I'll keep the slabs as simply supported for each balcony, this way the slab can simply bear on the angle each side.
4- I think the geometry and presence of partition and infill walls wouldn't allow for the erection, I'll try to solicit opinions from contractors though. They tend to be more creative than me when it comes to erection procedures.
5- Thanks for the advice! right on point. I think I got a decent idea on the structural system.


1503-44 - You're right, specially in SW-FL, according to the Xactimate report the cost is slightly less than 50% the appraisal.

MSc in Structural Engineering, PE,SE,PMP®

 

[Aesur]

@OLDSOUL22 - Out of curiosity, is location included in the value of building for IEBC purposes? I highly doubt the insurance carrier would pay out the value including prime location, only the actual cost of the building itself. Seems to be a grey area, so curious what others think.

 

[OLDSOUL22]

Aesur - The insurance will not pay for anything other than the repairs/ replacement cost of covered items ( different between Wind & Flood), up to the policy max.
what I was referring to when I mentioned the prime location is the number used to evaluate against the 50% rule. The rule compares the cost of repairs to 50% of the market value of the property.
the market value according to FEMA is defined as: "The price at which a property will change hands between a willing buyer and a willing seller, neither party being under compulsion to buy or sell and both having reasonable knowledge of relevant facts.". This value is usually obtained from the County Property Appraiser’s or Independent sources.

MSc in Structural Engineering, PE,SE,PMP®

 

[Aesur]

@OLDSOUL22 - Thanks for the clarification.

 

[OLDSOUL22]

@XR250 - Indeed, Some companies filled for bankruptcy after Hurricane Ian! Flood insurance is different though, It's a cooperative undertaking between the private insurance industry and FEMA. Not really sure who take the loss but I'd think the Federal government will help somehow.

MSc in Structural Engineering, PE,SE,PMP®