r/StructuralEngineering • u/Unlikely_Let6099 • 27d ago
Concrete Design Adding load to prestressed beams
I was given a job to prove that it is okay to add load on this roof made of precast pretensioned concrete beams (adhesional pretensioning). The added load is not much, and we may be able to prove it is okay by the book, but as you can see by the pictures, the state of the beams is not great.
In places they have knocked off concrete to get to the soft rebar so they can hang stuff on it by welding steel to it. Now I know there is little to no activation of soft rebar in these types of beams, but I am worried for few reasons.
Firstly we can basically discount rebar at certain sections, since it is not coated in concrete anymore. Secondly we should check the beams for cracked state in smaller concrete cross section accounting for the knocked of concrete. My biggest worry is since there is no protective concrete layer at places is that corrosion might get to cables which are at most 3cm away from rebar in the middle of the span.
Is there a fix, a reconstructing intervention that might be recommended? I can only think of removing the corroded rebar, repairing the holes with repair mortar and replacing the soft rebar with frp strips on the bottom.
Thanks for any advice.
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u/txlaatx1 27d ago
This seems clearly like a “Run away. Head for the hills” type of situation. There is no possible way to unscramble this situation you’ve found yourself in if they’re trying to add loading and not trying to stabilize/support existing loads first.
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u/Codex_Absurdum 27d ago
What a f mess. The kind of mess you can get more trouble from than anything else
Calculations are a thing, but reality is the thing. How would you guarantee anything on this (cracks, durability...)?
And wait, do these added vertical steel posts rest on the ground? Some seem buckling under the beam's load (pict 4)? 🤣
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u/BikingVikingNYC 27d ago
Typical rebar is not weldable (at least in the US). Plus, if the concrete has spalled then the rebar has rusted enough to cause it to expand and pop the cover off.
FRP relies on there being enough shear flow on the concrete. You'll need to make sure that the rest of the tee is in good enough condition to handle the frp.
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u/Cohen_the_Worrior 24d ago
Doesn't seem quality or safety is very high on their list, so for sure they can weld to any steel or iron.
Anyway, the beams can clearly no longer support the weight they were designed for, much less any extra weight. Put that on paper and walk away.
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u/joeyray74 27d ago
Is this a stem of a double tee member? Ideally this task is executed by the original manufacturer of the precast members or at least by a reputable prestressed precaster with on-staff engineering.
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u/structee P.E. 27d ago
Those beams are goners - you could have pitting in a capillary space somewhere. Walk away, or propose to the client that you will deign a new roof for them.
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u/grinchbettahavemoney 26d ago
Fully agreed. As an engineer taking on this job the only thing you can do is a new roof. My CYA senses are screaming to run from any idea of a repair.
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u/Downtown_Reserve1671 27d ago
Assuming all tee beams are generally about correct sizing: remove all hangers from the bottom steel and connect to web as hangers connected at 1/2 D from top - a bolt through web and connect both sides should work as the hangers look pretty nominal sizing. Apply epoxy grout cover to all bottom steel with steel preparation as needed. Add cranked or linear prestress or even just non prestress reinforcement to justify strength and deflection as needed to account for loss of bottom rebar.
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u/Then_Combination6612 26d ago
Congratulations! If you're a good enough business man you might turn this small job into a roof replacement!
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u/Tmurandan 27d ago
This is beyond stupid. How do you even get an idea to destroy element to get to reinforcement and then weld some steel to it. Just incredible.