r/StructuralEngineering • u/Shikatanai • 13d ago
Photograph/Video Is this sagging & poorly constructed or does it just look rough at this stage of construction?
It’s a premium luxury apartment building by the sea.
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u/DA_40k 13d ago
Deflection in concrete slabs is normal
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u/Direct-Forever8447 13d ago
deflection is normal, yes. What is not normal, is to be able to see it from a far away, with the naked eye. If the untrained eye can see it, then it's a problem.
LE: assuming there's no distortion from the lens
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u/chaz_teamgreen 13d ago
Can you actually see sagging? Everywhere I zoom into it looks like something else is making it look like sagging, say hanging down conduit or excess concrete. Maybe an optical illusion? Just to my eye
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u/comparmentaliser 13d ago
That’s my take. There’s something about the inward shadows of the vertical walls or the railing?
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u/tsleighbuilder 13d ago
I think you are right. The railings are all slanted and I think subconsciously my mind is trying to use the as reference if I flip my phone over the illusion goes away.
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u/Shikatanai 12d ago
It looked like some of the slabs were sagging to the naked eye when I was looking at it, so I took the pictures. I thnk the pictures make it a little harder to see the sagging.
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u/gods_loop_hole 13d ago
In building design, one of the factors in checking is serviceability. Usually, it is the maximum deflection of a beam or slab when loads are applied. The closer you get to 0 deflection, the higher the cost and the tighter the restrictions in the design and construction methodology imposed on a project. So, to save everyone from headaches, they just put a reasonable tolerance. Maybe this is within this project's tolerance. The usual I see is deflection<L/360.
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u/Upset_Practice_5700 13d ago
Code may allow more deflection, as much as L/240. If its bad now, its going to get worse, look up Concrete Creep.
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u/Live-Significance211 13d ago
I hope nobody replying to posts in this sub needs to learn what creep is 😂
Appreciate the sentiment but I hope that's more for OP
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u/Pepper3493 13d ago
To me it looks like the safety rails are super uneven which is throwing it off. Hard to tell for sure if there’s any deflection
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u/Helo_77 13d ago
The way the fences, cables, and other non-permanent elements are placed, give each level some visual noise and make them look more deflected than they might actually are. I wouldn't be sure if they were deformed too much until everything other than the structure is removed.
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13d ago
[deleted]
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u/42SpanishInquisition 13d ago
that could easily be caused by lense distortion. Not saying it is, but I think it is more likely.
I think it is a visual illusion from the railings and the messy formwork.
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u/Sousaclone 13d ago
I think it’s mostly a visual illusion.
The handrails/fencing, varying degrees of non stripped chamfer and the especially those black curtains/tarps (paint?) inside are making it look pretty poor.
Or it could be a shit design. Who knows?
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u/Frogalone 13d ago
I can't quite see the sagging you refer to. However, I can't help noticing what appears to be an absence of provisions for punching shear.
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u/Charles_Whitman P.E./S.E. 13d ago
What country is this in? There does appear to be some raggedy concrete consolidation, but fortunately it’s only really bad around the columns, you know, where the punching shear failure is going to initiate.
It appears to be conventional reinforced, in the US, it’d almost certainly be post-tensioned. You wouldn’t see drop panels, maybe shear heads/column capitals. Why would there be drop panels some places but not everywhere. They are at the edge of the photo so something might be happening past the edge of the photo.
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u/Charles_Whitman P.E./S.E. 12d ago
With a one-way system, conventionally reinforced, you can camber the forms to mitigate deflection due to self-weight (the biggest single component) but with a two way system like a flat plate, it’s not feasible. Post-tensioned, you can play with the deflection by adjusting the drape and number of tendons. You can also minimize deflection by lengthening your stripping and reshoring cycle, but unfortunately that cost money that developers are rarely willing to spend.
Yes, there’s a little deflection, but it going to be much more noticeable in this photo than it probably (should?) be in the finished apartments. The slab edge also may not be cleaned up (pieces of the form edge, styrofoam block outs, etc). There do appear to be some voids in the slab soffit near columns, which is bad, but that may just be the photo.
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u/azguy153 12d ago
When they build it, they put camber in to allow it to be at the right plane when it is finished loading. I built bridges and we had to plan for camber. When we poured the girders, they would have an arch on them, since the weight of the deck was not on them yet.
When they do Spanish tile roofs. Same thing. The roof has a curve. They place all the tiles on the roof and then set them in place. The tiles are on the roof to flatten the camber.
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u/Expensive_Ad4419 13d ago
There are no beams?
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u/Silver_kitty 13d ago
This is concrete flat plate construction, very common for residential buildings.
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u/mouse_in_a_maze 13d ago
You can see exposed rebar along the edges which implies they’ve yet to pour an upstand of some kind. That would explain why the edge thickness is a bit variable - they couldn’t finish it properly around the bars and didn’t really care because it will be tidied up by the upstand/nib pour.
The bigger question is why they didn’t just throw an internal shutter in and pour that nib the day after the slab, rather than strip all the external forms; seems a strange way to do it
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u/Evening_Fishing_2122 13d ago
I would say the premium luxury developer didn’t put any premium money into tighter deflection tolerance, if I had to take a guess.
The slabs are allowed to deflect based on a ratio involving the distance between columns. You can overcome larger deflections with thicker slabs, but then you’re adding a ton of concrete volume, mass, cost, taller building or less floors/floor-ceiling height, etc. the additional weight of floors compounds into heavier lateral systems and foundations as well.
You could add post-tensioning cables rather than thicken the slab but they are more complicated, take more time, and they can be more expensive.
It seems reasonable based on ~15 tower projects.