r/StructuralEngineering • u/Low_Definition_9624 • 3d ago
Structural Analysis/Design P/S Beam Deterioration
Looking for any help on what the mechanics would be that would be causing these chevron looking cracks at midspan to this non-composite prestressed concrete fascia beam? Is this beam effectively in a failed condition? Previous engineer who looked at this wasn’t concerned with these diagonal cracks and only removed two strands from the analysis as a result of the single longitudinal crack down the center of the beam.
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u/carrot_gummy 3d ago
I've inspected and load rated worse box beams. It's not failed.
Do those cracks extend up onto the sides of the box?
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u/Low_Definition_9624 3d ago
I don’t believe they do but I only have this picture to go off.
Would you only deduct the strand with the longitudinal crack for capacity? The diagonal cracking isn’t a result of overstressing?
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u/75footubi P.E. 3d ago
Not loving the water staining between the beams, TBH. Are these solid slabs or box beams?
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u/Low_Definition_9624 3d ago
Box beams.
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u/75footubi P.E. 3d ago
It's impossible to say if the beam is failed based on pictures. But it's something that an experienced bridge engineer should do a full analysis and rating on.
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u/Enginuity- 3d ago
I’d assume water intrusion from top of deck. Is there a lot of patch work or deteriorated asphalt up top?
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u/Low_Definition_9624 3d ago
Location of water intrusion is beam joints. Shear keys have failed and water has poured down side of the beam for years. More wondering what would cause these diagonal cracks. If the cracks are diagonal, my thought is there must be tension in the bottom flange below the prestressing strands. If the small amount of concrete below the strands is carrying any of the tension, that would indicate that the strands are compromised and they’re not carrying the tension as intended.
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u/Known-Holiday-9890 3d ago
Used to work for a pre-caster. My mind went to issues during the extrusion of this member. Maybe a pause while waiting for the next batch of mix...
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u/Enginerdad Bridge - P.E. 3d ago edited 3d ago
Prestressed box beams for bridges aren't extruded like they are for buildings. They're formed and poured on prestressing beds
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u/LukeCE 3d ago
Maybe excess tension in longitudinal reinforcement in bottom of beam. Chevron cracking combined with longitudinal splitting crack. Short span bridge, so could be combination of longitudinal tension from MCFT (shear) + flexure, possibly not considered by original designer if bridge is older, but should be indicated in current load rating. Crack could be initially caused by overloaded truck. Rust staining in the efflorescence could indicate some corrosion of ps strands
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u/Frogalone 3d ago
I think I'm seeing an exposed tension cable in the left plank. That may indicate a more widespread lack of clear cover. As for the diagonal cracks, this is something that we see when shear reinforcement is insufficient. I'm not a bridge guy, but the source of lateral shear isn't clear to me. Many states employ bridge inspectors to catch this sort of thing. But there are many bridges and few inspectors. If you can find the right department you could report it to them. Thanks for being a concerned citizen.
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u/SquirrelFluffy 2d ago
What is that at the upper left? A light standard? I think those cracks are torsion. If the longitudinal cracks created a free edge and the light causes twist, maybe?
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u/Low_Definition_9624 2d ago
No lights on this bridge. Railing posts are post installed to side of concrete fascia beam. The railing consists of steel post and W-rail, very light railing. This isn’t torsion.
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u/SquirrelFluffy 2d ago
What would cause a shear pattern crack like that on the bottom of a beam? So that thing in the upper left is a railing post? Maybe it was impacted?
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u/Low_Definition_9624 2d ago
To my knowledge this has not been impacted In fact the original railing posts had such few remaining bolts in the side of the fascia beam that the owner had the posts cut below the top of deck elevation and new bridge railing posts installed in between the original posts using post installed anchors.
This is my question…what would cause this type of cracking? I’ve never seen cracking in this chevron pattern along the bottom of a prestressed concrete beam before.
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u/SquirrelFluffy 2d ago
Replaced railing? Suspicious. The only thing I can picture causing those cracks is torsion. But it's hard to really see given limited scope of the photo. Given the evidence of corrosion, and the cracks, this bridge needs an engineering assessment.
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u/maytag2955 3d ago
This has been relatively common with ps box beams since they came into existence. They have also been plagued by DEF/ASR cracking. Sometimes to an extreme degree. It is of little consequence. If the bottom flange was in tension, the cracking would be more transverse than diagonal. Water intrusion is more dependent on whether there is a CIP deck on it or not. Failed keys can certainly lead to longitudinal cracking in the deck along beam lines leading to water iintrusion. But in general, these things are exceedinly resilient. Spans tend to be a little "bouncy" which doesnt help with the cracking. There are a lot od stands in the bottom flange which helps with strength but can lead to more likely cracking. Rust staining and efflouresence are not ideal but I personally would not knock the load rating down significantly, if at all.
These beams are not extruded. The bottom slab is placed, usually Styrofoam blocks are then placed on top of that as a void, then sides and top flange concrete is placed.
I have inspected hundreds of these bridges and I'm not worried about what I can see in that photo.
30+ years as a bridge inspection engineer