Bending of storm water box grating
Bending of storm water box grating
(OP)
Earlier this week I was talking to a industrial client who showed me some storm water boxes (on site) that had the grating on them permanently bent. (Fortunately, I had nothing to do with the design of these things.....but he wanted to complain to me nonetheless.) He was looking for a explanation.....and I really didn’t have one. In my experience, the people who fab these things do it based on a ASSHTO wheel load. (Just about the worst case for a highway if memory serves.) But you look around just about any road (and I've looked since then) and just about all of 'em are bent.
So does anyone know why this is? Is it because these grates are seeing a excessive load? Perhaps loaded by a smaller area than anticipated? The repetitive nature of the load?






RE: Bending of storm water box grating
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
https://www.facebook.com/AmericanConcrete/
RE: Bending of storm water box grating
According to the client: no. (I thought of that myself.) The dip seems excessive for that purpose anyway. (Compared to new ones I've seen.) It's creating quite a bumpy ride for the plant entrance road.
RE: Bending of storm water box grating
The convex grating can have a fairly large curve:
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
https://www.facebook.com/AmericanConcrete/
RE: Bending of storm water box grating
Correct.
Thanks....but the situation I observed was so unsymmetrical (in its deflection), I doubt it was cast like that.
RE: Bending of storm water box grating
About the only thing I can think is you have non-traffic (pedestrian) or medium duty grating in there.
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
https://www.facebook.com/AmericanConcrete/
RE: Bending of storm water box grating
That's something I hadn't considered.......need to measure it the next time I'm out there.
Also need to consider crane or forklift traffic. I doubt either could exceed the AASHTO loads....but I've caught these guys moving some crazy loads before.
RE: Bending of storm water box grating
How are they oriented w.r.t. the roadway, w.r.t. the normal (most common) wheel path for heavy wheels. Do out-of-path areas show less damage? What are their span lengths, boundary conditions, and are they individual oriented in their most favorable (strongest) structural orientation for worst loading/spanning? Are they located someplace, ‘at the gate’ where a heavy truck sits on them, vibrating for ten minutes (sixty times a day) while their gate paper work is gone over? That’s almost like a high cycle fatigue problem, a shake down problem, or vibratory stress relieving, if casting leaves them with some funny high residual stresses. I agree that if they are cast iron, they don’t do much yielding, they are strong enough or they fail/fracture. Cast steel will yield, but relatively rough castings, such as these, still as likely fracture without real much yielding. Is there any way to relate these to one casting lot where something unusual took place? What are their chemistry and materials properties? Are they someone’s new design of grating to save material, or using a new material grade? You really, probably, have to involve the supplier in solving this problem, not go looking for some way to stick it to them. Some storm drain gratings are bent down intentionally (manufactured) to conform with various trough shapes of slip formed conc. gutter-n-curb shapes.
RE: Bending of storm water box grating
The only thing I've ever seen require special consideration beyond that of H20 is aircraft landing gear loading.
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
https://www.facebook.com/AmericanConcrete/
RE: Bending of storm water box grating
Cast iron storm grates are manufactured by companies such as Neenah. Their products look like the examples above and since you didn't disagree with those, I'm guessing that's what you have. I've never seen these deform, but there's a first time for everything. I'm sure they'd be interested in any bad experiences. They have engineering departments that could get to the bottom of it. I'm sure a couple of pictures and/or dimensions would allow them to tell you real quick what is wrong.
Unless these were some kind of counterfeit grates (very possible), they're most likely the wrong grate for the application.
RE: Bending of storm water box grating
I suspect the crane possibility. They've actually managed to break cranes before out there from excessive loads. (I'm not talking flipping them either.)
Thanks for the help.