Seismic Loading on Grated Walkways

[Trillers]

I will start this by saying I believe this is one of those issues in the "Does the CM know anything about construction?" category that consistently pops up in construction.

We have a contractor client who is constructing a wastewater treatment plant with grated walkways over the pump station channels, influent channels and UV disinfection channels. The walkways are to sit on angle beams (2 1/2" x 2 1/2") seated and anchored in the formed insets on the concrete walls for these channels. The beams are hot-dipped galvanized, and the walkway grating is constructed of aluminum. The grates are not to be bolted to the angle beams, they will be clipped together, but sit freely with no restraints. See sketch attached.

The CM has directed the Contractor to submit a set of "structural" calculations for the angle beams seated and anchored to the concrete wall and the walkway to show they can resist seismic forces.

My first question and helpless plea is how do you determine the seismic responses for this type of system. The only things fixed to the structure are the angle beams.
Second question - if we can actually evaluate the walkway for seismic capacity where would I begin? I don't believe this system is addressed in ASCE Chapters 12, 13 or 15 because it can't be classified as a "similar to building" system or even "not similar to building system", and I could not figure out what the Fundamental Period from either 15.4.4 or 12.8.2. because I have no idea how to classify a walkway of this type.

Even the exclusions listed in Ch 15 do not seem to apply.

I could use ASCE 7-16 15.4.2 "Rigid Nonbuilding Structures" to determine V, but this only applies to nonbuilding structures with T less than 0.06s, which again I have no idea what this would be for the walkways.

I would appreciate your thoughts and sage advice.

 

[jayrod12]

I wouldn't even classify those as beams. Those are embedded bearing angles. They are just going along for the ride with the concrete walls.

 

[MotorCity]

As you have surmised, calculating the seismic load on the grating is ludicrous. But if one were to calculate it, take a look in ASCE 7 for a section on seismic loads of components, Fp. The seismic load is based on a percentage of the dead load (i.e. the weight of the grating). It is usually very low. If your grating is 10 psf and the Fp is 10%, you're talking about a lateral load equal to 1 psf * square feet of grating. That would be the seismic load transferred to the "angle beams".

Now if you're talking about the global seismic load of the entire concrete walkway, that is a very reasonable concern. I'd classify as a non building structure, not similar to a building and move forward.

 

[dhengr]

Trillers:
The way we operate these days is with lots of emails and texts back and forth, which may or may not be answered. Then even if answered it is usually with quick half-assed answers until about the forth cycle, rather than actually communicating, face to face or by phone, so we could ask, “what do your really mean, or what do you really want from us?” These exchanges are generally not to resolve the issue, they are just to throw the problem back on the shoulders of the other guy and get it off our back. What a hell of a way to expedite a project or solve each problem. Look how efficient we are, 17 exchanges, over a couple days, but not a damn thing resolved or really understood. It’s the same-old-same-old..., The Hurryier we go, The Behinder we get.

Why not talk to the CM, and find out what he really wants, or to explain why that isn’t needed, or how that’s already been accounted for, a real first-hand exchange and meeting of the minds. I wonder if he wasn’t asking..., can those light beams spanning these channels, etc. take the reaction loads btwn. the two walls during an EQ? I rarely get a request or question like that without it generating 6 more questions from me, so I am sure I understand what he really wants. We can see here on E-Tips, that engineering questions are asked so poorly these days that it takes the first 30 questions to start to understand what they are really talking about, what they really mean, what they really need.

 

[Trillers]

Thanks Guys,

We were able to convince the Owner (much to the chagrin of the CM) that these walkways are non-structural, non-building elements.

Our computations were as described by MotorCity - so seismic weight was actually generated by a hypothetical LL of 50#/SF per width of walkway plus the grating and framing weight which still produced negligible shear especially since the frames are only clipped and not bolted to the frames.

Unfortunately we will have to deal with a "pissed-off" CM for the remainder of the project.

Trillers