What lateral loading do I apply to a playground structure in an existing building (Space Frame)? 2

[Mike C]

The plan checker is requesting a 3D analysis of a playground structure that weighs 4k and is located on the second story of a two-story concrete slab-column building in the Bay Area (California). The frame elements are galvanized pipe sections with base plates anchored into the elevated slab. I need to know what seismic loading to apply in the model for this grid structure. Since it essentially has no diaphragms, I assume I should load it with the mass of each element. However, I have no experience with this type of lateral loading, so I would appreciate any quick and direct answers to help me analyze this structure and move forward. Thank you.

 

[XR250]

Ugh

 

[Mike C]

That's all you got for me? LOL. I agree.

 

[dik]

I'd use 100 psf in addition to the equipment.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[STpipe]

This would fall under the category of an amusement structure under ASCE 7. So for seismic R = 2.5, Omega = 2.5, Cd = 2.5.

I think as dik pointed out, 100 psf for your live load, and then use the appropriate fraction of that to add to your effective seismic weight as per the code. The problem with these types of structures is that you do not have a clearly defined SFRS, therefore everything ends up contributing to the lateral load resistance (and needs to be designed for these loads). As for how to apply the load, one of the ways to do it would be to apply the calculated seismic response coefficient as an acceleration.

 

[XR250]

100 PSF of kids on this - seriously?

 

[human909]

Even with bay area seismic conditions I'd be just as worried about serviceability deflection governing. I'd be looking for a stiff frame that doesn't have noticeable movement under body weight so that is 1-2mm maximum movement for 1kn. Sure this is kids but they move about alot!

Regarding ultimate load checks. I'd be looking at a serviceability level of users on the platform and applying the suitable seismic loads at the distributed platforms.

If you haven't performed analysis on structures without diaphragms then now is the time. It really isn't that different just like everything in structural engineering you need to calculate the loads, apply the loads and then follow the load paths.

 

[XR250]

Why would serviceability be an issue with this? What could get damaged?

 

[human909]

Why would serviceability be an issue with this? What could get damaged?

[URL unfurl="true"]https://en.wikipedia.org/wiki/Serviceability_(structur[/url]e)

"Damage" isn't the only item serviceability criteria.

 

[XR250]

@human, Can you provide a scenario where serviceability would be an issue with this structure?

 

[Lomarandil]

Absolutely. Kids start swinging, structure deflects. Kids think it's cool and everyone jumps in to see how much they can make it wobble. Moms think it's less cool and complain to the building owner. Or worse, moms contact the local news station for an "investigation" and threaten a lawsuit.

 

[XR250]

@Lomarandil,

The serviceability case was regarding seismic. I don't think any kids will be complaining.

 

[dik]

"100 PSF of kids on this - seriously?"

My error... I just noticed it was lateral loading.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[lexpatrie]

interesting question.

I got involved in a tangle between UC-Davis and the city regarding some shade structures quite a while ago, while there were issues there, the primary concern the department had was the anchors that were originally specified weren't rated for cracked concrete and this was outside exposed to weather (probably SDC D or so), we got through it eventually. But nobody got into a whole "attractive nuisance" discussion about a Fraternity pledge class climbing it. It would never endure fall protection harness loads and we stated so clearly.

One would presume there's staff there that would oppose any excessive mischief, and it's not like it's sitting exposed to the open are, this is inside a building, they have doors that lock.

Here? Don't you need the building acceleration? This is on the second floor of something, so the second floor is where the load "comes from." The bases need to resist that acceleration induced load.

I think you're looking at point loading (mass acceleration) for all the pieces, once you arrive at the effective lateral acceleration. I don't see any other option, it isn't like you can lump the mass and it will somehow move through the model, not realistically anyway, it may math through the analysis software but it's not realistic.

Accidental torsion, how's that going to happen on this structure? Is there some kind of exception for something like this that can't really develop accidental torsion?

Alternately, can this be anchored to the walls as needed like a bookcase? This isn't exactly a building, and it's not furniture, but it could hypothetically topple, and it would be awkward to extricate the children, who might be injured if it collapsed.

 

[Tomfh]

Can you provide a scenario where serviceability would be an issue with this structure?

Kids are pretty rowdy. They like shaking things, kicking things. They’ll often gang up to get things shaking.

There’s a brand new playground structure close to use, and the kids realised they could swing from the bars and kick out the side panels. Bye bye panels.

 

[XR250]

Again, this is a seismic discussion.

 

[human909]

Again, this is a seismic discussion.

Read my very first sentence for context. Also read the thread title. You seem to be viewing the discussion with an artificially narrow view.

The the application of lateral loads on the structure for the active live loads of children would be similar to the application of a seismic load. So I see the discussion as highly relevant. Given the question involves how to apply the loads to the structure. If they can't answer it for seismic the likely can't answer it for the lateral live load of kids playing on it. Certainly in most areas and for most structures/materials I'd expect the serviceability lateral loads of use to be dominant the design over seismic loads. There is a reasonable chance this would still be the case in a high seismic region unless your design is amazingly brittle.

 

[XR250]

I imagine the AHJ is only worried about this thing collapsing. And, honestly, the only practical way to determine if this thing shakes excessively when kids use it is to test it with actual kids. And if it does, then what do you do?

 

[Smoulder]

Looks proprietary so wouldn't worry about normal performance if that's the case. Just check ULS strength at the supports. Could justify the same vertical live load as the rest of the floor which might be 100psf anyway or a childcare centre loading.

 

[Greenalleycat]

I've dealt with similar stuff before - designed a freestanding abseil frame
The seismic loads are basically irrelevant IMO and should be easy to apply

Step 1 - model whole structure with proper sections and materials
Ensure joints are pinned or fixed as appropriate

Step 2 - calculate seismic Parts loading as appropriate for the 2nd floor
This will probably be conservative as I imagine the structure is flexible but I wouldn't apply any reduction for period

Step 3 - use the 'acceleration' loading option in your choice of software to apply this as a uniform acceleration to the whole thing

Step 4 - realise that the loads are sweet F all so seismic isn't governing

Step 5 - expand upon loading model to add in your various horizontal and vertical live load cases as appropriate for your local playground loading code

Step 6 - realise that live load massively governs
Design elements and connections for forces from this

Step 7 - check deflections and realise that the thing is deflecting like 50mm and that's not good
Try figure out how to stiffen the thing up to stop it wobbling uncontrollably during use

Step 8 - realise that SLS was the governing criteria all along