Another Tricky Question 1

[WpgKarl]

OK all you structural analysis & design experts....I have the joy of analyzing & designing a stage / canopy that looks like a fine piece of oragami (see attached)

Any suggestions on how to approach this one?

 

[WpgKarl]

Hmmmm, this one's so tricky, you can't even see it on the attachment - I'll try it again

 

[rb1957]

is it a white line drawing ?

 

[racookpe1978]

Sorry boss: Can't bring it up. Check this: I'm not seeing any recognized extension (.pdf, .jpg, .doc .pps) on the "canopy" file.

See if that helps.

Then again, an invisible orgami canopy is certainly lightweight, but you might have to worry about how to hold it down. 8<)

 

[WpgKarl]

I don't know what's wrong with the attachment, so I'll try a picture this time...

I need to analyze and do a preliminary design of this canopy / stage. The architect wants to use precast panels for the upper structure, and there will be a cast-in-place concrete slab on the base.

Funny, but when the architect built the model, they put a wire across the top to hold the one corner up (b/c it was tending to fall down), but we can't have any supports in the real structure, it is cantilevering out to the far corner.

I'll post one more picture after this one.

 

[WpgKarl]

Here's the other photo.

PS - the architect want panels & slabs to be as thin as possible.

We could put a rib on the outside edge if req'd and the end of the "box" can be solid, so I'm thinking a cantilevered triangular rib (along the outside edge), cast monolithically with a solid column / wall & a moment resisting base at the far end....

 

[racookpe1978]

What is the material of the "wings"? You mentioned concrete for the "base" - is that the bottom "floor" only, or what the edges of each wing will be attached to?

Indoor (stage only?) or exposed to real world wind and rain/snow/ice loads?

How tall to the points?

 

[WpgKarl]

Wings & walls are all precast, canopy is exposed to elements (freeze/thaw) etc., Max dims are 25ft x 25ft (walls), 10ft high to platform level +/-. Pad sits on the ground in a park.

My last attachment was a scanned pdf of photos, with mark-ups that explained all that, but I am having a "trouble with technology" day.

 

[WpgKarl]

I'll try the pdf one more time....

 

[BAretired]

To make it easier to discuss, I have numbered the nodes in the attachment. Maybe I should have used letters. If you wish, I can change it.

BA

 

[msquared48]

How can this Architect sleep at night?

Mike McCann
MMC Engineering

 

[WpgKarl]

Good idea BAretired. So the challenge is how to hold up point number 11, complicated by the fact that there is a change in slope between triangle 2-3-11 & 2-10-11 & that they want to use precast, which means joints (max. piece size around 12-15 ft, unless site-cast & erected).

I am most concerned with overall stability & possible wind vibrations of the corner @ 11, but using concrete it should be fairly heavy compared to light steel framing.

PS - For the analysis, I know somebody a lot smarter than me can model this with thousands of nodes in SAP, etc. but I am wL2/8, moment distribution / hand calculations kind of guy - if this problem was in the 1960's, how would you handle it with some relatively simple but accurate enough calcs?

PS - I was in diapers in 1975....

 

[prex]

I would start of course with the big upright panel 25'x25'.
Will it be erected as one piece? If not the joint will have to restore the continuity.
This one can be analyzed as a rectangular (square) plate with two free and two supported sides. It will likely be a plate of constant thickness, as I you can't have clamped sides. Determining the thickness under wind action and checking the frequency (higher than 5 Hz) will take you to a first check of feasibility.
I would also prevent the architect that, if necessary, some angles and inclinations could require changes: the angle between the panels joining at the line 2-11 is unclear in the photos, but seems quite small for transferring the load from panel 2-3-11 to panel 2-11-10.
Panel 2-3-11 is nearly a right triangle: Timoshenko has the formulae for a right triangular plate with two supported sides. Here too you could come up with a required thickness to start with some figures. BTW I don't think that your proposed rib would be very useful, unless you have loads on the edge (that you don't).
This structure doesn't seem so difficult, though it will require lengthy calculations. At a first sight it appears more or less stable, without requiring moment connections anywhere. Of course the free corner 16 need be analyzed firts for feasibility as above.

prex

http://www.xcalcs.com

: Online engineering calculations

http://www.megamag.it

: Magnetic brakes for fun rides

http://www.levitans.com

: Air bearing pads

 

[BAretired]

Mike,

He must sleep. This is his latest nightmare. I'll bet he sleeps in a "canopy" bed.

WpgKarl,

It might not be too bad for a hand calculations kind of guy. I don't believe it ought to be done by FEA. It is essentially a folded plate type of structure.

Gravity loads should be fairly straightforward. Wind loads would be tricky if this is outside. But I assume it is inside, right?

I would try to get more support for nodes 11 and 16. They appear to be flapping in the breeze.

BA

 

[COEngineeer]

Node number 9 is going to get crushed lol. tell the arch. that it wont be pointy like that, plus you need to anchor it down

Never, but never question engineer's judgement

 

[COEngineeer]

just wondering .. what is the purpose of node 3,4,7, 8? People walk thru there? I assume it is pretty tall? My concern is, node 2,9, and 10 do not even meet ADA spec because a blind person will hit his head since his walking stick will not hit anything before he hits his head! That is what you should ask the architect first before you draw you model.

Never, but never question engineer's judgement

 

[racookpe1978]

Why allow him to "demand" concrete?

LOTS of weight, no tension resistance, no "assistance" across the pivot points (joints) from panel to panel .... And not really very pretty: concrete is about as drab and generic as you can possibly get = rather ugly ordinary typical government-paid-for urban "art" at its Euro-kulture worst.

So, if the arch. needs to paint the concrete anyway, why not use a lighter fiberglass or plastic "foam-filled" panel? The wind loads, snow and ice loads are the same, but the deadweight loads are 1/10 as much. The panels aren't going to be "sidewalks" or "bridges" that need to resist live loads - other than vandals or skateboarders.

 

[WpgKarl]

Thanks to all for the posts....The architect wants the look of concrete and I think it would be better to have some mass to the structure to avoid vibration problems.

Of course the pointy node 9 will get crushed - we would square it off a bit. The open part is about 10 ft high and is a storage closet.

racookpe1978: "And not really very pretty: concrete is about as drab and generic as you can possibly get = rather ugly ordinary typical government-paid-for urban "art" at its Euro-kulture worst."

Awww, c'mon, concrete is beautiful!

 

[racookpe1978]

OK, OK - If you (like my dad) actually "like" concrete ....

Decide on the thickness of the wings (or plates) - but make them all a uniform thickness, or vary the thickness of each based on the twisting load. Make each plate out of C channel frame (C3 or C4 inch if you have to, the smaller sizes will be prettier and "lighter" in appearance. TS2-1/2x2-1/2 or TS3x3x1/4 would also work, but doesn't surround the edges of the concrete as thoroughly.

Weld 3/8 or 1/2 rebar across the fame between the channels or TS members to make a web - wire mesh won't be stiff enough. Add a few diagonals for rigidity. Lay on a smooth form and pour: Use as light a aggregate as possible. The wings and panels are now stiff.

Your problem now becomes attaching (welding) the wing edges together with enough bead in enough length to resist bending. Paint the steel, whitewash the concrete, hang the architect ... You're good to go.

 

[Teguci]

Create a 3-7-11 upturned L-Beam (cantilevered)- This can also double as a guard. Use the 2-9-10 plate as another beam. Run Stadia beams (simply supported) between these points to act as bleachers and steps.

For the far wall - consider vertical doubletees cantilevered from the ground.

Be extremely careful with how you detail the welded connections. The exterior exposure will require stainless steel connections. For the cantilevered beams I suggest NMB splices or sim.