Metal base for sculpture - questions about sizing & anchoring 6

[DCStructures]

I am designing a base for a 6ft tall x 5ft wide x 1/2in thick sculpture that is to be located outside in a plaza for 1 year. The artist prefers to use aluminum for the sculpture itself, but is open to using stainless steel if it's preferable structurally. We are not allowed to attach the sculpture or it's base to the plaza, and the base needs to be able to be disassembled, so the pieces cannot all be welding together. Finally, the artist would like to avoid having to get a crane, so it's preferable for the sculpture and the base to each be lightweight enough for 2-4 people to carry each and attach on site.

I ran some preliminary numbers on overturning and sliding at the base due to wind to get a feel for how big the base needs to be. The sculpture isn't solid (there's a big hole through the middle), so the wind load isn't tremendous. But I'm still coming up with a stainless steel base that would need to be 3.5ft x 5.5ft x 1in and which would weigh 800 lbs. I don't have any statistics on this, but I know personally I can only lift MAYBE 100 lbs, so I'm doubtful that 800 lbs is lightweight enough. What do you all think?

To connect the sculpture to the base, we're thinking there will be a small connecting plate that can be welding to the bottom of the sculpture ahead of time and then we'll use flat countersunk bolts to connect this smaller plate to the base. Does this seem like the easiest way to make this connection, or is there a simpler way that I'm not thinking of? Where can I find info. for designing the countersunk bolts? Is there info readily available online somewhere, or should I just use AISC steel manual and find standard bolt dimensions from a manufacturer online? Is it going to be possible to make this connection with a base that is only 1" thick steel? That doesn't offer much thickness to engage the threads and all that. But making the base even thicker will increase the weight even more.....Ideas?

 

[DCStructures]

After I wrote the above posted description and questions, some additional ideas occurred to me-

Is there any reason structurally why this 800 lbs base couldn't be 2 separate 400 lbs pieces? The sculpture is only 1/2in thick, so it's really only taking wind in one direction. Would I want the split to be parallel directly under the sculpture or perpendicular along the CL of the sculpture?

How hard is it to separate something that's welded together? Could the base connection just be welded and then after a year when they need to move it could the welds be chiseled apart?

If the 1" base isn't thick enough for the countersunk bolts, maybe I could add an additional plate sandwiched between the sculpture connecting plate and the base plate. This plate could also serve the purpose of lapping over the joint if I split the base into 2 separate pieces.

 

[DCStructures]

Further brainstorming-

If base itself is aluminum, I won't have to deal with dissimilar metals and galvanic corrosion. Plus aluminum is lighter weight. It'll need to be bigger and thicker to counteract overturning and sliding, but larger footprint is acceptable and thicker base will make bolted connections easier.

One more question: are there any posted values for coefficient of friction between aluminum and concrete? I searched, but couldn't find any.

 

[canwesteng]

You could try and hide sandbags or something inside the base for extra weight

 

[DCStructures]

Canwesteng, that is a thought. The artist was envisioning a flat base but something more along the lines of a pedestal may be acceptable. Thanks!

 

[MikeHalloran]

Further to that, if the base is a hollow box, it doesn't need to weigh much.
If you make it waterproof and put a pipe coupling in it near the bottom, you can unscrew a plug to remove the water that you add through another pipe coupling near the top, so you don't have to worry about shoveling sand. Or forget the drain plug and use a siphon hose to drain it.

If the base has substantial depth in the vertical direction, it could have a socket into which a mating feature of the statue extends, so gravity holds them together. ... allow also for a concealed padlock.

... further to that, forget the fill coupling, and pour the water into the base through the socket for the statue.




Mike Halloran
Pembroke Pines, FL, USA

 

[dhengr]

DCStructures:
Anything that can be accidentally or maliciously removed, or can leak or evaporate does not make for good reliable structural ballast. Freezing could be bad too. The base plate which is welded to the sculpture and is part of it must be strong enough, thick enough and sized to take the moments (any direction) from any and all lateral loads. It can be bolted down the steel plate stability base which rests on the plaza surface. I’d use mild steel plate for the stability base and a good prep. job and a good quality paint. That will last a year, plus, without any rust problems, and be far less costly. Make this 3, 4 or 5 plates (layers) thick, of a thickness which 4 guys can handle. Make each layer 3-4" smaller on a side, as you move down in the stack, and it’ll look like it is floating on the plaza. Put 8 bolt holes in the plates; the center 4 to match the bolts on the sculpture base plate, and one near the outer corners of the smallest (lowest) stability base plate. The outer four bolts hold the stability base plates together at the corners and the center four bolts go through all the plates and the sculpture base plate to provide the stability to the sculpture. Use plow bolts welded to the underside of the bottom plate, or it might not be all bad if the entire thing sat on 8 bolt heads on the plaza. Round the edges and the corners of the top stability plate so people don’t nick their ankles on it. All of this stuff could likely be handled with a small boom on the delivery truck or a small fork lift, and save the need for many plates and parts.

 

[wannabeSE]

Just a thought, have you considered putting a thin neoprene rubber sheet (or other elastomer) between the base and the pavement? I am guessing that it will help prevent sliding. Also, if you take Dhengr's suggestion of a carbon steel base, it will provide additional protection against rust stains on the pavement.

 

[DCStructures]

Great ideas, everyone. Thank you.

Welding the plow bolts to the bottom plate is a clever idea. Are plow bolts weldable? I have not used them before. I suppose we would need to use some type of cover to hide the nuts at the top to prevent someone from removing them. Any suggestions on what to use?

A neoprene surface layer between the bottom plate and the plaza pavers is a great idea. Both for sliding and to protect the plaza itself. Are there coefficient of friction values available for steel on neoprene and neoprene on concrete? I searched but could not find any.

 

[EdStainless]

If you use neoprene or a soft urethane the numbers will be similar to rubber on pavement.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[DCStructures]

Thank you edstainless!