Calculating loading
Calculating loading
(OP)
Now, this is out of my area of expertise so bear with me. I want to install a 22" concrete pad over buried instrument/electrical conduit. The top of the conduit is about 48" below ground. This leaves a fill space of about 26" between the bottom of the pad and the top of the conduits. I am not sure what the diameters are of the conduit in the duct bank. The soil is clay or rock. If you go down 5' you're supposed to run into sandstone. This is Lemont, Illinois. It may be be that the duct either runs through rock or right above it.
The pad will contain a 11,000 gallon liquid nitrogen tank and vaporizer: 175,000 lbs. for the tank, equipment, and pad. Add about 57,000 lbs. for liquid when filled.
How can I work this out. My first thought was to construct a sleeve around the duct bank made of steel with the foot carved into the rock below. Any ideas? Am I worried for no reason? Will the soil disperse the weight. The pad is about 30'X18'. Another idea was to cast the concrete around the conduit. It just means more concrete and that's cheaper than steel.
The pad will contain a 11,000 gallon liquid nitrogen tank and vaporizer: 175,000 lbs. for the tank, equipment, and pad. Add about 57,000 lbs. for liquid when filled.
How can I work this out. My first thought was to construct a sleeve around the duct bank made of steel with the foot carved into the rock below. Any ideas? Am I worried for no reason? Will the soil disperse the weight. The pad is about 30'X18'. Another idea was to cast the concrete around the conduit. It just means more concrete and that's cheaper than steel.





RE: Calculating loading
Mike McCann
MMC Engineering
RE: Calculating loading
So, what would you use as a sleeve. I should add that I think there is a cross over duct underneath so the sleeve might be a regular shape.
You said that the soil will disperse the load: 232,000 lbs.? Is that possible if the soil is only 26" deep above the duct bank?
Here's another idea I had: support the 22" pad on 4-6 steel columns, pylons, sunk into the rock below 5'. That would support the weight directly on the rock and relieve the pressure on the soil. What do you think?
Dirk
RE: Calculating loading
Rather than the steel columns, I would consider concrete piers to the sandstone and designing the slab as structural.
At a questileonable four feet to the sandstone, overexcavation may be too much. Then again, if you used CDF for backfill with the regular concrete slab over it, it could be feasible. Just verify the allowable bearing pressures to the CDF and sandstone layers.
Mike McCann
MMC Engineering
RE: Calculating loading
RE: Calculating loading
RE: Calculating loading
RE: Calculating loading
RE: Calculating loading
Unfortunately, relocating the pad won't work. I've presented 6 alternative locations in a crowded refinery. Besides, now that they've fallen in love with this location they want me to make it work for them. It's one of the dangers of the business we're in. You guys should know from working with architechs.
Moving the duct bank
Moving the conduit means shutting down the plant to pull the wires. This is definitely not going to happen.
What do you guys think of Mike McCann's solution? I was thinking about concrete pylons or piers (symantics to me) before he responded. Obviously, I don't know the load that will be bearing on the rock below. How much can it bear? What kind of footing do I need to each column? Should I design for 4 or 6, or even 8 columns? How do I connect the column to the concrete beam? Do I modify the concrete beam for support off from the column?
Of course, I will have to get the vendor of the tank involved. They will want their structural engineer involved.
I want to thank all of you for your help so far. If you ever need any environmental or chemical engineering help, look me up.
RE: Calculating loading
RE: Calculating loading
I doubt that piers will work. You probably have bedrock just several feet below the surface. The duct bank is probably laying right on top of the bedrock. Over in Lockport, the bedrock is 3-4 feet down. The bedrock is typically capable of 10,000 psf.
You might put the tank on the pad portion where it will bear on the bedrock, while the evaporator may be placed over the duct bank.
Investigate reducing the size of the unit if that will help.
RE: Calculating loading
Pad above ground: 3"; Pad thickness: 22"; Fill depth: ~ 2'8" below pad.
Ground level: 591'; Top of conduit duct bank: 587';
Bottom of conduit duct bank: 583'; Bedrock: 585'.
Obviously, the problem is not the loading. At only 432 pounds per square foot (psf), this is less than the 500 psf mentioned earlier. The problem is that the load is over a bank of old electrical conduit. The electrical engineer is not sure what the loading capacity is. I, for one, would not want to match old rolled steel, even in a bunch, against concrete and a filled 10,000 gallon tank pressing down against bedrock.
Another issue came up. I looked at one so-called exploratory excavation report. I guess that's what they do around here rather than using a ditch witch. The report says there is a sidewalk ductbank less than a foot down. This makes me nervous. Still, I can't believe there would be a duct less than a foot below the surface. I am going to recommend another dig where we want to put the pad.
Right now, I am still leaning towards piers. I planned 6 and estimated spirally reinforced piers at 6" diameter; I drew 12" on my CAD drawing. This is now something I will leave to the civil engineers. I just wanted to make sure I left at least 2 ft clearance for the footings. I want to go back to my other assignments.
10,000 psf for bedrock? That's pretty high. I'll have to remember that one.
Thanks guys
RE: Calculating loading
It is wise to do a geotechnical investigation for any tank, especially in an area with disturbed soils. The cost is minimal and it will proabably save money in the design of the foundation. Do yourself a favor and talk to SEECO.
RE: Calculating loading
Hopefully the concrete pad is properly reinforced.
RE: Calculating loading