Concrete/rebar cage vs Lally column??
McDesign
| Posted in Construction Techniques on
Got a design to quote. Part of it is a ~10′ x 20′, 6-column pergola by a pool, with two hanging “porch swings”, one on the rear crossmember of each bay. Columns will be 8″ or 10″ diameter hollow PolyCast (fiberglas or concrete composite).
Obviously, the swings, with kids, are going to put a lot of cyclic bending load on the columns. Two possibilities –
Option 1 – 6″ steel pipe columns within the back three columns, bolted to the piers, and the crossbeam bolted to them.
Option 2 – weld up a little “cage’ of rebar, welded to 4-6 rebar stubs extending out of the piers. Slide the column over it, plumb it, the vibrate the fool out of it and fill with concrete.
Anyone got preferences either way?
Forrest
Replies
Hey - thanks for all the replies.
Had another thought. What if I got the columns long, and embedded them a foot or so in the piers? (checking with the manufacturer first, of course. A 10" tube would be mega strong in bending.
Hmmmmmmm?
Forrest
thanks for all the replies
What, you expected answers? <G>
From this buncH? <G>
Easy, use sips & icf unless you're a loon, in which case use solid concrete, but only if a basement present, and you are using left-handed pulls on frameless cabinets . . . <g>
The first question may have been too detailed. Were you asking about the rebar in the pier, or reinforcing the column the pergola is upon? Or both?
If you go with a tube column under the column cover, it's a hair easier to spec anchor bolts hooked into the pier rebar. Or, a plate that matches the future column's base plate can have threaed anchor bolts welded on, which is very slick, as you can set that in the top of the pier when it's poured and later shim the column as needed.
Me, I'd spec a U-strap to be welded to the steel column to engage the pergola cross beam--hard to get the assembly much stronger than that, and still be able to set it to plumb & level too. (Which is why I'd not bury a couple feet of column in the pier, Murphy comes along goes, "hey this is pretty strong" while leaning on the column, until it moves a skosh, and Murph then uses non-calibrated eyeballs to get it back to plumb & true to the nearest 1/2" . . . )Occupational hazard of my occupation not being around (sorry Bubba)
Actually Mike my first reply was going to be if you are re-engineering something that has been drawn. That was unclear to me from your post. I decided that I would wait and see what other info came out about the project. Is it a design build?
Pre-engineered?
Does it use factory components that have attached engineering? Now having asked all those questions I will also admit to not answering because I don't have enough of an Engineering background to answer so anything I said would be a WAG.
They can't get your Goat if you don't tell them where it is hidden.
No pre-existing design. Just noodling this one out myself, from the client's description of what she wants -
"six painted classical columns; lattice-work natural beams with shaped ends as a pergola on top; two 5' porch swings on the back long beam; two fixed benches, one along each short side"
http://www.hbgcolumns.com/pccdimensions.pdf Here are the columns I'll be using. Probably the 10" diameter; the vertical BEARING load each is #14,000. With six columns, they'll support #84,000, likely a tad more than my pergola top, even is the cypress is wet. So, no vertical load worries - just don't want the structure to move laterally, even with vigorous swinging.
No external bracing is allowed (aesthetics and all)
Yes, I did just repair two sets of six 6" columns just like these last summer. We post-tensioned the hell out of them, with interior 5/8" all-thread from the piers, through 6" square X 3/8" steel plates, and a big nut at the top, all cranked down hard. Given all that, you can push and pause and push and pause and get a rhythmic wobble in the whole pergola - given the 96" height to 3" radius leverage, the all-thread is actually stretching.
It's certainly okay for that application, but they're just to sit under, and resist the wind.
Hence my concern.
Forrest
Well my thought (which has absolutely no value given what little I know about these things) would be to go with the concrete and cage. I would also suggest incorporating the side benches into the design in terms of a brace panel of some sort. If nothing else it would reduce the length of the column potentially affected by sway. As I said before I am no engineer. I either "seat of the pants" things or call one of the people who really do know how forces affect structure.
They can't get your Goat if you don't tell them where it is hidden.
Didn't you already do a project similar to that?? At FIL and MIL's place on the Chesapeake or something?
Re the steel pipe, I'd have to wonder about eventual rust unless you got galvanized pipe.
Re the rebar cage, I'm not sure I see the need to weld anything. Seems like if you just embed your 10' or whatever rebars in your pier footers, after words tie up whatever extra bar you want, slip on your perma cast and then fill with concrete and imbed anchor bolts in the top of the wet concrete. As you say, after checking with permacast to see if their product is suitable for a form... I mean it's basically a swing set - it's not like you are gonna be hanging dump trucks off it.
If the perma cast people say no (I'd say likely) just go with the pipe idea, imbedded in the footers, and just make sure it's either galvanized or painted real well especially right at grade.