Glulam post on concrete slab

[murak]

I am planning to attach glulam posts, 115x115, to a carport with anchor irons embedded in a concrete slab. Is it correct to have ~30mm of air between the concrete slab and the post? Meaning the posts hang in the anchor irons. This way, they can dry properly, and it's even possible to touch up the paint. The posts will be painted (primer oil, primer paint, cover paint oil acrylic. Extra carefully on the end grain.

Or is it better from a strength perspective to have the post resting on the slab? It feels like moisture will contact the end grain then. Some kind of spacer perhaps? Large square washer, galvanized? Some plastic spacer?

Thanks in advance!

 

[pacman42]

In my carport (now garage) which is 30 years old, the posts stand in foundation steel about 1cm above the asphalt, and they have no noticeable moisture damage. According to my neighbor, who is a carpenter, there are no issues with the strength either (there is an unbroken laminated beam that rests on top of all the posts, so the weight is distributed evenly).

The posts have not been exposed to any moisture other than ground moisture. I would not place directly on the concrete (even with tar paper) unless I was completely sure that I could keep moisture away in all situations.

 

[murak]

In my carport (now a garage) which is 30 years old, the posts are in base iron about 1cm above the asphalt and they have no noticeable moisture damage. According to my neighbor who is a carpenter, there are no issues with the strength either (it's a continuous glulam beam that sits on top of all the posts, so the weight is evenly distributed).

The posts haven't been exposed to other moisture than ground moisture. I wouldn't have placed directly on the concrete (not even with tar paper) unless I was absolutely sure I could keep moisture away at all times.

Thanks for the quick reply! That sounds reassuring. So like this but with a 10mm gap:



And the posts are secured with through bolts? M10? Should I use lock nuts?

EDIT: I should mention that there is a 4.3m wide opening on the long side of my carport, so one of the posts will have a fairly large load.

 

[pacman42]

It looks good. There's nothing stopping you from adding a plastic block in the gap underneath if you want extra durability. I would probably do that to prevent the wood from flexing in the screw holes. I would have used double bolts. A locking nut is overkill, the bolt will not move. I discovered when I tore down parts of my carport during renovation that they hadn't even put any nut at all. It stayed where it always had...

Regarding the span, the most important thing is to dimension for it otherwise. The attachment of the post should be the least of your problems if the concrete is properly cast.

 

[murak]

It looks good. There's nothing stopping you from adding a plastic block in the void underneath if you want extra durability. I would probably do it to avoid the wood flexing at the screw holes. I would put in double bolts. Lock nuts are overkill, the bolt won't move. I discovered when I dismantled parts of my carport during reconstruction that they hadn't even put a nut on at all. It stayed where it always had...

Regarding the span, the most important thing is that you dimension it properly otherwise. The post attachment should be the least of your problems if only the concrete is properly cast.

Thanks! Double bolts, absolutely. Regarding the plastic piece, I'm imagining in my head something that looks like a hockey puck, but made for the purpose. The question is whether plastic holds up year in and year out, or if it's a kind of rust-protected steel piece that's needed?

EDIT: Found this at Biltema: http://www.biltema.se/sv/Bygg/Fastelement/Ovriga-skruvar-och-tillbehor/Mellanlagg-2000032409/

I want one like that, but I don't need 220, four will suffice. I'll keep looking...

The span should be well-dimensioned. I've had good help from another forum member (justus) and the beam supplier has also calculated on it.

I'm planning a special with the anchor irons, cutting them to the right length, drilling holes, and then fitting them onto the edge beam's rebar:


No iron closer to the outer edge of the concrete than 40mm. The iron will sit down in the concrete about 180mm and above the slab about 220mm.

Attached is a picture of the whole setup so you get a complete picture: