Steel shed - supporting an existing column

[RonOz]

I have a damaged steel shed. The roof has collapsed. Construction was with C-section steel that’s commonly used in shed structures. The walls remain standing and are in good condition. My intention is to reinforce the existing posts using SHS fitted inside the existing C-section posts (sealed on both ends with welded plate). The new posts will be bolted to the existing posts. New trusses will be supported by the SHS rather than the C-section steel.

The existing C-section posts have concrete footings, but there is no concrete floor in the shed. Following my repair, I will have a concrete floor added.

Here’s the issue: I was considering not bolting the new SHS posts to the existing footings. They will be bolted to the existing post instead. The new posts will be about 10 or 15 mm above the existing footings (ie. an air gap). The surface of the footings are rough finished. It seems to me that if I were to rest the new posts on the footings there may only be a single point of contact, which might subsequently crush and then subside when the roof is installed. Also, any significant sized air gaps that remain under the posts following the addition of the concrete floor may result in corrosion.

Here’s what I was thinking: Following the installation of the new posts, I’ll pour a quantity of floor leveller around the base of the new post to just cover the base. This product is very fluid and would likely easily flow under the post filling any air gaps. In so far as I’m aware this product is as tough as concrete and being alkaline in nature would prevent corrosion as concrete does.

Any thoughts?

 

[Greenalleycat]

That's pretty much standard practice isn't it?
Here, typical column bases have 10-20mm allowed for drypack or, if they're critical, high-flow/non-shrink epoxy grout

Alternatively, could you just pour the floor leveler first to get a flush base then bolt onto that?

 

[RonOz]

"standard practice"

Really, I didn't realise that. I have to admit that I was stuck for a solution. Then came up with the floor lever notion. It was my own idea. It seemed a good solution to me, but subsequently wondered, not having an engineering background, that maybe I was in cloud cuckoo land to consider it.

Well, thanks. Most reassuring.

 

[Greenalleycat]

Just to make sure I understood correctly, I'm assuming a solution that will look something like this:

 

[RonOz]

Not quite. I'll try and show a sketch. With Easter around the corner, I'll post something next week.

 

[RonOz]

The photo of an actual shed post is a little dark. It was the best I could do, but it does provide perspective. Note that there is no concrete floor, as yet.

The next image is a copy of the photo. The existing post is bolted to a plate that extends from the footing. The C-section is not in contact with the footing, and that aspect is shown more clearly in my sketch.

I was planning on bolting a 75x75x4 SHS post to the existing post. The new post will support the new truss. The old post will then only secure the wall sheeting (and provide some additional rigidity to the new post).

My concern is that the upper surface of the footings is not flat and smooth. They are rough finished and because of that, I don't believe bolting the new posts to the existing footings is possible (or wise). My feeling is that by keeping the new post above the existing footing and then using floor lelveler to fill the gap, this particular product will ensure the new post does not rust and will transfer the load of the roof to the footing

The above is what currently exists, and how the floor might appear when concrete is poured over the footings.
The following is my planned repair. It shows the new post, the floor leveler notion I am considering, and once again, how it would appear when the concrete floor has been poured.

 

[Greenalleycat]

Seems fine but I still don't understand why you don't just pour the leveler first then stick the post on top of it
Allow a 20mm standoff and dry pack if you like - that's standard construction practice

The potential advantage I see is not pouring wet leveler against the base of your post - perhaps this is a corrosion risk?
Also, an advantage of a standard 10-20mm dry pack standoff is for water management - it stops the base of the post being in a potential pool
If you use floor leveler then you won't have any fall away from the post

 

[human909]

I have a damaged steel shed. The roof has collapsed.

I'd be asking a big WHY first.

Your shed most likely doesn't meet code probably by a significant margin. Lots of these 'kit' shed in Australia don't meet code. If you are fixing this problem are you throwing good money after bad?

Your proposed fix is almost certainly not addressing the issue. Adding a 75 SHS has decent axial load capacity but it doesn't address the moment capacity issue or the portal connection which is almost certainly what caused the failure. Wind is your enemy here and an SHS sistered onto undersized C purlin isn't a solution.

New trusses will be supported by the SHS rather than the C-section steel

The SHS is weaker in bending than the C-section. And bending capacity is what you need here.

 

[RonOz]

The floor leveler begins to set within about 10 minutes. That can be useful but can also be a disadvantage. There will no doubt be a degree of fiddling with each post pair in getting them vertical and ensuring the truss is level. This will take time. I was planning on having all posts and trusses secured and braced, locked up solid, before pouring the leveler.

Regarding ‘fall away’ the existing footings are slightly raised above the ground. Only by about 25mm but will probably be enough to keep things dry.

Meeting code. Good point. The shed was on the property when I bought it. It’s old but seems in good condition. Surprising, I know. Considering its age. It was built about 20 years ago. The condition is probably due to things being galvanised. Anyway, it lasted about 5 years before a heavy snowfall pulled down the roof. That must have been a sad day for the owners.

The shed wasn’t suitable in regions where there is a risk of snow. That was the problem. My solution is to have seven trusses instead of the four it had previously, and include some good bracing. It didn’t seem to have any bracing on the roof. Some on the walls, but not much. Perhaps the builder relied on the sheeting for wall bracing.

The 75x75 is the biggest I can fit into the C-section. I would prefer something more rectangular for the reason noted, but I can’t see it working.

 

[human909]

Fair enough. Seems like you have it sorted in the backyard handyman way. With the information available, I don't believe base corrosion or the floor mounted bracket will be the weak point in the structure.

If the shed roof is completely gone I'd consider looking at a complete replacement if you want it to last. But if that requires council approval the fixing what is there is the easiest backyard repair.

As an engineer I can't recommend it though. ;-)

 

[RonOz]

Yeah!

I expect (most) others would have it demolished and start again. Way too expensive. You're quite correct - backyard handyman. The roof will be removed and replaced. It'll be over-engineered. I have trade training in welding, which will help.

Thanks for your comments. Very welcome.