Steel beam question

[crazyal]

So I'm thinking of building a gazebo next summer. It'll be a 14' octagon about 8' off of the ground. I'm going to use steel tube on top of cement tube footings. There's going to be white lattice around the base and I'll use the space to store firewood. All the weight from the gazebo structure will rest directly on the steel posts. My question is about the sub floor. I was thinking of using C channel. Te flooring will be Tigerwood (to match my deck) and can span up to 24" between joists. This picture is kind of rough but it gives an idea how I was planning on laying out the c channels. The load this should see would be maybe 3000 lbs (12 people and the weight of decking) spread across the entire deck. My local steel supplier can answer basic questions about deflection but this is out of his comfort zone. Each side would be approx 6' and also would be c channel. This, of course, will be welded together. Only one channel will be a solid piece (it could be a S beam) but the rest would weld to it. I'm planning on welding a plate to the bottom of the beams where the joints are. So what size c channel would be needed?

 

[CurlyDave]

How far is the sub floor going to be above grade?

Most people would design the floor of the gazebo to the same criteria as a deck, which would be about 100 lb/square foot in most jurisdictions.

If it were a 14' square, it would have 196 sq feet of area, and I would design for a 20,000 lb load. This is a lot more than the 3000 lb load you are thinking about.

While I like the idea of the joist structure you show, it is not particularly easy to calculate. I would think long and hard about a more conventional framing plan with joists on 16" centers, which would be a lot easier for the average engineer to understand and calculate.

 

[s219]

Yeah, I have to agree -- what you're proposing would take Finite Element Analysis on a computer to figure loads and size the members. Try to simplify it if you can. And why are you building out of steel? Gazebos are easily made from PT lumber. Built a few dozen myself.

 

[EddieWalker]

Pretty design, but not very practical. With only a 14 foot span, I would build three beams and run your joists on top of the beams. This gives your joists a 7 foot span, which makes it very strong

 

[LD1]

I agree, you are over complicating the joists.

And even so, it is impossible for anyone to calculate that out because you did not say where the posts are going to go, and how many of them, etc.

I am "assuming" one post at each point of the octagon? But I dont like to make assumptions then give bad advise based off a wrong assumption

 

[Budweiser John]

I don't see any advantage to steel framing in this case. Conventional wood framing is cheaper and easier to attach the decking to. The finished flooring pattern will drive the joist lay out and setting the assembly on concrete piers is superior to treated wood posts set in the ground.
To further preserve the framing assembly I always treat cut ends by brushing on "Coppertox" before assembling. Cutting strips of roofing ice and water shield or using a window wrap such as "Vicore" placed on top of the joists provides a seal between the decking and joist adding years to life expectancy of the structure.
Ramblings from years of building new decks and rescuing old structures B/4 potential collapse.
B John

 

[NorEaster]

The reason joists are run that way in a gazebo is to allow the decking to be placed in a pattern, picture it as 8 triangles, whose points are at the center.

 

[BHD]

put a support in the center of the floor joist and your problems are solved, the max span then is 7 foot,

for the floor, just run the joist across the unit at the desired spacing and do not worry about the fancy web type construction, much like this picture, http://www.diyonline.com/graphics/gaz-fig6.gif

unless your desire if for a fancy pattern in the floor planks,

I my self is cheap enough I would most likely pour a concrete deck, (possibly stained or colored and stamped, or rolled), made one of these a number of years ago, does a nice job, Noble Coat on Quick Imprint Rolled Concrete | Quick Imprint Systems LLC

A center support would solve 98% of your worries if you used any kind of joist system, and the cost would be considerable less as lighter materials could be used,

 

[Mysfyt]

You are way over thinking this. Spiderman would be proud of your design I'm sure!



Something like this is all you need if you really want to make it out of channel. I'm not an engineer, just a guy who works with this material every day. Good luck!

 

[LD1]

The reason joists are run that way in a gazebo is to allow the decking to be placed in a pattern, picture it as 8 triangles, whose points are at the center.

View attachment 439690

That makes sense why you want the fancy joists now.

I may try to do some load figuring.

However, as nice as that design looks, I dont like it for practical reasons. When I build a deck, I try not to have ANYWHERE on the deck where 2 boards are butting up. Cause over time, as things change, they never seem to line up and cause trip hazards. I like full length deck boards.

IE: IF I was doing a full length of the house deck, like say an 8'x40' or a 12' x 40', my joists run the long span, and deck boards will run the short span as to not have any butt ends meeting in the middle. Your design is going to create ALOT of that.

 

[farmer2009]

Could you not run a simple design joist pattern with the steel. Then come back over it with say 2x4's and make your nailers to hold your floor pattern. I say 2x4's based off a 16" joist spacing. Then add steel blocks where you have lots of joints to handle that stress.

I would have to agree with LD1 though. Lots of trip hazards.

 

[LD1]

To do the joist pattern like you have designed, without a post in the middle, you are going to need a substantial carrier beam. Whatever one is the full 15.15' length.

And the design of all the other joists tying RIGHT into the CENTER of that beam, puts a point load right at midspan. A point load roughly 1/2 of the total load on the gazeebo deck.

I would not design for less than a 50psf live load and 10psf dead load.

162sq ft with 1/2 of that loading right on the midspan of the long beam @ 60PSF is 4860#. Lets just call it 5k.

So, you need a beam to span 15.15' and carry a load of 5k at midspan and not deflect more than 1/2" (L/360)

Minimum I see is a C9x13.4 channel. Could drop to a 8" with a C8x18.75 but gain more weight (and will cost more). And only way to get under 8" depth is with an Ibeam. S7x20 would work. But again, heavier and more $$$

I didnt figure any loading on the rest of the beams, as it isnt going to be too significant. But wanted to see if this is still what you are planning on doing.

 

[LD1]

Could you not run a simple design joist pattern with the steel. Then come back over it with say 2x4's and make your nailers to hold your floor pattern. I say 2x4's based off a 16" joist spacing. Then add steel blocks where you have lots of joints to handle that stress.

I would have to agree with LD1 though. Lots of trip hazards.

Or just a simple joist pattern with treated 2x10's. On 16" centers they are good for spanning almost 16' with a 50PSF load.

Then lay out that crazy pattern with 2x4's laid flat (like roof purlins) which sould be quite stout with 16" joist spacing.

 

[LD1]

If you wanted to do channel on 2' centers for the joists, like farmer2009 describes or like how I described with 2x10's

For steel channel it would require c5x6.7 at a minimum for the joists. If you want to do steel, this is how I would do it. Instead of a large beam in the middle, then 3 smaller ones on each side radiating out from the middle, then even smaller ones for the remainder of the bracing. Just seems way more complicated than it needs to be.

I know you said 8' up. What is going under it? is there a reason you want the fancy pattern on the under side?

 

[farmer2009]

The advantage of steel here would be the floor thickness. With steel you would be less than 8" total. With 2x10's you would be about 12". The disadvantage would be the building weight.

He said something about firewood storage underneath. So it seams like a one roof two purpose situation. Which I understand.

One thing to throw out to the OP. In 10-15 years are you going to want to climb steps to sit on the gazebo. Or is the wife going to be able to climb up to it while pregnant. I don't know your situation but might think about some more of those scenarios. What ever fits you.

 

[crazyal]

The reason joists are run that way in a gazebo is to allow the decking to be placed in a pattern, picture it as 8 triangles, whose points are at the center.

View attachment 439690

This is exactly what I had in mind and why the joist layout is "crazy".

So here's a picture of what I have now. The wife has never liked the woodshed. I've seen much worse looking ones but since we both want a gazebo I thought I could move the shed and put it there. My plan is to have a bridge from the corner of the deck to the gazebo and also a set of stairs coming down towards where I took this picture from. The idea is to use the space under the gazebo to store firewood wrapped with lattice to hide the wood but allow plenty of air flow. The firewood would already be seasoned, no green would be stored under it. The shed is 12' wide and 8' deep and the opening is about 7' high.

I found a picture on the net that may give a better idea of what I'm looking to do. When I get a chance I'll use one of the on-line deck designers and do a layout. If possible I would like the floor of the deck to be at the same height as the gazebo floor, one step up would be acceptable. Because I would like to store wood under it I would like at least 6' between the floor for the firewood (this could just be wood simply laid on the ground and the bottom of the joists. A center post isn't out of the question but I figured I would see just what size channel was needed without a center post first. Also the location of the outside posts doesn't have to be exactly at the edge. But the posts will be carrying the load of the gazebo's walls and roof so they should be close, say within a foot. Otherwise I think it would be adding even more calculations to the load mix. Part of the reason I like the idea is because it's complicated. I knew figuring out would be more work but that just makes it more rewarding when you look at it. Anyone can do simple.

 

[LD1]

but I figured I would see just what size channel was needed without a center post first.

A heavier 8" or lighter 9" channel is the minimum as per my post above.

If thats acceptable, go for it.

But if all you are gonna do is store wood under neath it, Id go more conventional then make your nailers to get that fancy floor

 

[farmer2009]

Those pictures help explain why you want what you do. I think it will look really nice. If that's all the bottom will be used for I wouldn't worry about the underneath and use the double layer to frame the floor and a center post.

With the bridge out to it you won't have to worry about access.

Keep pictures coming along the way.

 

[s219]

I built gazebos for many years (built in factory, sold alongside storage sheds) and we could do the octagonal pattern floor without any of the complexity shown in the OP's plan. Lots of ways to do it. Might want to research some plans for wood gazebos and see what pops up.

 

[crazyal]

Right now I'm just in the coming up with a plan on how I want to build it. Nothing is set. Since it's basically free standing I want the posts to be steel. Two of the 3 6x6 pressure treated posts are warped on my deck. It's tied to the house and a cement footing so it's not structurally a problem but doesn't look as nice as it could. Everything from the gazebo floor up will be wood with the pressure treated 4x4 posts having a vinyl cover like I did on the deck. I've also debating using 7" c channel with a pt 2x4 laying flat on top of it around the outside edge and 2x10 for joists and blocking as needed for the decking boards. I like the idea of making the octagon out of steel because I can lay it out exactly on my shop floor and weld it up and then place it on top of the posts.

Here's a basic layout of what it will look like. It's not exact and and it just show an octagon deck, not a gazebo but it at least gives an idea.