[scott brunsdon]

With my carport out the front about a quarter finished, I thought I should start another project.
I'm building an awning off my shed - I thought it was time to get all my steel stuff outside.
It's going to be open sided with a fibreglass sheet roof i.e. lightweight.
Given the span (3.5metres), for the rafters I should use 6 x 4 timber (pine/oregon).
I would rather use steel - smaller profile.
What angle would give me the same strength?
Not sure if there is some sort of table that gives comparisons?
Scott

 

[clive]

Where I live all houses are now steel framed, what you need are "C" purlins or "Z" purlins, both are strong, I personally prefer the "C" purlins as they are better to drill into. Their profile is that of a digital C so to my mind they are more rigid than the Z which is more like an L with a bit added. These are tec screwed down and we have a highest cyclonic rating here, plus they dont get termites.
I suggest you google steel building frames or steel purlins.
Clive

 

[enlpck]

Strength of wood: A few common cases are given in Machinery's, but you are best off going to a good lumber yard (construction supply, not home depot), as they will have the tables for both standard lumber and the engineered products that they stock.

See also the USDA web site for the wood handbook: http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/fplgtr113.htm
This has everything you could ever want to know about wood, including tables showing strength. A little calculation and there you go.

You can also find the info on standard steel profiles in machinery's, but for this, I'd second the steel purlins. CHeaper, lighter. Again, the supplier will have the data for the products they sell.

 

[Rojodiablo]

Get a span chart, for lumber. See what the static load is, and then match the load with a steel chart. If I were to guess, with an awning with no load or only a light load on it, I'd say 2x4 C channel in a .90 or greater thickness should be adequate, at 2ft. on center. Sorry, don't know exactly how many centimeters that is!!!

 

[DDA52]

My shop roof is 8"x 2.5" 14 ga c purlins on 4' centers and a 24' span on one side and a 21' on the other. The sheet is 24ga metal panels. We had three guys up on it and didn't come crashing down.:cool2: :laugh:

 

[MoonRise]

Well, it depends. It depends a lot on what the loading is, what the geometry is, and what the building codes may require.

3.5 m = ~ 12 ft, so general lumber span tables would say you need 2x12 #2 16" O-C or so for floor joists to span 12 feet. A flat roof rafter to span 12 ft could be 2x8 #2 24" O-C and meet code by me. You could squeak by with 2x6 #2 19.2" O-C for a flat roof rafter and still meet code by me.

I would say do NOT use angle iron for structural purposes. Its failure mode(s) can be sudden and catastrophic, mainly because just a little twist and the angle leg is no longer perpendicular to the load and when that happens .... FAILURE!

Steel isn't bothered by termites, but it is bothered by water. It rusts.

C-channel or c-purlins should be able to do what you want. Have the steel yard give you a handbook with the section properties, etc on steel and also have them check their dat charts for what you want. I don't have my handbook(s) handy for those light sections, but my guess (worth what you paid for it, this is not professional advise, objects in mirror may be bigger than they appear, flammable objects may be inflammable, etc, etc, etc) is 3x4 C-channel on 2 ft centers is in the ballpark of what you need.

 

[scott brunsdon]

Thanks guys. I'll check out those C purlins - steel is cheaper than timber over here.
I didn't realise that risk with angles. Good to know.
Scott

 

[scott brunsdon]

Hard to explain, but c purlins are tricky with the part completed structure.

The recommended timber size is 2" x 6".

I can't find what would give the equivalent strength over 3.3 metres in RHS? (You guys might call it 'rectangular tube'.)

Would 3" x 2" with a wall thickness of 3mm do the trick?

Scott

 

[phila.renewal]

You can figure out the loading and use that to size the steel (with the help of a steel supplier).

For rafters, the span is the horizontal distance between the support points (not the slope distance). I'm assuming you have the span listed correctly.

Typical roof design loads would be 10 lbs. per sq. ft. "dead" load (what it is assumed to weigh) plus 30 lbs. per sq. ft. live load for a total design load of 40 lbs. per sq. ft. Under those design load conditions, typical building codes for residential construction in the states would allow 2x6 rafters (any commonly available lumber species, grade #1 or better) 16" on center.

You would need to check the design load criteria where you are for wind loads and snow loads (if it snows where you are).

For structural steel framing, a true "c" channel is commonly used. In the states, for the same design load conditions as described above, typical residential code requirements would call for a minimum of a 550S162-43 spaced 16" on center. Translating that, it's an 18 gauge true C channel (with lips that wrap around) at least 5 1/2" deep, 1 5/8" wide, with 1/2" lip.

Here, using anything else would require an engineer's stamp. Many large metal suppliers and lumber yards have engineers on staff that can spec. the steel you need. Many will even FAX their stamped engineering report right to you to show the local building inspector.

 

[scott brunsdon]

Thanks Phila,
Over here, lumber and steel suppliers are reluctant to give that sort of advice. None that I know of have engineers on staff.
While I can get load specs for timber and C channel, I can't get anything for Rectangular Hollow Section. The latter will work better for me than C channel.
2x6 is the right timber, but I'm hoping to use RHS for a smaller profile.
Scott

 

[phila.renewal]

Thanks Phila,
Over here, lumber and steel suppliers are reluctant to give that sort of advice. None that I know of have engineers on staff.
While I can get load specs for timber and C channel, I can't get anything for Rectangular Hollow Section. The latter will work better for me than C channel.
2x6 is the right timber, but I'm hoping to use RHS for a smaller profile.
Scott

Hmm, that's a tough one. I'm not an engineer so I can't really get much farther on this one. All I can say for sure is that if you use a minimum 18 gauge, 5 1/2" x 1 5/8" rectangular tube spaced 16" on center for your rafters, they will safely hold a 40psf load distributed above it. That's the only substitution I can make for the C channel. I don't know enough engineering at this point to make substitutions of smaller dimensions on the tubing for thicker tubes (and I guess that's why they make us hire the engineers). I can do it for wood using fiber bending strength and modulus of elasticity of any species of wood, but I never went so far as to learn it for light steel framing. I'll keep looking and if I find something I'll let you know.

 

[scott brunsdon]

Thanks Phila.
I'm trying to make this awning thing look as light as possible, so it's not a ponderous structure in the backyard.

 

[Sandy]

You ever get high winds? Snow? What are your expectations?

Ya know in the car port & awning realm there are several concessions based on the assumption that this structure (ha) is intended for private use only and occassional use only and that use will not occur during adverse conditions, ie., recreational and not considered real shelter.

Look at some of the so called car ports and you'll see some pretty light (thin) gage material put together with socket joints and screws.

Just some thoughts.

 

[scott brunsdon]

Hi Sandy,
Nope no snow.
No high winds.
I'm putting translucent sheeting on it so no dills will walk on it.
It's basically something off the side if my shed to get my steel (which is currently rusting) and welding bench under cover. And a place I can work out of the sun.
Scott

 

[Sandy]

I'm putting translucent sheeting on it so no dills will walk on it.
It's basically something off the side if my shed to get my steel (which is currently rusting) and welding bench under cover. And a place I can work out of the sun.

Yeh, you can see where I'm leaning with this. Not that I (ahem) recommend cheap cheesy construction, but you won't be taking up residence under this thing or sleeping under it or taking up refuge under there soooo.

Also keep in mind that while greater wall thickness in your desired box size does mean stronger per single stick and given load it doesn't necessarily mean less deflection (spelled sag). You might play with the thought of let's say 3"x2" by maybe .090 wall or even .072 wall but closer spacing. I dunno. I've got a couple of sticks of 1"x2" something thin wall out there in the shop. Tommorrow I will prop them up at 10 foot spacing just for giggles.

 

[scott brunsdon]

Yeh, you can see where I'm leaning with this. Not that I (ahem) recommend cheap cheesy construction, but you won't be taking up residence under this thing or sleeping under it or taking up refuge under there soooo.

Yeah, my partner here at work is an ex builder. I made the mistake of saying to him that I wanted to extend my shed, as opposed to bunging an awning on the side. If I had done what he said I should do I would have a structure stronger than my house. He always worked in timber, so isn't any good at steel.
Scott

 

[TEK]

He always worked in timber, so isn't any good at steel.
Scott

Yep, termites and metal manglers are different species, for sure!

 

[Sandy]

Went out to the shop. Took a section of 1"x2" by .072 wall box, put it on edge across two high stands spaced at 12'. Being a little porky I weigh in at about 190lbs. I sat on this affair in the center and measured the deflection at 2 inches. Each time it returned to it's original height off the floor.

 

[phila.renewal]

How many times can you do that before it fails?

How many gusts of wind will load it similarly and beyond over the course of a year or several?

Might last forever . . . or . . . might fail, blow away and hit somebody in the next big storm. If it was near me, I'd want to know which for sure.

Reminds me of a story. A neighbor tied a tarp on his roof and down the front of his house just before a big storm expected to generate gusts upwards of 50 to 60 mph. He attached the lower portion of the tarp to two concrete masonry units (a/k/a cinder blocks) at each corner. The tarp was about 20' square.

When I saw this disaster about to happen I asked him if he thought 10 pounds seemed like a good guess for how hard a 1 foot square piece of plywood would push on you if you held it out the window of a car moving at 50 or 60 mph. He said sure or maybe even 20 pounds or more. OK, lets go easy on him and say 10 pounds per square foot of wind load on a 400 square foot tarp = 4,000 lbs of force when the wind blows. The blocks would be blowing around the front of his house like wrecking balls at every gust.

He must have listened to some of this (but maybe not enough) 'cause he got rid of the blocks and tied each bottom corner of his tarp to a tree instead of forgetting the whole crazy tarp idea. The rope he used was rated at something like 2,200 pounds breaking strength. The next day following the storm, the tree was still there . . . but both anchoring ropes had snapped.

 

[scott brunsdon]

190 pounds? You are a big fella, Sandy. Now, if you could just keep bouncing for a year that would be great. Thanks for experimenting.

Phil, I take your point.

That guy with the cinder blocks may be related to me. When my shed was temporarily without a side wall, I had a had a plastic tarp there to keep the rain out. It was tied on the top edge, but to keep the bottom down I had a couple of big angle offcuts tied to short lengths of rope.
A big storm came through and I heard a couple of big bangs out the back. I went out and those big bits of angle were whipping around like kites as the sheet caught the wind. They punched half a dozen holes in the metal roof before I was able to catch them and cut the rope.

Scott