mark vernon wrote:
..... rectangular steel tubing of 1.5mm wall thickness,
....... 4 mil thickness ( I think that is 26 gauge?), ........ C parlins are 1.2mm wall thickness, ........The beams will be welded to rebar coming out of the concrete tie beam. ...... the same 2x4 steel tubing beam act as a central ridge Using 2 x 20ft lengths welded together
.......7 trusses, spaced at 4.5 feet apart.
..........Yes, I want an attic space at the end of the house (about 10 ft x 20ft across),
but that will not be hanging from the structure..3/4 inch plywood will sit on the concrete tie beam, with the same 2x4 tubular beams supporting it.........
The rebar is 12mm (1/2 inch), ........
The steel tubes will be welded to 2 rebars at each point in the wall, plus both rebar connections will be bent over the bar itself
I notice you refer to something as "4 mil" in industrial practice saying "4mil" 0.004" inches or 4 thousandths of an inch, one mil being a thousandths, as in a 4 mil polyethylene sheet. But I have run across some people who think 4 mil means 4mm which is 0.157 inches ! since you are using a mixed metric imperial system I suggest you be more precise in your specification.
mark vernon wrote:We tend to mix both metric and imperial here - I wish they would go one way or the other,.
mark vernon wrote:The steel tubes will be welded to 2 rebars at each point in the wall, plus both rebar connections will be bent over the bar itself
Please be very careful with this.
A. remove all galvanizing before making the weld.
B. Placing a round bar against a flat and welding it there is a very CRAPPY weld design. Yes I know that some people do it and get away with it but it is still a crappy design because where the round lays against the flat there is a steadily tapering crevice that makes the weld size very deceptive. I have seen some heavy steel come apart this way on a walking beam oil pump and kill somebody who was inspecting it.
C. In the case you are describing there is a severe mismatch in the thickness of the two pieces being welded whch means that the thin one will be destroyed or the real thickness of the weld is much thinner than it looks and if the thin piece is not burned through or away the thick piece did not get enough heat to make a good weld.
If this kind of geometry must be welded, (thin flat to thick round) it would be better to put a thick flat between the pieces and make two welds, E.g. in your case put a 1/4-3/8 in thick flat piece against the thin flat tube wall and weld with a small weld at the edges and end, then put the rebar against the flat piece and put in enough heat to melt well into the rebar.
Just sticking ther round against the flat side of your tube and tack welding it would be ok for a tack weld. So if you are bending the rebar around the tube in a way that would hold without the weld and then just tack welding it to prevnt slippage is probably ok.
mark vernon wrote:I realize I could use a rafter going across the entire width of the structure, and might have to go with that...but would be nice to still keep the cathedral style ceilings..
I have some concern for your trusses versus the walls.
As I understand your truss design there is no link between the bottom end of the rafters. when they are supported at the bottom ends of the rafters and a load is placed atop the roof (e.g. wind load or ice load etc) the bottom ends of the truss will tend to move apart. even a very slight movement could prove troublesome for the walls. Even a simple rather small tie rod between those ends would greatly reduce this. Nor would the tie rod have to be on every truss Such tie rods could prove useful for suspending a light fixture or ceiling fan. but if any significant weight is to be hung then the connection should be something sufficient to constitute a ceiling/floor joist
a single rebar dangling from the rafter above and attached to the tie rod could provide a very strong support for a point load and properly painted and done with an eye for appearance could look rather appropriate. If there is any HVAC unit to be put somewhere this might prove very practical.
My ice storm experience with 4 foot spans of well ribbed sheet metal might be cause to question if you even need purlins. In my case the concrete slab was poured with a 1/4 inch per foot slope for easy cleaning and in this climate that is quite sufficient to run the water off the end of the roof. by cutting the end of the pulins into tabs they could be butted against the rafters and attached with sheet metal screws. This might prove more stable. Also there are reasons to prefer "Z" purlins to "C" purlins. butted purlins would provide more support to the rafters for more stiffness in the direction of the ridge pole.
mark vernon wrote:After a bit of discussion, we will probably stick to a skillion shed roof, with a 2ft rise over 20 ft...enough for the rain to come off into the gutters at one side...10 x tubular steel beams at 1 meter spacing, and C parlins at 600 or 900mm spacing. I will keep the more complex stuff for my next project..
I think that there are pluses and minuses to such a change but I have not given that much thought so far.
Apologies for typo's I am being rushed.